BASIC THEORY OF IC ENGINES
UNIT – I
INTRODUCTION TO IC ENGINES
Internal Combustion Engine
In an internal combustion engine, the chemical energy of fuel is released as heat by way of
combustion inside the engine cylinder where power is produced. The heat thus produced is
supplied to a medium which is nothing but the products of combustion By expansion of this hot
medium inside the engine cylinder, heat energy is converted into mechanical work. All heat
engines operated directly by combustion gases are sometime called Internal combustion engines.
Examples :
1. Gasoline or Petrol engine (SI)
2. Gas engine (SI)
3. Diesel engine (CI)
4. Wankel engine (SI, CI)
5. Open cycle gas turbine
6. Jet engine
7. Rocket
External Combustion Engine
External combustion engines are steam engines and steam turbines. In these units, heat
energy is produced during combustion of fuel in a boiler furnace. This energy is used to generate
steam under pressure in the boiler. The steam expands in an engine or turbine and there by does
work. In this, case, power is produced in a unit other than the one where heat is generated.
Examples
1. Steam engine
2. Steam turbine
3. Stirling or hot air engine
4. Closed cycle gas turbine
Top, Bottom, Inner and Outer dead Centers
The top most position of the piston inside the cylinder ie when it is at the maximum
distance from the crankshaft axle is known as top dead center (TDC). The bottom most position of
the piston in the cylinder is called bottom dead center (BDC). These refer to a vertical engine. In
the case of a horizontal engine, the innermost position of the piston in the cylinder is called inner
dead center (IDC), and the outermost position is called outer dead center (ODC). At the two dead
center positions, the piston necessarily comes to a stand still before the direction of motion gets
reversed.
Comparison of Internal and External combustion engines
The temperature of the steam and of boiler surfaces must be kept far below that of the
furnace gases. If not, the metal parts of the boiler will melt. Because of this fact, only moderately
high temperatures can be obtained in the steam engine cylinder or steam turbine. In an internal
combustion engine, only a relatively small fraction of the heat energy in the cylinder gases is
transferred to the metal parts of the engine. The high gas temperatures and large temperature
drops available in an internal combustion engine make possible higher efficiencies in these
engines. As a unit for converting heat into work, the most efficient internal combustion engine is
more than twice as efficient as the most efficient steam power plant.
Components of SI engine
Spark Ignition engine is called as SI engine. The gasoline / petrol engines are widely used in
motor cycles and motor cars. In the four stroke engine, a piston moves up and down totally four
times executes four strokes in a cylinder, to complete the cycle of operations. The four stroke
engine is also called sometime as four cycle engine. The main components of the four stroke spark
ignition are piston assembly, connecting rod, cylinder, cylinder head, crankcase, crankshaft, inlet
and exhaust valve assembly, valve operating mechanism and ignition system, full system, cooling
system etc.
Internal combustion engines are widely used when compared to external combustion engine
An external combustion engine viz a steam power plant requires furnaces, boilers and
condensors. Absence of these cumbersome auxiliary apparatus and higher efficiency make an
internal combustion engine relatively light and compact for the required output, therefore IC
engine are widely used.
Significance of stroke bore ratio
The distance between the two dead centers is known as stroke or piston travel. The stroke
is usually measured in mm. The piston completes its stroke during one half revolution of the
crankshaft axle (ie. 1800 crank travel). The diameter of the cylinder is called bore, and is measured
in mm. The bore, stroke and stroke bore ratio decide the size of the engine. When the stroke is
equal to bore, the engine is called a square engine. An over square engine has a bore large than
the stroke, then the bore to stroke ratio will be greater than one. This is the most common engine
design. A large bore and short stroke allows for higher engine speeds as is needed in an
automotive engine. An under square engine has a stroke that is large than the bore. Then the bore
to stroke ratio will be less than one. Large industrial engines and tractor engines are sometime
under square because they operate at low rpm.
Clearance Volume, Stroke Volume and Cylinder Volume Swept Volume.
The space above the piston, when the piston is at TDC is known as clearance volume. The
volume above the piston, when the piston is at BDC is called volume. The volume swept through
by the piston, as it moves from one dead center to the other dead center is called stroke volume or
swept volume of the piston. The cylinder volume is the sum of the clearance volume and stroke
volume.
Compression ratio and its important
Compression ratio is the ratio of the cylinder volume to the clearance volume. This is
designated by the letter r. During the operation of an engine, the charge is sucked into the
cylinder. The charge is compressed into the clearance volume. Hence, the compression ratio of
the engine is the ratio of the volume occupied by the charge at the beginning of compression to the
volume occupied by the charge at the end of compression.
Compression ration r = (Vc +Vs) / Vc = 1+Vs/Vc.
Where Vs is stroke volume and Vc is clearance volume.
The compression ratio for different engines are as follows:
Spark ignition engines : 4-8, Gas engines 5-10.
Cubic Capacity of an Engine
The cubic capacity of an engine or engine displacement or engine size is the product of
stroke volume in one cylinder and the number of cylinders in the engine.
Working Cycle
For an engine to work continuously the cycle of operations, ie suction and compression of
charge, ignition and combustion of charge, expansion and exhausting of products of combustion
must be regularly repeated in the engine cylinder. A complete cycle of these operations is known
as working cycle of the engine. As the working fluid undergoes a cycle of operations, power is
produced inside the engine cylinder. The working cycle is repeated again and again and the
engine works continuously.
Relative Merits of Horizontal and vertical engine
An engine may be erected on its foundation or mounting, with the cylinder axis either
horizontally or vertically. With such orientation of the cylinder, the engine is called a horizontal
engine or vertical engine respectively. A horizontal engine occupies greater floor space but
requires less head room. A vertical engine requires less floor space and large head rook.
Stationary engines can be either horizontal or vertical type. Transportation engines are mostly
vertical orientation because of space consideration.
Torque and power of engine and its Important
Torque is the turning moment as indicated in figure. In the case of an engine it is usually
referred at the crankshaft of the engine, how much torque an engine can develop at different
speeds is an important factor. The torque refers to the average turning moment (or rotative
moment) exerted on the crankshaft by the gases acting on the piston during a cycle. Torque (T) is
measured in kgf.m. or Nm. The rotative force diagram for a four stroke, single cylinder, single
acting, slow speed engine can be seen in figure.
Figure: Torque or turning moment
Torque = Rotative force x crank radius
Power is defined as the time rate of doing work. Thus, power P = 2 NT. The horse power
(HP) and kilowatts (Kw) are units of power.
Torque is a measure of the ability of an engine to do work, while power is a measure of the
rate at which work is done. In a simplified form, we can say, torque determines whether an
engine fitted to a vehicle can drive the vehicle through sand or other obstacles, whereas the power
determines how quickly the vehicle progresses over the obstacles.
IHP, BHP and FHP and mechanical efficiency
Power output of an engine is referred in two ways: indicated power output and brake
output. Indicated power output called indicated horse power or IHP is the power developed
within the engine cylinder by the gases on undergoing the cycle of operations.
A portion of the indicated power is utilized in overcoming the friction between the moving
parts, sucking the charge and exhausting the combustion products, and in driving the engine
components (such as valve train, coolant pump, fan, dynamo, ignition system in a SI engine or
fuel injection system in a CI engine. The power used to perform these tasks is called friction horse
power. (FHP). Rest of the power only is available at the crankshaft to perform external work.
This shaft as shaft horse power (SHP) or merely delivered horse power or BHP. Sometime, the
BHP is stated as shaft horse power (SHP ) or merely delivered horse power. BHP = IHP – FHP.
Sometime, the indicated work per cycle is defined in two ways: (1) Gross indicated work
per cycle, this is the work delivered to the piston over the compression and expansion strokes
only. (2) Net indicated work per cycle, this is the work delivered to the piston over the entire
cycle. The ratio of BHP to IHP is called mechanical efficiency of the engine.
Indicator Diagrams
The indicator diagram shows the variation of pressure inside the engine cylinder during a
cycle of operations. It can be in two forms:
(i)
(ii)
Pressure-volume diagram (P-V diagram)
Pressure crankangle diagram (P - diagram)
PV diagram
Pressure –volume diagram indicates the cylinder gas pressure (shown vertically) against
the respective piston position i.e. cylinder volume (shown horizontally). The PV diagram for a
four stroke engine and a two stroke engine are slightly different. From the PV diagram, the
indicated mean effective pressure and hence the indicated work output can be calculated. The
indicator diagrams of a four stroke engine and a two stroke engine of spark ignition type can be
seen in figs respectively.
Pressure Crank Angle Diagram
Pressure – crank angle diagram indicates the cylinder gas pressure (shown vertically)
against the respective crankshaft rotation in degrees (shown horizontally). From this diagram, the
rate of pressure rise, dp /d, and the peak pressure can be determined. When the rate of pressure
rise is below a certain value, the engine will run smoother. The peak pressure decides the design
and durability of the engine. Pressure crank angle diagram for a petrol engine and a diesel engine
can be seen in figure
Figure: Pressure crank angle diagram for a petrol engine and a diesel engine
Engines are constructed with more number of cylinders
Industrial engines and automotive engines (except for very small power output) are of
multi cylinder type. This is because when greater number of cylinders are used, the power output
of the engine can be increased. This becomes possible without increasing its height and width but
with an increase in length. This results in reduced weight of the engine per horse power.
Even in the case of multi cylinder engines it is usually desirable to have a large number of
small cylinder rather than a small number of large cylinders. An engine having large number of
cylinders results in smoother and lesser variation in torque, compact engine, lower engine weight
and higher thermal efficiency.
Thermodynamic Air Standard Cycle
Different IC engines work on various thermodynamic cycles. In order to compare the
efficiencies of these thermodynamic cycles, it becomes necessary to eliminate the effect of the
calorific value ie heat value of the fuels used. To do this, air is assumed to the working medium
inside the engine cylinder.
Air is assumed to be heated during certain strokes by a hot body and then cooled during
certain other strokes by the action of the cold body, applied to the cylinder end. Thus the air in the
cylinder alternately absorbs and rejects heat during the cycle and the engine can be considered to
be working as a hot air engine. Throughout the cycle, the working medium (air) is assumed t
behave as a perfect gas. Further, the specific heats of air are considered as constant and no heat
exchange takes place between the working medium and the engine walls during compression and
expansion. These processes are assumed as adiabatic and reversible. A process is said to be
thermodynamically reversible if it can be reversed and can return the medium and all other
substances involved to their original condition existing before the process occurred.
The whole conception is theoretical only. The efficiency thus obtain is known as air
standard efficiency. It is sometime called ideal efficiency.
Otto cycle.
The first successful engine embodying the principle of BEAU DE ROCHAS was built in
1876 by NIKOLAUS A OTTO, a German, from which came the term Otto cycle. This is the basic
cycle for all engines working on spark ignition principle. The cycle is shown in figure.
In the air standard Otto cycle, air is compressed adiabatically and reversibly from 1 to 2.
Heat is added to the compressed air during the constant volume heating process from 2 to 3.
Adiabatic reversible expansion occurs from 3 to 4. The air is finally cooled from 4 to 1. This
process returns the air to the initial condition.
DIFFERENT TYPES OF CYLINDER ARRANGEMENT
Line diagrams of ‘U’ type cylinder arrangement.
‘U’ Type Engine: The ‘U’ type is a variation of opposed piston arrangement.
Line diagram of X type cylinder arrangements.
‘X’ Type Engine: The design is a variation of ‘V’ type. It has four banks of cylinders attached to a
single crankshaft.
Line diagram of Radial type engine.
Radial Engine: Radial engine is one where more than two cylinders in each row are equally
spaced around the crankshaft. The radial arrangement of cylinders is most commonly used in
conventional aircooled aircraft engines where 3,5,7 or 9 cylinders may be used in one bank and
two to four banks of cylinders may be used. The odd number of cylinders is employed from the
point of view of balancing. Pistons of all the cylinders are coupled to the same crankshaft.
‘H’ type cylinder engine.
The ‘H’ type is essentially two ‘opposed cylinder’ type utilizing two separate but interconnected
crankshafts.
‘H’ type Engine : The ‘H’ type is essentially two ‘opposed cylinder’ type utilizing two separate but
interconnected crankshafts.
Opposed cylinder engine diagram.
This engine has two cylinder banks located in the same lane on opposite sides of the crankshaft. It
can be visualized as two ‘in – line ‘ arrangements 180 degrees apart. It is inherently a well
balanced engine and has the advantages of a single crankshaft. This design is used in small
aircrafts.
Opposed piston engine diagram.
When a single cylinder houses two pistons, each of which driving a separate crankshaft, it
is called an opposed piston engine. The movement of the pistons is synchronized by coupling the
two crankshafts. Opposed piston arrangement, like opposed cylinder arrangement, is inherently
well balanced. Further, it has the advantage of requiring no cylinder head. By its inherent features,
this engine usually functions on the principle of two – stroke engines.
Delta type engine cylinder arrangement.
The delta type is essentially a combination of three opposed piston engine with three crankshafts
interlinked to one another.
Advantages of diesel engine over the petrol engine
Diesel engines are being widely used because of the following advantages they posses over
other power plants.
1. Better fuel economy
2. Lower emissions
3. Reduced maintenance
4. Greater reliability
5. Good torque characteristics
6. Easy to supercharge
7. Longer service life
8. Higher power per unit weight of the engine
9. Lower fire hazard.
10. High sustained torque.
Classification of CI engine according to the method of fuel injection
Method of fuel injection In the original (low speed) engines, fuel was injected into the engine
cylinder by a blast of high pressure (compressed) air. Hence these engines are called air injection
engines. Air injection equipment is too heavy and complicated for small bore, high speed engines.
These engines use various types of airless or mechanical injection systems. At present mechanical
injection system is used for all types of diesel engines of different sizes.
Classification of CI engine according to the speed of operation.
Speed of operation Diesel engines are classified according to their speed of operation as low
speed (upto 300 rpm), medium speed (from 300 to 1000 rpm) and high speed (above 1000 rpm)
engines. The speed factor influences the design and operation of engines, their maintenance and
life compared to petrol engines, the speed of operation of diesel engines are lesser.
PV diagram of Diesel cycle constant pressure cycle and their process.
In 1892, Diesel, a German proposed compression of air alone until a sufficiently high
temperature was attained to ignite the fuel which as to be injected at the end of the compression
process. The cycle proposed by him is called Diesel cycle. This is the basic cycle for the slow speed
compression ignition oil engines. The air standard diesel cycle is shown in fig.
Qin
PVK
=e
P
4
Qout
1
V
In this cycle, air is compressed adiabatically for 1 to 2. Heat is added to the compressed air
from 2 to 3, at constant pressure. Adiabatic reversible expansion occurs from 3 to 4. Heat is
rejected from 4 to 1. This process returns the air to the initial condition.
Four stroke SI/ gasoline / petrol engine
The four strokes of the engine are intake or suction stroke, compression stroke, expansion
or power stroke and exhaust stroke. Admission of the charge into the cylinder is controlled by the
intake valve, and exhausting of the products of combustion from the cylinder is controlled by the
exhaust valve. The opening and closing of these valves and the operation of the engine can be
seen in figure. Let us now see how these operations take place and the engine works.
Fig. OPERATION OF A FOUR STROKE PETROL ENGINE
Intake or suction stroke: when the piston is moving upward and is close to top dead center
(TDC), intake valve is opened by the valve operating mechanism. As the piston moves
downward, performing intake stroke, vacuum is created in the cylinder. Air fuel mixture is
sucked in.
Carburetor supplies the correct quantity and quality mixture according to the engine
requirements. The quality of the mixture is indicated by the term air fuel ratio (on weight basis).
Air fuel ratio for a petrol engine is usually 14 to 15. During suction stroke, work is done by the
entering charge on the piston. The downward moving piston after reaching bottom dead center
(BDC) begins to rise. At this point, the inlet valve is closed, and the exhaust valve is already in
the closed position.
Compression stroke: With both valves in the closed position, as the piston moves upward, air fuel
mixture gets compressed. The compression ratio ( ie volume before compression to volume after
compression) usually ranges from 6 to 8. Due to compression, temperature and pressure of the
mixture increase. During compression stroke, work is done by the piston on the working medium
in the cylinder. At several degrees before the end of this compression stroke, ignition system
causes a spark to jump across the gap between the spark plug electrode terminals. This spark
ignites the compressed hot air fuel mixture. The force of explosion rapidly increases the pressure
in the cylinder to 4 to 5 time what it was just before ignition.
Power stroke: as the piston crosses TDC, the high pressure gases tend to expand and thereby
push the piston downward. Work is now done by the working medium on the piston. Thus,
power stroke occurs. Now, the crankshaft is rotated by this power impulse. As the piston
approaches the end of the power stroke, the exhaust valve is opened.
Exhaust stroke: After the power stroke, piston rises up from BDC. The rising piston forces the
burnt mixture ie products of combustion out of the engine cylinder. This constitutes the exhaust
stroke. During exhaust stroke, work is done by the piston on the working medium (ie products of
combustion). As the piston reaches the end of the exhaust stroke, ie close to and before TDC, the
intake valve is opened. Fresh charge of air fuel mixture begins to enter into the engine cylinder.
During the following intake stroke, as the piston moves downward, air fuel mixture is drawn into
the engine cylinder.
The cycle of operations mentioned above, namely, suction, compression, expansion and
exhaust repeats again and again. As long as the cycle is repeated continuously, the engine
continues to work.
Engine Components and Basic Engine Nomenclature
Figure shows the cross-section of a single cylinder spark-ignition internal combustion
engine. The cylinder is supported in position by the cylinder block at the top end is covered by
cylinder head. In the cylinder a piston travels in reciprocating motion. The space enclosed
between the upper part of the cylinder and the top of the piston during the combustion process is
called the combustion chamber.
A mixture of air and fuel enters the cylinder through the carburettor in spark-ignition
engine via the inlet manifold i.e. the pipe which connects the Inlet port of the engine to the air
intake. In carburettor a throttle is provided to control the mass of mixture entering the
combustion chamber. In the cylinder head are inlet valves for taking the charge in the cylinder
and exhaust valves for discharging the products of combustion.
Figure: Cross-section of spark – ignition engine
A spark plug near the top of the cylinder initiates the combustion. The energy of the
expanding gas is transmitted by the piston (having piston rings to prevent leakage) through the
gudgeon pin to the connecting rod. The connecting rod and the crank arm of the crankshaft
translate the reciprocating motion of piston into rotational motion of the crankshaft. The
crankshaft is supported in bearings attached to the crankcase. The crankshaft is supported in
bearings attached to the crankcase. The crankshaft is supported in bearings attached to the
crankcase. The crankcase is the main body of the engine to which the cylinder is attached. The
products of combustion leave through exhaust port and exhaust manifold. Both the intake and
exhaust valves are operated by the valve mechanism. A camshaft is driven by the crankshaft
through timing gears. Lobed cams on the camshaft actuate the push rods and rocker arms for
opening the valves against the force of valve springs.
IC Engine Classification
The IC engine can be classified on the basis of cycle operation in cylinder, type of fuel,
method of supply of fuel, type of ignition, etc.
1. Basic engine design: Reciprocating engines, rotary (Wankel) engines.
2. Working cycle: engines working on Otto cycle (spark-ignition or S.I. engines), and engines
working on diesel cycle (compression-ignition or C.I. engines).
3. Number of strokes : Four-stroke engines and two-stroke engines (both SI and CI engines).
4. Fuel: Gasoline (or petrol), compressed natural gas (CNG), liquefied petroleum gas (LPG),
diesel oil (light, diesel oil, LDO and high speed diesel oil, HSD), fuel oil, alcohols
(methanol, ethanol).
Dual fuel and multi-fuel engines have also been developed.
5. Fuel supply and mixture preparation.
(a) Carbureted types, fuel supplied through carburetor.
(b) Injection type:
(i) Fuel injected into inlet ports or inlet manifold.
(ii) Fuel injected into the cylinder just before ignition.
6.
7.
8.
9.
Method of ignition. In S.I. engines battery ignition or magneto ignition.
Method of cooling: Water cooled or air cooled.
Cylinder arrangement. Inline, V, radial, opposed.
Valve or port design and location: Overhead (I head), side valve (L head) valves; in two stroke
engines: cross scavenging, loop scavenging, uniflow scavenging.
10. Application: Automotive engines for land transport, marine engines for propulsion of ships,
aircraft engines for aircraft propulsion, industrial engines, prime movers for electrical
generators.
Fig. IC Engine Classification
Basic type of cylinder arrangements
1. In-line engines. In – line engine is an engine with one cylinder bank, i.e., all cylinders are
arranged linearly, and transmit power to a single crankshaft. This type is very popular with
automobiles where 4 and 6 cylinder in-line engine are quite common.
2. ‘V’ engines. An engine with two cylinder banks (i.e., two in-line engines) inclined at an angle
to each other and with one crankshaft. Most of the bigger automobiles use the 8-cylinder Vengine (4-cylinder in-line on each side of the V).
3. Opposed cylinder engine. An engine with two cylinder banks located in the same plane on
opposite sides of the crankshaft. It can be visualized as two ‘in-line’ arrangements 180 degrees
apart. It is inherently well-balanced and has the advantages of a single crankshaft. This design
has been used in small aircrafts.
4. Opposed piston engine. When a single cylinder houses two pistons, each of which drives a
separate crankshaft, it is called an ‘opposed piston’ type of engine. Opposed piston
arrangement like opposed –cylinder arrangement is inherently well balanced. Further, it has
the advantage of requiring no cylinder head. In addition the relative piston velocity rate of
change of volume) is doubled for a given crank and piston speed. As shown, this arrangement
lends itself to cylinder porting and straight flow-through of gases for scavenging, where the
openings of the inlet and exhaust ports are controlled, by the position of pistons. In Junker’s
de sign both the pistons drive the same crankshaft, the father piston being connected to the
crankshaft by two side connecting rods.
5. Radial engine. Radial engine is an engine with more than two cylinders in each row equally
spaced round the crankshaft. The radial engine is most commonly used in conventional aircooled aircraft engines where three, five, seven, or nine cylinders may be used in one bank and
tow or three banks may be used. The radial engine presents the problem of fastening 3,4,7,or 9
connecting rods to a single crank. A master rod is guided by the crank and articulated rods are
6.
7.
8.
9.
attached to the master rod. It should be noted that the master rod executes the same motion as
the connecting rod in other conventional engines, while an articulated rod follows a slightly
different path since the point of attachment is not at the centre of the crankpin. Vertical shaft
radial engines are used in large stationary power plants with vertical shaft generators mounted
below. An odd number of cylinders per bank is necessary with alternate-cylinders firing in
successive revolutions for four-stroke cycle radial engines, but any number of cylinder can be
used for two-stroke engines. Besides the above important types of cylinder arrangements, the
other types which have been used are as follows:
‘X’ type. This design is a variation of ‘V’ type. It has four banks of cylinders attached to a
single crankshaft.
‘H’ type. The ‘H’ type is essentially tow ‘opposed cylinder’ types, utilizing two separate, but
interconnected crankshafts.
‘U’ type. The ‘U’ type is a variation of opposed piston arrangement
Delta type. The delta type is essentially three opposed pistons with three crankshafts.
Piston controlled port tow stroke SI engine:
The piston controlled port two stroke SI engine has three main moving parts, namely,
piston, connecting rod and crankshaft. The engine can be seen in figure.
Figure: Two stroke engine with piston controlled ports
Ports of different sizes are located in the cylinder wall at different levels and locations. The
intake port is located in the lower half of the cylinder opposite the exhaust port. The transfer port
is located at the bottom of the cylinder where the cylinder surface matches with the crankcase.
The exhaust port is located at a slightly higher level compared to the transfer port. These ports are
opened and closed by the moving piston. The functions of the various ports are as follows:
Intake port admits air fuel mixture, supplied by the carburetor into the airtight crankcase.
Transfer ports connect the crankcase and the cylinder bottom portion. Thus they provide
passages for the mixture to flow from the crankcase into the cylinder. These ports are also called
scavenge ports. Exhaust port permits the products of combustion to flow out from the cylinder
into the exhaust pipe and finally into the atmosphere. The inlet, exhaust and transfer ports can be
seen in figure
Figure: Inlet, exhaust and transfer ports in the cylinder
Now let us see how the various operations of a cycle take during one revolution of the
crankshaft, beginning with the piston at TDC.
The piston tends to descend from TDC, performing expansion or power stroke. At some
point of its downward travel, piston crown opens the exhaust port. The expanding combustion
products rush out through the exhaust port. This is called blow down. Simultaneously, the
downward movement of the piston compresses the air fuel mixture that was previously sucked
into the crankcase.
Further downward movement of the piston, causes the piston crown to open the transfer
ports. The mixture compressed to some extent and confined in the crankcase, nor rushes through
the transfer ports and fills the cylinder. During this operation, the entering mixture sweeps the
combustion products that remain in the cylinder. As such they push the combustion products out
through the exhaust port. This operation is called scavenging by crankcase charge or simply
crankcase scavenging.
The piston, after reaching BDC, begins to ascend. As it rises, its crown first closes the
transfer ports. From this instant onwards, the upward movement of the piston creates a partial
vacuum in the crankcase.
At the piston moves further upward, at some instant the bottom of the piston uncovers the
intake port. Since the upward movement of the piston has produced vacuum with in the
crankcase, air fuel mixture is sucked in the crankcase. As the piston continues to ascend, the
piston crown closes the transfer ports first and then the exhaust port. Now onwards, the upward
moving piston beings to compress the mixture which is above it and trapped in the cylinder.
When the piston approaches TDC, ignition of the mixture is caused by an electric spark.
Force of explosion pushes the piston downward. Piston once again descends to being the next
cycle of operations.
As illustrated ignition and combustion of air fuel mixture occurs every 3600 of crankshaft
rotation. Power is produced during each revolution.
Valve arrangements in SI engines.
The SI engines (as well as CI engines) may also be classified by valve location. The
classification of SI engines by valve location is shown in figure.
Figure: Classification of S.I. engines by valve location
The T-head design shown is now obsolete. The side valve, or L-head design was quite
popular up to 1960. The most popular design today is the overhead valve design, which is also
called I-head or valve – in – head engine. A combination of side valve design and overhead valve
design is occasionally made to give a F-head. Here the intake valve is located in the head (over
head) while the exhaust valve is located in the block (overhead).
Firing order for 3, 4 and 6 cylinder engines.
Firing order applies to multi cylinder engines. Firing order refers to the sequence in which
the charge in the various cylinders of a multi cylinder engine is burnt. In this order, the power
impulses are produced and act on the crankshaft.
In the case of an in line engine, the firing order is so chosen to fire the charge in the various
cylinders, as far as possible, at the alternate ends of the crankshaft. This enable the crankshaft to
be stressed more or less uniformly along its length during two revolutions of the crankshaft in a
four stroke multicylinder engine. Arranging the firing order as above reduces vibration and makes
the engine to run smoothly. The firing order used in four cylinder in line engines are 1-3-4.2 or 1-24-3. Two firing orders used in six cylinder in line engines are 1-5-3-6-2-4 or 1-4-2-6-3-5. The firing
order for 3 cylinder in line engine is 1,2,3.
In the case of V engines, the firing order is so chosen: (1) To alternate the firing between the
ends of the crankshaft and the cylinder block on each bank. (2) To distribute the forces around
over the engine. (3) To avoid concentrating subsequent explosions near one location of the
crankshaft.
Four stroke diesel engine
A four stroke diesel engine has intake, compression, power, and exhaust strokes for each
operating cycle. These four operations of the cycle are carried out in the four strokes of the piston.
Each stroke corresponds to the complete up and down movement of the piston. The resulting four
strokes are equal to two complete crankshaft revolutions or 720o crank travel.
Intake process Intake or suction stroke begins just before the piston reaches top head center
during its upward movement in the cylinder. The intake stroke begins at about 20-30 degrees
before top dead center (BTDC). At this point intake valve begins to open. As the inlet valve opens.
Piston goes past TDC and begins to move downward in the cylinder, low pressure is crated inside
the cylinder during suction stroke. Work is done by the entering air on the piston. Intake
operation ends shortly after the piston reaches the bottom of its travel (ie BDC) and begins to
move up in the cylinder. In terms of crankshaft rotation, the intake process ends at about 30 to 40
degrees after bottom dead center (ABCD). At this point, intake valves closes.
Compression, fuel injection and combustion processes Compression stroke begins once the
intake valve closes and thus seals off the cylinder space. Compression of the entrapped air
continues for the next 150 degrees or so of the crankshaft rotation, until the piston reaches TDC.
Depending on the compression ratio, the volume of air in the cylinder is reduced to the extent of
14 to 24 times. During compression stroke, work is done by the piston on the air trapped inside
the engine cylinder.
Air drawn in during the intake stroke is squeezed tighter and tighter. This forces the
vibrating molecules of air to come closer and closer. Due to high compression, temperature of air
in the combustion chamber goes up. By the time the piston has reached TDC, air temperature rises
to 700-900oC. At the same time air pressure also goes up to 35-55 Kscm.
Fuel is injected late in the compression stroke. High pressure fuel injector opens up at about
20 to 30 degrees BTDC and continues to spray the fuel into the combustion chamber until TDC or
a little longer. This gives the fuel sufficient time to vapourize and start burning. The compressed
air is so hot that it ignites the fuel without the need for a spark. As such diesel engines are
sometime called self ignited engines. Glow plugs and other heating devices are sometime used to
help starting an engine in cold weather.
Expansion process Expansion or power stroke begins after TDC when the piston is being actively
pushed down in the cylinder by the hot high pressure gases. Power stroke begins after all (or
nearly all) of the fuel is burnt close to TDC.
Piston is pushed done in the cylinder by the expanding gases produced by combustion.
Nearly constant pressure is crated on the top of the piston until about 60-70 degrees after TDC.
This is the point at which the orientation of the piston, connecting rod and crankshaft gives the
greatest mechanical advantage and hence the gases exert the maximum force on the crankshaft.
During expansion stroke, work is done by the hot products of combustion on the piston.
Exhaust process Exhaust stroke occurs as the piston moves from BDC to TDC. The exhaust valve
begins to open before the end of the power stroke ie before BDC. After the piston moves past BDC
and as the piston moves up in the cylinder, the combustion products are pushed out through the
exhaust port. During exhaust stroke, work is done by the piston on the products of combustion in
expelling the same from the cylinder.
It should be remembered that during a stroke of the piston, the speed of the piston in the
cylinder is not constant. It accelerates from rest at one end of the cylinder until it reaches a certain
speed and then it decelerates back to rest at the other end of the cylinder. Therefore, as the piston
approaches the end of the exhaust stroke, it is slowing down. This causes the pressure in the
clearance space to slightly less than the pressure of the outside atmosphere. This in turn results in
the entry of air.
The work done during a cycle is equal to the work done by the gases on the piston during
expansion and suction strokes minus the work done by the piston on air during compression and
the work done by the piston on the products of combustion during exhaust stroke.
Fig. Operation of a four stroke Diesel engine
Valve Timing Diagram of Four stroke SI engine
Actual valve timing of four stroke petrol engine: Valve timing is the regulation of the points in
the cycle at which the valves are set to open and close. As described above in the ideal cycle inlet
and exhaust valves open and close at dead centres, but in actual cycles they open or close before or
after dead centres as explained below.
There are two factors, one mechanical and other dynamic, for the actual valve timing to be
different from the theoretical valve timing.
(a). Mechanical factor. The poppet valves of the reciprocating engines are opened and closed by
cam mechanisms. The clearance between cam, tappet, and valve must be slowly taken up and
valve slowly lifted, at first, if noise and wear is to be avoided. For the same reasons the valve
cannot be closed abruptly, else it will ‘bounce’ on its seat. (Also the cam contours should be so
designed as to produce gradual and smooth changes in directional acceleration). Thus the valve
opening and closing periods are spread over a considerable number of crankshaft degrees. As a
result, the opening of the valve must commence ahead of the time at which it is fully opened (i.e.
before dead centres). The same reasoning applies for the closing time and the valves must close
after the dead centres. Figure shows the actual valve timing diagram of a four –stroke engine in
relation to its pressure-volume diagram.
(b) Dynamic factor. Besides mechanical factor of opening and closing of valves, the actual valve
timing is set taking into consideration the dynamic effects of gas flow.
Intake valve timing: Intake valve timing has a bearing on the actual quantity of air sucked during
the suction stroke i.e. it affects the volumetric efficiency. Figure shows the intake valve timing
diagram for both low speed and high speed SI engines. It is seen that for both low speed and high
speed engine the intake valve opens 100 before the arrival of the piston at TDC on the exhaust
stroke. This is to insure that the valve will be fully open and the fresh charge starting to flow into
the cylinder as soon as possible after TDC.
Suction
Figure: Four-stroke petrol engine valve timing diagram in relation to the pressure volume diagram.
At the piston moves out in the suction stroke, the fresh charge is drawn in through the
intake port and valve. When the piston reaches the BDC and starts to move in the compression
stroke, the inertia of the entering fresh charge tends to cause it to continue to move into the
cylinder. To take advantage of this, the intake valve is closed after TDC so that maximum air is
taken in. this is called ram effect. However, if the intake valve is to remain open for too long a
time beyond BDC, the up-moving piston on the compression stroke would tend to force some of
the charge, already in the cylinder, back into the intake manifold. The time the intake valve
should remain open after BDC is decided by the speed of the engine speed the charge speed is
low.
Figure: Valve timing for low and high speed four-stroke SI engine
And hence the intake valve should close relatively early after BDC for a slow speed engine
(say about 10 after BDC). In high speed engines the charge speed is high and consequently the
inertia is high and hence to induct maximum quantity of charge due to ram effect the intake valve
should close relatively late after BDC (up to 600 after BDC). For a variable speed engine the
chosen intake valve setting is a compromise between the best setting for low and high speeds.
Exhaust valve timing. The exhaust valve is set to open before BDC (say about 25 0 before BDC in
low speed engines and 55 before BDC in high speed engines). If the exhaust valve did not start to
open until BDC, the pressures in the cylinder would be considerably above atmospheric pressure
during the first portion of the exhaust stroke, increasing the work required to expel the exhaust
gases. But opening the exhaust valve earlier reduces the pressure near the end of the power stroke
and thus causes some loss of useful work on this stroke. However, the overall effect of opening
the valve prior to the time the piston reaches BDC results in overall gain in output.
Typical valve timings for four-stroke SI engines
Actual
Position
Theoretical
Low speed
engine
High speed
engine
Inlet valve opens (IVO)
Inlet valve closes (IVC)
Inlet valve is open for
TDC
BDC
1800
100 b TDC
100 a BDC
2000
100 b TDC
600 a BDC
2500
Exhaust valve opens
Exhaust valve closes
Exhaust valve is open for
BDC
TDC
1800
250b BDC
50 a TDC
2100
550 b BDC
200 a TDC
2550
Valve overlap
Spark
Nil
TDC
150
15 b TDC
300
30 b TDC
0
0
Note: Valve timing is different for different makes of engines
b
before
a
after
TDC top dead centre
BDC Bottom dead centre.
The closing time of exhaust valve effects the volumetric efficiency. By closing the exhaust
valve a few degrees after TDC (about 150 in case of low speed engines and 200 in case of high
speed engines) the inertia of the exhaust gases tends to scavenge the cylinder by carrying out a
greater mass of the gas left in the clearance volume. This results in increased volumetric
efficiency.
Note that there may be a period when both the intake and exhaust valves are open at the
same time. this is called valve over-lap ( say about 150 in low speed engine and 300 in high speed
engines). This overlap should not be excessive otherwise it will allow the burned gases to be
sucked into the intake manifold, or the fresh charge to escape through the exhaust valve.
Two stroke SI engine
Figure shows the simplest type of two – stroke engine – the crankcase scavenged engine.
Figure shows the typical valve timing diagram of a two-stroke engine. The air or charge is sucked
through spring – loaded inlet valve when the pressure in the crankcase reduces due to upward
motion of the piston during compression stroke.
Ignition
occurs
TDC
20
Figure: Crankcase-scavenged two –stroke engine
After the compression, ignition and expansion takes place in the usual way. During the
expansion stroke piston uncovers the exhaust ports, and the cylinder pressure drops to
atmospheric as the combustion products leave the cylinder. Further motion of the piston uncovers
transfer ports, permitting the slightly compressed air or mixture in the crankcase to enter the
engine cylinder. The top of the piston sometimes has a projection to deflect the fresh air to sweep
up to the top of the cylinder before flowing to the exhaust ports. This serves the double purpose
of scavenging the upper part of the cylinder of combustion products and preventing the fresh
charge from flowing directly to the exhaust ports. The same objective can be achieved without
piston deflector by proper shaping of the transfer port. During the upward motion of the piston
from bottom dead centre, the transfer ports and then the exhaust port close and compression of
the charge begins and the cycle is repeated.
Compare the SI and CI engine in all aspects in a Tabular form.
Table Comparison of SI and CI Engines
S.No
Description
1
Basic cycle
2
Fuel
3
Introduction
of fuel
4
Load control
5
Ignition
SI Engine
CI Engine
Works on Otto cycle or constant
volume heat addition cycle.
Gasoline, a highly volatile fuel. Self –
ignition temperature is high.
Works on Diesel cycle or constant
pressure heat addition cycle.
Diesel oil, a non – volatile fuel. Self
–
ignition
temperature
is
comparatively low.
Fuel is injected directly into the
combustion chamber at high
pressure at the end of the
compression stroke. A fuel pump
and injector are necessary.
The quantity of fuel is regulated.
Air quantity is not controlled.
Self-ignition occurs due to high
temperature of air because of the
high compression. Ignition system
A gaseous mixture of fuel air is
introduced during the suction stroke.
A carburetor and an ignition system
are necessary. Modern engines have
gasoline injection.
Throttle controls the quantity of fuelair mixture introduced.
Requires an ignition system with
spark plug in the combustion
chamber. Primary voltage is provided
6
by either a battery or a magneto.
and spark plug are not necessary.
Compression 6 to 10. Upper limit is fixed by 16 to 20. Upper limit is limited by
antiknock quality of the fuel.
weight increase of the engine.
ratio
7
Speed
8
Thermal
efficiency
9
Weight
Due to light weight and also due to
homogeneous combustion, they are
high speed engines.
Because of the lower C
R, the maximum value of thermal
efficiency that can be obtained is
lower.
Lighter due to lower peak pressures.
Due to heavy weight and also due
to heterogeneous combustion, they
are low speed engines.
Because of higher CR, the
maximum value of thermal
efficiency that can be obtained is
higher.
Heavier due to higher peak
pressures.
Compare and Tabulate the Four Stroke and Two Stroke engine in all aspects.
S.No
Four-Stroke Engine
1
The thermodynamic cycle is completed in four
strokes of the piston or in two revolutions of
the crankshaft. Thus, one power stroke is
obtained in every two revolutions of the
crankshaft.
2
Because of the above, turning moment is not
so uniform and hence a heavier flywheel is
needed.
3
Again, because of one power stroke for two
revolutions, power produced for same size of
engine is less, or for the same power the
engine is heavier and bulkier.
4
Because of one power stroke in two
revolutions lesser cooling and lubrication
requirements. Lower rate of wear and tear.
5
Four-stroke engines have valves and valve
actuating mechanisms for opening and closing
of the intake and exhaust valves.
6
Because of comparatively higher weight and
complicated valve mechanism, the initial cost
Two-Stroke Engine
The thermodynamic cycle is completed in
two strokes of the piston or in one
revolution of the crankshaft. Thus one
power stroke is obtained in each
revolution of the crankshaft.
Because of the above, turning moment is
more uniform and hence a lighter
flywheel can be used.
Because of one power stroke for every
revolution, power produced for same size
of engine is twice, or for the same power
the engine is lighter and more compact.
Because of one power stroke in one
revolution greater cooling and lubrication
requirements. Higher rate of wear and
tear.
Two-stroke engines have no valves but
only ports (some two-stroke engines are
fitted with conventional exhaust valve or
reed valve).
Because of light weight and simplicity
due to the absence of valve actuating
of the engine is more.
7
8
9
Volumetric efficiency is more due to more
time for induction.
Thermal efficiency is higher; part load
efficiency is better.
Used where efficiency is important, viz., in
cars, buses, trucks, tractors, industrial engines,
aeroplanes, power generation etc.
mechanism. Initial cost of the engine is
less.
Volumetric efficiency is low due to lesser
time for induction.
Thermal efficiency is lower; part load
efficiency is poor.
Used where low cost, compactness and
light weight are important, viz., in
mopeds, scooters, motorcycles, hand
sprayers etc.
UNIT – I
QUESTIONS
PART – A
1. What is an internal combustion engine? Give examples.
2. What is an External combustion engine? Give examples.
3. What are called Top Bottom inner and outer dead centers?
4. Write the comparison of internal and external combustion engines.
5. What is an SI engine? Mention the major components of an ordinary SI engine.
6. Why internal combustion engines are widely used compared to external combustion engine?
7. What is the significance of stroke bore ratio?
8. What are called clearance volume, stroke volume and cylinder volume swept volume.
9. What is compression ratio? How is it important?
10. What is the cubic capacity of an engine?
11. What is a working cycle?
12. What are the relative merits of Horizontal and vertical engine?
13. Define Torque and power of engine. Why they are important?
14. Explain briefly IHP, BHP and FHP and mechanical efficiency.
15. What is an indicator diagrams? How it is represented?
16. What is PV diagram?
17. What is pressure crank angle diagram? How it is represented?
18. Why engines are constructed with more number of cylinders?
19. What is a thermo dynamic air standard cycle?
20. Explain briefly otto cycle.
21. Draw the line diagrams of ‘U’ type cylinder arrangement.
22. Draw the line diagram of X type cylinder arrangements.
23. Draw the line diagram of Radial type engine.
24. Draw the ‘H’ type cylinder engine.
25. Draw the opposed cylinder engine diagram.
26. Draw the opposed piston engine diagram.
27. Draw the Delta type engine cylinder arrangement.
28. What are the advantages of diesel engine over the petrol engine?
29. Mention the classification of CI engine according to the method of fuel injection.
30. Mention the classification of CI engine according to the speed of operation
31. Draw the PV diagram of diesel cycle constant pressure cycle and mention the process.
PART – B
1. Briefly describe the operation of a four stroke SI/ gasoline / petrol engine with neat sketches.
2. With the help of neat sketches explain in detail about the construction and working at different
engine components?
3. Explain briefly the different classifications of IC Engines.
4. Draw and explain with neat sketches the different arrangement of engine cylinders and their
relative merits.
5. Briefly describe the operation of a piston controlled port two stroke SI engine with neat
sketches.
6. Draw and explain the different types of valve arrangements in SI engines.
7. What is meant by firing order? Why firing order is important? Mention the firing order for 3, 4
and 6 cylinder engines.
8. Explain the working of four strokes diesel engine.
9. Draw the valve timing diagram of a four stroke SI engine and explain.
10. Draw the port timing diagram of two stroke SI engine and explain
11. Compare the SI and CI engine in all aspects in a Tabular form.
12. Compare and Tabulate the Four Stroke and Two Stroke engine in all aspects.
UNIT – II
PERFORMANCE OF IC ENGINES
Some Terms Pertaining to the Engine:
Top Dead Centre (TDC) is the farthest point of forward travel of the piston in the cylinder.
Bottom Dead Centre (BDC) is the lowest point of backward travel of the piston in the cylinder.
Stroke (l) is the distance, between TDC and BDC, travelled by the piston in the cylinder. One
forward and backward strokes of the piston make one revolution of the crankshaft.
Bore (d) is the inside diameter of the cylinder.
Throw is the distance between the centre of crankshaft main bearing to the centre of the crank pin
or connected rod bearing. The thro is the half of the stroke length.
Clearance Volume (vc) is the volume of cylinder above the piston when the piston is at TDC.
Piston Displacement (dp) is the volume displaced by the piston between TDC and BDC.
Therefore, dp = (/r) /d2l, where d is cylinder diameter, and l is stroke length.
Total Volume (vt) is the volume of the cylinder above piston when the piston is at BDC.
Therefore, vt = vc + dp
Compression Ration (r) is the ratio of total volume of the cylinder to clearance volume, therefore,
r = vt / cc
Power and M.E.P:
(a) Indicated Power (i.p.) is the net power actually developed at the piston face during the events
of mechanical cycle. It is so named because it is determined by the use of an instrument called an
‘engine indicator’. It can be expressed as,
i. p 
pilAn
, for single cylinder engine, kW.
60,000
pilAn

 no. of cylinders, for multi - cylinders engine, kW
60,000
Where, l is length of stroke, m
A is cross – sectional area of cylinder, m2
n is number of power impulses / min or number of working strokes / min
pi is indicated mean effective pressure (i.m. e.p.), Pa.
(b) Indicated Mean Effective Pressure (i.m.e.p. or simply i.m.p.) is the algebraic sum of the mean
pressures acting on the piston during each stroke over one comple cycle. The pressures are
positive when acting in the direction of the piston movement and negative when opposite to the
direction of movement of piston. It is measured through indicator diagram drawn with the help
of an engine indicator. Thus,
i.m.e.p., pi 
Area of indicator diagram
N/m 2
Length of indicator diagram  Spring scale index'
(c) Brake Power (b.p) is the actual work output of an engine or the actual work available at the
crankshaft and is termed so because it can be obtained by absorbing (also be transmission
dynamometer) the power output by means of some form of brake. This is also termed shaft
power (s.p).
Figure illustrates the principle of rope – brake absorption type dynamometer. The b.p can
be expressed as,
Figure: Schematic of rope – brake dynamometer
b.p 
2 NT
, kW
60,000
Where N is rpm of crankshaft or engine
T is the torque or resisting torque in the dynamometer Nm
Resisting torque, T = (W – S) D/2, Nm
Where W = dead load applied, N
S = Spring tension, N
D = Diameter of flywheel, N
If there is no missing cycle, then,
n = N/2 for single acting, four – stroke engine
= N for single acting, two – stroke engine
= N for double acting, four – stroke engine
= 2N for double acting two – stroke engine.
The engines cannot be compared on the basis of their relative powers as power depends not
only on size but also on speed.
(d) Brake Mean Effective Pressure (b.m.e.p) is the m.e.p., which could have developed power
equivalent to the b.p. calculated above if acted on the piston. It is a comparative measure of the
powe capabilities of engine, which operate with the same engine displacement and speed, and
forms a basis for the index of performance.
b.m.e.p., pb=
60,000
, Pa
lAN
The b.m.e.p. unlike i.m.e.p. cannot be measured directly. Both b.m.e.p. and i.m.e.p. of a
petrol engine increase with the compression ratio up to the limit of compression fixed by the
detonating properties of the fuel used.
(e) Friction Power (f.p.) of an engine is less than its i.p. owing to frictional losses at the working
surfaces like bearings, piston rings and valves. The power lost in this way is known as friction
power. Thus,
f.p. = i.p. – b.p.
The i.p. of an engine can be determined by adding the f.p. to its b.p. This method is suitable
for high power engines where the f.p. is comparatively high and the measurement of i.p. by
engine indicator is not possible. The f.p. of the engine is obtained by motoring it by use of an
electric dynamometer operating as a motor. The f.p., thus, obtained includes both mechanical
frication and fluid friction (pumping losses of the exhaust and intake processes).
(f) Engine Torque. The torque and the b.m.e.p. of a given engine are linearly related. Since,
Therefore,
Where
pblAn
2 NT

 b.p., kW
60,000 60,000
lAn
T
pb  Cpb
2 N
lAn
C
is a constant for a given engine.
2 N
Thus, when brake torque and b.m.e.p. are plotted against rpm., the shapes of the resulting
curves should be similar. This is not a good index of performance as it depends on the size of the
engine. But for automobile engines, the torque is significant from driving viewpoint.
(g) Piston Speed. The piston speed of an engine is the total travel made by a piston in one minute,
i.e. piston = 2N  stroke, for double acting two – stroke engine.
Engine Efficiencies and Fuel Consumption:
(a) Theoretical Efficiencies: Air Standard Efficiencies (A.S.E.) is a function of the compression
ratio and method of combustion. Theoretical cycles are based on air as a working substance. The
efficiency of the cycle, which is the ratio of work output to the energy input as heat, is evaluated
for all the three cycles as follows:
(i) Otto Cycle.
A.S.E. = (q2 – 3 – q4 – 1) / q2 – 3
= 1 – (T4 – T1) / (T3 – T2)
There, q2 – 3 energy intake = Cv (T3 – T1)
q4 -1, energy rejection = Cv (T4 – T1)
and
Cv is the specific heat at constant volume.
 1
T2  T1  v1/ v2 
For air,
 T1 r -1
Where r is the volumetric compression ratio, v1/v2.
(ii) Diesel Cycle:
A.S.E. = 1 – q4 – 1/q2 – 3
= 1 – Cv (T4 – T1) / [Cp (T3 – T2)]
= 1 – (T4 – T1) / [ (T3 – T2)]
Now,
T2 = T1 r - 1
T3 = T2 ; where  is cut – off ratio = v3/v2
T4 = T3 / (r/) - 1
Substituting,
1
A.S.E. of diesel cycle = 1   
r 
 1
1     1
    1 
Air standard efficiency of diesel cycle depends on cutoff ratio, in addition to compression
ratio and decreases as the cut off ratio increases for fixed compression ratio. The quantity
(-1)/[(-1) is greater than unity for values  > 1. Therefore, the diesel cycle efficiency is less than
that of an Otto cycle with the same compression ratio. However, due to high compression ratio
diesel engine efficiency is practically higher than that of petrol engine running on the Otto cycle.
Diesel cycle efficiency reaches a minimum when cutoff ratio equals compression ratio.
(iii) Duel Cycle:
A.S.E. = 1 – q5 -1 (q2-3+q3-4)
=1 –(T5-T1) (T3-T2) +  (T4 –T3)
where, T2  T1r  1
T3  T2 ( p3 / p2 )  T1r  1 ; where  is the pressure or explosion ratio
T4  T3   T1r  1 
T5  T4 ( r /  )1  T1 
Substituting,
1 
A.S .E. of duel cycle - 1 -  
r 
 1


  1


   1      1 
The duel cycle efficiency lies between the efficiencies of Otto and diesel cycle. Both diesel
and duel cycle efficiencies decrease with increasing , but the duel cycle efficiency increased with
increasing .
The ideal efficiency is evaluated without considering the heat losses to the walls, but same
working substances as in the actual engines is taken into consideration. The variable specific heats
and the conditions of chemical equilibrium are also taken into account.
(b) Thermal Efficiency. The thermal efficiency forms a basis upon which the performance of all IC
engines is compared. It is the ratio of the useful work obtained to the heat supplied to engine.
The factors on which the thermal efficiency depends are,
(i) Compression Ratio. Thermal efficiency increases with increases in compression ratio,
(ii) Engine Speed. Thermal efficiency increases with the engine speed up to the most economical
speed. In modern car engines this speed is 20 to 30% less than the maximum power output speed.
(iii) Loads. At part loads thermal efficiency is less than when running at full load
(iv) Mixture Strength. Thermal efficiency depends upon mixture strength when speed, throttle
opening and other influencing factors remain constant.
(v) Nature of Fuel, i.e. fuel with low or high-octane value. It increases with increase in octane
number.
(vi) Temperature of Cylinder walls. The greatest thermal efficiency is obtained at a certain
minimum value of temperature and beyond this thermal efficiency decreases.
When the thermal efficiency of an engine is calculated on the basis of i.p, it is known as
indicated thermal efficiency and when it is based on b.p. it is termed brake thermal efficiency.
Indicated Thermal Efficiency , i
The indicated thermal efficiency shows what fraction of the heat supplied is converted into
indicated work.
Indicated work in heat units
60i. p
i.e.
i 

Energy supplied
 fH .V
Where, f is fuel supplied, kg/min
H.V. is heating value of fuel, KJ/kg
Brake Thermal Efficiency, b
The brake thermal efficiency indicates the fraction of the heat supplied that is transformed
engine shaft work.
Therefore,b 
Brake work done in heat units
60b. p

Energy supplied
 f H.V.
(c) Fuel Consumption. The total consumption of fuel by an engine under test conditions given
time is determined by measuring its volume or weights. The specific fuel consumption (s.f.c) is
defined as the total fuel consumption per hr per kW developed. In other words, s.f.c is the rate of
fuel consumption per kWh. When i.p. is used to calculate s.f.c , it is known as indicated specific
fuel consumption (i.s.f.c) and when b.p. is used, it is termed brake specific fuel consumption
(b.s.f.c). Thus,
60 f
3600

, kg / i.k Wh
i. p
H .V .i
60f
3600
b.s.f.c=

, kg / b.kWh
b.p
H .V .b
i.s. f .c 
(d) Mechanical Efficiency, m. The ratio of the power delivered by the engine (b.p.) to the total
power developed within the engine (i.p) is known as the mechanical efficiency. Thus
m 
b. p. 
b. p  b.m.e. p. b

 or

i. p  b. p  f . p  i.m.ep. i
The mechanical efficiency is indicative of the mechanical losses in a machine. It depends
upon the operating conditions especially speed, power output and lubrication.
The losses in a machine can be put into four main groups as follows:
(i)
(ii)
(iii)
(iv)
Friction losses in pistons, bearing, gears, valves and valve mechanisms. These losses
vary from 7 to 10% of i.p.
Ventilating action of the flywheel. This loss varies from 1 to 3 % of i.p
The work of charging absorbed during the suction and exhaust strokes in four stroke
engine or by scavenges pumps in two-stroke engines. These losses vary from 2 to 6 of
i.p
Power absorbed by different auxiliaries such as fuel pumps, lubricating pumps
circulating pumps, radiator fans, magneto and distributor drives and electricgenerators
vary from 1 to 9% of i.p.
Therefore, all these mechanical losses together vary from 11 to 28% of i.p, causing the variation
of mechanical efficiency from 72 to 89%.
(e) Volumetric Efficiency, v. It is defined as the ratio of the actual weights of air induct by the
engine in the intake stroke to the theoretical weight of air that should have been inside due to
piston displacement at intake temperature and pressure. Thus neglecting the present of fuel in
the mixture,
Actual air capacity
Ideal air capacity
Weight of air actually induced
=
Weight of air equivalent to the piston displacement
Volume of air aspirated at intake conditions
=
Swept volume
Volume of air aspirated at intake conditions per min
=
Stroke volume  no. of cylinders  n
 
As the volume of petrol present in the mixture is negligible in petrol engines, its present
may be over-looked without getting appreciable errors in . In case of gas engines, constitutes a
significant part of the volume of mixture supplied to the engine, hence its influence on  cannot
be overlooked.
Sometimes  is defined with reference to S.T.P conditions instead of intake conditions
Sometimes Therefore, =
Volume of mixture aspirated expressed at S.T.P conditions
Swept volume
The actual weight of air aspirated under maximum output conditions is always less the
theoretically possible weight due to following reasons.
(i)
(ii)
(iii)
Long and tortuous inlet passage and carburetor.
Insufficient inlet and exhaust valve area.
Excessive friction of the mixture due to passage through rough surfaces and sudden
changes in section of the inlet pipe and carburetor.
(iv) Premature heating of the mixture by induction manifold, valves and ports,
combustion chamber and cylinder walls, before inlet valve closes.
(v)
Heating of the residual exhaust gases in clearance volume.
(vi) Excessive back pressures due to exhaust gases. Cooling water temperature in
cylinder block passages.
(vii) Poor valve design causing insufficient valve lift.
(viii) Incorrect valve timing i.e. the opening and closing points are incorrect.
Volumetric efficiency is a measure of the breathing ability of an engine. In supercharged
engines, the volumetric efficiency is dependent upon the following factors.
(i)
Engine Speed. The volumetric efficiency, after attaining a maximum value at a
certain speed, falls with any further increase in speed. This generally happens n the
case of petrol engines.
(ii)
Compression Ratio. tends to fall with increase in compression ratio.
(iii) Mixture Strength, is a maximum for correct and slightly weak mixtures but is a
maximum for rich mixtures.
(iv) Temperature of Inlet Air,  decreases as the air temperature increases.
(v)
Temperature of Cooling Water,  increases slightly with a reduction in the
temperature of cooling water.
(f) Efficiency Ratio (or Relative Efficiency). The efficiency ratio indicates the degree by
which the actual thermal efficiency approaches the ideal cycle efficiency. Further, the
actual thermal efficiency may be either brake or indicated thermal efficiency. Therefore,
Indicated thermal efficiency
Air standard efficiency of the relevant cycle
Brake thermal efficiency
Brake relative efficiency =
Air standard efficiency of the relevant cycle
Indicated relative efficiency=
The efficiency ratio depends on the same factor as thermal efficiency. The indicated relative
efficiency attains values from 0.85 to 0.95 in present engines.
Engine Rating
Generally three methods are adopted to define the rated power of an automobile engine.
(a) Maximum load carried by the engine continuously. This load is indicated their the basis
of the mean effective preesure, kPa or by the piston displacement in mL per kW per sec.
However, both are identical. Automobile engines using petrol, the m.e.p. varies from 40
kPa. Every engine should be capable of withstanding overload of 10 to 20% in case of
emergency.
(b) Maximum power developed by the engine. In this case the engines are rated in terms of
their maximum capacity, i.e. maximum b.p. that can be developed.
(c) Using Conventional formula (R.A.C. Rating). For taxation purpose, the Royal Automobile
Club made certain assumption for finding out the b.p. for a four- stroke automobile
engines. The assumptions are
Piston speed = 1000 ft/min
m.e.p. = 90 psi
Mechanical efficiency = 75%
Using these values the nominal b.p of the engine is calculated by the formula:
b.p. = (d2n) /2.5 where, d is diameter of the cylinder, inches; and n is number of cylinder.
NOTE: The above definition of R.A.C rating is taken from British Standard. If SI is introduced , te
expression changes to:
Piston speed 305 m/min, m.e.p 620 kpa, and Mechanical efficiency 75%
Therefore , b.p. = 460 (d2n), where d in meter.
This b.p. which is much less than obtained in case (b) , represents the R.A.C. rating engine.
Morse Test
The i.p and the mechanical efficiency of a multi-cylinder auto engine is found out in a very
short time by this test. During the test the engine is run at a constant speed and at same throttle
opening. First the b.p of the engine with all cylinder operative is measured by the dynamometer.
Next, the b.p of the engine is measure with each cylinder rendered in one by one by shorting the
park plug in case of petrol engine or by cutting off the fuel in case of diesel engine.
When any cylinder is rendered inoperative, the speed abruptly goes down. Before taking
any reading, the initial speed must be restored by adjusting the load.
It is assumed that the f.p. of the inoperative cylinder remains the same as it were when
cylinder was operative. Considering the case of a 4- cylinder engine,
Let ,
B = b.p. of the engine with all cylinders operative.
B1= b.p. of the engine with cylinder no.1 inoperative.
B2 = b.p. of the engine with cylinder no.2 inoperative.
B3 = b.p. of the engine with cylinder no.3 inoperative
B4 = b.p. of the engine with cylinder no.4 inoperative
I1, I2, I3 and I4 = i.p of cylinder 1,2,3 and 4 respecitvely.
F1,F2, F3 and F4 =f.p of cylinders 1,2,3 and 4 respectively.
Therefore,
B = (I1-F1)+(I2-F2) +I3-F3) + (I4-F4)
= (I1+I2 +I3 +I3) – (F1 +F2+F3+F4)
When cylinder NO.1 is rendered inoperative, it does not develop any power, on the contrast some
power is lost due to movement of piston insider the cylinder. Then,,
1
=(I2+I3+I4) –(F1+F2+F3+F4)
From the above equation, B –B1 = I1
Similarly,
B –B2 = I2
B – B3 = I3
B –B4 = I4
Therefore, total i.p. of the engine = I1 + I2 +I3 +I4 =I (say) and mechanical efficiency = B/I
When the Morse test is carried out:
(i)
The b.p should be measured as soon as possible after making cylinder inoperative.
(ii) The dynamometer load should be adjusted soon to bring the speed to its constant value for
the test; otherwise the engine may race. In order to plot i.p b.p. and  a series of tests
should be conducted at predetermined engine speeds because b.p varies with load and
speed.
Performance Curves
The word performance for an engine is generally used for designating the relationship
between power, speed and fuel consumption. In variable speed engines (like automobile engines),
the rated power at a particular speed does not provide enough information. Under such
situations, the performance curves help to obtain necessary information.
Typical performance curves for Internal combustion engines (both petrol and diesel
engines) used in automobiles are shown in figure.
Figure(1) shows performance curves for high output, V-8 multiple carburetor 3 –dual throat
carburetors) automotive petrol engine with 7  10-3 m3 displacement.
Figure shows performance curves for typical automotive CI engine having 6-cylinder (100
mm 135mm) and compression ratio 15:1 that uses 50 cetane fuel.
Figure(1) : Typical performance curves of automotive SI engine.
Figure(2): Typical performance curves of automotive CI engine.
The two figures reveal that in diesel engines, fuel consumption per kWh is less and marked
so at the usual range of part-load operation. The torque for diesel engine remains fairly unifold
over a wider range of operating speeds than for petrol engine. This results in bettertop gear
performance, as the engine is more flexible over a wider speed range. Moreover, a high values of
the torque at lower engine speeds in diesel engine enables the vehicle to run much more slowly on
top gear. The CI engine has an appreciable higher thermal efficiency than the petrol engine
because (a) it has a higher compression ratio and (b) it uses higher air/fuel ratios in which order to
avoid incomplete combustion and smoky exhaust. On the other hand, the use of high
compression ratios in petrol engines creates the problem of combustion knock. In petrol engine,
the brake power curve takes a peak, but in CI engine it does not peak because the top speed is
limited to their heavier reciprocating masses. The friction power curve goes up rapidly at higher
speeds in both cases as it includes fluid friction.
It is common practice to give the automotive diesel engine three ratings; (a) the maximum
rating for short interval of operation (b) the rated output for larger period of operation of the
former and (c) the continuous output rating for operating with no time limit.
Factors Affecting the Engine Performance
The factors due to which the indicated power developed by actual engines differs from the
of ideal engines are as follows:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
The working media is not air but mixture of air and fuel in case of actual engines
The chemical composition of working media change during combustion
The process of combustion is never at constant volume or at constant pressure
The process of compression and expansion are not adiabatic.
The specific heats of gases of working media vary considerably with temperature.
The combustion may be incomplete.
The residual gases changes the composition, temperature an actual amount of fresh
charge.
(viii) The amount of fresh charge is decreased due to pumping losses.
Heat Transfer
The heat is exchanged in both directions between the gases and engine cylinder walls and
the other parts of the engine coming in contact with the gases. During combustion, expansion,
exhaust and the later part of the compression, heat transfer take place from the gases to the walls
and from the wall to the cooling water or ambient air. During suction and the earlier part of the
compression, heat transfer take lace from the walls to the gases. The heat lost to the walls during
latter part of compression is almost equal to the heat received by the gases from the walls during
early part of compression. The amount of heat lost during exhaust stroke is avoidable and
unavailable. The heat lost during combustion and expansion lowers the thermal efficiency of the
engine. The factors that affect the heat losses to the walls are as follows.
(i)
(ii)
Duration of combustion of the charge. This increases the heat loss
Temperature of combustion. This is turn depends upon the fuel, compression ratio
(iii)
(iv)
(v)
(vi)
and the load on the engine. The temperature increases with load and compression
ratio. This increases the thermal loss.
Speed of the engine. The increase of the engine speed decreases the duration of
combustion hence decreases the heat loss.
Shape of the combustion space. The increase in ratio of combustion chamber
surface to volume decreases the heat loss. However, turbulence and flame
propagation also effect the heat transfer to combustion chamber wall.
Size of the cylinder. The effect of cylinder size is rather complicated. An increase in
the cylinder size decreases the ratio of surface to volume but increases the frame
travel. This increases the combustion duration and hence engine speed is decreased.
Ignition timing in S.I. engines and fuel injection timing in C.I. engines. Proper
ignition and injection timings give rise in quicker combustion with les after burning
and hence less heat loss. The heat flow from the walls to fresh charge during suction
stroke increases the temperature of the charge and hence decreases the quantity of
charge. This decreases the power that the engine can develop.
Residual Gas
The residual gases left in the compression space from the previous cycle dilute the fresh
charge by increasing the amount of inert gases in it. This affect the ignition and combustions .
The residual gases also lower the volumetric efficiency of the suction stroke and raise the
temperature of the charge. Both these lower the amount of fresh charge induction.
Valve Resistance
In theoretical cycle of four-stroke engines, it is assumed that the exhaust and intake
pressure are equal to atmospheric. But the exhaust pressure is higher and the suction pressure is
lower than atmospheric pressure due to the resistance in exhaust and intake manifolds and valves.
The valve resistance affects the volumetric efficiency. The valve resistance causes the pumping
losses, which is the negative loop on the indicator diagram. The pumping losses increases with an
increase in speed. In two-stroke engines, the power consumption of scavenge and charging
pumps corresponds to the pumping losses in four-stroke engines.
Valve Timing.
In ideal cycle it is assumed that opening and closing of intake and exhaust valves take place
on dead centres. In actual case the exhaust valve closes and intake valve open approximately on
TDC, but the opening of the exhaust valve and closing of the intake valve vary considerably from
the BDC, depending principally on the desired speed. The net result due to derivations of valve
opening and closing other than at dead centres is that the indicator diagram is rounded at the
exhaust corner. This reduces the work output by 1 to 2%.
Combustion Time
In ideal cycle it is assumed tht the time of combustion is zero for constant-volume process
and combustion occurs at a rate necessary to maintain constant pressure during the constant
pressure process. Actually combustion process requires an appreciable amount of time, which
depends upon various factors. The increase in the combustion time decreases the ideal efficiency
by 2 to 3%.
Incomplete Combustion
A volumetric analysis of the constituents of the products of combustion indicates as
incomplete combustion that amounts to about 2% of the heating value of the fuel. Mixture with
excess air trends to reduce this loss to zero, on the other hand rich mixtures result in considerable
unburnt fuel due to oxygen deficiency.
Atmospheric Conditions.
The temperature of air, humidity of air and barometric pressure affect the air charge. The weight
of the air charge found to be inversely proportional to the square root of the temperature
especially in high-speed automobile engines.
For obtaining the performance at the standard conditions, the following correction on
pressure, temperature and humidity are to be adopted.
Pressure. The standard pressure is taken as 760 mm of Hg. Adopting correction on observed b.p.
 b. p c  b. p.
760

Where  is the pressure in the test house, mm of Hg
Temperature. The standard temperature is taken as 25C
273  t
273  t
 b. p.
273  25
298
Where t is temperature of the test house, C.
Thus,
(b.p.)c  b. p
Humidity. The correction for water vapour pressure present in atmosphere is to be made for
getting accurate results. The vapour pressure can be obtained by knowing the wet-bulb and dry –
bulb temperature and using psychometric chart.
If  is the vapour pressure in the test house in mm of Hg, then the corrected barometric
pressure of the test house is  -
Thus the formula with the above corrections becomes;
 b. p c  b. p
273  t
760

298
  
It can be seen that the effect of change of pressure is to increase or decrease the output
power as the level in the barometer rises or falls. The b.p. varies inversely as the absolute
temperature of the intake air.
Note: The units used in these expressions are the one actually used for the measurement the
parameters.
Energy Losses (Heat Balance)
Only a part of the energy supplied to the engine is transformed into useful work whereas
the rest is either wasted or utilized for heating purposes. The main part of the anutilized heat goes
to exhaust gases and to the cooling system. In order to draw a heat balance chart for an engine,
tests should be conducted to give the following information.
(i)
(ii)
(iii)
(iv)
(v)
Energy supplied to an engine which is known from the heating value of the fuel
consumed.
Heat converted to useful work
Heat carried away by cooling water.
Heat carried away by exhaust gases
Heat unaccounted for (radiation etc).
It is expected that that heat balance results of CI engine must differ from that of petrol
engine due to much higher compression and expansion ratios in the former. The higher
compression results in lower exhaust gas temperature and also lower flame temperature that in
turn causes lower heat loss to the cylinder walls in CI engines.
The utilization of the fuel’s heat energy is also higher in CI engines because of its higher
compression ration.
Although the actual value of heat utilization is dependent upon a number of factors like
compression ratio, engine load, fuel injection quantity, timing etc. some average figures for heat
balances for both the engines are given below.
Item
Heat converted to useful work (i.p)
Heat carried away by cooling water
Heat carried away by exhaust gases
Heat unaccounted for
Total (=energy supplied)
S.I.Engine
25 to 32%
33 to 30%
35 to 28%
7 to 10%
100%
C.I. Engine
36 to 45%
30 to 28%
29 to 20%
5 to 7%
100%
If the shaft work (b.p) is considered instead of useful work, the mechanical losses are to be
accounted for are generally included in the cooling water heat.
Example: 1. An eight-cylinder automobile engine of 85.7mm bore and 82.5mm stroke with
compression ratio of 7 is tested at 4000 r.p.m on a dynamometer which has a 0.5335 m arm.
During a 10 minutes test at a dynamometer scale beam reading of 400 N, 4.55 kg of gasoline for
which the heating value is 46,000 kJ/kg are burnt, and air at 294K and 10  104N/m2 is supplied
to the carburetor at the rate of 5.44 kg per min. Find (a) the b.p. delivered, (b) the b.m.e.p.(c) the
b.s.f.c (d) the specific air consumption , (e) the brake thermal efficiency, (f) the volumetric
efficiency, (g) the air-fuel ratio.
Solution:
2 NT 2  4000  400  0.5335

 89.34 kW
6000
60000
N
8pblA
2
(b) b.p=
60000
b.p.  60000
89.34  60000
b.m.e. p
p6=

 704.36kPa
2
4lAN
  85.7 
4  0.0825 
 4000
4 106
(a) b.p.=
(c) fuel consumed in one minute, f = 0.455kg.
Therefore, b.s.f.c=
f  60
b.p

0.455  60
 0.306kg / bkWh
89.34
(d) Air consumption in one minute, a = 5.44 kg
f  60 5.44  60
b.s.a.c =

 3.65kgb.kWh
b.p
89.34
(e)
Brake thermal efficiency
b 
b. p  60
89.34  60

 25.6%
 f H.V 0.455  46000
( f ) Piston displacement =

4
 0.0857   0.0825  4.76 104 m3 / cycle.
2
At 40000 r.p.m and four stroke eight-cylinder engine,
476 4000

 8m3 / min  7.62m3 , / min .
6
10
2
 RT 5.44  287.1 294
Volume of air used at intake conditions = 0

 4.56m3 / min
4
p
10 10
Piston displacement
4.56
100%  60%
7.62
( g ) Air : Fuel = 5.45L 0.455
Volumetric efficiency,  
A/F = 11.96
Example. 2. The following were noted for a 4-cylinder, 4-stroke engine:
Diameter = 101 mm
Stroke = 114 mm
Speed = 1600 r.p.m
Fuel Consumption = 0.204 kg/min
Heating value of fuel = 41800 kJ/kg
Difference in tension on either side of brake pulley = 378 N
Brake circumference = 3.35 m
Assume a mechanical efficiency = 83%
Calculate:
(a)
Brake thermal efficiency
(b)
Indicated thermal efficiency
(c)
Mean effective pressure of cylinder
(d)
Petrol consumption per b.k.Wh
Solution:
b. p 
(a)
2 NT  2 R  NW 3.35 1600  378


 33.77kW
60000
60000
60000
Brake thermal efficiency,
b. p.  60
33.77  60
b 

 23.7%
 fH .V . 0.204  41800
(b) Indicated thermal efficiency,
i 
b 0.237

100%  28.5%
m 0.83
N
2
i.p=
60000
2p lAN
b.p. i
60000
4pilA
(c )
Now, i.m.e.p.,
pi 
60000  b. p

2lANm
60000  33.77
2  0.114 
  0.101
4
2
1600  0.83
60000  33.77
 835.75 kPa
0.114 10.2 103  800  0.83
f  60
b.s.f.c =
b.p
0.204  60
=
 0.36 kg/b. kWh
33.77
QUESTIONS
=
(d)
UNIT – II
PART – A
1. Define the terms throw, piston displacement, and compression ratio?
2. What is the difference between (a) IP, BP, FP (b) IMEP and BMEP?
3. Show that the engine torque is linearly related to B.M.E.P.
4. Derive the expression for air standard efficiency for dual cycle. What are the differences
between Otto, diesel and dual cycle?
5. What are the factors that effect thermal efficiencies of the IC engines?
6. Define indicated thermal efficiency, brake thermal efficiency and mechanical efficiency?
7. Why actual air aspirated under maximum output condition is always less than theoretical
value?
8. On what factors, does the volumetric efficiency dependent on supercharged engine?
9. How is engine rating defined?
PART – B
1. How is Morse test uded to fine out i.p. and mechanical efficiency of an engine and under what
conditions?
2. Plot typical performance curve for diesel and petrol engines?
3. What are the factors that affect the performance of an engine?
4. Draw heat balance for petrol and diesel engine.
UNIT – III
CARBURETION
PART – A
Fuel feed system of an automobile
The fuel system of a car uses a fuel pump to lift fuel from the tank and to supply the same
to the carburetor or fuel injection system. Mechanically operated diaphragm type fuel pumps are
commonly employed.
Fuel strainer
Some of the fuel tanks include a fuel filter at their exit or a sump at the bottom, to collect
dirt and water and prevent them from reaching the pump and carburetor. Some tanks are
provided with a drain plug on the bottom for emptying the tank. Fuel tanks are often provided
with vertical baffles to prevent surging of the fuel.
An additional fuel strainer is usually placed in the fuel line either before the pump as
shown in the figure or after pump to filter gasoline before it enters the float chamber. The unit
serves to catch any water or foreign particles that were not filtered out previously, in the fuel tank
filter unit. One type of filter is shown in figure. The fuel that enters into the glass bowl, passes
through a ceramic filter (strainer) to reach the inside section that is connected with the exit. The
water and sediment are collected in the bowl which is made removable for cleaning.
Some engines use a disposable fuel filter in the fuel feed system. The entire unit may be
replaced whenever it becomes inoperative, simply by loosening snap clamps.
Carburetion
Carburetion is the process of measuring, mixing and supplying to a spark ignition engine
continuously a suitable combustible mixture of fuel and air. This mixture supply must be in
accordance of engine speed and load requirements. Carburetor supplies the mixture to an engine.
Carburetion stages
Carburetion involves the following stages:
1. Continuous measuring out of liquid fuel and air in correct proportions for combustion in
the cylinder.
2. Atomization or breaking up of the fuel jet into a very fine spray.
3. Intimate and uniform mixing of the fuel spray with the air that flows through the intake
system into the engine cylinder.
4. Supplying the necessary latent heat of vaporization to the fuel spray and air mixture. This
becomes essential to make a homogenous fuel vapour air mixture ready for ignition, after
compression in the engine cylinder.
Carburetor and their functions
Carburetor is a device fitted in the intake system, which does the above job. MAYBACK
discovered the carburetor principle. With a defined mode of drawing fuel out of a jet supplied by
a constant fuel level,
Functions of the carburetor
The carburetor has to do the following functions:
1. Meter the correct amount of fuel into the air stream, under all operating conditions.
2. Atomize the fuel jet into droplets.
3. Control the mixture (air and fuel) flow into the engine cylinder and thereby regulate engine
speed and power developed by the engine.
4. Control the vacuum operated devices such as distributor (spark) vacuum advance.
In practice, realizing the above objectives becomes difficult due to the following aspects:
1. The two fluids handled (air and fuel) are of different nature. Air is a mixture of gases. Fuel
is more or less a volatile liquid.
2. Mechanical characteristics of the engine like valve setting, intake arrangement etc. affect
mixture flow.
3. Load on the engine and speed of operation of the engine may vary very much. As such,
there is great and rapid fluctuations in the demand for fuel by the engine.
4. There is variation in the temperature and humidity of the atmosphere which supplies air to
the carburetor.
Air fuel ratio, theoretical air fuel ratio or chemically correct air fuel ratio or stochiometric air
fuel ratio
The air fuel ratio is the ratio of the weight of air and the weight of fuel supplied to an
engine cylinder, during each suction stroke or during a cycle of operations.
Theoretically, the air fuel ratio necessary for complete combustion of any given fuel
depends only upon the composition of the fuel. But practically it depends also upon how
thoroughly the air and fuel are mixed so that their particles can combine properly.
Theoretically, for complete combustion of 1 kg of fuel (gasoline) about 14.5 kg of air is
required. Thus, the air fuel ratio 14.5:1 is known as the theoretical air fuel ratio or chemically
correct air fuel ratio or stochiometric air fuel ratio.
Equivalence ratio, Rich mixture and lean mixture.
If the air fuel ratio, supplied to an engine is greater than the theoretical air fuel ratio, then
the mixture is said to be lean mixture. On the other hand, if the air fuel ratio supplied to an engine
is less than the theoretical air fuel ratio, then the mixture is called rich mixture.
Equivalence ratio – Sometime the mixture strength is indicated by the term equivalence
ratio as given below:
Equivalence ratio = Stoichiometric air fuel ratio / Actual air fuel ratio
When the mixture is rich, the equivalence ratio is greater than one. When the mixture is
lean, the equivalence ratio is less than one.
Mixture strength on engine performance
1. Too lean – Poor engine power. Missing, especially at cruising speeds. Burned valves and
burned pistons. Scored cylinders. Spark knock or ping.
2. Slightly lean – High gas kilometer coverage. Low exhaust emissions. Reduced engine power.
Slight tendency to ping or knock.
3. Stoichiometric – Best all round performance.
4. Slightly rich – Maximum engine power. Higher emissions, Higher fuel consumption, Lower
tendency to ping or knock.
5. Too rich – Poor gas kilometer coverage. Missing Increased air pollution. Fouled spark plug.
Oil contamination.
Types of Carburetors
Carburetors used in SI engines may be updraft, downdraft and side draft type. This
classification is based on the direction of air flow into the carburetor and air fuel mixture flow at
the carburetor outlet to the inlet manifold. Carburetors may also be classified into another group,
namely, constant choke carburetor and constant vacuum carburetor.
In the constant choke carburetor, the air and fuel flow areas are always maintained to be
constant. But the pressure difference or depression which causes the fuel flow and air flow are
being varied as per the demand on the engine. Solex and Zenith carburetors belong to this class.
In the constant vacuum carburetor, the air and fuel flow areas are being varied as per the
demand on the engine, while the depression or vacuum is maintained to be always constant. SU
and Carter carburetors belong to this class. Constant vacuum carburetor is also called a variable
choke carburetor.
Constant Choke Carburetor
In the constant choke carburetor, the air and fuel flow areas are always maintained to be
constant. But the pressure difference or depression which causes the flow of fuel and air are being
varied as per the demand on the engine. Solex and Zenith carburetors belong to this class.
Constant Vacuum or Variable Choke Carburetor
In the constant vacuum carburetor, (sometimes called variable choke carburetor) air and
fuel flow areas are being varied as per the demand on the engine, while the vacuum is maintained
to be always same. The S.U. and Carter carburetors belong to this class.
Deficiencies of a Simple carburetor
Some of the deficiencies of the simple, elementary carburetor are as follows:
1. At low loads, the mixture becomes leaner. But the engine requires the mixture to be
enriched at low loads.
2. At intermediate loads, the mixture equivalence ratio increases slightly as the airflow
increases. But the engine requires an almost constant equivalence ratio.
3. With wide open throttle, as the air flow approaches the maximum, the equivalence ratio
remains essentially constant. However, the equivalence ratio should increase to 1.1 or
greater to provide maximum engine power.
4. During starting and warm up, the mixture must be made richer. Elementary carburetor
cannot do this. The elementary carburetor cannot compensate for transient phenomena in
the intake manifold.
5. At higher altitudes, the air density decreases. The mixture will become richer. The
elementary carburetor cannot adjust the mixture strength to the proper value, according to
the changes in ambient air density.
Fig: Two Wheeler Carburetor
To overcome the above problems, some arrangements are incorporated in the carburetor
unit which aim to compensate the quality of the mixture at different loads.
Requirements of an automotive carburetor
The spark ignition engines fitted to automotive vehicles have to operate under variable
speed and load conditions. These engines present the most difficult and stringent requirements to
the carburetors. They are as follows:1.
2.
3.
4.
5.
6.
7.
8.
9.
Ease of starting the engine, particularly under low ambient conditions.
Ability to give full power quickly after starting the engine.
Equally good and smooth engine operation at various loads.
Good and quick acceleration of the engine.
Developing sufficient power at high engine speeds.
Simple and compact in construction.
Good fuel economy.
Absence of racing of the engine under idling conditions.
Ensuring full torque at low speeds.
The operation of a SI engine depends primarily on the quality (A/F ratio) and the quantity
of the air fuel mixture delivered to the engine cylinder. A good carburetor must produce the
desired air fuel mixture ratio and supply the mixture to the engine at all speeds and loads and
must do the same automatically.
Parts (or construction) of a carburetor
The simple carburetor can be seen in figure. The carburetor consists basically of a float
chamber and a metering cum mixing chamber. Let us see the details of the various parts.
1. Air horn or carburetor throat – A large opening that passes through the carburetor body. Air
flows through the air horn. Fuel is sprayed into this air mass by the carburetor.
2. Float chamber – Also called float bowl. A storage space for the fuel before it is being drawn
into the air horn.
3. Float assembly – Device designed to control the entry of fuel into the float chamber. This
mechanism consists of a hollow float, a hinge, needle valve and needle seat. As the float
rises up and down on the top of the fuel, operates the needle valve. The upward and
downward movements close and opens respectively the fuel passage to the float chamber.
The float device thus maintains the level of the fuel in the float chamber and hence in the
fuel jet as constant. This level is kept just below the exit of the fuel jet. By keeping the fuel
level as above, leakage or spilling of fuel, when the engine is not operating is prevented.
During cornering, acceleration or braking the vehicle, there is a tendency for the fuel to
surge in the float chamber, the fuel may temporarily drain away from the main metering
circuit. This will cause disruption of fuel supply to the engine. To minimize these effects, in
large sized carburetors, one or two parts of interconnected floats may be used to straddle
multiple mixing chambers. Alternately, a single float may be used and be centrally
disposed between multiple mixing chambers.
4. Bowl vent – An opening is provided in the float chamber. This hole prevents pressure or
vacuum from building up in the bowl. As the carburetor gets heated up, fuel vaporization
occurs in the float bowl. This may cause pressure building up. Venting the float chamber
prevents this pressure build up.
5. Main discharge pipe – This tube or opening connects the bottom of the float chamber with the
center of the air horn. It allows the fuel to move from the float chamber to the air horn
during engine operation and causes it to mix with the air rushing into the engine.
6. Main discharge jet – A small brass fitting, with a carefully sized hole called orifice is fitted
over the end of the main discharge tube. This orifice controls the quantity of fuel flow into
the main stream. The exit of the fuel jet is located at the throat of the venturi.
7. Venturi – A narrow portion of the air horn. The venture creates vacuum as the flowing air is
speeded up, stretched and swirled by the restriction.
8. Secondary or boost venturi – A smaller venturi or restriction, incorporated in some
carburetors in the middle of the primary venturi. It increases air speed, vacuum created and
hence the fuel flow.
9. Throttle valve – The throttle valve is a round disc mounted on a shaft. It is located between
the venture and the engine inlet of the induction system. Thus valve can be tilted to various
angles in the carburetor throttle valve body. The throttle valve is connected by a suitable
linkage to the accelerator pedal in the drivers cabin. Depressing the accelerator pedal,
opens the throttle valve and permits an increased amount of air fuel mixture to reach the
engine intake manifold.
Fuel supply by fuel jet in the carburetor
The fuel supply by the simple carburetor can be seen in figure. The suction stroke of an
engine reduces the pressure in the cylinder. This causes a pressure gradient, from outside
atmospheric pressure to the pressures that exist in the carburetor, inlet manifold and engine
cylinder. This pressure drop causes air to flow through the inlet system. The restricted passage in
the air horn i.e. venture increases air velocity and thus pressure drop at this point. The vacuum at
the throat of the venturi is called carburetor depression. Now the pressure at the venturi i.e. at the
fuel jet in the venturi is less than the pressure in the float bowl. Remember, the float bowl is
vented to atmosphere. As such the fuel in the float bowl is subjected to atmospheric pressure. The
pressure difference thus created causes the fuel to be ejected out through the fuel jet located at the
throat of the venturi.
The reduction in pressure must reach about 12 mm and only then the fuel will flow out
from the jet. This is because the level of the fuel is below the top of the jet when the engine is not
operating and the viscosity of the fuel tends to prevent the fuel flow from the jet.
The quantity of air flow through the venturi can be increased or decreased by increasing or
decreasing the extent of opening of the throttle valve. When the air quantity is increased,
carburetor depression also increases. This in turn increases fuel flow from the fuel jet.
Venturi arrangements
The narrow portion of the air horn is the venture. Different venturi arrangements and fluid
flow direction can be seen in figure. The venturi creates vacuum as the flowing air is speeded up,
stretched and swirled by the restriction, the fuel is sprayed into the air stream which flows
through the venturi.
Fig. Different venturi arrangements
Most passenger car carburetors are of downdraft type. The updraft type was used
considerably in the past because, when it was installed low at the side of the engine, it was well
adapted to the gravity fuel feed system then in use. This arrangement also reduces engine bonnet
height. The down draft type permits a manifold of a larger cross section because the fuel flows
down into the manifold instead of being lifted into it. The location of the carburetor above the
engine is more accessible for inspection, adjustment, or repair. The air entering the carburetor is
also cooler. Side draft carburetor is intermediate between these two.
SU Constant Vacuum Carburetor
The widely used SU carburetor with a variable jet device is shown in figure. A plunger or
cylinder integral with the air disc is rigidly attached to the piston rod. The piston rod slides in the
guide cylinder at the center of the housing. When the piston rod moves down to its lowest
position, it closes the air passage or air orifice to the throttle barrel. The air orifice or choke is the
rectangular opening of constant width and variable height formed below the plunger. The
depression in the throttle chamber is communicated through a hole and acts on the top portion of
the air disc. The atmospheric pressure is communicated through the vent hole and acts on the
bottom portion of the air disc. Because of the difference in pressures, the air disc rises from its
lowest position and thereby changes the height of the air orifice formed below the plunger.
It can be seen that the depression necessary to maintain the disc and the cylinder or plunger
valve from falling to their lowest position is determined only by the weight of the plunger, air disc
and piston rod assembly and the top and bottom surface areas of the air disc. As such, this
depression is a constant one.
The position which the moving members take up, and therefore the choke area, is such that
this area, when subject to the foregoing constant depression is capable of passing the charge
demanded by the engine. Remember the charge demanded by the engine is determined by the
combination of throttle (valve) opening and the rate of engine piston displacement.
A taper needle is rigidly attached to the bottom of the moving plunger. This needle goes
inside the fuel jet tube as shown in the figure. As the plunger moves up and down, varying fuel jet
is formed by the annular space between the jet tube and the taper needle. By this arrangement, as
the plunger moves up and down, the air flow area and the fuel flow are altered.
Fig. SU Constant Vacuum Carburetor
When the needle is ground to the proper profile, the jet area will be correctly related to the
choke (air orifice) area for all positions of the moving plunger and thereby ensures correct mixture
(both in quality and quantity) flow to the engine. It should be remembered that in general, the
profile of the needle will not be a straight taper, and any desired air fuel mixture relationship
(ratio) can be obtained by a suitable choice of the needle profile.
The clamping screw fixes the jet needle to the bottom of the plunger. The jet needle can be
rised or lowered to some extent in the hole within the plunger and clamped. This also helps to
vary the mixture strength.
Provision is also there for raising or lowering the jet tube relatively to the needle by means
of a lever (not shown in the figure). This lever can be operated from the dashboard. By this means,
the mixture can be weakened or enriched throughout the operating range. The jet tube is usually
lowered to ensure rich mixture supply which is required for cold starting.
Light springs are used above the disc in some horizontal models, to compensate for the
lighter weight of the aluminium alloy used for the air disc assembly.
In some designs the piston rod is made hollow, and the dash pot damper device is
incorporated within it. The damper device checks the rise of the air disc on sudden opening of the
throttle. This delay results in enrichment of the mixture strength as in a fixed choke carburetor.
Solex Carburetors
The solex carburetor is famous for its case of starting, good performance and reliability. It is
made in various models and is used in many automobile engines. The solex carburetor as shown
in figure is a downdraught carburetor. This has the provision for the supply of richer mixture
required for starting and weaker mixture during cruising the vehicle. It consists of various fuel
circuits such as starting, idling or low speed operation, normal running, acceleration, etc.
Figure gives the line sketch of a solex carburetor. It incorporates a device called bistarter
which is unique for this carburetor. This device is very useful for cold starting of the engine. The
various components and the circuits for air and fuel are explained below for various ranges of
operation.
Normal Running: A float (1) with a tapered needle valve at the top face of the float is fixed in
the float chamber. This tapered valve takes care of the level of fuel in the float chamber. The main
metering jet (2) supplies fuel and the air comes through the venturi (3). The fuel from the main jet
goes into the well of the air-bleed emulsion system. The emulsion tube has lateral holes (4) as
shown in the figure. Air correction jet (5), calibrates the air entering through it and ensures
automatically the correct-balance of air and fuel. The metered emulsion of fuel and air is supplied
through the spraying orifice or nozzles (6). These nozzles are drilled horizontally in the vertical
stand pipe in the middle of the choke tube or the venturi. The conventional butterfly valve throttle
valve is shown by (7).
Cold Starting and Warming: The uniqueness of solex carburetor is the incorporation of a BiStarter or a progressive starter. The starter valve is in the form of a flat disc (8) with holes of
different sizes. These holes connect the starter gasoline jet (9) and, starter air jet sides, to the
passage which opens into a hole just below the throttle valve at (10). Smaller or bigger size holes
come opposite the passage depending upon the position of the starter lever (11). The starter lever
is operated by flexible cable from the dash board control. Initially, for starting richer mixture is
required and after the engine starts, the mixture has to be progressively leaned. In the start
position bigger holes are in operation. The throttle valve being in the closed position, whole of the
engine suction is applied to the starting passage (11), inducting gasoline from jet 9) and air from
jet (10). The jets and passages are so shaped that the mixture provided to the carburetor I rich
enough for starting.
Idling and Slow Running: From the well of the emulsion system a hole leads to the pilot jet
(13). During idling, the throttle is practically closed and therefore the suction created by the engine
on suction stroke gets communicated to the pilot jet(13). Fuel is inducted from there and mixed
with little quantity of air coming from the small pilot air-bleed orifice (14). This form an emulsion
which is sent down the vertical tube to below the throttle valve, but through the idling volume
control screw (15). The idle running adjustment is done by the idle adjustment screw (15). The
idling speed can be thus varied and set to a desired value.
In order to change over smoothly from the idle and low speed operation to the main jet
operation without a flat spot, there is a by-pass orifice (17) on the venturi side of the throttle valve.
As the throttle is opened, the suction at idle port (16) is reduced. But the suction pressure is
exerted at a slow speed opening (17). This off sets the reduction of suction at the idle port (16).
Thus flat spot is averted.
Acceleration: In order to avert flat spot during acceleration a diaphragm type acceleration
pump is incorporated. This pump supplies extra fuel needed for acceleration through pump
injector (18). Pump lever (18) is connected to the accelerator. When the pedal is pressed by foot the
lever moves towards left and presses the pump diaphragm towards left. This forces the gasoline
through pump jet (20) and injector (18). On releasing the pressure on the pedal, the lever moves
the diaphragm back towards right and in so doing, creates vacuum towards left. The vacuum so
created opens the pump inlet valve (21), and gasoline from float chamber enters the pump.
Compensating devices in a carburetor
In the case of the simple carburetor, there is a tendency to increase the richness of the
charge with increase of load. This is because of the reduction in air density as the vacuum in the
inlet system increases (as load increases i.e. airflow increases). Various compensating devices are
used for maintaining more or less a constant air fuel ratio at different loads.
1. An auxiliary air valve.
2. A compensating jet (a) unrestricted air bled jet and (b) restricted air bled jet.
3. Tapered metering pin in the fuel jet orifice.
4. Combination of variable air passage and tapered metering pin.
Auxiliary air valve compensation and air bled jet compensation arrangements can be seen in
figure.
In the auxiliary air valve carburetor, there is a spring loaded air valve. As vacuum increases
at the throat of the venturi, air valve opens and permits entry of air, proportionately. The resulting
mixture is thus a compensated one.
Another compensating system incorporates an unrestricted air bled jet or a restricted air
bled jet. In these systems, petrol is drawn through two orifices, ultimately mixing in the inlet tract
with the air stream.
In this system there is a main jet and a compensating jet. The compensating jet delivery
tube surrounds the main jet delivery tube. The main jet is fed directly from the float chamber in
the usual way. As suction increases, this jet gives an increasingly rich mixture. The compensating
orifice discharges into the intermediate well which is in direct communication with the
atmosphere.
When the petrol in the well is level with that in the float chamber, there is no flow through
the compensating orifice. This is because the petrol in the well and in the float chamber are subject
to the some pressure (atmosphere). As the throttle is opened, the resulting depression in the choke
(venturi) causes petrol to flow from the well through the compensating jet delivery tube. Because
of this the petrol level in the well falls. The rate of fuel flow through the compensating orifice
depends upon its size and the difference of level of petrol in the well below that in the float
chamber.
As the throttle valve is opened, the well gets emptied at quire a small depression. The flow
through the compensating orifice reaches a maximum. Thereafter the flow remains at this
maximum value itself, no matter how great the depression at the choke may become. This is
because the pressure in the well as well as in the float chamber are atmospheric.
Further increase in depression in the choke will, however increase the airflow (and not fuel
flow). This makes the strength of the mixture effected by the compensating jet to be leaner. The
strength of the mixture effected by the main jet becomes richer. The combined mixture strength is
a compensated one.
The two jets (orifices) working together, properly proportioned, compensate one another,
and give a mixture which may be constant or become leaner or richer with increase of load,
depending on the design.
The bled air flows through the compensating well into the auxiliary fuel jet. In the
unrestricted air bled system, the compensating well is located by the side of the float chamber and
is open to the atmosphere. In the case of restricted air bled system, the compensating well is
located in the air horn at the venturi.
In another system, a tapered metering pin that reaches into the fuel nozzle is lowered or
lifted. By doing so the annular space through which the fuel could be discharged is varied. As
such, the quantity of fuel drawn into the air charge gets changed.
UNIT – III
QUESTIONS
PART – A
1. What is the function of fuel feed system of an automobile?
2. Where is the fuel filter fitted in the fuel system? Why is it so necessary?
3. What is meant by Carburetion?
4. What is a Carburetor? What are the functions of a carburetor? Mention the practical difficulties
in realizing the objectives of efficient carburetion?
5. What is air fuel ratio, theoretical air fuel ratio or chemically correct air fuel ratio or stochiometric
air fuel ratio?
6. Define equivalence ratio, Rich mixture and lean mixture.
7. Briefly mention the effect of mixture strength on SI engine performance and represent them
graphically.
8. Mention the different types of carburetors.
9. What is Constant Choke Carburetor?
10. What is Constant Vacuum or Variable Choke Carburetor?
11. What are the deficiencies of a Simple carburetor?
PART – B
1. What are the requirements of an automotive carburetor? Draw and explain the different parts of
a simple carburetor.
2. Mention the different Venturi arrangements with sketches.
3. Draw and explain the functions of a Constant Vacuum Carburetor or Variable Choke
Carburetor with neat sketches.
4. Draw and explain the functioning of a “Solex” carburetor with its various circuits.
5. Briefly explain the compensating devices in a modern carburetor.
UNIT – IV
FUEL INJECTION SYSTEM
Two distinctive methods of fuel injection used in diesel engines
Many fuel injection systems are available in the field. Differences exist in the metering
Principle, in the injectors and in the injection principle. Fuel injection system used in C1 engines,
can be grouped under the two categories, namely(1) air injection system and (2) airless or solid or
mechanical injection system. The second system includes (a) constant pressure or common rail
system(b) distributor pump and (c) jerk pump system.
Air injection system applies to systems which inject fuel into the combustion space with the
aid of compressed air. Solid injection system applies to systems which inject only fuel by means of
a pumping device. No air is injected with the fuel. Present day CI engines, particularly those in
automotive and marine fields use solid injection system.
The functions of diesel injection pump
The fuel injection pump of a diesel engine has to perform the following critical function is:
1.
2.
3.
4.
5.
Meter the fuel to each injection.
Develop extremely high fuel pressure.
Inject fuel through fuel lines and nozzles.
Time fuel injections, to meet speed and load of the engine.
Allow operator to control power output.
6. Control engine idle speed and maximum engine speed.
7. Help close the injector nozzle after fuel injection.
8. Provide a means of shutting engine off.
Components of the injection pump
One of the widely used pump is the bosch type fuel injection pump. The main components
of this pump and similar pumps include the following:
1.
2.
3.
4.
5.
6.
7.
8.
9.
Injection pump camshaft being driven by the engine camshaft.
Roller tappets riding on each of the camshaft lobes.
An advance or retard timing device connected to the end of the camshaft.
Plunger type pumping elements riding on the top of each tappet and moving up and down
within the pump barrel. The barrel contains ports for entry and exit of fuel.
A control rack running the length of the upper end of the pump. This device engages
pinions (gear teeth) on each plunger of the pump to control the amount of fuel injected and
thereby determines engine power output.
A governor connected to the control rack, acting as an intermediatory mechanism between
control rack and accelerator pedal.
A fuel gallery or supply chamber located in the top part of the pump and connected to the
ports in each fuel pump barrel.
A spring loaded delivery valve at the top of each plunger pump.
High pressure fluid lines connecting each pump delivery end with an injector nozzle
located in each cylinder.
“Single acting feed pump” for CI engines.
Single acting feed pump a revolving cam or eccentric presses the plunger of the feed pump
“downwards” by means of the roller tappet and pressure spindle. A portion of the fuel present in
the suction chamber is delivered through the pressure valve to the pressure chamber. Now the
plunger spring gets compressed. Towards the end of this stroke the spring loaded pressure valve
closes again. As soon as the cam or eccentric has passed its maximum stroke, plunger, pressure
spindle and roller tappet move upward due to the pressure exerted by the plunger spring. A
portion (quantity delivered per stroke) of the fuel present in the pressure chamber is thereby
delivered to the injection pump through the filter. During this period, fuel is also sucked from the
fuel tank into the suction chamber through the preliminary filter and the suction valve.
Fuel filtration
The fuel injection equipment must be supplied with sufficiently clean fuel to prolong its
useful life. The components of the fuel injection unit are made to a high degree of precision. The
clearances provided between the plunger and the barrel in the fuel injection pump and between
the needle and the nozzle holder are of the order of 2 to 5 microns (thickness of the human hair is
about 100 microns). Therefore, the impurities (gritty matter) if any carried along with the fuel will
cause rapid wear in the injection system and in the cylinder. Worn components of the fuel
injection unit results in the following:
1.
2.
3.
4.
5.
6.
After dripping of fuel at the nozzle tip.
Defective spray characteristics i.e. Poor atomization and improper spray.
Affects fuel consumption.
Poor starting due to leakage and consequent reduction in the quantity of fuel supplied.
Irregular idling operation.
Spoiling the nozzle and the pump elements with a short period.
As such, incorporation of efficient filters in the fuel injection system becomes essential.
Elaborate filtering and centrifuging is necessary for successful use of heavy fuels in diesel engines
fitted to marine diesels.
The problem of keeping out of foreign matter is much more difficult in the case of CI engine
fuels than in the case of gasoline. This is because the much higher viscosity of the heavier fuel has
a greater tendency to hold solid particles in suspension
Fuel nozzles.
The different types of nozzles tried and used in diesel engines are
1. Closed nozzles –(a) single hole nozzle, (b) multi hole nozzle, (c) pintle nozzle, and (d)
pintaux nozzle. 2. Open nozzle
Open type nozzle
The open type nozzle has the fuel orifice or orifices and part of the passage way open to
cylinder pressure at all times. There is a check valve in the unit. The fuel injection begins when
the fuel injection pump rises the pressure sufficiently to seat the check valves. The fuel injection
ends, when the fuel pressure falls below the combustion pressure. In this type of injection, the
pressure difference that is causing the fuel flow is flow is low at the beginning and at the end of
injection. This causes poor atomization during these periods of injection. Hence, the present day
diesel engines use closed nozzles.
Closed type nozzle
The closed nozzles are so called because the nozzle is closed with a spring loaded needle
valve after each injection of fuel into the combustion chamber. The spring loaded needle valve is
opened either mechanically or hydraulically. Different types of closed nozzle used in diesel
engines can be seen in fig,
Fig: Different types of closed nozzles
The mechanically operated injection valve is connected to the common rail (constant
pressure) system. The injection valve is opened at the desired time. The fuel is sprayed through
the orifices in the nozzle tip due the fuel pressure in the common rail. The amount of fuel injected
may be regulated by keeping the valve off its seat to the required duration or by the variation of
the fuel pressure in the common rail.
The hydraulically operated injection nozzle has a spring loaded needle valve with a
differential valve stem. The valve opens inwards, when the fluid (fuel) pressure exceeds the
spring force.
Atomization of fuel jet:
Atomization of the liquid fuel jet is the disintegration of the fuel stream issuing from the
injector nozzle into droplets of different sizes. This disintegration presents a large surface for a
given quantity of fuel. The large surface helps quicker heating and vapourization of the drops.
Atomization of the fuel jet is caused by friction between the stream of fuel and air in the
combustion chamber
Swirl injector nozzle
A swirl injector nozzle was developed by Toyota Research Group. This nozzle has two
spiral grooves on the outer surface of its needle body. The swirl nozzle exhibited spray
characteristics of weak penetration and wide spray angle compared to the conventional single
hole nozzle. The system provides low fuel consumption and smoke emission, especially at low
speed range.
Mechanical fuel pump
The fuel pump can be seen in figure. It consists of a spring loaded flexible diaphragm
actuated by a rocker arm. The rocker arm is actuated by the eccentric. Non return valves are there
in the inlet and outlet of the pump. These valves ensure flow of fuel in the proper direction. The
internal parts of the pump are made of suitable materials to withstand exposure to fuel, oil, low
and high temperatures and also wear and tear, vibration etc.
As the rocker arm is moved by the eccentric, the diaphragm is pulled down against the
spring tension. This movement causes a partial vacuum in the pump chamber. Because of the
construction, the deliver valve remains closed while the suction valve opens. This admits fuel into
the pump chamber. At the maximum position of the eccentric, the diaphragm reaches the end of
its stroke. After this, further rotation of the eccentric will release the rocker arm. Now the rocker
arm will simply follow the eccentric by the action of the return spring. The diaphragm spring will
now push the diaphragm upwards and force the fuel to flow out, opening the delivery valve into
the delivery tube. Now the suction valve remains closed. This action is repeated as the eccentric
revolves.
In these pumps, the downward movement of the diaphragm is caused by the rocker arm,
while the delivery stroke is achieved by the tension of the diaphragm spring. The diaphragm
spring is so designed that the fuel pressure is suitably balanced by the buoyancy of the float
system of the carburetor. As such, when the needle valve closes with carburetor bowl becoming
full, the fuel pump cannot deliver fuel to the carburetor. In this case, the rocker arm simply
continues to rock while the diaphragm remains at or near its end of travel. However, as the
carburetor uses the fuel the needle valve opens to admit fuel. Now the diaphragm moves down by
the rocker arm action and sucks the fuel to deliver back the same when required. This self
regulating feature helps the pump to deliver the correct quantity of fuel at all operating condition.
Distributor fuel Injection system
Fig. Basic elements of bicera common rail system and Cummins pt common rail system
There are several types of distributor fuel injection system. One type provides a high
pressure metering pump with a distributor which delivers fuel to the various cylinders. Another
design provides a low pressure metering and distributor. High pressure needed for injection is
provided by the injection nozzles which are cam operated. In these systems, the metered fuel is
directed to the proper cylinder by the rotating distributor with drilled passage ways. The
distributor is driven by the camshaft of the engine.
Constant pressure or common rail system:
Fig (b) High pressure pump, common rail injector system
The constant pressure common rail system was developed by M/S. Vickers company. The
basic elements of BICERA common rail system and the CUMMINS PT common rail fuel injection
system can be seen in fig. This system consists of a high pressure pump which distributes fuel to a
common rail or header to which each injector is connected. A spring loaded bypass valve on the
header maintains a constant pressure or 330 to 530 kscm in the system and returns all excess fuel
to the supply tank.
The fuel injectors are operated mechanically. The metering and timing of fuel injection are
accomplished by the spray valve. The amount of fuel injected into the cylinders is controlled by
the lift of the needle valve in the injector. The duration of injection depends on the length of time
the valve is off its seat. The quantity of fuel injected depends on the duration, size and number of
holes in the nozzle tip and fuel pressure and air pressure in the cylinder.
The nozzles must therefore be closely matched to ensure equal distribution among the
cylinders. The common rail system tends to be self governing. That is if the speed falls, an
increased quantity of fuel is injected (since more time is now available). Remember, the supply
pressure is independent of engine speed.
Constant pressure or common rail system:
Fig : Basic elements of bicera common rail system and cummins pt common rail system
The constant pressure common rail system was developed by M/S. Vickers company. The
basic elements of BICERA common rail system and the CUMMINS PT common rail fuel injection
system can be seen in fig. This system consists of a high pressure pump which distributes fuel to a
common rail or header to which each injector is connected. A spring loaded bypass valve on the
header maintains a constant pressure or 330 to 530 kscm in the system and returns all excess fuel
to the supply tank.
The fuel injectors are operated mechanically. The metering and timing of fuel injection are
accomplished by the spray valve. The amount of fuel injected into the cylinders is controlled by
the lift of the needle valve in the injector. The duration of injection depends on the length of time
the valve is off its seat. The quantity of fuel injected depends on the duration, size and number of
holes in the nozzle tip and fuel pressure and air pressure in the cylinder.
The nozzles must therefore be closely matched to ensure equal distribution among the
cylinders. The common rail system tends to be self governing. That is if the speed falls, an
increased quantity of fuel is injected (since more time is now available). Remember, the supply
pressure is independent of engine speed.
Jerk pump injection system:
The jerk pump controlled system has a single reciprocating type pump for each fuel
injector. The pump is separately mounted on the engine block. The pump is driven by an
accessory shaft. Injector is connected to the pump by suitable tubing. The injector is opened by
lifting the needle valve automatically by fuel pressure. When the fuel pressure in the system falls
below a certain value, the spring loaded injector needle valve terminates fuel injection. This
system can be seen in fig.
Construction of the injection pump
A typical fuel injection pump is shown in fig. In the inline injection pumps, the camshaft is
assembled in an aluminum housing. It is connected to the driving device of the engine either
directly or through a coupling or a timing device. Above the cam of the camshaft is the roller
tappet with spring seat. The spring seat provides a non positive connection between the pump
plunger and the roller tappet.
Fig. Jerk type fuel injection pump
A steel plunger is located inside the pump barrel. The pump plunger and barrel form an
assembly called pump element. The steel plunger has a helical groove on the cylindrical surface.
The helical groove is connected to the top of the plunger by a vertical slot on the surface or by an
axial hole at the center of the plunger. The plunger bottom is kept pressed on a cam, by a spring.
The cam is rotated by the engine crankshaft through gears. Rotary motion of the cam causes the
plunger to move up and down. The pump plunger has a lug at its lower end. This engages a slot
in a sleeve. A toothed wheel or quadrant is clamped to the sleeve. A toothed rack known as
control rack is meshing with the toothed quadrant. By moving the toothed rack to and fro, the
quadrant and hence the plunger can be rotated axially. The pump barrel has ports. The fuel flows
into and out of the barrel through these ports.
A spring loaded valve is mounted at the top of the pump barrel. This valve called delivery
valve closes the deliver passage. The delivery valve is of conical type. The value has a collar on
its stem. This collar exactly fits into the bore in the valve body. The stem diameter below the
conical portion s lesser for a smaller length. In one design, the stem has flutes on its surface, These
flutes are in communication with the space below the collar. In another design, the stem is
hollow. This hollow space is connected to the space below the collar by two inclined holes. A
high pressure tube connects the delivery end of the pump to the injector.
Operation of the pump
The operation of the pump causes pressure build up as well as quantity measurement.
When the plunger is at the bottom of its stroke, inlet and spill ports are uncovered by the top end
of the plunger. Fuel is forced through these ports into the barrel by the lift pump via the fuel
filters. The lift pump supplies fuel at a pressure 0.8 to 1.0 kscm.
The volumetric efficiency of the pump depends upon the clearance between pump plunger
and barrel, injection pressure, number of strokes of the plunge per minute, viscosity of fuel and
the pressure at which fuel is supplied to the fuel injection pump.
The fuel entry into the pump barrel, compression of the fuel and spilling of the fuel to
terminate fuel injection can be seen in fig. As the pump cam lifts the plunger, ports are closed by
the plunger. This phase of the plunger stroke is termed “Prestroke”. The fuel above the plunger is
now trapped within the barrel. In the further upward movement of the plunger, the fuel is slightly
compressed i.e. due to its elastic characteristics the fuel is incompressible. This delivery stroke
following the pre stroke is known as the retraction stroke. After the retraction stroke comes the
actual effective stroke. Further rising of the plunger exerts pressure on the fuel. Pressurized fuel
now flows past the delivery valve into the high pressure tube. This tube is already full of oil. As
such, extra oil pumped in at the pump end causes an increase in pressure throughout the fuel in
the tube. This rise is fuel pressure lifts the injector needle against the spring force. The fuel thus
gets sprayed into the combustion chamber.
Fig. Pump plunger movement during pumping of fuel
The fuel injection into the combustion chamber takes place until the (lower) helical groove
on the plunger uncovers the barrel ports. When this happens, high pressure fuel above the
plunger escapes through the vertical slot into the inlet passage. This in turn, reduces fuel pressure
in the pump barrel. Now the delivery valve and the injector needle valve snap back on their
seats. The plunger which is still moving upward, completes its stroke without pumping anymore
oil. After this residual stroke and after the reversal of direction at top dead center, the fuel initially
flows back into the pump barrel through the vertical slot/hole until the helix again closes the
inlet/spill port. As the plunger moves further down in the barrel, vacuum is created. When the
inlet ports have been opened by the top of the plunger, fuel under pressure from the feed pump,
flows from the suction gallery into the high pressure chamber. As the cam rotates, the plunger is
ret turned to its bottom dead center by the spring. The plunger is now ready for the next cycle of
operations. During the operation of an engine, this cycle of pump operations repeat again and
again.
Unit injector
A unit injector combines the pump and the injection nozzle. There is no high pressure tube
connecting the pump and the injector. This construction eliminates the pressure wave
phenomenon which produce erratic fuel discharges under some conditions, when the system is
not properly turned. Sometimes, pressure surges in the high pressure line connecting the injector
and the fuel injection pump may cause after injection.
With a unit injector, the injection velocity of fuel through the orifice is proportional to the
pump plunger speed. The plunger speed itself s proportional to engine speed. On the other hand,
injection velocity is proportional to the square root of the different between the injection pressure
and compression pressure. Assuming that the compression pressure is almost a const, the
injection pressure increases as the square of the engine speed. However, in actual practice, the
injection pressure does not increase as fast above, due to compressibility of fuel, elasticity of metal
parts and leakage of fuel.
Fig. Poppet covered orifice nozzle-outward opening type
The only objection to the application of unit injector is that the fuel pump drive and the low
pressure fuel lines are somewhat more complicated.
Fuel Injection Nozzles
The successful operation of the diesel engine depends to a large extent on the functional
efficiency of the fuel injector/fuel nozzle ie. To direct, distribute and atomize the metered fuel into
combustion chamber. The combustion chamber design dictates the type of nozzle, droplet size
and spray required to achieve complete combustion within the available time and space.
Constructional features of a nozzle
The fuel injector is shown in fig. It consists of the spring loaded needle valve, injector body
and nozzle holder. The needle valve has a conical end and this end rest on the conical seat in the
nozzle holder. The needle valve which closes the nozzle orifices is held tight on its sear by a
spring. A predetermined tapper is provided in the needle valve above the needle valve sear. This
portion of the needle valve is exposed to oil pressure that exists in the fuel chamber.
Fig. Spring loaded, hydraulically operated fuel injector
The nozzle body bore and the needle valve are lapped to a close tolerance and are matched
sets. The nozzle body seat is commonly 1.5 smaller than the angle of the needle valve conical
seat. The machined and drilled passages within the nozzle body connect the fuel inlet with the
fuel pressure chamber or gallery of the injection nozzle. The leak off fuel (which is responsible for
the lubrication of the needle valve) surrounds the spindle and fills the adjusting device space and
returns via the leak off connection to the fuel tank.
The nozzle body and the needle valve are made of high grade alloy steel. The fuel injector
holder assembly positions and holds the fuel injection nozzle in the cylinder head. Grooves and
stress raisers and avoided in the design and construction of the nozzles.
Operation of fuel injector
Fuel from the high pressure fuel pump enters the injector body and nozzle holder through
the fuel inlet and fuel passage. On the tapered portion of the needle valve fuel pressure acts.
When the fuel pressure on the difference in the cross sectional area of the two exposed stem parts
plus the gas pressure on the end of the valve exposed to the cylinder gas is more than the
downward force exerted by the spring, the needle valve lifts off its seat. The fuel pressure at
which the needle valve lifts off its seat is called injection pressure or nozzle opening pressure.
The nozzle opening pressure can be varied by varying the extent of compression of the
spring acting on the valve. The spring force can be varied the extent of compression of the spring
acting on the valve, The spring force can be varied by the adjustment mechanism at the top of the
nozzle.
As soon as the nozzle opens and the fuel escapes through the orifices, the fuel pressure
drops slightly within the pressure chamber at the bottom of the valve. This slightly reduced
pressure is now exerted against a larger area of the valve. This facto causes the valve to open
appreciably.
During the period of fuel injection, the energy of compression in the fuel is converted into
energy of velocity at the spray holes. This causes the fuel to be emitted from the orifice in the
form of a fine spray.
When the fuel injection pump spills back the fuel into the suction side, delivery from the
pump ceases. Now the pressure in the nozzle pressure chamber drops instantly. The injector
spring snaps the needle valve on to its seat and thereby prevents fuel from leaving the nozzle.
Now no dribbling of fuel from the nozzle tip should occur
Types of fuel nozzles
The different types of nozzles tried and used in diesel engines are
1. Closed nozzles –(a) single hole nozzle, (b) multi hole nozzle, (c) pintle nozzle, and (d) pintaux
nozzle. 2. Open nozzle
Open type nozzle – the open type nozzle has the fuel orifice or orifices and part of the passage
way open to cylinder pressure at all times. There is a check valve in the unit. The fuel injection
begins when the fuel injection pump rises the pressure sufficiently to seat the check valves. The
fuel injection ends, when the fuel pressure falls below the combustion pressure. In this type of
injection, the pressure difference that is causing the fuel flow is flow is low at the beginning and
at the end of injection. This causes poor atomization during these periods of injection. Hence, the
present day diesel engines use closed nozzles.
Closed nozzle – The closed nozzles are so called because the nozzle is closed with a spring loaded
needle valve after each injection of fuel into the combustion chamber. The spring loaded needle
valve is opened either mechanically or hydraulically. Different types of closed nozzle used in
diesel engines can be seen in fig
Fig: Different types of closed nozzles
The mechanically operated injection valve is connected to the common rail (constant
pressure) system. The injection valve is opened at the desired time. The fuel is sprayed through
the orifices in the nozzle tip due the fuel pressure in the common rail. The amount of fuel injected
may be regulated by keeping the valve off its seat to the required duration or by the variation of
the fuel pressure in the common rail.
The hydraulically operated injection nozzle has a spring loaded needle valve with a
differential valve stem. The valve opens inwards, when the fluid (fuel) pressure exceeds the
spring force.
Single hole nozzle – The single hole nozzle has a hole drilled centrally in the nozzle body, the
hole is closed by the needle valve. The single hole nozzle produces one fuel jet. This fuel jet has a
good penetration, but requires a high injection pressure to effect fine atomization. The single hole
nozzle can be used in engines where the combustion chamber shape creates high turbulence (air
motion) as in the swirl chamber engines.
In the case of a single hole nozzle the orifice area and the injection pressure are chosen
judiciously. A large orifice area could circumvent the high injection pressure. But it involves a
problem. At low speeds, the closed nozzle will not discharge until the pressure is sufficiently
high to open the valve. Discharge then occurs at a greater rate than that of the pump, Now the
pressure falls, and the needle valve snaps on its seat. The pressure is built up again by the pump
and the valve opens. The operation is repeated, possibly several times in one injection period,
and a multiple injection takes place. However, the multiple injections by the opening and closing
of the valve at low speeds and wide open throttle are not desirable, because dribbling is possible.
For this reason, there is a definite limit to the size of the orifice that is feasible as the means for
limiting the maximum pressure at high speeds, and high injection pressures are unavoidable.
Multi hole nozzle – A multi hole nozzle can have number of holes, from 3 to as 18 for large bore
engines. The number of hole depends on the engine requirements. The multi hole nozzle is
mostly used open combustion chamber engines, where it is necessary to distribute the fuel spray
to all parts of a wide shallow combustion chamber. Remember, the air motion is less vigorous in
these chambers. Multi hole nozzles give good atomization. However, the small size holes may
get easily clogged up with foreign particles. The injector valve opening pressure usually varies
from 150 – 250 kscm.
Pintle or delay nozzle – Some needle valves have an extension known as Pintle. The pintle
extends through the orifice in the nozzle body, when the valve is in the closed position. The
shape of the pintle is according the spray pattern desired. The pintle may be either cylindrical or
conical in shape. The pintle may be designed to produce various cone angles 0 to about 60. The
pintle may be designed to produce a throttling effect during the initial phase of fuel injection. As
such, this type nozzle is called Delay Nozzle. The throttling effect reduces the amount of fuel
injected for a given orifice and causes a lag in the injection of the principal amount of fuel. This
phenomena reduces diesel knock effect. The pintle nozzle promotes atomization at the expenses
of penetration. Furthermore, the pintle keeps the spray hole free from carbon deposits.
Fig. Flow areas and valve lifts for throttling and non throttling pintle nozzles
The flow area and valve lifts for the throttling and non-throttling pintle nozzles can be seen
in fig. The rate of fuel discharge of the throttling and now throttling nozzles can be seen in fig.
Pintaux nozzle - Pintaux nozzle is a development of the pintle nozzle. This type of nozzle is
shown in fig (b). This has an extra small auxiliary spray hole drilled to bypass the pintle and to
assist easy starting, under cold conditions. During cold starting, only this auxiliaries spray hole
comes into operation as the main orifice is blocked by the pintle. For normal running fuel will be
supplied through the pintle hole fig. shows the main and auxiliary pintaux nozzle hole deliveries
for different pump speeds.
Fig (b) Pintaux Nozzle
fuel delivery from main and auxiliary holes of a
pintaux nozzle
Fig (c)
1. Normal type of commet injection with
pintle injector
2. Pintaux Nozzle spray under engine
starting condition
3. Pintuax nozzle spray under engine
running condition
In the swirl combustion chambers, the auxiliary hole of the pintaux nozzle is so located in
the injector tip, that the fuel is injected across the combustion chamber and towards the throat of
the chamber as shown in fig (c). With the arrangement, the hottest air will come in contact with
the injected spray. This helps easy starting. When the engine is warmed up and operating at the
rated load, the auxiliary spray from the pintaux nozzle is in small amount and is entirely
vapourised (or broken) by the air swirl . Thus, it serves as the focal points for ignition (premixed
flame) with combustion then spreading to the main spray.
Engine speed governing
The quantity of fuel injected which decides the speed of operation and power output of an
engine is regulated by mechanical or pneumatic governors. The governor operates the control
rack of fuel injection pump. Mechanical governors of the centrifugal type are usually used for this
purpose. For in line fuel injection pumps, there are also pneumatic governors.
Vehicle engines which are of variable sped type do have governors. By means of the
accelerator pedal, the driver sets a desired speed through the speed control lever of the injection
pump assembly. The task of the governor is now to vary the fuel delivery of the injection system
so that the set speed is reached and maintained within a narrow range of speed. Therefore, the
fuel delivery is varied until the set speed and the actual speed are equal.
If the vehicle is subjected to a variation in load (i.e. uphill gradient), there is a drop in
engine speed. However, within the control range, the governor now changes fuel delivery
(increases in this case) and the set engine speed is again reached.
Governors:
The governor regulates the speed of the diesel engine. It ensures that within the control
range, the engine speed does not drop below a specific value, since otherwise the engine will stop.
Also the engine speed should not exceed a specific value, to prevent damage to the engine. A part
from this, the governor also has other functions, such as automatic releasing or controlling of the
starting fuel delivery, correction of the full load delivery as a function of engine speed (torque
control), charge air pressure and altitude compensation. The various functions result in the
following governors.
 Maximum speed governors – limits the maximum speed
 Minimum and maximum speed governors – are used predominantly in motor vehicles. The idle
and maximum speeds are governed, not the range in between where the quantity of fuel injected
is controlled by the accelerator pedal.
 Variable speed governors – govern both the idle and maximum speeds as well as the range in
between.
Mechanical Governor in CI Engine
Fig. Mechanical Governor
UNIT – IV
QUESTIONS
PART - A
1.
What are the two distinctive methods of fuel injection used in diesel engines?
2.
Enumerate the main requirements which a fuel injection system should fulfill.
3.
What are the functions of diesel injection pump?
4.
Briefly mention the various components of diesel injection pump.
5.
Briefly mention about the “Single acting feed pump” for CI engines.
6.
Why diesel is to be filtered?
7.
Mention the types of fuel nozzles.
8.
What is Open type nozzle?
9.
What is closed type nozzle?
10.
What is “Atomization of diesel fuel?
11.
What is Swirl injector nozzle?
PART - B
1. Sketch a mechanical fuel pump and describe its working.
2. Give a diagrammatical sketch of a distributor pump system used in CI engines and brief
shortly:
3. Give a diagrammatical sketch of a distributor pump system used in CI engines and brief
shortly:
4. Briefly explain the constant pressure or common rail diesel injection system of a Cummins
PT common rail system with a neat sketch.
5. What are the essential parts in jerk pump?
6. Briefly explain the construction of a diesel injection pump with a sketch.
7. Briefly explain the operation of a diesel injection pump plunger with sketches.
8. Draw and explain the function of an “Unit injector” used in diesel engines.
9. Draw and explain the operation of a diesel fuel injector /Nozzle Holder.
10. Draw and explain single hole nozzle.
11. Sketch and explain multi hole nozzle
12. Draw and explain the functioning pintle or delay nozzle.
13. Sketch and explain pintaux nozzle.
14. What is engine “Speed Governing”?
15. What is “Governor” used in CI engine, mention the different governor types?
16. Sketch explain the operation of a “Mechanical Governor in CI engine.
UNIT - V
COMBUSTION OF FUELS
Ultimate Analysis
It involves the determination of percentage of
Carbon and hydrogen contents
Nitrogen content
Sulphur content
Ash content
Oxygen content
1. Carbon and Hydrogen contents
A known amount of the coal sample is burnt in a current of O2 in a combustion apparatus.
The carbon and hydrogen, present in the coal sample, are converted into CO2 and H2O
respectively according to the following equations.
C + O2 --- CO2 
H2 + ½ O2 --- H2O
The liberated CO2 and H2O vapours are absorbed respectively in KOH and anhydrous
CaCl2 tubes of known weights. The increase in weight of KOH tube is due to the formation of
CO2 while increases in weight CaCl2 tube is due to the formation of H2O. From the weights of
CO2 and H2O formed, the % of carbon and hydrogen present in the coal can be calculated as
follows.
Calculations
2KOH  CO2  K 2 CO3  H2 O
CaCl 2  7H2 O  CaCl 2 .7H2 O
Let
m = weight of the coal sample taken
x = increase in weight of KOH tube
y = increase in weight of CaCl2 tube
(a) % of carbon
C O 2  CO 2
12
44
44 gms of CO2 contains, 12 gms of carbon
12  x
 x gms of CO2 contains = 44 gms of carbon
12  x
m gms of coal contains = 44 gms of carbon
12  x 100

m gms of carbon
100 gms of coal contains = 44
12  x 100

44
m
% of carbon in coal =
(or)
Increase in weight of KOH tube 12
  100
Weight
of
coal
sample
taken
44
% or carbon in coal =
(b) % of hydrogen
H2  1 2 O 2  H2 O
2
18
18 gms of water contains 2 gms of hydrogen
2y
 y gms of H2O contains = 18 gms of hydrogen
2y
m gms of coal contains = 18 gms of hydrogen
2  y 100

m gms of hydrogen
100 gms of coal contains = 18
2  y 100

m
% of hydrogen in coal = 18
(or)
Increase in weight of CaCl 2 tube 2
  100
Weight
of
coal
sample
taken
18
% of hydrogen =
2. Nitrogen content
The determination of nitrogen content is carried out by Kjeldahl’s method. A known
amount of powdered coal sample is heated with con. H2SO4 in presence of K2SO4 (catalyst) in a
long necked flask (called Kjeldahl’s flask). Nitrogen in the coal is converted into ammonium
sulphate and a clear solution is obtained.
2N  3H2  H2 SO 4  (NH4 )2 SO4
The clear solution is then heated with excess of NaOH and the liberated ammonia is
distilled over and it is absorbed in a known volume of standard N/10 HCl.
(NH4 )2 SO4  2NaOH  2NH3  Na 2 SO4  2H2 O
NH3  HCl  NH4 Cl
The volume of unused N/10 HCl is then determined by titrating it against standard N/10
NaOH. Thus the amount of acid neutralized by liberated ammonia from coal is determined.
From this the percentage of nitrogen is calculated as follows.
Calculation
Let, the weight of the coal sample taken = m gms
Initial volume of
N
10 HCl = V1 ml
Volume of unused
N
10 HCl = V2 ml
 The acid neutralized by ammonia = (V1 – V2) ml
We know that
1000 ml of 1 N HCl  1 mole of NH3
[ HCl  NH 3  NH 4 Cl]
1 mole
1 mole
 14 gms of N2 [or 17 gms of NH3]
 (V1  V2 ) ml of
N
14  (V1  V2 )
HCl 
gms of N2
10
1000  1N
m gms of coal sample contains
14  (V1  V2 )  N / 10
1000  1
gms of N2
% of N2 in coal

14  Volume of acid consumed  Normality
1000  weight of coal sample
(or) % of N2 in coal

1.4  Volume of acid consumed  Normality
weight of coal sample
3. Sulphur content
A known amount of coal sample is burnt completely in a bomb calorimeter. During this
process sulphur is converted into sulphate, which is extracted with water. The extract is then
treated with BaCl2 solution so that sulphates are precipitated as BaSO4. The precipitate is filtered,
dried and weighed. From the weight of BaSO4 obtained, the sulphur present in the coal is
calculated as follows.
Calculation
Let the weight of coal sample = m gms
Weight of BaSO4 obtained
= x gms
BaCl 2
S  2O 2  SO 42 
 BaSO 4 
32
233 gms of
233
BaSO 4 contains 32 gms of sulphur
 x gms of BaSO4 contains =
m gms of coal sample contains =
32  x
233 gms of S
100 gms of coal sample contains =
 % of sulphur in coal =
(or) % of sulphur in coal =
32  x
233 gms of S
32  x 100

233
m
gms of S
32  x
233 100
32  weight of BaSO4 obtained
 100
233  weight of coal sample
4. Ash content
Determination of ash content is carried out as in proximate analysis
5. Oxygen content
The percentage of oxygen is calculated as follows
% of oxygen in coal = 100 - % of (C + H + N + S + ash)
Significant (or( Importance of Ultimate Analysis
(i) Carbon and hydrogen contents
Higher the % of carbon and hydrogen, better is the quality of coal and higher is its calorific value.
The % of carbon is helpful in the classification of coal.
Higher % of carbon in coal reduces the size of combustion chamber required.
(ii) Nitrogen content
Nitrogen does not have calorific value, and its presence in coal is undesirable.
Good quality coal should have very little nitrogen content.
(iii) Sulphur content
Though sulphur increases the calorific value, its presence in coal is undesirable because
The combustion products of sulphur, i.e., SO2 and SO3 are harmful and have corrosion effects on
equipments.
The coal containing sulphur is not suitable for the preparation of metallurgical coke as it
affects the properties of the metal.
(iv) Oxygen content
1. Lower the % of oxygen higher is its calorific value.
2. As the oxygen content increases its moisture holding capacity increases, and the calorific
value of the fuel is reduced.
CARBONISATION
When coal is heated strongly in the absence of air (called destructive distillation) it is
converted into lustrous, dense, porous and coherent mass known as coke. This process of
converting coal into coke is known as carbonization.
Caking coals and coking coals:
When coals are heated strongly, the mass becomes soft, plastic and fuses to give a coherent
mass. Such type of coals are called Caking Coals.
But if the mass so produced is hard, porous and strong then the coals are called Coking
Coals. Coking coals posses lower volatile matter and are used for the manufacture of
metallurgical coke.
Thus all coking coals are caking coals but all caking coals are not coking coals.
Metallurgical Coke:
When bituminous coal is heated strongly in the absence of air, the volatile matter escapes
out and the mass becomes hard, strong, porous and coherent which is called Metallurgical Coke.
Requisites (or) characteristics of good metallurgical coke
1. Purity
The moisture, ash, sulphur and phosphorous contents in metallurgical coke should be low.
Moisture and ash reduce the calorific value. Sulphur may contaminate the metal.
2. Porosity
Coke should be highly porous so that oxygen will have intimate contact with carbon and
combustion will be complete and uniform.
3. Strength
The coke should have very high mechanical strength inorder to with stand high pressure of
the overlying material in the furnace.
4. Calorific value
The calorific value of coke should be very high.
5. Combustibility
The coke should burn easily.
6. Reactivity
The reactivity of the coke should be low because low reactive cokes produce high
temperature on combustion.
7. Cost
It should be cheap and readily available.
Manufacture of metallurgical coke:
There are so many types of ovens used for the manufacture of metallurgical coke. But the
important one is Otto – Hoffman’s by product oven.
Otto – Hoffman’s by – product oven:
Inorder to
1. Increase the thermal efficiency of the carbonization process and,
2. Recover the valuable by products (like coal gas, ammonia, benzol oil, etc) Otto – Hoffman
developed modern by product coke oven.
The oven consists of a number of silica chambers. Each chamber is about 10 – 12 m long, 3
– 4 m height and 0.4 – 0.45 m wide. Each chamber is provided with a charging hole at the top, it is
also provided with a gas off take valve and iron door at each end for discharging coke.
Figure: Otto – Hoffmann’s by product oven
Coal is introduced into the silica chamber and the chambers are closed. The chambers are
heated to 12000 C by burning the preheated air and the producer gas mixture in the interspaces
between the chambers.
The air and gas are preheated by sending them through 2nd and 3rd hot regenerators. Hot
flue gases produced during carbonization are allowed to pass through 1st and 4th regenerators
until the temperature has been raised to 10000 C. While 1st and 4th regenerators are heated by hot
flue gases, the 2nd and 3rd regenerators are used for heating the incoming air and gas mixture.
For economical heating, the direction of inlet gases and flue gases are changed frequently.
The above system of recycling the flue gases to produce heat energy is known as the regenerative
system of heat economy. When the process is complete, the coke is removed and quenched with
water.
For economical heating, the direction of inlet gases and flue gases are changed frequently.
The above system of recycling the flue gases to produce heat energy is known as the regenerative
system of heat economy. When the process is complete, the coke is removed and quenched with
water.
Time taken for complete carbonization is about 12 – 20 hours. The yield of coke is about
70%.
The valuable by products like coal gas, tar, ammonia, H2S and benzol, etc. can be recovered
from flue gas.
Recovery of by – products:
(i) Tar
The flue gases are first passed through a tower in which liquor ammonia is sprayed. Tar
and dust get dissolved and collected in a tank below, which is heated by steam coils to recover
back the ammonia sprayed.
(ii) Ammonia
The gases are then passed through another tower in which water is sprayed.
ammonia get converted to NH4OH.
Here
(iii) Naphthalene
The gases are again passed through a tower, in which cooled water is sprayed. Here
naphthalene gets condensed.
(iv) Benzene
The gases are passed through another tower, where petroleum is
gets condensed to liquid.
sprayed. Here benzene
(v) Hydrogen Sulphide
The remaining gases are then passed through a purifier packed with moist Fe2O3. Here
H2S is retained.
The final gas left out is called coal gas which is used as a gaseous fuel.
Advantages of Otto Hoffman’s Process:
1. Valuable by products like ammonia, coal gas, naphthalene etc., are recovered.
2. The carbonization time is less
3. Heating is done externally by producer gas.
Synthetic Petrol:
The gasoline, obtained from the fractional distillation of crude petroleum oil, is called
straight run petrol. As the use of gasoline is increased, the amount of straight run gasoline is not
enough to meet the requirement of the present community. Hence, we are in need of finding out a
method of synthesizing petrol.
Hydrogenation of coal (or) Manufacture of synthetic petrol:
Coal contains about 4.5% hydrogen compared to about 18% in petroleum. So, coal is a
hydrogen deficient compound.
If coal is heated with hydrogen to high temperature under high pressure, it is converted to
gasoline. The petroleum of liquid fuels from solid coal is called hydrogenation of coal (or)
synthetic petrol.
There are two methods available for the hydrogenation of coal
1. Bergius process (or direct method)
2. Fischer-Tropsch process (or indirect method).
Bergius process (or) (direct method)
In this process, (figure) the finely powdered coal is made into a paste with heavy oil and a
catalyst powder (tin or nickel oleate) is mixed with it. The paste is pumped along with hydrogen
gas into the converter, where the paste is heated to 400 - 450C under a pressure of 200-250 atm.
Figure: Bergius process
During this process hydrogen combines with coal to form saturated higher hydrocarbons
which undergo further decomposition at higher temperature to yield mixture of lower
hydrocarbons. The mixture is led to a condenser, where the crude oil is obtained.
The crude oil is then fractionated to yield
1. Gasoline
2. Middle oil
3. Heavy oil
The middle oil is further hydrogenated in vapour phase to yield more gasoline. The heavy oil is
recycled for making paste with fresh coal dust. The yield of gasoline is about 60% of the coal used.
2. Fischer-Tropsch process (or) (indirect method)
Figure: Fischer-Tropsch process
In this process (Figure) coal is first converted into coke. Then water gas (CO + H2) is
produced by passing steam over red hot coke.
1200 C
C  H2 O 
 CO  H2
(Coke)
(Steam)
Water gas
The water gas is mixed with hydrogen and the mixture is purified by passing through
Fe2O3 (to remove H2S) and then into a mixture of Fe2O3 + Na2CO3 (to remove organic sulphur
compounds). The purified gas is compressed to 5 to 25 atm and then led through a converter,
which is maintained at a temperature of 200 - 300C. The converter is provided with a catalyst bed
consisting of a mixture of 100 parts cobalt, 5 parts thoria, 8 parts magnesia and 200 parts
keiselghur earth. A mixture of saturated and unsaturated hydrocarbon is produced as a result of
polymerization.
nCO  2nH2  Cn H2n  nH2 O
nCO  (2n  1)H2  Cn H 2n2  nH2 O
The outcoming gaseous mixture is led to a condenser, where the liquid crude oil is
obtained. The crude oil is fractionated to yield (i) gasoline and (ii) heavy oil. The heavy oil is used
for cracking to get more gasoline.
KNOCKING
Definition
Knocking is a kind of explosion due to a rapid pressure rise occurring in an IC engine.
Causes of knocking in S.I (Spark Ignition) Engine [Petrol engines]
In a petrol engine, a mixture a gasoline vapour and air at 1:17 ratio is used as fuel.
This mixture is compressed and ignited by an electric spark. The products of oxidation
reaction (combustion) increase the pressure and push the piston down the cylinder.
If the combination proceeds in a regular way, there is no problem in knocking. But in some
cases, the rate of combustion (oxidation) will not be uniform due to unwanted chemical
constituents of gasoline. The rate of ignition of the fuel gradually increases and the final portion
of the fuel-air mixture gets ignited instantaneously producing an explosive sound known as
“Knocking”. Knocking property of the fuel reduces the efficiency of engine. So, a good gasoline
should resist knocking.
Chemical structure and knocking
The knocking tendency of fuel hydrocarbons mainly depends on their chemical structures.
The knocking tendency decreases in the following order.
Straight chain paraffins > Branched chain paraffins Cycloparaffins > Olefins > Aromatics
Thus olefins of the same carbon-chain length possess better anti-knock properties than the
corresponding paraffins.
Improvement of antiknock characteristics
The octane number of fuel can be improved by
Blending petrol of high octane number with petrol of low octane number, so that the octane
number of the latter can be improved,
The addition of anti-knock agents like Tetra-Ethyl Lead (TEL)
Now a days aromatic phosphates are used as antiknock agent because it avoids lead pollution.
OCTANE NUMBER (or) OCTANE RATING
Octane number is introduced to express the knocking characteristics of petrol. It has been
found that n-heptane knocks very badly and hence, its anti-knock value has been given zero. On
the other hand, iso-octane gives very little knocking and so, its anti-knock value has been given
100.
Definition
Thus octane number is defined as ‘the percentage of iso-octane present in a mixture of isooctane and n-heptane.
IMPROVEMENT OF ANTI KNOCKING CHARACTERISTICS OR LEAD PETROL
The anti-knock properties of a gasoline can be improved by the addition of suitable
additives. Tetra ethyl lead (TEL) (C2H5)4 Pb is an important additive added to petrol. Thus the
petrol containing tetra ethyl lead is called leaded petrol.
Mechanism of knocking
TEL reduces the knocking tendency of hydrocarbon. Knocking follows a free radical
mechanism, leading to a chain growth which results in an explosion. If the chains are terminated
before their growth, knocking will cease. TEL decomposes thermally to form ethyl free radicals
which combine with the growth free radicals of knocking process and thus the chain growth is
stopped.
Disadvantages of using TEL
When the leaded petrol is used as a fuel, the TEL is converted to lead oxide and metallic
lead. This lead deposits on the spark plug and on cylinder walls which is harmful to engine life.
To avoid this, small amount of ethylene dibromide is added along with TEL. This ethyl dibromide
reacts with Pb and PbO to give volatile lead bromide, which goes out along with exhaust gases.
But this create atmospheric solution. So a nowadays aromatic phosphates are used instead
of TEL.
CETANE NUMBER (or) CETANE RATING
Cetane number is introduced to express the knocking characteristics of diesel. Cetane (hexa
decane) (C16H34) has a very short ignition lag and hence its cetane number is taken as 100. On
the other hand -methyl naphthalene has a long ignition lag and hence its cetane number is taken
as zero.
Definition
Thus the cetane number is defined as “the percentage of hexa decane present in mixture of
hexa decane and -methyl naphthalene, which has the same ignition lag as the fuel under test”.
The cetane number decreases in the following order.
Straight chain paraffins > Cycloparaffins > Olefins > Branched paraffins > Aromatics
The cetane number of a diesel oil can be increased by adding additives called dopes.
Important dopes: Ethyl nitrate, Iso-amyl nitrate.
Table Comparison of gasoline oil and diesel oil
S.No
1.
2.
3.
4.
5.
6.
7.
Gasoline Oil
Low boiling fraction of petroleum
contains C5 – C9 hydrocarbons
Fuel for SI engine
Knocking tendency is measured
in octane rating.
Knocking is due to premature
ignition
Anti-knocking is improved by the
addition of TEL
Its exhaust gases contain higher
amounts of pollutants
More consumption, lower thermal
efficiency
Diesel Oil
High boiling fraction of petroleum
contains C15 – C18 hydrocarbons
Fuel for CI engine
Knocking tendency is measured in
cetane rating
Knocking is due to ignition lag
Anti-knocking is improved by doping
with ethyl nitrate
Its exhaust gases contain lesser amount
of pollutants
Less consumption, higher thermal
efficiency
GASEOUS FUELS
COMPRESSED NATURAL GAS (CNG)
When the natural gas is compressed, it is called Compressed Natural Gas (CNG). The
primary component present in CNG is methane. It is mainly derived from natural gas.
The natural gas can either be stored in a tank of a vehicle as compressed natural gas (CNG)
at 3,000 or 3,600 psi or as liquefied natural gas (LNG) at typically 20-150 psi.
Properties
1.
2.
3.
4.
5.
Uses
CNG is the cheapest, cleanest and least environmentally impacting alternative fuel.
Vehicles powered by CNG produce less carbon monoxide and hydrocarbon (HC) emission.
It is less expensive than petrol and diesel.
The ignition temperature of CNG is about 550C.
CNG requires more air for ignition.
CNG is used to run an automobile vehicle just like LPG.
Comparison of emission levels between CNG driven vehicles and petrol driven vehicles
Pollutants
CO (gm/km)
HC (gm/km)
Emission levels
Petrol driven vehicle
CNG driven vehicle
0.92
0.05
0.36
0.24
PRODUCER GAS
It is a mixture of CO & N2 with small amount of H2. Its average composition is as follows
Constituents
CO
N2
H2
CO2 + CH4
Percentage (%)
30
51-56
10-15
Rest
Its calorific value is about 1300 kcal/m3
Manufacture
The reactor used for the manufacture of producer gas is known as gas producer. It consists
of a tall steel vessel inside of which is lined with refractory bricks. It is provided with cup and
cone feeder at the top and a side opening for producer gas exit. At the bottom, it is provided with
a inlet pipe for passing air and steam (Figure).
Figure: Manufacture of producer gas
When a mixture of air and steam is passed over a red hot coke maintained at about 1100 in
a reactor, the producer gas is producer.
Various Reactions
The reactions of producer gas production can be divided into four zones as follows
(i) Ash Zone
This is the lowest zone consists mainly of ash. The incoming air and steam mixture is preheated in
this zone.
(ii) Combustion of Oxidation Zone
This is the zone next to ash zone. Here the coke is oxidized to CO andCO2. Both the
reactions are exothermic. Hence, the temperature of the bed reaches around 1,100C.
C  1 2 O2  CO ; exothermic
C + O2  CO 2
(iii) Reduction Zone
; exothermic
This is the middle zone. Here both CO2 steam are reduced.
C  CO2  2CO ; endothermic
C + H2 O  CO  H2 ; endothermic
The above reactions are endothermic. Hence the temperature of the coke bed falls to
1000C.
(iv) Distillation or Drying Zone
This is the upper most layer of the coke bed. In this zone (400 - 800C) the meaning coke is
heated by the outgoing gases.
Uses
1. It is used as a reducing agent in metallurgical operations.
2. It is also used for heating muffle furnaces, open-hearth furnaces etc.
WATER GAS
It is a mixture of CO and H2 with small amount of N2. The average composition of water gas is as
follows
Constituents
CO
H2
N2
CO2 + CH4
Percentage (%)
41
51
4
rest
Its calorific value is about 2800 kcal/m3
Manufacture
The water gas producer consists of a tall steel vessel, lined inside with refractory bricks. It
is provided with cup and cone feeder at the top and a side opening for water gas exit. At the
bottom it is provided with two inlet pipes for passing air and steam (Figure).
When steam and little air is passed alternatively over a red hot coke maintained at about
900 - 1000C in a reactor, water gas is produced.
Figure: Manufacture of water gas
Various Reactions
The reactions of water gas production involves the following two steps.
I-Step
In the first stage, steam is passed through the red hot coke, where CO and H2 gases are
produced. The reaction is endothermic. Hence, the temperature of the coke bed falls.
C  H2 O  CO  H 2 ; endothermic
II-Step
In the second stage, in order to raise the temperature of the coke bed to 1000C, the steam
supply is temporarily cut off and air is blown in. The reaction is exothermic
C  O2  CO2
; exothermic
Thus the steam-run and air-blow are repeated alternatively to maintain proper
temperature.
Uses
1.
2.
3.
4.
It is used for the production of H2 and in the synthesis of ammonia
It is used to synthesis gasoline in Fischer-Tropsch process
It is used as an illuminating gas and a fuel.
It is also used in the manufacture of power alcohol and carbureted water gas (water gas +
oil gas).
CALORIFIC VALUE
The efficiency of a fuel can be understood by its calorific value. The calorific value of a fuel
is defined as “the total amount of heat liberated, when a unit mass of fuel is burnt completely.”
Units of calorific values
The quantity of heat can be measured by the following units:
Calorie.
Kilocalorie
British Thermal Unit (B.T.U).
Centigrade Heat Unit (C.H.U).
Calorie: It is defined as the amount of heat required to raise the temperature of 1 gram of
water through 1C (15 to 16C).
HIGHER AND LOWER CALORIFIC VALUES
Higher (or) Gross calorific value (GCV)
It is defined as the total amount of heat produced, when a unit quantity of the fuel is
completely burnt and the products of combustion are cooled to room temperature.
When a fuel containing hydrogen is burnt, the hydrogen is converted into steam. If the
combustion products are cooled to room temperature, the steam gets condensed into water and
latent heat is evolved. Thus, the latent heat of condensation of steam is also included in gross
calorific value.
Lower (or) Net Calorific Value (NCV)
It is defined as the net heat produced, when a unit quantity of the fuel is completely burnt
and the products of combustion are allowed to escape.
 NCV = GCV – Latent heat of condensation of water vapour produced.
= GCV – Mass of hydrogen  9  Latent heat of condensation of water vapour.
1 part by weight of H2 produces 9 parts by weight of H2O as follows. The latent heat of
steam is 587 cal/gm.
H  O  H2O
2
2gms
1
16gms
8
18gms
9
Thus
9
H  587
100
= GCV - 0.09H  587
NCV  GCV 
Where
H = % of H2 in the fuel
THEORETICAL CALCULATION OF CALORIFIC VALUE
Dulong’s formula
Dulong’s formula for the theoretical calculation of calorific value is
GCV (or) HCV 
1 
 H  O   2240 S  kcal / kg
8080
C

34500



100 
8


Where C, H, O and S represent the % of the corresponding elements in the fuel.
It is based on the assumption that the calorific values of C, H, and S are found to be 8080,
34500 and 2240 kcal, when 1 kg of the fuel is burnt completely. However, all the oxygen in the
fuel is assumed to be present in combination with hydrogen in the ratio H : O as 1:8 by weight. So
H
the surplus hydrogen available for combustion is
O
8
9
 N C V (or) L C V = HCV H  587  kcal / kg
100


Problems based on calorific value
PROBLEM 1
Calculate the gross and net calorific values of coal having the following composition
carbon = 85%, hydrogen = 8%, Sulphur = 1%, Nitrogen = 2%, ash = 4%, latent heat of steam = 587
cal/gm.
Solution
(i) Gross calorific value (GCV)

1 
%O 


8080

%C

34500
%H


  2240  %S  kcal / kg

100 
8 


1 
0

8080  85  34500  8    2240  1 kcal / kg

100 
8


1

[6, 86, 800  2,76, 000  2240] kcal/kg
100
1
=
[9, 65, 040] kcal/kg
100
= 9,650.4 kcal/kg

(ii) Net Calorific Value (NCV)
9
H  587 kcal/kg
100
9
= 9650.4  8  587 kcal/kg
100
= 9650.4 - 422.64
 GCV 
= 9227.76 kcal/kg.
Problem 2
Calculate the gross and net calorific values of a coal sample having the following
composition C = 80%; H = 7%; O = 3%; S = 3.5%; N = 2.5% and ash 4.4%.
Solution
(i) GCV

1 
%O 


8080

%C

3400
%H


2240

%S


 kcal / kg
100 
8 


1 
3

8080  80  34500  7    2240  3.5  kcal / kg

100 
8


 [8080  0.80  345(7  0.375)  2240  3.5]kcal / kg

 [6464  2285.6  78.4]kcal / kg
 8828.0 kcal/kg.
(ii) NCV
 GCV  [0.09H  587]kcal / kg
 8828  [0.09  7  587]kcal / kg
 [8828  369.8]kcal / kg
 8458.2kcal / kg.
Problem 3
On analysis, a coal sample has the following composition by weight; C = 85%; O = 3%; S
= 0.5% and ash = 3%. Net calorific value was found to be 8,400 kcal/kg. Calculate the
percentage of hydrogen and gross calorific value of coal.
Solution
GCV  [NCV  0.09H  587]kcal / kg
= [8,400 + 0.09H  587] k cal/kg
= [8,400 + 52.8 H] k cal/kg
GCV 
..........(1)
1 
%O 


8080

%C

34,
500
%H


2249

0

%S


 kcal / kg
100 
8 


1 
 H  3   2240  0.5  kcal / kg
8080

85

34,
500



100 
8


1
=
[686800  34500H  12937.5  1120]kcal / kg
100
= [6,868 + 345H - 129.4 + 11.2] kcal/kg
=
= 6,749.8 + 345 H kcal/kg
.............(2)
Equation (2) is substituted in equation (1)
6749.8  345H  8400  52.8H
345H  52.8H  8400  6749.8
292.2 H = 1650.2
1650.2
H=
292.2
H = 5.647
% of H = 5.647
.............(3)
Equation (3) is substituted in equation (1)
 GCV  (8400  52.8  5.647)
= 8698.16 kcal/kg
CALCULATION OF MINIMUM QUANTITY OF AIR REQUIRED FOR THE COMPLETE
COMBUSTION OF A FUEL
For efficient combustion, it is essential that the fuel must be brought into intimate contact
with sufficient quantity of air. The combustible constituents usually present in a fuel enter into a
process of combustion are C, H, S and O. But non-combustible constituents N, CO2 and ash
present in the fuel do not take any O2 during combustion.
The amount of oxygen and air required for the complete combustion of a given quantity of
fuel can be calculated by taking the following points into consideration.
1. Substances always combine in definite proportions, which are determined by the molecular
weights of the substances.
Examples
(a) Combustion of carbon
C O 2  CO 2
12
32
(by weight)
44
12 parts by weight of C require 32 parts by weight of O2.
(Or)
1 part by volume of C requires 1 part by volume of O2 for complete combustion
 C parts by weight of carbon require

32  C
parts by wt. of O2
12
(b) Combustion of hydrogen
2H 2  O 2  2H 2 O
22
32
36
(by weight)
4 parts by weight of H2 require 32 parts by weight of O2
(Or)
2 parts by volume of H2 require 1 part by volume of O2
 H parts by weight by hydrogen require

32  H
4
parts by weight of O2
Some of the hydrogen is present in the combined form with oxygen (ie., as H2O). This
combined hydrogen does not take part in combustion reaction. Therefore, the quantity of
combined hydrogen must be deduced from the total hydrogen in the fuel.
2H 2  O 2  2H 2 O
4 : 32
1 :8
Now 1 part by weight of H2 combines with 8 parts by weight of O2
O
H 2   H  
8

So the available
H  O 


8  parts by weight of H2 requires

O
32   H  
8

4
=
parts by weight of O2
Volume (or) weight of oxygen required for complete combustion of some combustible matters.
(i)
S  O2  SO2
32
1
32
1
( by weight)
(by volume)
(ii)
CH4  2O2  CO2  2H2 O
16
1
64
2
( by weight)
(by volume)
(iii)
C2 H6  7 2 O2  2CO2  3H2 O
30
1
112
3.5
( by weight)
(by volume)
(iv)
C2 H4  3O2  2CO2  2H2 O
28
1
(v)
96
3
(by weight)
(by volume)
C2 H2  5 2 O2  2CO2  H2 O
26
1
80
2.5
( by weight)
(by volume)
(vi)
CO 1 2 O2  CO2
28
1
16
0.5
( by weight)
(by volume)
(vii)
S  O2  SO 2
32
1
( by weight)
(by volume)
2. Amount of O2 required by the fuel will be given by subtracting the amount of O2 already
present in the fuel from the total or theoretical amount of O2 required by fuel.
Net amount of O2 required = Total amount of O2 required – O2 present in the fuel.
3. Air contains 21% of O2 by volume and 23% of O2 by weight. Hence from amount of O2
required by the fuel, the amount of air required can be calculated.
100
 min imum O2
Minimum weight of air required = 23
100
 min imum O2
21
Minimum volume of air required =
4. Molecular mass of air is taken as 28.94 g/mol
5. Density of air at NTP = 1.29 kg/m3
6. 22.4 litres (or 22,400 ml) of any gas at NTP (ie 0C and 760 mm of Hg) has a mass equal to its 1
mol (gram molecular weight).
Thus 22.4 litres of CO2 at NTP will have a mass of 44 g (44 is the molecular weight of CO2)
7. Excess air for combustion
It is necessary to supply excess air for complete combustion of the fuel. It is found out from
the theoretical amount of air as follows.
The amount of air required if excess air is supplied

Theoretical amount of air
 [100  %Excess air]
100
PROBLEMS BASED ON COMBUSTION REACTION
Important steps on combustion reactions
The amount of air required for complete combustion of a fuel can be calculated by
following the various steps given below.
Step 1: Write the equation for the combustion reaction
Step 2: From the equation calculate the amount of oxygen required for the elements or compound
present in the fuel.
Step 3: If oxygen is mentioned in the problem, subtract the weight or volume of oxygen already
present in the fuel from the total volume of oxygen required by the other elements or compounds.
Step 4: Since N2, CO2 and H2O are non-combustible, they do not require any oxygen (air).
Therefore, their values, if mentioned in the problem, can be ignored.
Step 5: Finally calculate the amount of air required by the fuel by multiplying 100/21 (if volume%)
with total amount of oxygen required, and 100/23 (if weight %) with total amount of oxygen
required.
Step 6: Excess amount of air for combustion
Excess amount of air is necessary than the theoretical amount of air to achieve complete
combustion.
This excess air required is expressed in percentage of theoretical air as
 Theoretical air 
100 + Excess air
100
Example 1
Calculate the volume of air (volume % of O2 in air = 21) required for complete
combustion of 1 litre of CO.
[B.D.U – Apr.96 MDS-Apr. ‘94]
Solution
The combustion equation of CO is written as follows
CO 1 2 O 2  CO2
1 vol
0.5 vol
One volume (lit) of CO requires 0.5 volume (lit) of O2 for complete combustion.
We know that
21 lits of O2 is supplied by 100 lits of air
0.5 lit of O2 is supplied by
100  0.5
21
lits of air
= 2.38 lits of air
The volume of air required for complete combustion of 1 lit of CO = 2.38 lits
Example 2
What is the volume of air required for complete combustion of 1 m3 of mixture
containing 80% CH4 and 20% C2H6?
Solution
1 m3 of mixture contains
80
 0.8 m 3 of CH4
100
The combustion equations of CH4 and C2H6 are written as follows
CH4  2O 2  CO 2  2H 2 O
1 vol
2 vol
i.e., 1 volume (or m3) of CH4 requires 2 volumes (or m3) of O2 for complete combustion
 0.8 m3 of CH4 requires = 2  0.8m3 = 1.6m3 of O2
C2 H6  7 2 O2  2CO 2  3H 2 O
1 vol
3.5 vol
i.e., 1 volume (or m3) of C2H6 requires 3.5 volume (or m3) of O2 for complete combustion
 0.2 m3 of C2H6 requires = 3.5  0.2 m3 = 0.7 m3 of O2
Total volume of O2 required = 1.6 + 0.7 = 2.3 m3
We know that
2lm3 of O2 is supplied by 100 m3 of air
 2.3m3 of O2 is supplied by =
100  2.3
21
= 10.95 m3
The volume of air required for complete combustion of 1 m3 of the mixture = 10.95 m3
Example 3
Calculate the minimum volume of air required for the complete combustion of 1 m3 of a
gaseous fuel containing the following composition by volume. CO :23% ; H2 : 12%; CH4 :3%;
CO2 : 5%; N2 : 55%; and O2 : 2%
Solution
1. 1m3 of fuel contains
23
100
12
100
3
100
5
100
55
100
2
100
 0.23 m 3 of CO
 0.12m 3 of H 2
 0.03m 3 of CH 4
 0.05m 3 of CO 2
 0.55m 3 of N2
 0.02m 3 of O2
2. N2 and CO2 are non-combustible constituents they do not burn and do not require any O2
3. The combustion equations of the remaining constituents are written as follows
CO 1 2 O 2  CO2
(a)
(b)
(c)
1 vol
0.5vol
H2  1 2 O2  H2 O
1 vol
0.5 vol
CH4  2O 2  CO 2  2H 2 O
1 vol
2vol
(a) 1 m3 of CO requires 0.5 m3 of O2
0.23 m3 of CO requires =
0.5  0.23
 0.115 m 3
1
of O2
(b) 1m3 of H2 requires 0.5 m3 of O2
0.12 m3 of H2 requires =
0.5  0.12
 0.06
1
m3 of O2
(c) 1 m3 of CH4 requires 2m3 of O2
2  0.03
 0.06
1
0.03 m3 requires
m3 of O2
Total volume of O2 required = 0.115 + 0.06 + 0.06
= 0.235 m2 of O2
 Net volume of O2 required = Total volume of O2 required
- O2 already present in the fuel
= 0.235 – 0.02 = 0.215 m3
We know that
21 m3 of O2 is supplied by 100 m3 of air
 0.215 m 3of O2 is supplied by =
100  0.215
21
= 1.02m3 of air
The volume of air required 

for the complete combustion   1.02 m3

of 1m 3 of the gaseous fuel 
Example: 4
A fuel contains C = 75%; H = 4%; O = 5%; S = 7%; remaining ash. Calculate the minimum
quantity of air required for complete combustion of 1 kg of the fuel.
Solution:
1. 1 kg of the fuel contains
75
100
4
100
5
100
7
100
 0.75 kg of carbon
 0.04 kg of hydrogen
 0.05 kg of oxygen
 0.07 kg of sulphur
2. The combustion equations of the various elements presents in the fuel are as follows
 a  C + O2
12kg
----------> CO 2
32 kg
 b  H2 + 1 2 O 2
2kg
16 kg
 c  S + O2
32kg
----------> H2O
---------->
SO 2
32 kg
(a) 12 kgs of carbon requires 32 kgs of oxygem
32  0.75
12
=2 kg of oxygen
 0.75 kg of carbon requires =
(b) 2 kgs of hydrogen requires 16 kgs of oxygen
 0.04 kg of hydrogen requires =
16  0.04
2
(c) 32 kgs of sulphur requires 32 kgs of oxygen
32  0.07
32
= 0.07 kg of oxygen
 0.07 kg of sulphur requires =
Total amount of O2 required = 2 + 0.32 + 0.07 = 2.39 kgs
But, the amount of O2 already present in the fuel = 0.05 kg
 Net amount of O2 required = Total amount of O2 – O2
already present in the fuel.
= 2.39 – 0.05 = 2.34 kgs
We know that
23 kgs of O2 is supplied by 100 kgs of air
100  2.34
2.34 kgs of O2 is supplied by 
23
= 10.174 kgs of air
 The minimum amount


of air required for complete   10.174kgs
combustion of 1 kg of a fuel 
Example: 5
Calculate the minimum amount of air needed for the complete combustion of 100 kgs of
coal containing 80% carbon, 5% hydrogen, 5% oxygen, 2% sulphur and the rest nitrogen by
weight.
Solution:
1. 1 kg of the coal contains
80
 0.8 kg of carbon
100
5
 0.05 kg of hydrogen
100
5
 0.05kg of oxygen
100
2
 0.02kg of sulphur
100
2. Nitrogen is a non – combustible constituent, they do not burn and do not require any O2.
3. The combustion equations of the remaining constituents are written as follows.
 a  C + O2
12kg
32 kg
 b  H2 + 1 2 O 2
2kg
----------> H2O
16 kg
 c  S + O2
32kg
----------> CO 2
---------->
SO 2
32 kg
(a) 12 kgs of C requires 32 kgs of O2
32  0.8
 0.8 kg of C requires =
= 2.13 kg of O 2
12
(b) 2 kgs of H2 requires 16 kgs of O2
16  0.05
 0.05 kg of H2 requires =
 0.4 kg of O 2
2
(c) 32 kgs of S requires 32 kgs of O2
32  0.02
 0.02 kg of S requires =
32
= 0.02 kg of O2
 Total amount of O2 required = 2.13 + 0.4 + 0.02
= 2.55 kgs
But, amount of O2 already present in the fuel = 0.05 kg
 Net amount of O2 required = 2.55 – 0.05 = 2.50 kgs
We know that,
23 kgs of O2 is supplied by 100 kgs of air
100  2.50
 2.50 kgs of O2 is supplied by 
23
= 10.869 kgs of air
The amount of air required for 1 kg of coal = 10.869 kgs
 The amount of air required for the 
  10.869  100 kgs
complete combustion of 100 kgs of coal 
=1087.00 kgs of air
Example: 6
Calculate the minimum amount of air required for complete combustion of 50 kgs of fuel
containing 80% carbon, 6% hydrogen, 2% sulphur and the rest nitrogen by weight.
Solution:
1. 1 kg of the fuel contains
80
 0.8 kg of carbon
100
6
 0.06 kg of hydrogen
100
2
 0.02 kg of sulphur
100
2. Nitrogen is a non – combustible constituent, they do not burn and do not require any O2.
3. The combustion equations of the remaining constituents are written as follows.
 a  C + O2
12kg
----------> CO 2
32 kg
 b  H2 + 1 2 O 2
2kg
16 kg
 c  S + O2
32kg
----------> H2O
---------->
SO 2
32 kg
(a) 12 kgs of C requires 32 kgs of O2
 0.8 kg of C requires =
32  0.8
=2.13 kg of O 2
12
(b) 2 kgs of H2 requires 16 kgs of O2
 0.06 kg of H2 requires =
16  0.06
 0.48 kg of O 2
2
(c) 32 kgs of S requires 32 kgs of O2
 0.02 kg of S requires =
32  0.02
 0.02 kg of O 2
32
 Total amount of O2 required = 2.13 + 0.48 + 0.02 = 2.63 kgs
We know that
23 kgs of O2 is supplied by 100 kgs of air
100  2.63
23
= 11.43 kgs of air
The minimum amount of air required for the complete combustion of 1 kg of fuel = 11.43 kgs
 2.63 kgs of O2 is supplied by 
 The minimum amount of air required for 
  11.43  50 kgs = 571.5 kgs
the complete combustion of 50 kgs of fuel 
Example: 7
A coal sample on analysis gives C = 80% ; S = 1% ; H = 4.5%; O = 2% and the rest ash.
Find the theoretical amount of air required per 2 kg of the coal burnt.
Solution:
1. 1 kg of the coal contains
80
 0.8 kg of carbon
100
1
 0.01 kg of sulphur
100
4.5
 0.045 kg of hydrogen
100
2
 0.02 kg of oxygen
100
2. The combustion equations of the various elements present in the coal are as follows.
 a  C + O2
12kg
----------> CO 2
32 kg
 b  S + O2
32kg
----------> SO2
32 kg
 c  H2 + 1 2 O2
2kg
----------> H2 O
16 kg
(a) 12 kgs of C requires 32 kgs of O2
 0.8 kg of C requires =
32  0.8
 2.13 kgs of O2
12
(b) 32 kgs of S requires 32 kgs of O2
32  0.01
32
= 0.01 kg of O2
 0.01 kg of S requires =
(c) 2 kgs of H2 requires 16 kgs of O2
 0.045 kg of H2 requires =
16  0.045
 0.36 kgs of O2
2
 Total amount of O2 required = 2.13 + 0.01 + 0.36
= 2.5 kgs
But, the amount of O2 already 
  0.02 kg
present in the coal

 Net amount of O2 required = Total amount of O2  O2 already present in the coal
= 2.5 – 0.02 = 2.48 kgs
We know that,
23 kgs of O2 is supplied by 100 kgs of air
100  2.48
23
= 10.78 kgs of air
 2.48 kgs of O2 is supplied by =


air required for the complete   10.78 kgs
combustion of 1 kg of the coal 
Minimum amount of
 Minimum amount of


air required for the complete   10.78  2 kgs = 21.56 kgs
combustion of 2 kgs of the coal 
Flue Gas Analysis (Orsat Method)
The mixture of gases (like CO2, O2, CO, etc) coming out from the combustion chamber is
called flue gases. The analysis of a flue gas would give an idea about the complete or incomplete
combustion process. The analysis of the flue gases is carried out by using orsat’s apparatus.
Description of orsat’s apparatus:
It consists of a horizontal tube. At one end of this tube, U – tube containing fused CaCl2 is
connected through 3 – way stop cock. The other end of this tube is connected with a graduated
burette. The burette is surrounded by a water – jacket to keep the temperature of gas constant.
The lower end of the burette is connected to a water reservoir by means of a rubber tube.
Figure: Orsat’s apparatus
The level of water in the burette can be raised or lowered by raising or lowering the
reservoir figure.
The horizontal tube is also connected with three different absorption bulbs I, II, and III for
absorbing CO2, O2 and CO.
I – bulb: It contains ‘potassium hydroxide’ solution, and it absorbs only CO 2.
II – bulb: It contains ‘alkaline pyrogallol’ solution, and it absorbs CO2 and O2.
III – bulb: It contains ‘ammoniacal cuprous chloride solution’ and it absorbs CO2, O2 and CO.
Working:
The 3 – way stop – cock is opened to the atmosphere and the reservoir is raised, till the
burette is completely filled with water and air is excluded from the burette. The 3 – way stop –
cock is now connected to the flue gas supply and the flue gas is sucked into the burette and the
volume of flue gas is adjusted to 100 cc by raising and lowering the reservoir. Then the 3 – way
stop cock is closed.
(a) Absorption of CO2
The stopper of the absorption bulb – I, containing KOH solution, is opened and all the gas
is passed into the bulb – I by raising the level of water in the burette. The gas enters into the bulb
– I, where CO2 present in the flue gas is absorbed by KOH.
The gas is again sent to the burette. This process is repeated several times to ensure
complete absorption of CO2. The decrease in volume of the flue gas in the burette indicates the
volume of CO2 in 100 cc of the flue gas.
(b) Absorption of O2
Stop – cock of bulb – I is closed and stop cock of bulb – II is opened. The gas is again sent
into the absorption bulb – II, where O2 present in the flue gas in absorbed by alkaline pyrogallol.
The decrease in volume of the flue gas in the burette indicates the volume of O2.
(c) Absorption of CO
Now stop – cock of bulb – II, where O2 present in the flue gas is absorbed by alkaline
pyrogallol. The decrease in volume of the flue gas is absorbed by ammoniacal cuprous chloride.
The decrease in volume of the flue gas in the burette indicates the volume of CO. The remaining
gas in the burette after the absorption of CO2, O2 & CO is taken as nitrogen.
Significance (or) uses of flue gas analysis
1. Flue gas analysis gives an idea about the complete or incomplete combustion process.
2. If the flue gases contain considerable amount of CO, it indicates that incomplete
combustion is occurring and it also indicates that the short supply of O2
3. If the flue gases contain considerable amount of O2, it indicates that complete combustion is
occurring and also it indicates that the excess of O2 is supplied.
QUESTIONS
1. Explain: Gross and Net calorific value.
2. How the flue gas analysis is carried out? Explain with neat diagram
3. Define gross and net calorific values of a fuel. How are they related.
4. Calculate the gross and net calorific value of coal having the following compositions. Carbon –
85%, hydrogren – 8%, sulphur – 1%, nitrogen – 2 %, ash – 4%
5. Describe the Otto – Hoffman by product oven process for the manufacture of coke.
6. Explain the various steps involved in the calculation of minimum quantity of air required for
the complete combustion of a fuel.
Exercise:
1. Calculate the volume of air required for complete combustion of 100 m3 of a gaseous fuel
having the following analysis composition by volume. (H2 = 50%; CH4 = 36%; N2 = 1.5%; CO =
6%; C2H4 = 4%; CO2 = 2.5 %.
2.
3.
A gaseous fuel has the following composition by volume. H2 = 12%, CH4 = 2%, CO = 24%, CO2
= 5%, O2 = 2% and the rest N2. Calculate the volume of air needed for the complete
combustion of 100 m3 of the fuel.
A sample of coal was found to have the following percentage composition. C = 75%, H 2 =
5.2%, O2 = 12.8 %, S = 12.8%, S = 1.2% and the rest ash. Calculate the amount of air needed for
the complete combustion., if 1 kg of the coal is burnt with 30% excess air.
*****************