Internal
Combustion
Engine
CHAPTER 1
OVERVIEW OF THERMODYNAMICS OF FUEL-AIR
CYCLES AND REAL CYCLES
(Roll No.: - 072 BME 601, 602,603, 604, 605, 606)
Syllabus
1.1Otto cycle, Diesel cycle, Atkinson cycle, Stirling cycle, Brayton cycle
1.2Assumption in fuel air cycle analysis
1.3Composition of cylinder gases
1.0 Introduction to IC Engine
Internal Combustion engine is a type of heat engine that converts chemical energy in a fuel into
mechanical energy through heat energy formed by combustion of fuel inside of the engine.
Examples of IC engine are Petrol engines, Diesel engines, Gas engines etc. Jet engines, rocket
engines are also internal combustion engines but they are not studied with reciprocating IC
engine.
History
Locomotives were started to use in later half of 1700s. Lenoir invented atmospheric engine in
around 1860 – 1865. Otto and Langen invented another atmospheric engine with higher
efficiency than that of the Lenoir. In atmospheric engine, the air gas mixture is drawn into at
atmospheric pressure. The charge is then ignited with a spark, which result in rise in pressure
early in the outward stroke to accelerate a free stroke to accelerate a free piston and rack
assembly so its momentum would generate a vacuum in the cylinder. The atmospheric pressure
then pushes the piston inward.
In 1876, Otto made a prototype of spark ignition, four stroke engines. This engine had a very low
weight and volume for the same power development compared to corresponding atmospheric
engine. In this way, we can consider that, Otto was the inventor of the modern IC engine. By the
1880s many engineers had developed two-stroke IC engines.
In 1992 Rudolf Diesel, a German engineer patented the diesel engine in which, combustion is
initiated by the heat from the compression of the gases inside the engine.
Air pollution from the IC engine become apparent from the 1940s and the codes and standards
about the pollution were developed in 1960s. These days there is attempt to displace IC engine
with more clean energy conversion technology.
1.1Cycles
A thermodynamic cycle consists of a linked sequence of thermodynamic processes that involve transfer
of heat and work into and out of the system, while varying pressure, temperature, and other state
variables within the system, and that eventually returns the system to its initial state
For the analysis of the IC engine, the processes in the IC engine can be broken down into
different processes forming cycle. There are various cycles used to represent different thermal
machines including engines.n the process of passing through a cycle, the working fluid (system)
may convert heat from a warm source into useful work and dispose of the remaining heat to a
cold sink, thereby acting as a heat engine. Conversely, the cycle may be reversed and use work to
move heat from a cold source and transfer it to a warm sink thereby acting as a heat pump.
Glimpse of thermodynamic processes used in cycles
1.0
Adiabatic: No energy transfer as heat (Q) during that part of the cycle would amount to
δQ=0. This does not exclude energy transfer as work.
2.0
3.0
4.0
5.0
6.0
Isothermal: The process is at a constant temperature during that part of the cycle (T=constant,
δT=0). This does not exclude energy transfer as heat or work.
Isobaric: Pressure in that part of the cycle will remain constant. (P=constant, δP=0). This
does not exclude energy transfer as heat or work.
Isochoric: The process is constant volume (V=constant, δV=0). This does not exclude energy
transfer as heat or work.
Isentropic: The process is one of constant entropy (S=constant, δS=0). This excludes the
transfer of heat but not work.
Isenthalpic: process that proceeds without any change in enthalpy or specific enthalpy
Polytropic: process that obeys the relation:
8.0 Reversible: process where entropy production is zero
7.0
Otto Cycle
An Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical
spark ignition piston engine. It is the thermodynamic cycle most commonly found in automobile
engines. Temperature-Entropy Otto cycle is a gas power cycle that is used in spark-ignition
internal combustion engines (modern petrol engines).
The thermodynamic analysis of air standard Otto cycle is as follows:
An Otto cycle consists of four processes:
Two isentropic (reversible adiabatic) processes
Two isochoric (constant volume) processes
Process 1-2: Isentropic compression in this process, the piston moves from bottom dead
center(BDC) to top dead center(TDC) position. Air undergoes reversible adiabatic (isentropic)
compression. We know that compression is a process in which volume decreases and pressure
increases. Hence, in this process, volume of air decreases from V1 to V2 and pressure increases
from p1 to p2. Temperature increases from T1 to T2.
Process 2-3: Constant Volume Heat Addition: Process 2-3 is isochoric (constant volume) heat
addition process. Here, piston remains at top dead centerfor a moment. Heat is added at constant
volume (V2 = V3) from an external heat source. Temperature increases from T2 to T3, pressure
increases from p2 to p3 and entropy increases from s2 to s3.
Process 3-4: Isentropic expansion in this process, air undergoes isentropic (reversible adiabatic)
expansion. The piston is pushed from top dead center(TDC) to bottom dead center(BDC)
position. Here, pressure decreases from p3 to p4, volume rises from v3 to v4, temperature falls
from T3 to T4 and entropy remains constant (s3=s4).
Process 4-1: Constant Volume Heat Rejection The piston rests at BDC for a moment and heat is
rejected at constant volume (V4=V1). In this process, pressure falls from p4 to p1, temperature
decreases from T4 to T1 and entropy falls from s4 to s1.
The thermodynamic analysis of air standard Otto cycle is as follows:
Heat suppllied at constant volume = 𝐶𝑣 (𝑇3 − 𝑇2 )
Heat rejected at constant volume = 𝐶𝑣 (𝑇4 − 𝑇1 )
work done = heat supplied-heat rejected
= 𝐶𝑣 (𝑇3 − 𝑇2 ) − 𝐶𝑣 (𝑇4 − 𝑇1 )
𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
Efficiency =ℎ𝑒𝑎𝑡 𝑠𝑢𝑝𝑝𝑙𝑖𝑒𝑑
𝐶𝑣 (𝑇3 −𝑇2 )−𝐶𝑣 (𝑇4 −𝑇1 )
=
=1-
𝐶𝑣 (𝑇3 −𝑇2 )
(𝑇4 −𝑇1 )
𝑇3 −𝑇2 )
…………………. (i)
𝑣
Let compression ratio 𝑟𝑐 (= 𝑟) = 𝑣1
2
And expansion ratio = 𝑟𝑒 (𝑟 =) =
𝑣4
𝑣3
(these two ratios are same in this cycle)
As
𝑇2
𝑇1
𝑣
𝛾−1
= (𝑣1 )
2
Then, T2 = T1.(r)ϒ-1
𝑇
𝑣
𝛾−1
Similarly,𝑇3 = (𝑣4 )
4
3
T3 =T4.(r)^(ϒ-1)
Inserting the values of T2 and T3 in equation eq(i) we get,
𝜂𝑜𝑡𝑡𝑜 = 1 −
𝑇4
ϒ−1
T4. (r)
=1−
𝑇4 − 𝑇1
(𝑇4 − 𝑇1 )
r ϒ−1
=1−
𝑊=
− 𝑇1
− T1. (r)ϒ−1
1
𝑟 𝛾−1
𝑃1 𝑉1
[(𝑟 𝛾−1 − 1)(𝑟𝑝−1 )]
𝛾−1
𝑃1 𝑟[(𝑟 𝛾−1 − 1)(𝑟𝑝−1 )]
Pm =
(ϒ − 1)(𝑟 − 1)
Diesel Cycle
The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it,
fuel is ignited by heat generated during the compression of air in the combustion chamber, into
which fuel is then injected. Diesel cycle is a gas power cycle invented by Rudolph Diesel in the
year 1897. It is widely used in diesel engines.
Diesel cycle is similar to Otto cycle except in the fact that it has one constant pressure process
instead of a constant volume process (in Otto cycle).
Processes in Diesel Cycle:
Diesel cycle has four processes. They are:
1)Process 1-2: Isentropic Compression in this process, the piston moves from Bottom Dead
Centre (BDC) to Top Dead Centre (TDC) position. Air is compressed isentropically inside the
cylinder. Pressure of air increases from p1 to p2, temperature increases from T1 to T2, and
volume decreases from V1 to V2. Entropy remains constant (i.e., s1 = s2). Work is done on the
system in this process
2)Process 2-3: Constant Pressure Heat Addition in this process, heat is added at constant
pressure from an external heat source. Volume increases from V2 to V3, temperature increases
from T2 to T3 and entropy increases from s2 to s3.
3)Process 3-4: Isentropic Expansion Here the compressed and heated air is expanded
isentropically inside the cylinder. The piston is forced from TDC to BDC in the cylinder.
Pressure of air decreases from p3 to p4, temperature decreases from T3 to T4, and volume
increases from V3 to V4. Entropy remains constant (i.e., s3 = s4). Work is done by the system in
this process
4)Process 4-1:Constant Volume Heat Rejection in this process, heat is rejected at constant
volume (V4 = V1). Pressure decreases from P4 to P1, temperature decreases from T4 to T1 and
entropy decreases from s4 to s1.
𝜂𝑑𝑖𝑒𝑠𝑒𝑙 =
cp(T3 −T2)−Cv (T4 −T1 )
𝑤𝑜𝑟𝑘 𝑑𝑜𝑛𝑒
=
heat supplied
cp(T3 −T2 )
= 1−
𝜂𝑑𝑖𝑒𝑠𝑒𝑙 = 1 −
((T4 − T1 )
γ(T3 − T2 )
1
𝜌𝛾 − 1
[
]
𝛾(𝑟)𝛾−1 𝜌 − 1
The net-work for diesel cycle can be expressed in terms of pvas follows
𝑝1 𝑣1 𝑟 𝛾−1 [𝛾(𝜌 − 1) − 𝑟 1−𝛾 (𝜌𝛾 − 1)]
W=
(𝛾 − 1)
Mean effective pressure 𝑃𝑚 is given by
Pm =
𝑃1 𝑟 𝛾 [𝛾(𝜌 − 1) − 𝑟 1−𝛾 (𝜌𝛾 − 1)]
(𝛾 − 1)(𝑟 − 1)
Dual Cycle
None of the actual engine have actual characteristics of burning in constant volume and constant
pressure condition. The combustion can be better modeled by the partial combustion in both the
constant volume and in constant pressure.
The dual combustion cycle (also known as the mixed cycle, Trinkler cycle, Seiliger cycle or
Sabathe cycle) is a thermal cycle that is a combination of the Otto cycle and the Diesel cycle.
Heat is added partly at constant volume and partly at constant pressure, the advantage of which is
that more time is available for the fuel to completely combust. Because of lagging characteristics
of fuel this cycle is invariably used for diesel and hot spot ignition engines. It consists of two
adiabatic and two constant volume and one constant pressure process. Efficiency lies between
Otto and diesel cycle.
The dual cycle consists of following operations:
Process 1-2 is reversible adiabatic compression
Process 2-3 is constant volume heat addition
Process 3-4 is constant pressure heat addition
Process 4-5 is reversible adiabatic expansion
Process 5-1 is constant volume heat rejection
i)
ii)
iii)
At point 1, Air enters the cylinder or air is completely filled inside the cylinder.
From point 1 to point 2, compression of air takes place & it is reversible adiabatic
compression.
During Compression process, there is decrease in volume with corresponding
increase in pressure keeping the entropy constant.
iv)
At point 2, heat addition (fuel addition) starts & it is added at constant volume is
constant but pressure & temperature increases.
v)
At point 3, heat addition starts at constant up to point 4. During process 3 to 4
pressure is constant but volume & temperature increases.
Once the heat addition process has completed, there is isentropic expansion of air .
Expansion takes place from point 4 to 5 & during the expansion process, entropy
remains constant but there is drop in pressure & temperature of air.
After expansion, heat – rejection starts from points and heat is rejected at constant
volume from point 5 to point-1. During this process both temperature & pressure
decrease.
vi)
vii)
The relations for the efficiencies, work done and map is as follows.
𝜂𝑑𝑢𝑎𝑙 = 1 −
Work done is given by
1
(𝛽. 𝜌𝛾 − 1)
∗
(𝑟)𝛾−1 [(𝛽 − 1) + 𝛽𝛾(𝜌 − 1)]
𝑝1 𝑣1 𝑟 𝛾−1 [𝛽𝛾(𝜌 − 1) + (𝛽 − 1) − 𝑟 𝛾−1 (𝛽𝜌𝛾 − 1)]
W=
𝛾−1
Mean Effective pressure 𝑃𝑚 is given by
Pm =
𝑝1 (𝑟)𝛾 [𝛽(𝜌 − 1) + (𝛽 − 1) − 𝑟 1−𝛾 (𝛽𝜌𝛾 − 1)]
(𝛾 − 1)(𝑟 − 1)
Brayton (Joule) Cycle:
The Brayton cycle is a thermodynamic cycle named after George Bailey Brayton that describes
the workings of a constant pressure heat engine. The original Brayton engines used a piston
compressor and piston expander, but more modern gas turbine engines and airbreathing jet
engines also follow the Brayton cycle.
A simple open cycle gas turbine consists of a compressor, combustion chamber and a turbine .
The compressor takes in ambient fresh air and raises its pressure. Heat is added to the air in the
combustion chamber by burning the fuel and raises its temperature. The heated gases coming out
of the combustion chamber are then passed to the turbine where it expands doing mechanical
work. Some part of the power developed by the turbine is utilized in driving the compressor and
other accessories and remaining is used for power generation. Fresh air enters into the
compressor and gases coming out of the turbine are exhausted into the atmosphere, the working
medium need to be replaced continuously. This type of cycle is known as open cycle gas turbine
plant and is mainly used in majority of gas turbine power plants as it has many inherent
advantages.
The Process that take place in Brayton cycle are.
•
•
•
•
process 1-2 Isentropic compression (in a compressor)
Process 2-3 Constant pressure heat addition
Process 3-4 Isentropic expansion (in a turbine)
Process 4-1 Constant pressure heat rejection
Closed Brayton cycle
QH
heat
exchanger
2
3
Wnet
compressor
1
turbine
4
heat
exchanger
QL
Advantages:
 These continuous flow machines offer a high-power density, and a respectable efficiency
at full load on the order of 30% for the open cycle, and even higher for the closed.

Brayton cycles are fairly insensitive to fuel quality. The sizes considered for this study
also have a distinct advantage over other methods in that the rotating components can be
supported by air bearings in any orientation.

no auxiliary lubricating fluid or oil is needed.
Disadvantages:
 The primary disadvantage is cost.
 Although expensive alloys are now used, the cost will decrease as less expensive
materials are used
 Due to less expensive material used more units are produced.
Atkinson Cycle
•
•
•
•
•
•
•
Operates on a four-stroke cycle
Intake, compression, combustion, and exhaust strokes occur in a one revolution of
crankshaft
Combustion stroke is longer than compression stroke allowing more expansion of
combustion gases
Greater efficiency when compared to Otto engines
Biggest disadvantage is reduction in power density (power/unit volume) arising from the
reduction in air intake.
Atkinson cycle engine can be supplemented with electric motor to provide more power if
Needed
Electric motors can be used in combination or independent of Atkinson cycle engines to
provide the desired power output most efficiently.
.
1. Reversible adiabatic compression
2. Heat addition at constant volume
3. Isentropic expansion
4. Heat rejection at constant pressure
Thermal efficiencyof Atkinson
cycle
The Atkinson cycle is a variation
on the Otto cycle which
effectively increased the engine's
expansion ratio compared with
the compression ratio by using a
complex crankshaft linkage. This
enables the exhaust stroke to be
longer than the induction stroke
and hence the swept volumes
are different. The greater
expansion allows more energy to
be extracted from the fuel charge
and allows the engine to run
cooler. It provides better
efficiency at the expense of
power density.
Advantages
• Reduces pumping losses in the engine by using a more
efficient combustion cycle
• Reduces the amount of fuel-air to be compressed, resulting in
greater fuel efficiency
• Allows the expansion stroke to be longer than the compression
stroke, improving thermal efficiency
Disadvantages
• Requires a more complex linkage system
• Some engine power is lost due to backflow of fuel-air mixture
Stirling Cycle
A Stirling cycle is like an Otto cycle, except that the adiabats are replaced by isotherms.
The idealized Stirling[5] cycle consists of four thermodynamic processes acting on the working fluid
(See diagram to below):
1.
2.
3.
4.
3-4 ISOTHERMAL Heat addition (expansion).
4-1ISOCHORIC Heat removal (constant volume).
1-2 ISOTHERMAL Heat removal (compression).
2-3 ISOCHORIC Heat addition (constant volume).
Advantages of Stirling Cycle
•
Potentially high thermal efficiency.
•
Fairly insensitive to fuel or fuel quality.
•
No lubricants needed.
•
the working fluid can support bearing loads.
Disadvantages
•
High cost, which may decrease as less expensive materials are used and more units are
produced.
•
Low power density and low power-to-weight ratio.
•
Working fluid leakage and sealing.
Simple Comparison between all cycles
1.2 Assumption
in Fuel Air Cycle Analysis
Fuel-Air cycle is defined as the theoretical cycle that is based on the actual properties of the
cylinder gas. The results obtained from analysis are much greater than the actual performance.
For example, an engine with CR=7 has a thermal efficiency (based on air cycle analysis) equals
to 54% while the actual value does not exceed 30%.
Following are the assumption mad in the analysis of the Fuel Air cycle:
1. The gas mixture is treated as ideal gas. During first half of the cycle only 7% of the
mixture is fuel vapour. Even in the second half the gas composition is mostly CO2, H2O
and N2. Air is treated as an ideal gas as this doesn’t created large errors.
2. By assuming that the exhaust gas is fed back to the intake the system is assumed to be the
closed cycle. Since the composition of both the exhaust and intake is same the
assumption is fair.
3. Since there is no external source of heat used the combustion of the fuel inside is
considered as Heat addition from the external source.
4. Similarly, open exhaust process takes away large amount of heat which is assumed as
heat rejection.
5. Although small amount of heat is lost from the cylinder walls it is assumed that no heat
loss takes place in the process.
6. All the process is considered to be reversible.
7. Exhaust blowdown is approximated by a constant volume process.
8. Compression and expansion process is idealized as isentropic process.
9. No chemical reaction takes place in the engine cylinder.
10. There is neither friction nor turbulence.
Real engine cycle differs from ideal engine cycle as:
1. Heat Transfer: Due to heat transfer from the wall of the cylinder, piston and cylinder
head the energy is lost which make the real air standard efficiency less than the air
standard efficiency.
2. Finite combustion time: In the theoretical approach the combustion takes in an
infinitesimal time but in the real life it takes certain time for the combustion to occur.
3. Exhaust blowdown loss: The exhaust blows down loss causes the usable amount of
heat energy to go to the surrounding without doing any work. This decreases the
efficiency of the engine by a certain amount.
4. Crevice effects and leakage: As the pressure increases leakage takes place from the
crevice region of the piston, piston ring and cylinder wall.
5. Incomplete combustion: The combustion taking inside the cylinder is not perfect. The
combustion may be incomplete. In SI engine 5% of the fuel goes without combustion
while in CI engine the combustion efficiency is about 98%.
1.3 COMPOSITION OF CYLINDER GASES
Cylinder gases comprises intake gases and exhaust gases.
(a) Composition of intake gases:
• During intake stroke, fuel (gasoline mostly made up mostly of H and C) is sucked
in along with the oxidizer i.e. air (80% nitrogen and 20% oxygen).
• For combustion, every mole of O2 there are 3.76 moles of N2 in dry air.
• Stoichiometric reaction of octane is as:
C8H18
+ (12.5) (O2+3.76N2)
8CO2+9H2O+47N2
(b)Composition of exhaust gases:
•
Complete combustion would actually result in carbon dioxide (CO2) and water vapor
(H2O).
•
•
•
However incomplete combustion occurs in the cylinder and some gasoline (HC) vapor
does not burn which leaves the cylinder with some HC, carbon monoxide (CO) and
nitrogen oxides (NOx).
Carbon monoxide is formed due to insufficient oxygen.
Nitrogen oxides is the result of high temperature in the combustion chamber which
causes some of the nitrogen and oxygen to unite and form NOx.
HC + N + O2 = CO2 + H2O + CO +NOX +HC
 The gases like HC, CO and NOX are air pollutants. The amount of these can be
controlled in the vehicles.
 Crankcase emission control system: sends gases back through the engine to be burned
and prevents their escape into the atmosphere.
 Evaporative emission control system: traps the fuel vapors escaping from the air
cleaner, carburetor and fuel tank, and then returned to the engine.
 Exhaust emission control: emission control devices like catalytic converter are used to
reduce the pollutants in the exhaust gases.
Chapter 2: Engine Construction and Operation:
(Roll, 607,608,609,610,611,612)
2.1 WORKING PRINCIPLE OF SPARK-IGNITION ENGINE
SI engine is a type of ICE consisting of a spark plug, for the purpose of igniting the mixture of
air and fuel, mainly designed to run on petrol (gasoline). Usually, the air and fuel are mixed
together in the intake, either by using a carburetor or fuel-injection system (electronically
controlled). The ratio of mass flow of air to fuel must be approximately around 15 for reliable
combustion.
CYCLE ON WHICH SI ENGINE IS BASED
OTTO CYCLE
Otto cycle; an idealized thermodynamic cycle, describes the theoretical working of an SI engine.
It is a description of what happens to a mass of gas when it is subjected to a change of pressure,
temperature, addition of heat, removal of heat and volume.
F
i
g
:
P
V
a
n
d
T-S Diagram of SI Engine
Process 0-1: Intake stroke
Mass of air is drawn into the cylinder, at constant pressure, from the atmosphere.
Process 1-2: Compression strokePiston moves from BDC to TDC as the working fluid is
compressed isentropically. The compression ratio for SI engine should in the range of 6-12.
Process 2-3: Ignition or heat addition stroke
It is a constant volume heat addition process where the compressed mixture is ignited with the
help of spark plug.
Process 3-4: Expansion stroke (power stroke)
Increased pressure exerts a force on the piston causing it to move downwards to the BDC.
Expansion of the working fluid is isentropic, and work is done on the piston.
Process 4-1: Heat rejection stroke
Working fluid pressure decreases during a constant volume process as heat is rejected to an
idealized sink.
Process 1-0: Exhaust stroke
The exhaust valve opens, as the piston moves from BDC to TDC, and the gaseous mixture is
vented out into the atmosphere and the process starts anew.
CI Engine
The combustion process in a CI engine starts when the air-fuel mixture self-ignites due to high
temperature in the combustion chamber caused by high compression.CI engines are often called
DIESEL engines, especially in the non-technical community.
Working principle of CI engine:
In four-stroke cycle engines there are four strokes completing two revolutions of the crankshaft.
These are respectively: The suction, Compression, Power and Exhaust strokes.
Suction stroke:
In suction stroke, inlet valve is opened and exhaust valve is closed. Air, which has been drawn
into the cylinder during the suction stroke, is progressively compressed as the
piston
ascends. The compression ratio usually varies from 14:1 to 22:1. The pressure at the end of the
compression stroke ranges from 30 to 45 kg/cm2.
Fig: P-V diagram of CI Engine
Compression stroke:
Both valves are closed. As the air is progressively compressed in the cylinder, its temperature
increases, until when near the end of the compression stroke, it becomes sufficiently high (650800 C) to instantly ignite any fuel that is injected into the cylinder. When the piston is near the
top of its compression stroke, a liquid hydrocarbon fuel,
such as diesel oil, is sprayed into the
combustion chamber under high pressure (140-160 kg/cm2), higher than that existing in the
cylinder itself.
Power Stroke:
This fuel then ignites, being burnt with the oxygen of the highly compressed air. During the
fuel injection period, the piston reaches the end of its compression stroke and
commences to
return on its third consecutive stroke, viz., power stroke. During this
nitrogen left from the
compressed air expand, thus forcing the piston downward. This is only the working stroke of
the cylinder. During the power stroke the pressure falls from
its maximum combustion
2
value (47-55 kg/cm ), which is usually higher than the greater
value of the compression
2
2
pressure (45 kg/cm ), to about 3.5-5 kg/cm near the end of the stroke. The exhaust valve then
opens, usually a little earlier than when the piston reaches its lowest point of travel.
Exhaust Stroke:
The exhaust gases are swept out on the following upward stroke of the piston. The reciprocating
motion of the piston is converted into the rotary motion of the crankshaft.
GAS TURBINE
INTRODUCTION
A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has
first the compression section followed by combustion chamber and turbine section. It uses air
and fuel for working. Gas turbines are used to power aircraft, trains, ships, electrical generators,
and tanks
A simple gas turbine is comprised of three main sections a compressor, a combustor, and a
power turbine. The gas-turbine operates on the principle of the Brayton cycle, where compressed
air is mixed with fuel, and burned under constant pressure conditions. The resulting hot gas is
allowed to expand through a turbine to perform work.
WORKING PRINCIPLE
In an ideal gas turbine, gases undergo three thermodynamic processes: an isentropic
compression, an isobaric (constant pressure) combustion and an isentropic expansion. Together,
these make up the Brayton cycle. In gas turbine the mechanical energy is converted in pressure
and heat.Heat is added in the combustion chamber and the specific volume of the gas increases,
accompanied by a slight loss in pressure. During expansion through the stator and rotor passages
in the turbine, irreversible energy transformation once again occurs.
FIG: Brayton cycle (p-v and T-s diagram)
Gas turbine find use in jet engine of the aircraft and the specific operation of the turbine in jet
engine can be summarized as:
•
•
•
•
•
•
•
•
Air gets into the engine by forward motion of the engine and sucking effect of the compressor.
Diffuser increases pressure and temperature of this fluid to some extent, by converting some
part of kinetic energy to pressure energy.
After that, energy addition by compressor takes place and P and T increases even more.
At compressor outlet, very high pressure and temperature.
Compressor requires some power input to do that. This power is given by a turbine which is
situated right after the
combustion chamber.
Turbine absorbs some
amount of energy from
the high energy fluid
and transmits it to the
compressor.
Hence, production of
high velocity jet at
outlet is selfsustainable.
We will get supply of
high velocity jet and
thrust force to this aircraft due to synchronized working of these components.
Change in the level of energy of fluid from inlet of gas turbine to exit
1-2: Assuming 1-2 is an adiabatic reversible process, entropy remains same. Pressure and
temperature rises slightly.
2-3: In the compressor, pressure and temperature rises to a level where combustion process is
sustainable.
3-4: Heat addition to the fluid at constant pressure. Temperature of stream rises to a very high
level.
4-5: Turbine will absorb some amount of energy required by the compressor.
5-6: Nozzle produces high velocity jet. Entropy is constant. Internal energy is converted into
kinetic energy. Pressure expands to surrounding pressure.
Exit stream does not go back to inlet. Inlet sucks fresh stream of air, hence it is an open cycle
process. But, point 6 and point 1 has same pressure of air, thus we can assume a pseudoconstant pressure process like shown by dotted line in order to complete a cycle.
It is a Brayton Cycle.
Difference of Gas turbine and IC engine
Gas Turbine
Perfect balancing of rotating parts.
Mechanical efficiency is high (95%)
There is no need of flywheel as the torque on
the turbine shaft is continuous
The weight of gas turbine is 0.15 kg/KW
High rpm
Max pressure is 5 bar
Components of gas turbine are loghter
Gas turbine can use cheaper fuel
The exhaust gas is less polluting.
The starting of gas turbine is difficult.
Thermal efficiency is low (15-20%)
Temperature of gases supplied to the turbine
is limited to 1100k
IC engine
Difficult to balance perfectly
Mechanical efficiency is low (85%)
Flywheel is needed
The weight of IC engine is 2.5 kg/KW
Low rpm
Max pressure is 60 bar or more
Components of IC engine are heavier
High grade fuels are used to avoid knocking
The exhaust gas is polluting and needs
treatment.
Starting is relatively easier.
Thermal efficiency is high (25-30%)
Gas temperature can be higher
ADVANTAGES OF GAS TURBINE
•
Very high power-to-weight ratio, compared to reciprocating engines
•
Smaller than most reciprocating engines of the same power rating
•
Moves in one direction only, with far less vibration than a reciprocating engine
•
Fewer moving parts than reciprocating engines
•
Greater reliability, particularly in applications where sustained high power output is
required
•
Waste heat is dissipated almost entirely in the exhaust. This results in a high temperature
exhaust stream that is very usable for boiling water in a combined cycle, or
for cogeneration.
DISADVANTAGES
•
•
•
•
Cost is very high
Less efficient than reciprocating engines at idle speed
Longer startup than reciprocating engines
Less responsive to changes in power demand compared with reciprocating engines
MAJOR ENGINE COMPONENTS
Following is the list of major components in most of the reciprocating internal combustion (IC)
engines:
ENGINE BLOCK
It contains the cylinders of the engine and made of cast iron or aluminum. Water jacket cast is
made around the cylinders in water-cooled engines while on air-cooled engines, the exterior
surface of the block has cooling fins.
CAMSHAFT
Rotating shaft used to push open valves at the proper time in the engine cycle, either directly or
through mechanical or hydraulic linkage (push rods, rocker arm, tappets). They are generally
made of forged steel or cast iron and driven by crankshaft by means of belts, chains or timing
gears. Camshaft rotates at half the engine speed in four-stroke engine.
CARBURATOR
Blends air and fuel in an internal combustion engine in proper ratio for combustion. The speed of
air flow and thus its pressure determines the amount of fuel drawn into the airstream.
CATALYTIC CONVERTER
Chamber mounted in exhaust flow containing catalytic material that reduces emissions by
chemical reaction.
COMBUSTION CHAMBER
Region between piston head and engine head where combustion of air-fuel mixture occurs.
Fig: Components of petrol engine
COOLING FINS
Metal fins on the outer surface of engine in an air-cooled engine to improve conduction and
convection.
CRANKSHAFT
Rotating shaft through which engine work output is supplied to the external systems.Crankshaft
are usually made of forged steel or cast iron. Reciprocating pistons rotate the crankshaft through
connecting rods.
CYLINDERS
They are the central working part of reciprocating engine where piston reciprocates. The wall of
the cylinder has highly polished hard surface. Cross section is usually round.
EXHAUST MANIFOLD
Piping system that carries exhaust gases out of the engine cylinders, usually made of cast iron.
FAN
Engine driven and used to increase air flow through the radiator to facilitate waste heat removal
from the engine.
FLYWHEEL
Rotating mass with large moment of inertia, connected to the crankshaft, to store energy and
furnish a large angular momentum that keeps the engine rotating between power strokes and
smoothen engine operation.
FUEL INJECTOR
Pressurized nozzle that sprays fuel into the incoming air on SI engines or into the cylinder on CI
engines.
FUEL PUMP
It supplies fuel from fuel tank to the engine and is driven electrically or mechanically.
GLOW PLUG
Small electrical resistance mounted inside the combustion chamber of many CI engines, used to
preheat the chamber enough so that combustion will occur when first starting a cold engine. The
glow plug is turned off after the engine is started.
HEAD
Closes the ends of the cylinders, usually made of cast iron or aluminum and bolts to the engine
block. Sometimes it may contain clearance volume of the combustion chamber. Head contains
spark plugs in SI engines and fuel injectors in CI engines. Most modern engines have the valves
and many have camshaft as well in the head called overhead valves and overhead cam.
HEAD GASKET
Gasket which serves as a sealant between the engine block and head where they bolt together.
They are usually made in sandwich construction of metal and composite materials. Some engines
use liquid head gaskets.
INTAKE MANIFOLD
Piping system that delivers incoming air to the combustion chamber and usually made of cast
metal or composite materials.
MAIN BEARING
Bearings connected to the engine block in which the crankshaft rotates.
OIL PAN
Oil reservoir bolted to the bottom of the engine block.
OIL PUMP
Pump used to distribute oil from the oil sump to required lubrication points.
OIL SUMP
Reservoir for the oil system of the engine. Commonly part of the crankcase. Some engines
(aircraft) have a separate closed reservoir called a dry sump.
PISTON
Cylindrical mass that reciprocates in the cylinder and transmits pressure force from the
combustion chamber to the crankshaft as work via connecting rods. The top of the piston is
called crown and the sides are called skirt. Some pistons have indented bowl in the crown that
makes large percentage of clearance volume. Pistons are made of cast iron, steel or aluminum
and have low thermal expansion.
Fig: Piston and Connecting part components
PISTON RINGS
They fit into the circumferential grooves on the skirt of the piston and form a sliding surface
against the cylinder walls. Two or more compression rings are placed in the grooves near the
crown to form seal between the piston and the combustion chamber and to prevent the highpressure gases in the combustion chamber from leaking past the piston into the crankcase. Oil
ring is placed in groove below the compression rings to assist in lubricating the cylinder walls
and scrape away excess oil to reduce oil consumption.
CONNECTING ROD
The rod connecting piston with the rotating crankshaft that moves piston up and down in the
cylinder. It is usually made of steel or alloy forging or aluminum.
CONNECTING ROD BEARINGS
Bearing where connecting rod fastens to crankshaft.
RADIATOR
Heat exchanger used for cooling engines via engine coolant, usually mounted in front of the
engine. Flow of air as automobile moves forward carries away the heat from the radiator.
SPARK PLUG
Initiate combustion by high voltage discharge across the electrode gap, usually made of metal
surrounded with ceramic insulation.
STARTER
Most engines are started by electric motor geared to the engine flywheel where energy is
supplied to the motor from battery. For large engines as in tractor and construction equipment,
small IC engines are used as starters which are first started with normal electric motor. The small
engine then engage gearing on the flywheel of the large engine, turning it until the large engine
starts. Compressed air is also used to start some large engines.
SUPERCHARGER
Mechanical compressor used to compress incoming air, powered by engine output.
THROTTLE
Butterfly valve used to control the amount of air flow into the engine.
TURBOCHARGER
Turbine compressor used to compress incoming air as done by supercharger but is powered by
exhaust flow.
VALVES
Allow air or exhaust flow into or out of the combustion chamber at proper time in each engine
cycle, usually made of forged steel. Most common valve in engines is Poppet Valves which are
spring loaded. Rotary Valves and Sleeve Valves are sometimes used.
WATER JACKET
Liquid flow passage around the cylinders, usually constructed as part of engine block and head.
Coolant flows through the jacket and keeps the cylinder from overheating.
WRIST PIN
Pin fastening the connecting rod to the piston, also called piston pin.
2.3 Working of Different Types of Engine
2.3.1 Four Stroke SI Engine
Figure: Four stroke engine
Working of four stroke SI engine is described in following steps:
1. Intake stroke
In this stroke piston travels from TDC to BDC with intake valve open and exhaust
valve closed. This creates a vacuum inside the cylinder and sucks the air. When air is
sucked, fuel is also added and mixed with the help of carburetor.
2. Compression stroke
When piston reaches BDC, inlet valve closes and piston travels back to TDC. This
process compresses fuel mixture raising temperature and pressure. At the end of this
stroke spark plug is fired and combustion is started.
3. Combustion
Combustion of fuel mixture occurs for very short interval of time. This process
increase temperature and pressure. In this stroke fuel mixture is changed to exhaust
product.
4. Power stroke
In this process all valves remain closed and pressure created in combustion process
pushes the piston away from TDC. In this stroke work output is produced and volume
is increasedso pressure and temperature is decreased.
5. Exhaust stroke
In this stroke exhaust valve is opened and exhaust blowdown occurs due to pressure
difference created between exhaust and atmosphere. Here exhaust gas carries one third
amount energy which lowers efficiency.
2.3.2 Four Stroke CI Engine
Working of four stroke CI engine is described as follow:
1. Intake stroke
In this stroke piston travels from TDC to BDC with intake valve open and exhaust
valve closed. This creates a vacuum inside the cylinder and sucks the air. In this
stroke no fuel is added to the incoming air.
2. Compression stroke
When piston reaches BDC, inlet valve closes and piston travels back to TDC. This
process compresses air raising temperature and pressure. Then fuel is added to the
combustion chamber, where it mixes with the hot air. This cause fuel to evaporate
and self ignite.
3. Combustion
After self ignition of fuel combustion process is started. This process occurs at
constant pressure.
4. Expansion stroke
Power stroke continues as combustion process ends. In this stroke all valves remain
closed and work output is produced which cause piston to move from TDC to BDC.
5. Exhaust stroke
In this stroke exhaust valve is opened and exhaust blowdown occurs due to pressure
difference created between exhaust and atmosphere. Here exhaust gas carries one
third amount energy which lowers efficiency.
2.3.3 Two Stroke SI Engine
Figure: Two stroke engine
Working of two stroke SI engine is described as follows:
1. Combustion
When piston is at TDC combustion occurs raising temperature and pressure at
constant volume.
2. Expansion stroke
In this stroke piston moves downward due to pressure created by combustion
process. Expanding volume of combustion chamber causes pressure and
temperature to drop.
3. Exhaust stroke
At 75obBDC, the exhaust port opens and blowdown occurs. After blowdown the
cylinder remains filled with exhaust gas at lower pressure.
4. Intake and Scavenging
When blowdown is complete at 50obBDC, intake port opens on the side of
cylinder and fuel mixture enters under pressure. Incoming mixture pushes much
of remaining gases out from exhaust port and fills with combustible fuel mixture
which is called scavenging.
5. Compression stroke
With all valves closed, piston travels towards TDC which compresses the fuel
mixture to higher temperature and pressure. Near the end of compression stroke
spark plug is fired and combustion occurs.
Animated GIF for Two Stroke Engine
2.3.4 Two Stroke CI Engine
Working of two stroke engine is described as follows:
1. Combustion
When piston is at TDC combustion occurs raising temperature and pressure at
constant volume.
2. Expansion stroke
In this stroke piston moves downward due to pressure created by combustion
process. Expanding volume of combustion chamber causes pressure and
temperature to drop.
3. Exhaust stroke
At 75obBDC, the exhaust port opens and blowdown occurs. After blowdown the
cylinder remains filled with exhaust gas at lower pressure.
4. Intake and Scavenging
When blowdown is complete at 50obBDC, intake port opens on the side of
cylinder and air enters under pressure. Incoming air pushes much of remaining
gases out from exhaust port and fills with air which is called scavenging.
5. Compression stroke
With all valves closed, piston travels towards TDC which compresses the air to
higher temperature and pressure. Near the end of compression fuel is added
through fuel injector which is vaporized which cause self ignition of fuel and
combustion occurs.
2.3.5 Difference Between Four Stroke and Two Stroke Engine
S.N. Aspects
Four stroke engine
1.
Completion of cycle Completed in 4 stroke.
2.
Flywheel
3.
Power produced for
same size
Cooling and
lubrication
requirement
Valve mechanism
4.
5.
6.
7.
8.
9.
Initial cost
Volumetric
efficiency
Thermal efficiency
Applications
Motion is not so uniform
so heavy flywheel is
required.
Low
Two stroke engine
Completed in two
stroke.
Motion is uniform so
lighter flywheel is
required.
High
Low
High
Fitted with valve and
valve mechanism.
Higher
More
Fitted with port.
Higher
Cars, bus, truck,
aeroplane etc.
Lower
Scooter, lawn mover
etc.
Lower
Less
DIFFERENCE BETWEEN SI AND CI ENGINE
S.N
1.
Parameter
Definition
SI engine
It is an engine in which spark
is used to burn the fuel.
2.
Fuel used
Petrol is used as fuel.
3.
Operating cycle
It operates on Otto cycle.
4.
Compression ration
Low compression ration
CI engine
It is an engine in
which heat of
compressed air is
used to burn the
fuel.
Diesel is used as
fuel.
It operates on Diesel
cycle,
High compression
5.
Thermal efficiency
High thermal efficiency
6.
Method of ignition
Spark plug is used to produce
spark for the ignition.
7.
8.
Engine speed
Pressure generated
High speed engines
Low pressure is generated
after combustion
9.
Constat parameter during cycle Constant volume cycle
10.
11.
Intake
Weight of engine
Air+fuel
SI engine has less weight
12.
Noise production
It produces less noise.
13.
Production of hydrocarbon
Less hydrocarbon is produced.
14.
Starting
Starting of SI engine is easy.
15.
16.
17.
18.
19.
20.
Maintenance cost
Vibration problem
Cost of engine
Volume to power ratio
Fuel supply
Application
Low
Less
Less cost
Less
Carburetor
Used in light commercial
vehicles like motorcycle, cars,
etc.
ratio.
Less thermal
efficiency.
Heat of compressed
air is used for the
ignition.
Low speed engines.
High pressure is
generated after
combustion.
Constant pressure
cycle.
Only air
CI engine are
heavier.
It produces more
noise.
More hydrocarbon
is produced.
Starting of CI
engine is difficult.
High
Very High
High cost
High
Injector
Used in heavy duty
vehicles like bus,
trucks, ships, etc.
CHAPTER 3. Engine Fuels
Submitted by
072BME613,
072BME614
072BME615
072BME616
072BME617
072BME618
Submitted to:
Dr. Ajay Kumar Jha
Department of Mechanical Engineering
Pulchowk Campus
Engine Fuels (6 hours)
3.1 Basic requirements of engine fuels:
3.2 Chemical structure of petroleum
3.3 Heat value of fuels.
3.4 Rating of SI Engine fuels,
3.5 Rating of CI engine fuels
3.6 Combustion equation for hydrocarbon fuels
3.7 Properties and ratings of petrol and diesel fuels
3.8 Fuel supply systems of SI and CI engines
3.9 Non-conventional fuels for IC engines; LPG, CNG, Methanol, Ethanol, Non-edible
vegetable oils, Hydrogen.
3.1 Basic Requirement Of Engine Fuel
Knocking characteristics: There is always a time lag between the injection of the fuel and
burning of the fuel. When the fuel is finally burnt, excessively large amounts of energy is
released, which produces extremely high pressure inside the engine. This causes the knocking
sound inside the engine, which can be clearly heard. Thus the engines should have a short
ignition lag so that the energy is produced uniformly inside the engine and there is no abnormal
sound. The ignition of the fuel also affects starting, warming, and production of exhaust gases in
the
engine.
The knocking capacity of the fuel is measured in terms of octane rating and cetane rating of the
fuel. The fuel you are using for your CI engine should have a cetane number high enough to
avoid knocking of engine.
2) Volatility of the fuel: Thorough mixing of the fuel and air when fuel is injected in the
cylinder head ensures uniform burning of the fuel. The fuel should be volatile in nature within
the operating temperature range of the cylinder head so that it gets converted into a gaseous state
and mixes thoroughly with air.
A highly volatile ( of low molecular weight ) fuel generates a rich fuel air ratio at low starting
temperature, to satisfy the criteria at the starting of the ignition. But, it will create another
problem during running operation, it creates vapour bubble which choked the fuel pump delivery
system.
This
phenomenon
is
known
as vapour
lock.
A vapour lock thus created restricts the fuel supply due to excessive rapid formation of vapour in
the
fuel
supply
system
of
the
carburettor.
High volatility of fuel can also result in excessive evaporation during storage in a tank which will
also
pose
a
fire
hazards.
Low volatile fuel like kerosene and distillates can be used for SI engines for tractors.
3) Starting characteristics of the fuel: The smooth starting of the vehicle depends greatly on
the fuel used for the vehicle. For easy starting of the vehicle it is important that the fuel has good
volatility so that it mixes with the air uniformly and it readily forms into the combustible
mixture. The high cetane number of the fuel ensures that the ignition of the fuel will be fast,
which in turn will lead to faster starting of the vehicle.
4) Smoke produced by the fuel and its odor: The exhaust gases produced from the fuel should
not have too much smoke and odor.
5) Viscosity of the fuel: The fuel should have a viscosity low enough so that it can easily flow
through the fuel system and the strainer at the lowest working temperatures.
6) Corrosion and wear: The fuel used for the IC engine should not cause corrosion of any
components of the engine before or after combustion.
7) Easy to handle: Large quantities of fuel for a IC engine have to be transported from one place
to the other, hence it should be easy to handle and transport. The fuel should have a high flash
point and high fire point to avoid it catching fire during transport.
8) Cleanliness
The fuel should be free from any unwanted impurities.
SUMMARY
 It should readily mix with air to make a uniform mixture at inlet, i.e it must be volatile.
 It must be knock resistant.
 It should not pre-ignite easily ( In case of SI Fuel)
 It should not tend to decrease the volumetric efficiency of the engine.
 It should not form gum and varnish.
 Its sulphur content should be low as it is corrosive.
 It must have a high calorific value.
3.2 Chemical Structure Of Petroleum
Petroleum or crude oil is a complex mixture of hydrocarbons and other chemicals. The
composition varies widely depending on where and how the petroleum was formed. In fact, a
chemical analysis can be used to fingerprint the source of the petroleum. However, raw
petroleum or crude oil has characteristic properties and composition.
HYDROCARBONS IN CRUDE OIL
There are four main types of hydrocarbons found in crude oil.
 paraffin (15-60%)- alkanes
 naphthenic (30-60%)-cycloalkanes
 aromatics(3-30%)
 asphaltic (remainder)
 The hydrocarbons primarily are alkanes, cycloalkanes and aromatic hydrocarbons.
 ELEMENTAL COMPOSITION OF PETROLEUM
 Although there is considerable variation between the ratios of organic molecules, the
elemental composition of petroleum is well-defined:
 Carbon - 83 to 87%
 Hydrogen - 10 to 14%
 Nitrogen - 0.1 to 2%
 Oxygen - 0.05 to 1.5%
 Sulfur - 0.05 to 6.0%
 Metals - < 0.1%
 The most common metals are iron, nickel, copper and vanadium.
3.3 Heat value of fuels
Introduction
Heating value of the fuel is also known as calorific value of fuel. Heating Value of a fuel is the
amount of heat energy released during the combustion of a specified amount of the fuel in the
presence of excess of oxygen. It is generally measured in the unit like KJ/kg or MJ/Kg. Heating
value of gasoline is 44MJ/Kg and that of diesel is 42MJ/Kg. Bomb calorimeter is used to
measure the value of calorific value of fuel.
A bomb calorimeter is a type of constant-volume calorimeter used in measuring the heat of
combustion of a particular reaction. Bomb calorimeters have to withstand the large pressure
within the calorimeter as the reaction is being measured. Electrical energy is used to ignite the
fuel; as the fuel is burning, it will heat up the water outside the bomb vessel. The change in
temperature of the water allows for calculating calorie content of the fuel.
Fig:Bomb Calorimeter
Higher Heating Value (HHV)
The gross or high heating value is the amount of heat produced by the complete combustion of a
unit quantity of fuel.It is also known as gross calorific value.
The gross heating value is obtained when:
➢ All products of the combustion are cooled down to the temperature before the
combustion.
➢ The water vapor formed during combustion is condensed.
Lower Heating Value (LHV)
Lower heating value determined by subtracting the heat of vaporization of the water vapor from
the higher heating value. This treats any H2O formed as a vapor. The energy required to
vaporize the water therefore is not released as heat. Lower heating value (LHV) is calculated
with the product of water being in vapor form.It is also known as net calorific value.
Hv is the heat of vaporization of water,
nH2O,out is the moles of water vaporized
andnfuel,in is the number of moles of fuel combusted
3.4 Rating of SI engine fuel
Introduction
The fuel used in the spark ignition engine is petrol. When the temperature of the air fuel mixture
is raised high enough the mixture will self ignite without need of spark plug generating pressure
pulses which can damage the engine which is known as knocking.The compression ratio which
can be utilized depends on the fuel to be used and a scale has been developed against which the
knock tendency of the fuel can be rated.The rating is given as octane number.
fig. cylinder pressure vs time for different knocking intensities
Octane number
The octane number of the fuel is the percentage of octane in the reference mixture which knocks
under the same conditions of fuel. For example. The octane number of any fuel is 90 means that
it consists of 90% of iso-octane and 10% normal heptane by volume. High octane fuels can be
produced by refining process, but it can be done more cheaply, and more frequently, by the use
of anti-knock additives, such as tetraethyl lead.
 The antiknock value of an SI engine fuel is determined by comparing its antiknock
property with a mixture of two reference fuels, normal heptane (C7H18) and iso-octane
(C8H18)
Iso octane (excellent & score =100)
n heptane(poor & score=0)
Measurement methods of octane number
 Research Octane Number (RON):
▪
▪
At 600 rpm (most common rating method)
▪
determined by running the fuel in a test engine with a variable compression
ratio under controlled conditions, and comparing the results with those for
mixtures of iso-octane and n-heptane.
Motor Octane Number (MON):
▪
At 900 rpm engine speed
▪
With a preheated fuel mixture, higher engine speed, and variable ignition
timing to further stress the fuel's knock resistance
Avg of two = Road Octane Number
Addition of tetraethyl lead
 Addition of even a small amount is highly effective
 Octane number increase from 92 to 93 produces greater antiknock effect than a similar
increase from 32 to 33 octane number
 However, the antiknock effectiveness of tetraethyl lead, for the same quantity of fuel
added, decreases as the total content of the lead in the fuel increases
Graph betn octane no increase and TEL added
Advantages of high-octane number
•
•
•
The engine can be operated at high compression ratio and therefore, with high
efficiencies without detonation.
The engine can be supercharged to high output without detonation.
Optimum spark advance may be employed raising both power and efficiency.
3.5 Compression Ignition:
The concept behind compression ignition involves using the latent heat buildup by highly
compressing the air inside the combustion chamber in order to heat the air above the flash
point of the fuel to cause selfignition. It involves compressing air inside the combustion
chamber to a ratio about 21:1.Mist of precisely metered fuel is injected inside the
combustion chamber. Atomized fuel diffuses in the hot air bursting into controlled
explosion. It isalso referred to as diesel engine.
This contrasts with spark-ignition engines such as a petrol engine (gasoline engine) or gas
engine (using a gaseous fuel as opposed to petrol), which use a spark plug to ignite an airfuel mixture.
Benefits of compression ignition
•
Reduced wear and tear compared to spark ignition engine
•
Spark plug isn’t required
•
Increased efficiency hence better fuel economy
•
Diesel engines are more efficient than gasoline (petrol) engines of the same power rating,
resulting in lower fuel consumption.
•
While a higher compression ratio is helpful in raising efficiency, diesel engines are much
more efficient than gasoline (petrol) engines when at low power and at engine idle.
Unlike the petrol engine, diesels lack a butterfly valve (throttle) in the inlet system, which
closes at idle. This creates parasitic loss and destruction of availability of the incoming
air, reducing the efficiency of petrol engines at idle.
•
Quiet operation and requires less maintenance
•
Diesel engines produce more torque than petrol engines for a given displacement due to
their higher compression ratio.
•
Since the diesel engine uses less fuel than the petrol engine per unit distance, the diesel
produces less carbon dioxide (CO2) per unit distance.
•
Diesel fuel is a better lubricant than petrol and thus, it is less harmful to the oil film
on piston rings and cylinder bores as occurs in petrol powered engines
Rating of fuel
Generally fuels are rated based on their antiknock qualities. Gasoline is rated by a
parameter called Octane number. Diesel is rated by another parameter called Cetane
number. Knocking in SI engine is due to pre ignition of fuel whereas knocking in CI
engine is due to ignition lag. Hence different parameters are used to measure their
antiknock qualities.
Cetane number
In compression-ignition engines, the knock resistance depends on chemical
characteristics as well as on the operating and design conditions of the CI engine.
Therefore, the knock rating of a diesel fuel is found by comparing the fuel under
prescribed conditions of operation in a special engine with primary reference fuels. The
reference fuels are normal cetane (C16H34), which is arbitrarily assigned a cetane number
of 100 and α-methyl napthalene (C11H10) with an assigned cetane number of 0. Thus,
cetane number is defined as the percentage, by volume, of cetane in a mixture of cetane
and alpha-methyl-naphthalene that produces the same ignition lag as the fuel being
tested, in the same engine and under the same operating conditions. Higher the cetane
number better is the antiknock quality of the fuel and vice versa.Since ignition delay is
the primary factor in controlling the initial auto ignition in CI engine, it is reasonable to
conclude that knock should be directly related to the ignition delay of the fuel. Knock
resistance property of diesel oil can be improved by adding small quantities of
compounds like amyl nitrate, ethyl nitrate or ether.The cetane number of diesel oil, in
general, is 40 to 55
Benefits of high cetane number diesel fuel
The benefit of high cetane number diesel fuel is as follows:
•
Shortened ignition delay
•
Improved cold start
•
Reduced white smoke during warm up
•
Reduced emissions (HC, CO, NOx and PM)
•
Reduced engine noise
•
Reduced fuel consumption.
•
Fewer misfires
•
High cetane fuels may even provide more power. Fuels refined with a high cetane
number typically are lighter fuels and contain a lower British thermal unit (Btu) content.
Methods to improve Cetane number of diesel oil
The use of Cetane Number Improvers (CNI) enables the ignition quality of distillate stocks to be
simply and economically increased. The additives increase the Cetane Number of distillate fuels
by improving fuel ignition characteristics in the combustion chamber during the compression
stroke.
Some Cetane number improvers are:
•
CI-0801 is a 2-Ethylhexyl nitrate (2-EHN)
•
CI-0808 is a blend of Di-tert-butyl peroxide in solvent and is typically used at 0.05% to
0.50% volume.
•
Amyl nitrate, ethyl nitrate or ether.
3.6 Combustion equation for hydrocarbon fuel
Hydrocarbon combustion refers to the type of exothermic reaction where a hydrocarbon
reacts with oxygen to create carbon dioxide, water and large quantity of heat.
Hydrocarbon are the compounds that contains only carbon and hydrogen. During
combustion the reactants are completely converted into the products. During the
combustion hydrocarbons and oxygen are used as reactants. After the complete
combustion the carbon from the fuel is completely converted into co2 and hydrogen into
h20. The large amount of thermal energy is also produced. The amount of thermal
energy produced depends upon the calorific value of the fuel. Calorific value of a fuel is
defined as the amount of heat released by a unit weight or unit volume of a substance
during complete combustion. The thermal energy generated in a combustion reaction is
converted into mechanical energy by an engine. Due to imperfections and nature of
conversion process, significant amount of energy is lost to the surrounding. In general,
the combustion reaction can be written as:
CxHy + N (O2) = x (CO2) + (y/2) H2O + heat
In the above stoichiometric reaction, the oxygen used is the atmospheric air to react with
the fuel. Atmospheric air is made up of different composition of gases like about 78% of
nitrogen,21% of oxygen,1% of argon and traces of CO2, Ne, CH4, He, H2O etc.
Nitrogen and argon are chemically neutral and do not react in the combustion process.
Their presence however does not affect in the combustion process. For simply in the
Calculation process we modeled oxygen as 21% and nitrogen as 79% of mixture in the
air. So for every 0.21 moles of oxygen in the air there is also presence 0.79 moles of
nitrogen. So for every mole oxygen needed for combustion,4.76 moles of air must be
supplied: that is one mole of oxygen plus 3.76 moles of nitrogen.
Let us consider an example as,
Stoichiometric combustion of methane with air is:
CH4 + 2O2 + 2(3.76)N2= CO2 +2H2O +2(3.76)N2
In the above reaction for every one mole of oxygen there is presence of 3.76 moles of
Nitrogen as discussed in above.
From the stoichiometric reaction during the combustion different parameters like
Air-fuel ratio, fuel-air ratio, equivalent ratio can be easily calculated.
For air-fuel ratio:
AF=mass of air/mass of fuel
=no. of moles*molecular mass
for fuel-air ratio:
FA=mass of fuel/mass of air
=no. of moles*molecular mass
In the above relation the molecular mass of air is taken as 29(standard).Combustion
Stoichiometry
It develops relations between composition of reactants (fuel and mixture) of a
combustible mixture and composition of products. Since it is based on conservation of
mass of each chemical element in the reactants, only relative elemental composition is
needed. If sufficient oxygen is available, the overall complete combustion equation is:
CaHb + (a+b/4)(O2 +3.773 N2) = a (CO2) + (b/2) H2O+ 3.773 (a+b/2) N2 +heat
3.7 Properties and Ratings of Petrol and Diesel Fuels
Petrol and diesel are two types of common fuels that are used to power vehicles.
However with the advancement in technology, RFG fuels have been used with fuel
ethanol content ranging from 10% to 85% by volume to improve efficiency, consumpton
and perfomance. The debate between petrol and diesel regarding which one is the better
fuel or which is more economically safe has been going on for years. Originally, Petrol
was considered better of the two; but with the advancement in technology diesel has now
becoming a cheaper and efficient choice of fuel for powering automobiles. However, the
choice between petrol and diesel depends on the user and his usage.
PETROL:
Chemical Composition: Petrol, also known as Gasoline, is a transparent fuel made from
the longer hydrocarbon chains found in crude oil: C5 to C12. The C5 to C12 hydrocarbon
chains are liquid at room temperature and are blended to create petrol. Petrol contains a
mixture of paraffins, napthenes, aromatics and olefins. Petrol is separated from crude oil
from 40°C to 205°C. Gasoline has a high volatile rate, which is controlled by blending it
with butane.
Density=750 kg/m3 (from 720 kg/m3 to 760 kg/m3 at 20 ºC).
Thermal expansion coefficient=900⋅10-6 K-1 (automatic temperature compensation for
volume metered fuels is mandatory in some countries).
Boiling and solidification points. Not well defined because they are mixtures. (e.g.
when heating a previously subcooled sample at constant standard pressure, some 10% in
weight of gasoline is in the vapour state at 300 K, and some 90% when at 440 K).
Viscosity=0.5⋅10-6 m2 /s at 20 ºC.
Vapour pressure. 50.90 kPa at 20 ºC, typically 70 kPa at 20 ºC.
Heating value. Average Eurosuper values are: HHV=45.7 MJ/kg, LHV=42.9 MJ/kg.
Theoretical air/fuel ratio: A=14.5 kg air by kg fuel. Octane number (RON)=92..98.
This is a measure of autoignition resistance in a spark-ignition engine, being the volume
percentage of iso-octane in aiso-octane / n-heptane mixture having the same antiknocking characteristic when tested in a variable-compression-ratio engine.
Cetane number: 5.20, meaning that gasoline has a relative large time-lag between
injection in hot air and autoignition, although this is irrelevant in typical gasoline
applications (spark ignition).
Composition: Gasoline composition has changed in parallel with SI-engine
development. Lead tetraethyl, Pb(C2H5)4, a colourless oily insoluble liquid, was used as
an additive from 1950 to 1995, in some 0.1 grams of lead per litre, to prevent knocking;
sulfur was removed at that time because it inhibited the octane-enhancing effect of the
tetraethyl lead.
Average molar mass: M=0.099 kg/mol, 87%C and 13%H (corresponds roughly to
C7.2H12.6).
About 8.91 kg of carbon dioxide (CO2) are produced from burning a (US) gallon) of petrol
that does not contain ethanol (2.36 kg/l).
Octane rating:
An octane rating, or octane number, is a standard measure of the performance of an
engine or aviation fuel. The higher the octane number, the more compression the fuel can
withstand before detonating (igniting). In broad terms, fuels with a higher octane rating
are used in high performance gasoline engine that require higher compression ratios. In
contrast, fuels with lower octane numbers (but higher cetane numbers) are ideal for diesel
engines, because diesel engines (also referred to as compression-ignition engines) do not
compress the fuel, but rather compress only air and then inject fuel into the air which was
heated by compression. Gasoline engines rely on ignition of air and fuel compressed
together as a mixture, which is ignited at the end of the compression stroke using spark
plugs. Therefore, high compressibility of the fuel matters mainly for gasoline engines.
Use of gasoline with lower octane numbers may lead to the problem of engine knocking.
DIESEL:
Diesel is a liquid fuel that is used in diesel engines.It is commonly derived from crude
oil.Petroleum diesel or petrodiesel is produced by distilling crude oil between 200 °C
(392 °F) and 350 °C (662 °F) at atmospheric pressure.
Composition: A complex mixture of hydrocarbons with 12 to 2 carbon atoms per
molecule, with the average being 15.
The average chemical composition, by percent, is:
30 percent alkanes (paraffins).
45 percent cyclic alkanes (naphthenes).
25 percent aromatics.
Density=830 kg/m3 (780-860 kg/m3 at 40 ºC).
Thermal expansion coefficient=800⋅10-6 K-1 .880 kg/m3 for biodiesel (860-900 kg/m3
at 40 ºC).
Boiling and freezing points:
Not well defined because they are mixtures. In general, these
fuels remain liquid down to −30 ºC (some antifreeze additives
may be added to guarantee that).
Viscosity=3⋅10-6 m2 /s (2.0⋅10-6-4.0⋅10-6 m2 /s at 40 ºC) for diesel; 4.0⋅10-6 -6.0⋅10-6
m2 /s for biodiesel.
Vapour pressure=1.10 kPa at 38 ºC for diesel and JP-4, 0.5,5 kPa at 38 ºC for kerosene.
Cetane number=45 (between 40-55); 60..65 for biodiesel. This is a measure of a fuel's
ignition delay; the time period between the start of injection and start of combustion
(ignition) of the fuel, with larger cetane numbers having lower ignition delays. This is
only of interest in compression-ignition engines, and only valid for light distillate fuels
(because of the test engine; for heavy fueloil, a different burning-quality index is used,
calculated from the fuel density and viscosity).
Flash-point=50 ºC typical (40 ºC minimum). In the range 310..340 K (370..430 K for
biodiesel).
Heating value. HHV=47 MJ/kg, LHV=43 MJ/kg (HHV=40 MJ/kg for biodiesel).
About 22.38 pounds (10.15 kg) of CO2 are produced from burning a (US) gallon (3.78l) of
diesel fuel (2.69 kg/l).
Cetane number:
Cetane number (or CN) is an inverse function of a fuel's ignition delay, and the time
period between the start of injection and the first identifiable pressure increase during
combustion of the fuel. In a particular diesel engine, higher cetane fuels will have shorter
ignition delay periods than lower Cetane fuels.
DIFFERENCES BETWEEN PETROL AND DIESEL
Petrol
Diesel
Made from
Petroleum/ Crude oil
Petroleum/ Crude oil
Energy content
38.6 MJ/liter
34.6 MJ/liter
Process
Fractional distillation
Fractional distillation
CO2 produced per litre
2.36 kg
2.67 kg
Calorific Value (megajoules per
kilogram)
45.8 MJ/kg
45.5 MJ/kg
Boiling Range
40°C to 205°C
250°C to 350°C
Torque (for 10L engine)
1000 Nm @ 2000 rpm
300Nm @ 4000 rpm
Power (for 10L engine)
490Hp @ 3500 rpm
600Hp @ 5500 rpm
Energy by Volume
33.7 MJ/liter
36.9 MJ/liter
Best suited for which type of car
Small, compact cars that has low
consumption
Large cars, 4WDs and cars with
high consumption
Fuel economy
Lower fuel economy
Better fuel economy
Purchase Price
Expensive
Cheaper
Environmental impact
High levels of CO2 and carbon
monoxide (CO); initially low
levels of NOx but can increase;
high levels of hydrocarbon and
does not produce Suspended
Particulate Matter
Low level of CO2 per kilometer;
does not produce CO, high level
of NOx; low level of
hydrocarbon; and high level of
Suspended Particulate Matter
Power
Runs at higher RPM
More torque at low speeds
Viscosity
No change
Increase at lower temperatures
Density
Ranges between 0.71–0.77 kg/l
(6.073 lb/US gal)
0.832 kg/l (6.943 lb/US gal)
Tetra-Ethyl
Additives
Lead
(TEL),
Methylcyclopentadienyl
manganese tricarbonyl (MMT),
Ethanol
,
Oxygenate
Blending(MTBE,
ETBE,
biobutanol)
Nitromethane,acetone, Xylene,
Toulene
Oxygenate
blendings(FAME,alcohols,
esters, ketones,ethers)
3.8 Fuel Supply system of SI and CI Engine
Fuel supply system of engine is the system which delivers the carburetor or fuel injection system
with sufficient fuel in all engine operating conditions. The fuel supply system is responsible for
maintaining a constant fuel output while being able to address sudden hike or drop in the intake
amount. The Fuel Supply system must be able to receive data about the engine output, exhaust
output, MAP output, O2 sensor data and thus finally regulate the amount of fuel sent to the FI for
being injected into the cylinder.
Main parts: The fuel supply system consists of
o Fuel tank
o Fuel pump (mechanical or electrical)
o Suction and pressure lines
o Fuel filter
o Air Cleaner
Fig: carburetor system
Fig: layout of fuel supply system of SI
COMBUSTION REQUIREMENTS
•
Compression
•
Air
•
Fuel
•
Spark
•
Air and fuel ratio needed to be 14.7 is to 1 for efficient combustion
IMPLEMENTING DEVICES
•
Carburetor( in SI engine):A carburetor basically consists of an open pipe through which the
air passes into the inlet manifold of the engine. The pipe is in the form of a venturi: it narrows
in section and then widens again, causing the airflow to increase in speed in the narrowest
part. Below the venturi is a butterfly valve called the throttle valve — a rotating disc that can
be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so
that it (almost) completely blocks the flow of air. This valve controls the flow of air through the
carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby
regulating engine power and speed. The throttle is connected, usually through a cable or a
mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on
a car, a throttle level in an aircraft or the equivalent control on other vehicles or equipment.
Fuel is introduced into the air stream through small holes at the narrowest part of the
venturiand at other places where pressure will be lowered when not running at full throttle.
Fuel flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel
path.
•
Electronic Fuel injection(in CI engine):The Electronic Fuel Injection system fitted to most
modern vehicles combines sophisticated computer controls with a high pressure fuel
delivery system to provide optimum power and fuel efficiency. The system is controlled
by an electronic control unit (ECU). These systems often have anything in excess of
thirty different engine and emission sensors all sending information continually to the
ECU. The ECU then monitors the information from the sensors and ensures the correct
amount of fuel and air is used to provide optimum fuel efficiency and performance and
also minimise exhaust emissions.
•
However, EFI is prone to problems & can be caused by dirty fuel or a blocked injector.
Sometimes however, the fault has to be found somewhere within the EFI system using
advanced diagnostic testers. Common symptoms include; poor fuel economy, backfiring,
‘running on’ when the car is turned off and rough idling.
•
Carburetion – mixing of petrol with air to get a combustible mixture
•
Vaporization – changing of liquid petrol into vapor by evaporation
•
Atomization - When liquid petrol is sprayed into the air passing through the carburetor
for very quick vaporization the sprayed liquid turns into many fine droplets. The breaking
of liquid inot small droplets is called atomization
•
Venturi effect – As the air flows through the venturi, a partial vacuum is produced in it
causing sprayed fuel to atomize and quick vaporize
•
Air-fuel ratio requirement - Air-fuel mixture varies depending on the driving condition.
–
Starting: very rich mixture (A/F ratio = 9:1)
–
Idling: rich mixture (A/F ratio = 13:1)
–
Acceleration: rich mixture (A/F ratio = 11:1)
–
Part throttling: stoichiometric mixture (A/F ratio = 15:1)
–
Full throttling: rich mixture (A/F ratio = 13:1)
• Air filter
A particulate air filter is a device composed of fibrous materials which removes solid particulates
such as dust, pollen, mould, and bacteria from the air. Filters containing an absorbent or
catalyst such as charcoal (carbon) may also remove odors and gaseous pollutants such as
volatile organic compounds or ozone.[1] Air filters are used in applications where air
quality is important, notably in building ventilation systems and in engines.
3.9 Non-conventional fuels for IC engines; LPG, CNG, Methanol, Ethanol, Nonedible vegetable oils, Hydrogen.
IV.LIQUEFIED PETROLEUM GAS – LPG
LPG is a by-product of natural gas processing or a product that comes from crude oil refining
and is composed primarily of propane and butane with smaller amounts of propylene and
butylenes.
Production:
LPG is a by-product of natural gas processing and petroleum refining. LPG is a by-product of
two sources: natural gas processing and crude oil refining.
Advantages of LPG:
1. Its cost is 60% of petrol with 90% of its mileage.
2. Has a higher octane number and burns more efficiently.
3. LPG has many of the storage and transportation advantages of liquids, along with the fuel
advantages of gases.
4. Saves on the maintenance costs.
Disadvantages of LPG:
in the initial stages of introduction of this fuel, issues like safety, storage & handling, extreme
volatility of the fuel, etc. needs proper attention.
CNG
Natural gas is a mixture of hydrocarbons mainly methane (CH4) and is produced either from gas
wells or in conjunction with crude oil production. Due to its low energy density for use as a
vehicular fuel, it is compressed to a pressure of 200-250 bars to facilitate storage in cylinders
mounted in vehicle and so it is called compressed natural gas (CNG).
The principal constituent of natural gas is methane (80 to 95% by volume).
Environmental Characteristics: Natural gas has low CO emissions, virtually no PM (particulate
matter) emissions, and reduced volatile organic compounds. Per unit of energy, natural gas
contains less carbon than any other fossil fuel, leading to lower CO2 emissions per vehicle mile
travelled.
Advantages
1. It is cheap
2. It is Engine-Friendly
3. It is safe
4. It’s clean, easy to trap and odourless.
Disadvantages
1. The storage cylinder takes a lot of space.
2. CNG gas stations are not widely available in Nepal.
Methanol
Methanol is an alcohol fuel. The primary alternative methanol fuel being used is M-85, which is
made up of 85 percent methanol and 15 percent gasoline. Methanol is mainly produced from
natural gas. Coal and cellulose consisting biomass like wood etc. may also be used to produce
methanol.
Methanol is not the cleanest gasoline alternatives but it has a distinct advantage in controlling
ozone formation.
Advantages:
o High octane and performance characteristics.
o Only smaller modifications are needed to allow gasoline engines to use methanol.
o There is a significant decrease of reactive emissions when using M-85. Operation
& Performance:
o Because of low energy content, mileage will be slightly lower.
o Power, acceleration and payload are comparable to those of equivalent internal
combustion engines.
o Methanol needs special lubricants.
o Compatible replacement parts are required. Methanol is mostly used in light-duty
vehicles.
Ethanol:
Ethanol is a cheap non-petroleum based fuel. As with methanol, E-85 is the primary ethanol
alternative fuel. The use of ethanol in vehicles is not a new innovation. In the 1880s, Henry Ford
built one of his first automobiles to run on ethanol. Ethanol (CH3CH2OH) is a group of chemical
compounds whose molecule contains a hydroxyl group, -OH, bonded to a carbon atom. Ethanol
made from cellulosic biomass materials instead of traditional feedstock (starch crops) is called
bio ethanol.
Production of Ethanol:
It can be produced by fermentation of vegetables and plant materials.
Advantages
○ Unlike petroleum, ethanol is a renewable resource
○ Ethanol burns more cleanly in air than petroleum, producing less carbon (soot) and carbon
monoxide
○ The use of ethanol as opposed to petroleum could reduce carbon dioxide emissions, provided
that a renewable energy resource was used to produce crops required to obtain ethanol and to
distil fermented ethanol.
○ cost effective
○ minimises global warming
○ minimisses dependency in fossil fuels
○ source of hydrogen fuel
Disadvantages:
○ Ethanol has a lower heat of combustion (per mole, per unit of volume, and per unit of mass)
that petroleum
○ Large amounts of arable land are required to produce the crops required to obtain ethanol,
leading to problems such as soil erosion, deforestation, fertiliser run-off and salinity
○ Major environmental problems would arise out of the disposal of waste fermentation liquors.
○ Typical current engines would require modification to use high concentrations of ethanol.
○ Spike in food prices
○ High affinity of water makes it incombustible.
VI.NON-EDIBLE VEGETABLE OILS
Non-edible vegetable oils is a cleaner burning diesel fuel made from natural, renewable sources
such as vegetable oils. Non-edible vegetable oils operates in compression ignition engines like
petroleum diesel.
Advantages of Non-edible vegetable oils:
The benefits of Non-edible vegetable oils are: - The lifecycle production and use of Non-edible
vegetable oils produces approximately 80% less carbon dioxide emissions, and almost 100% less
sulphur dioxide. Combustion of Non-edible vegetable oils alone produces over a 90% reduction
in total unburned hydrocarbons, and a 75- 90% reduction in aromatic hydrocarbons. Non-edible
vegetable oils further provides significant reductions in particulates and carbon monoxide than
conventional diesel fuel.
o Non-edible vegetable oils is the only alternative fuel that runs in any
conventional, unmodified diesel engine.
o Needs no change in refueling infrastructures and spare part inventories.
o Maintains the payload capacity and range of conventional diesel engines.
o Diesel skilled mechanics can easily attend to Non-edible vegetable oils engines.
o 100% domestic fuel.
o Neat Non-edible vegetable oils fuel is non-toxic and biodegradable. Based on
Ames Mutagenicity tests, Non-edible vegetable oils provides a 90% reduction in
cancer risks.
o Cetane number is significantly higher than that of conventional diesel fuel.
o Lubricity is improved over that of conventional diesel fuel.
o Has a high flash point of about 300 F compared to that of conventional diesel,
which has a flash point of 125 F.
● Disadvantages of Non-edible vegetable oils: Some of the disadvantages of Non-edible
vegetable oils are:
o Quality of Non-edible vegetable oils depends on the blend thus quality can be
tampered.
o Non-edible vegetable oils has excellent solvent properties. Any deposits in the
filters and in the delivery systems may be dissolved by Non-edible vegetable oils
and result in need for replacement of the filters.
o There may be problems of winter operatibility.
o Spills of Non-edible vegetable oils can decolorize any painted surface if left for
long.
V. HYDROGEN (H2)
Hydrogen (H2), when used in a fuel cell to produce electricity is an emissions-free alternative
fuel produced from diverse energy sources. Through retail dispensers, it fills passenger vehicles
in less than 10 minutes to provide a driving range of more than 300 miles. Research and
commercial efforts are under way to expand the hydrogen fuelling infrastructure and production
of fuel cell electric vehicles
Production:
Hydrogen can be produced from a number of different sources, including natural gas, water,
methanol etc.
Two methods are generally used to produce hydrogen:
(1) Electrolysis
(2) Synthesis gas production from steam reforming or partial oxidation.
Advantages
1. Hydrogen-air mixture burns nearly 10 times faster than gasoline-air mixture.
2. Hydrogen has high self-ignition temperature but requires very little energy to ignite it.
3. Clean exhaust, produces no CO2.
4. As a fuel it is very efficient as there are no losses associated with throttling.
Disadvantages
1. There is danger of back fire and induction ignition.
2. Though low in exhaust, it produces toxic NOx.
3. It is difficult to handle and store, requiring high capital and running cost.
TRIBHUVAN UNIVERSITY
Central Campus, Pulchowk
A Report On:
CARBURETOR AND FUEL INJECTION SYSTEMS
Submitted to:
Dr. Ajay Kumar Jha
Department of Mechanical Engineering
Submitted by:
Kshitiz Bohora(072BME620)
Manish Dahal (072BME621)
Manish Raj Aryal (072BME 622)
Nabin Neupane (072BME623)
Navin K. Mahato (072BME624)
Nayan Bista (072BME625)
Table of Contents
Topic
No.
4.1
4.2
4.3
Topics
Construction and
working of
carburetor
Inlet and exhaust
valve timings
Fuel feed and fuel
injection pumps
4.4
Petrol injection
4.5
Electronic Fuel
injection systems
(EFI)
Multi-point fuel
injection system
(MPFI)
4.6
Page
No.
3-7
7-9
9-10
1015
1518
1925
4.1 Construction and working of carburetor
A carburetor is an engine component which mixes air and fuel in
the correct amounts for efficient combustion. The carburetor bolts to
the engine intake manifold. The air cleaner fits over the top of the
carburetor to trap dust and dirt. It is used in a petrol engine where a
mixture of fuel and air is to be injected in the cylinder.
The pumping action of the pistons creates a vacuum which is
amplified by the venturi in the carburetor. This pressure drop will pull
fuel from the float bowl through the fuel nozzle. Unfortunately, there is
not enough suction present at idle or low speed to make this system
work, which is why the carburetor is equipped with an idle and low
speed circuit.
There is also an oil tank which is basically filled with oil. Also,
there is a floating chamber with is connected with the venturi whose
area is suddenly decreased and low pressure is created. The floating
chamber is named so because there is a valve that floats on oil from oil
tank. When the floating chamber is filled with oil the valve remains
closed and when there is deficiency of oil in floating chamber then the
floating chamber withdraws the oil until the value is completely closed.
The low pressure in venturi pulls the oil from floating chamber and
mixes with air at certain ratio.
Fig: Carburetor cross
section
The basic parts of a carburetor are as follows:
•
•
•
•
•
•
Carburetor body
Air horn
Throttle valve
Venturi
Main discharge tube
Fuel bowl
Air horn
The air horn is also called the throat or barrel. The parts which are
often fastened to the air horn body are as follows: the choke, the
hot idle compensator, the fast idle linkage rod, the choke vacuum
break, and sometimes the float and pump mechanisms. When we
pull the choke the choke valve in Air horn gets horizontal and
blocks the amount of supply air entering inside the carburetor
increasing the fuel to air ratio. Also, when the air to fuel ratio is
increased the engine starts easily since there is more fuel
contained in the mixture.
Throttle valve
This disc-shaped valve controls air flow through the air horn.
When closed, it restricts the flow of air and fuel into the engine,
and when opened, it allows air flow and fuel flow, and engine
power increase. Idle speed Adjustment is just above throttling
valve so that the air can flow pass when the throttle valve closes
through the small gap between Idle Speed Adjustment and
Throttle valve.
Venturi
The venturi causes sufficient suction to pull fuel out of the main
discharge tube due to the varying cross section of the venture
which results the pressure difference.
Figure shows that the basic carburetor is a venturi tube mounted
with a throttle plate (butterfly valve) and a capillary tube to input fuel.
It is usually mounted on the upstream end of the intake manifold, with
all air entering the engine passing first through this venturi tube. Most
of the time, there will be an air filter mounted directly on the upstream
side of the carburetor. Finally some carburetor also use second venturi
which basically looks like a little cone in between two primary venturi
which further increases the pressure differences between air cleaner
and the end of the venturi. Also. due to more pressure drop there will
be more fuel coming inside the venturi which ultimately increases the
engine power.
Main discharge tube
The main discharge tube is also called the main fuel nozzle. It is a
passage that connects the fuel bowl to the center of the venturi.
Fuel bowl
The fuel bowl is the storage of fuel (petrol). It’s not under the fuel
pump pressure. It provides fuel to the carburetor for mixing. Fuel bowl
is also connected with floating chamber from which carburetor gets
fuel or petrol via suction. If the floating chamber is filled with fuel then
fuel bowl don’t send anymore fuel to fuel bowl and vice versa.
4.2 Inlet and exhaust valve timings
For smooth operation of IC engine, it it necessary to open and
close inlet and exhaust valve at correct timing and for correct duration.
Inlet valve opens at TDC and closes at BDC during intake/suction stroke,
Exhaust valve opens at BDC and closes at TDC during exhaust stroke.
The opening and closing of the valves is controlled by cam shaft which
is connected to the flywheel. As the flywheel rotates, it drives the
camshaft and rocker arm and thus control the valves.
Fig: Operation of opening and closing of valve in an IC engine.
There is certain time lapse or mismatch in inlet and exhaust value
opening and is due to heat loss, friction as well as expansion due to
temperature. These factors generally affect the value timing.There in
certain delay in opening of inlet and closing of the value. Similarly,
same goes to exhaust value in opening and closing. Hence, there is
certain lag or lead in angle ranging from 10-30 degrees.
4.3 Fuel Feed and Injection Pump
Fuel Feed Pumps
The fuel feed pump used
for the diesel engine is similar
to that of a fuel lift pump for
the petrol engine. It delivers the
fuel from the tank to the
injection pump continuously
and at a reasonable pressure. It
is necessary because there is
possibility of formation of
vapour
bubbles
and
subsequently cavitation in the
pump due to suction of the
plungers of the injection pump.
This would lead to uncontrolled variations in the rate of delivery
of fuel to the cylinders, causing rough running and possibly even
mechanical damage to the engine. Also cavitation could cause
mechanical damage in the injection pump. Generally delivery pressures
of between about 29 and 98 kPa adequate for preventing vapour
formation on the suction side of in-line type injection pumps. This
pressure also ensures adequate supply of fuel for filling the plunger
elements at high speeds in a rotary or distribution pumps
Injection Pump
An Injection Pump is the
device that pumps diesel (as the
fuel) into the cylinders of a diesel
engine.
Traditionally,
the
injection pump is driven indirectly
from the crankshaft by gears,
chains or a toothed belt (often
the timing belt) that also drives
the camshaft. It rotates at half
crankshaft speed in a conventional four-stroke diesel engine.
Its timing is such that the fuel is injected only very slightly
before top dead centre of that cylinder's compression stroke. It is also
common for the pump belt on gasoline engines to be driven directly
from the camshaft. In some systems injection pressures can be as high
as 200 MPa (30,000 PSI)
4.4 Petrol Injection
Fuel injection is a very important part in an engine. Fuel is injected
through fuel injectors. Fuel injectors are nozzles that inject a spray of
fuel into the intake air. They are normally controlled electronically, but
mechanically controlled injectors which are cam actuated also exist. A
metered amount of fuel is trapped in the nozzle end of the injector, and
a high pressure is applied to it, usually by a mechanical compression
process of some kind. At the proper time, the nozzle is opened and the
fuel is sprayed into the surrounding air.
It needs to be injected at correct timing and in correct quantity
with proper amount of fuel and air so as to optimize engine
performance. Fuel injection is a system for introducing fuel into internal
combustion engines, and into automotive engines, in particular. The
functional objectives for fuel injection systems can vary. All share the
central task of supplying fuel to the combustion process, but it is a
design decision how a particular system is optimized.
Presently gasoline injection system is coming into vogue in SI engines
because of the following drawbacks of the carburetion:
1. Non uniform distribution of mixture in multi cylinder engines.
2. Loss of volumetric efficiency due to restrictions for the mixture
flow
3. the possibility of back firing
The primary difference between carburetors and fuel injection is
that fuel injection atomizes the fuel through a small nozzle under high
pressure, while a carburetor relies on suction created by intake air
accelerated through a Venturi tube to draw the fuel into the airstream.
Furthermore; fuel injection also dispenses with the need for a
separate mechanical choke, which on carburetor-equipped vehicles
must be adjusted as the engine warms up to normal temperature.
The injection of fuel into an SI engine can be done by employing any
of the following methods:
1. direct injection of fuel into the cylinder
2. injection of fuel close to the inlet valve
3. injection of fuel into the inlet manifold
There are two types of gasoline injection systems
1. Continuous Injection: Fuel is continuously injected. It is adopted
when manifold injection is contemplated
2. Timed Injection: Fuel injected only during induction stroke over a
limited period. Injection timing is a critical factor in SI engines.
Various injection schemes
1. Single point injection
Single-point injection (SPI) uses a single injector at the throttle
body (the same location as was used by carburetors).
2. Multipoint injection
Multipoint fuel injection (also called PFI, port fuel injection)
injects fuel into the intake ports just upstream of each cylinder's
intake valve, rather than at a central point within an intake
manifold.
3. Continuous injection
In a continuous injection system, fuel flows at all times from the
fuel injectors, but at a variable flow rate. This is in contrast to
most fuel injection systems, which provide fuel during short
pulses of varying duration, with a constant rate of flow during
each pulse. Continuous injection systems can be multi-point or
single-point, but not direct.
4. Central port injection
The system uses tubes with poppet valves from a central injector to
spray fuel at each intake port rather than the central throttle-body
5. Direct injection
In a direct injection engine, fuel is injected into the combustion
chamber as opposed to injection before the intake valve (petrol
engine) or a separate pre-combustion chamber (diesel engine).
6. Swirl injection
Swirl injectors are used in liquid rocket, gas turbine, and diesel
engines to improve atomization and mixing efficiency.
Benefits of fuel injection system
• There are several competing objectives in a fuel injection system
which are:
• Power output
• Fuel efficiency
• Emissions performance
• Ability to accommodate alternative fuels
• Range of environmental operation
The modern digital electronic fuel injection system is more capable at
optimizing these competing objectives consistently than earlier fuel
delivery systems (such as carburetors).
History and Development
Herbert Akroyd Stuart developed the first device with a design
similar to modern fuel injection, using a 'jerk pump' to meter out fuel
oil at high pressure to an injector. This system was used on the hot bulb
engine and was adapted and improved by Bosch and Clessie Cummins
for use on diesel engines .Fuel injection was in widespread commercial
use in diesel engines by the mid-1920s. The first electronic fuel
injection system was developed by Bendix Corporation and was offered
by American Motors Corporation (AMC).
Electronic Fuel Injection uses various engine sensors and control
module to regulate the opening and closing of injector valve. Because
of this, electronic fuel injection system is considered to be better than
the carburetor. Sensors are the components which Monitors engine
operating condition and reports this information to ECM (computer,
electronic control module). Sensors change resistance or voltage with
change in condition such as temperature, pressure and position. With
the data collected through sensors, ECM controls the operation of the
fuel injection system.
Some of the sensors used in an electronic fuel injection system are
given below:
Oxygen Sensor: measures the oxygen content in engine exhaust.
Throttle Position Sensor (TPS): reads the position and orientation of
the throttle valve.
Mass Air Flow Sensor (MAF): Measures the amount of outside air
entering the engine.
Inlet Air Temperature Sensor: Measures the temperature of air
entering the engine.
Crankshaft Position Sensor: reads the speed of engine.
4.5 Electronic Fuel injection systems (EFI)
Electronic Fuel Injection uses various engine sensors and control
module to regulate the opening and closing of injector valve to
maintain air fuel ratio. Because of this, electronic fuel injection system
is considered to be better than the carburetor. It is come into use from
1980’s.
Sensors are the components which Monitors engine operating
condition and reports this information to ECU (computer, electronic
control module). Sensors change resistance or voltage with change in
condition such as temperature, pressure and position. With the data
collected through sensors, ECU controls the operation of the fuel
injection system.
Some of the sensors used in an electronic fuel injection system
are given below:
Oxygen sensor
It is mounted in the exhaust manifoldand measures the oxygen
content in engine exhaust in order to maintain air-fuel ratio. It tells the
ECU wheather the air-fuel mixture is burning rich (less oxygen) or lean
(more oxygen).
Throttle Position Sensor (TPS)
It reads the position and orientation of the throttle valve. The
sensor is usually located on the butterfly spindle/shaft so that it can
directly monitor the position of the throttle.
Mass Air Flow Sensor (MAF):
It measures the amount of outside air entering the engine.
Inlet Air Temperature Sensor:
It measures the temperature of air entering the engine to start
the engine smoothly. It is also used to delay the release the exhaust gas
recirculation valve to warm up engine in less time.
Crankshaft Position Sensor
It reads the speed of engine to control the fuel injection or
ignition system timing.
Difference between EFI and carburator
EFI
CARBURATOR
It is a digital system
It is a mechanical
system
It react to the pressure
created by a engine
It builds its own
pressure to inject a fuel
to the engine
It’s working does not
depend
on
environmental
condition
It is efficient and
reliable than carburetor
It is more precise than
carburetor
It is expensive
It
is
difficult
maintenance
in
It’s working depend
upon
environmental
condition
It is less efficient and
reliable than EFI
It is less precise than
EFI
It is cheap
It
is
easy
maintenance
in
4.6 Multi-point fuel injection system (MPFI)
Fuel injection is a method or system for admitting fuel into
the internal combustion engine. From early 1940s many injection
system like single-point injection, continuous injection are introduced
in the market by the different companies. But presently the most used
injection system is MPFI in petrol engine and CRDI in diesel engine.
ADVANTAGES OF MULTI POINT FUEL INJECTION SYSTEM
•
•
More uniform air-fuel mixture will be supplied to each cylinder,
hence the difference in power developed in each cylinder is
minimum.
The vibrations produced in MPFI engines is very less, due to this
life of the engine component is increased.
•
•
•
•
No need to crank the engine twice or thrice in case of cold starting
as happen in the carburetor system.
Immediate response, in case of sudden acceleration and
deceleration.
The mileage of the vehicle is improved.
More accurate amount of air-fuel mixture will be supplied in this
injection system. As a result complete combustion will take place.
The main purpose of the multi-point fuel injection system is to
supply a proper ratio of gasoline and air to the cylinders. These system
function under two basic arrangements:
1. Port injection
2. Throttle body injection
PORT INJECTION
In this system, the injector is placed on the side of the intake
manifold near the intake port. The injector sprays gasoline into the air,
inside the intake manifold. The gasoline mixes with the air in a
reasonably uniform manner.
This mixture of gasoline and air then passes through the intake
valve and enters into the cylinder. Every cylinder is provided with an
injector in its intake manifold. If there are six cylinders, there will be six
injectors.
The gasoline port fuel injection is the most popular drive system
for gasoline engines worldwide. The power train system convinces with
low costs, reduced technology and new, innovative further
developments.
When using engines with a specific performance of approx.
60 kW/l and the result is a less complex injection control by means of
variances regarding the injection time frame. The robust combustion
process of the gasoline port fuel injection also tolerates fuel of lower
quality.
Fig: Port injection system.
THROTTLE BODY INJECTION
The throttle body is similar to the carburetor throttle body, with
the throttle valve controlling the amount of air entering the intake
manifold also fuel injection systems can be either timed or continuous.
In the timed injection system, gasoline is sprayed from the injector in
pulses. In continuous injection system, gasoline is sprayed continuously
from the injectors.
The port injection system and the throttle body injection system
may be either pulsed systems or continuous system. In both system,
the amount of gasoline injected depends upon the engine speed and
power demands.
The appearance of throttle body injection systems is similar to the
carbureted fuel system. Although not as efficient as multi-port systems,
it does offer better driveability and lower emissions than carbureted
systems.
The fuel injector(s) is mounted vertically above the throttle
plate(s). The throttle body assembly also houses the fuel pressure
regulator. These systems typically run at lower pressure compared to
multi-port systems.
This is mostly due to the fact that pressure in the intake manifold
does not have to be overcome. Since the injector(s) is mounted above
the throttle plate, fuel is actually drawn into the intake system. Other
than this, the actual operation of the throttle body injection system is
similar to the multi-port system.
Fig: Throttle body injection.
In another way, MPFI can be classified into D-MPFI and L-MPFI
D-MPFI
The D-MPFI system is the manifold fuel injection system. In this
type, the vacuum in the intake manifold is first sensed. Further, it
senses the volume of air by its density. Figure shows the block diagram
regarding the functioning of the D-MPFI system
As air enters into intake manifold, the manifold pressure sensor
detects the intake manifold vacuum and sends information to the ECU.
The ECU in turn sends commands to the injector to regulate the
amount of gasoline supply for injection. When the injector sprays fuel
in the intake manifold the gasoline mixes with the air and the mixture
enters the cylinder.
Fig: Block diagram of D-MPFI system.
L-MPFI system
It is a port fuel injection system. Here, the fuel metering is
regulated by the engine speed and the amount of air that actually
enters the engine. This is called air-mass metering or air -flow metering.
The block diagram of L-MPFI system is shown. As air enters into the
intake manifold, the air flow sensor measures the amount of air and
sends information to the ECU. Similarly, the speed sensor sends
information about the speed of the engine to the ECU. The ECU
processes the information received and sends appropriate commands
to the injector to regulate the amount of gasoline supply for injection.
When injection takes place, the gasoline mixes with the air and mixture
enters the cylinder.
Fig: Block diagram of L-MPFI.
CHAPTER 5 :COMBUSTION IN SI AND CI ENGINES
(Roll 072 BME 626,628,629,630,631,632)
5.1 IGNITION SYSTEMS
Ignition system: To start the automobile engine, typically for the spark ignition
engine, spark must be generated inside the cylinder while piston goes up from
bottom dead center to top dead center in compression stroke. The combustion in a
spark ignition engine is initiated by an electric discharge across the electrodes of a
spark plug, which usually occurs from 10 to 30 degrees before TDC depending
upon the chamber geometry and operating conditions.After the spark produced
inside cylinder, mixture of gas and the fuel burnat the predetermined position in
the engine cycle to produce the power stroke. So proper spark timing and system
must be fitted in the automobile engine. Basically, there are three types of ignition
system which are described below:
Battery ignition system:
Rechargeable acid lead battery is used to provide electrical energy for ignition. It is
recharge by dynamo which is driven by the engine. The battery is connected to the
primary winding of the ignition coil through the ignition switch. This switch is used
to turn the ignition system on or off. Under prolonged operation of the engine, the
temperature of the ignition coil increase which can be dangerous. To prevent this a
ballast resistor made of iron wire in provide with series with primary winding.The
primary consists of the battery, ammeter, ignition switch, primary coil winding,
capacitor, and breaker points.Ignition coil is the source of the ignition energy. Its
function is to step up the low voltage to the level required for producing electric
spark in the spark plug. Ignition coil consists of magnetic soft iron core and two
insulated conducting coil known as primary and secondary windings. Primary
winding consists of 200 to 300 turns with its both ends connected to external
terminal. Secondary winding consists of 21000 turns with its one end connected
with high tension wire that goes to distributor and second end connected to the
primary coil. Contact breaker is the mechanical device for making and breaking
the circuit. It consists of two metal points one fixed and other movable. While the
fix metal point is connected to the contact breaker assembly. The movable one is
connected to the spring-loaded pivot arm. The spring on this arm keeps the both
metal points in contact thereby closing the primary circuit. Now as the high point
of cam passes under the heel, the contact break and current flow through contact
breaker stops. A condenser connected in parallel with contact breaker to prevent
the burning of metal point.
A distributor is provided for distributing ignition surges to individuals spark plug at
correct sequence and correct time. It consists rotor in the middle and metallic
electrode on the periphery. These metallic electrode are directly connected to the
spark plug and also known as ignition harness. Secondary coil of the ignition coil
is connected to the rotor of the distributor which is driven by the cam shaft. The
secondary circuit converts magnetic induction to high voltage electricity to jump
across the spark plug gap, firing the mixture at the right time.As the rotor rotates
,it passes through high tension current to the ignition harness which then carries
these high tension current to the spark plug. Spark pluck is the output part of the
whole ignition system. It consists of two electrode one attached to the high
tension current carrying wires and other is grounded. The potential difference
between two electrode ionize the gap present between them thus the spark
produce.
Magneto ignition system:
In magneto
ignition system, battery and ignition coil are replaced by compact magneto. Basic
component of this systems are magneto, contact breaker, condenser, ignition
switch, distributor, spark plug lead and spark plug. All the parts except the
magneto are same as in battery ignition system. The function of the magneto is to
induced the current in primary and secondary circuit. It consists of two pole
magnet of soft iron core on which both primary and secondary coil winding is
done. As the magnet rotates , current produce in both primary and secondary
winding. Magneto can be either rotating armature type or rotating magnetic type.
In the former, the armature consisting of the primary and secondary windings all
rotate between the poles of a stationary magneto while in the second type, the
magneto revolves and the windings are kept stationary. When magnet rotates,
the magnetic field change through soft iron core. This continuous change in
magnetic field induces a varying voltage in both primary and secondary which
produce an alternating current. The magnet is directly connecting to the cam
which is tied in such a way that as the current in primary winding reaches in
maximum value. The breaker points on the contact breaker opens. As the breaker
points open, current now flows through the condenser and starts charging it. Thus,
current in a primary winding decrease to zero and varies the induced magnetic
field. When the condenser starts to discharge rapidly into the primary circuit
which reverse the direction of primary current and induced magnetic fields. This
rapid collapse and reversal of the magnetic field induces a very high voltage in the
secondary winding. It is then carried through the high-tension wire to the
distributor rotor where it is passes to one of the spark plug lead into the sparkplug
where spark is produced.The high powered, high speed spark ignition engines like
aircraft, sports and racing cars use magneto ignition system.
Electronic ignition system:
The disadvantage of the mechanical system is that it requires regular adjustment
to compensate for wear, and the opening of contact breakers, which is responsible
for spark timing, is subject to mechanical vibrations.
In addition, the spark voltage is also dependent on contact effectiveness, and
poor sparking may lower the engine efficiency. Electronic ignition has solved these
problems.
In the electronics ignition system breaker point is replaced by armature. Armature
sends the signal to the ignition module to make and break the circuit. When the
ignition switch is turn on, current flows through the battery through the ignition
switch to the coil of primary windings. When the reluctor or armature tooth comes
in front of the pickup coil, a voltage signal is generated. The electronic module
sense the signal produced by the pickup coil and stops the current to flow from the
primary circuit. A timing circuit inside the module turns ON the current flow when
the reluctor tooth rotates away from the pickup coil. Due to the continuous make
and break of the current, a magnetic field is generated in the ignition coil. Due to
the magnetic field , an emf is induced in the secondary winding, causing the
voltage increase up to 50000 volt. The high voltage is the transferred to the
distributor. And distributor works on same mechanism as it does on the battery
and magneto ignition system.
5.1: Stages of Combustion in Engines
The process of combustion in IC engines is very complex. This complex process is
attempted to describe using some models which gives near accurate
understandings. In SI engine, engine, uniform A uniform A uniform A: F mixture :
F mixture : F mixture is supplied, supplied, supplied, but in CI engine A but in CI
engine A but in CI engine A: F mixture is not mixture is not homogeneous and fuel
remains in liquid particles, therefore quantity of air supplied is 50% to 70% more
than stiochiometric mixture. The combustion in SI engine starts at one point and
gen The combustion in SI engine starts at one point and generated flame at the
point of ated flame at the point of ignition propagates through the mixture for
burning of the mixture, where as in CI engine, the combustion takes place at
number of points simultaneously and number of flames generated are also
many.The process of combustion in CI and SI engines are quite different. Hence
these are described separately below:
5.1.1: Combustion in SI Engines
It can be subdivided into three regions:
5.1.1.1: Ignition and Flame Development
5.1.1.2: Flame Propagation
5.1.1.3 : Flame termination
Ignition and Fame Development: This stage consumes about 5% of the total air
fuel mixture. During this phase, ignition occurs and the combustion process starts.
The combustion is initiated by an electric spark plug. This may occur between 10
to 30 degrees before TDC. When the air fuel mixture is ignited, the combustion
reaction spreads outward from there. Combustion starts very slowly because of the
high heat losses to the relatively cold spark plug and gas mixtures.There is a
certain time interval between instant of spark and instant when there is a noticeable
rise in pressure due to combustion. This time lag is called ignition lag.Flame can
generally be detected at about 6 degrees of crank rotation after spark plug firing.
This is a chemical process depending upon the nature of fuel, temperature and
pressure, proportion of exhaust gas and rate of oxidation or burning.
Fig: Mass fraction burned vs angle of rotation of crankshaft
After the flame has been well developed , it spreads fast which is called the flame
propagation region.
Flame Propagation: In this phase, the flame front moves quickly through the
combustion chamber. Due to the induced turbulence and swirl, flame propagation
is very fast.
The burning of gas makes the temperature and pressure inside the cylinder very
high. The burned gas being more hotter occupies more space than the to-be-burnt
gases at any instant this is described in graph below:
Fig: mass percent burned vs volume percent burned
This causes the compression of unburned gases and thus compression heating of
those occurs. Radiation heating from the combustion process further heats the
burnt and unburnt gases in the chamber. The temperature, by this means, reaches to
a maximum at the end of the combustion process.
However the temperature of burnt gases is not uniform throughout the combustion
chamber but is higher near the spark plug where combustion started. This is
because the gas here has experienced higher amount of radiation energy input from
later flame reaction.
The maximum temperature and maximum pressure of the cycle occurs somewhere
between 5 degree and 10 degree after TDC. The combustion in a real four-stroke
cycle SI engine is almost but a constant volume process. The following figure
shows how pressure rise with engine rotation.
Rate of flame propagation affects the combustion process in SI engines. Higher
combustion efficiency and fuel economy can be achieved by higher flame
propagation velocities
Factors Affecting Flame Propagation:
1. Type of fuel
2. Air fuel ratio
3. Engine speed
4. Compression ratio
5. Load on engine
The flame speed is slower for lean mixtures and is fastest for slightly rich
mixtures. It is maximum for most fuels at an AF ratio of 12. The exhaust
residual and recycled exhaust gas slows the flame speed. Flame speed
increases with engine speed due to higher turbulence, swirl, and squish.
These can be demonstrated by the figures below:
Fig: average flame speed vs air fuel ratio
Fig: engine speed vs flame speed
Flame Termination: At about 15 degree to 20 degree after TDC, 90-95% of the air
fuel mass has been combusted and the flame front has reached the extreme corners
of the combustion chamber, the flame starts to terminate. During the flame
termination period, self-ignition will sometimes occur in the end gas in front of the
flame front, and engine knock will occur. The temperature of the unburnt gases in
front of the flame front continues to rise during the combustion process, reaching
the maximum in the last end gas. This maximum temperature is often above self
ignition temp. because the flame front move slowly at this time, the gases are often
not consumed during ignition delay time, and self ignition occurs. The resulting
knock is usually not objectionable or even noticeable. This is because there is so
little unburn air-fuel left at this time that self ignition can only cause very slight
pressure pulses. Maximum power is obtained from the engine when it operate at
very slight self ignition and knock at the end of the combustion process. This
occurs when maximum pressure and temp exists in the combustion chamber and
knock gives a small pressure boost at the end of combustion.
5.1.2: Combustion in CI Engines
The process of combustion in a CI engine is quite different from that in SI engine.
In a CI engine, fuel is sprayed directly inside the engine and the ignition starts
spontaneously. The combustion process is very unsteady occurring simultaneously
at many spots in a very non-homogenous mixture.To burn the liquid fuel is more
difficult as it is to be re difficult as it is to be evaporated; it is to be elevated to
ignition temperature and it is to be elevated to ignition temperature and then burn.
The rate of combustion is controlled by fuel injection pattern. After injection, the
fuel goes through the following steps:
a) Atomization: fuel drops breaks into very small droplets.
b) Vaporization: the small droplets of liquid fuel evaporate to vapor due to
the high temperature condition created by high compression. As the first
fuel evaporates, the immediate surroundings are cooled by evaporation
cooling. This effects the subsequent evaporation.
c) Mixing: after vaporization, the fuel vapour mixes with to form a mixture.
The mixture should have an AF ration which is combustible.
d) Self ignition: at about 8 degree bTDC, 6-8 degree after the start of
injection, the AF mixture starts to self-ignite. Self ignition leads to
combustion process.
e) Combustion: combustion starts by self ignition simultaneously at many
locations in the rich zones of fuel jet. When combustion starts, multiple
flame fronts spreading from the many self-ignition sites quickly consume
all the gas mixture which is in a correct combustible AF ratio, even
where self-ignition wouldn’t occur. The combustion gives a very high
rise in temperature and pressure inside the cylinder within a short period
of time .
This graph shows the fuel injection flow rate, net heat release rate and cylinder
pressure for a direct injection CI engine.
5.4 Knocking and Pre-ignition
An engine is designed in such a way that the combustion process in the pistoncylinder must occur at specific point in the cycle. In normal operation, this process
occurs at that specific point in the piston stroke and hence is characterized as
normal combustion. However, cases may arise where the combustion of the airfuel mixture may occur somewhat outside the normal combustion front. This may
cause some abnormities in the normal functioning of the engine as premature
ignition or post cycle ignition prove to be inconsequential with the operation and
somewhat destructive.
Abnormal ignition is classified into two categories:
1. Knocking
2. Surface Ignition (Pre-ignition and Post-ignition)
Knocking
Knocking, in an internal-combustion engine, sharp sounds caused by premature
combustion of part of the compressed air-fuel mixture in the cylinder. In a properly
functioning engine, the charge burns with the flame front progressing smoothly
from the point of ignition across the combustion chamber. However, at high
compression ratios, depending on the composition of the fuel, some of the charge
may spontaneously ignite ahead of the flame front and burn in an uncontrolled
manner, producing intense high-frequency pressure waves. These pressure waves
force parts of the engine to vibrate, which produces an audible knock.
Knocking can cause overheating of the spark-plug points, erosion of the
combustion chamber surface, and rough, inefficient operation. It can be avoided by
adjusting certain variables of engine design and operation, such as compression
ratio and burning time; but the most common method is to burn gasoline of higher
octane number.
Octane Number (ON)
Octane number is the numerical indicator of how well a fuel self-ignites. It is a
scale that is based upon the actual performance of a standard fuel in a specific
engine at specific operating conditions. The two standard reference fuels used are
isooctane (2, 2, 4-trimethylpentane), which is given the octane number (ON) of
100, and n-heptane, which is given the ON of 0. The higher the ON, the less likely
it will self-ignite. Thus, a higher rated fuel will prevent knocking.
Figures: Relations between a) ON and Compression Ratio, b) Cylinder Pressure
and Crank Angle
Fuel
hexadecane
RON
< 30
n-octane
20
n-heptane (RON and MON 0 by definition)
0
diesel fuel
15–25
2-methylheptane
23
n-hexane
25
1-pentene
34
2-methylhexane
44
3-methylhexane
1-heptene
60
n-pentane
Figure:- commonly used fuels with their octane numbers
Detection of Knocking
1. By human ear
2. Knock detectors
3. Optical probes and ionization detectors
4. Piezoelectric pressure transducer
5. ion sensing at the spark plug gap is a modern method of measuring knock.
Prevention of Knocking
1. Reducing compression ratio in SI engines
2. Raising compression ratio in CI engines
3. Installing knock sensors in engines
4. Water injection - water is injected into the engine via the air/fuel intake. The
water suppresses the ignition point of the engine allowing a more complete
burn.
5. Increasing the inlet pressure of air (i.e. by supercharging) in CI engines
6. Increasing the turbulence of compressed air making it homogenous
7. Raising the temperature of the coolant that of the intake air as well as
cylinder head and combustion chamber
8. Using a fuel of high ON rating.
9. Crankshaft offset is another design tool which allows increased static
compression ratio while reducing knock tendency.
10. Reducing the engine’s temperature through EGR system and intercoolers.
Surface Ignition
Surface ignition is ignition of the fuel-air charge by any hot surface other than the
spark discharge prior to the arrival of the normal flame front. It may occur before
the spark ignites the charge (pre-ignition) or after normal ignition (post-ignition).
Out of these two, pre-ignition is the most damaging type. It causes the combustion
to occur faster than usual, before the spark plug fires, and causes higher heat
rejection. It occurs in most heated parts such as spark plugs, exhaust valves, metal
asperities such as edges of head cavities or piston bowls.
Detonation induced pre-ignition
Because of the way detonation breaks down the boundary layer of protective gas
surrounding components in the cylinder, such as the spark plug electrode, these
components can start to get very hot over sustained periods of detonation and
glow. Eventually this can lead to the far more catastrophic pre-Ignition as
described above.
While it is not uncommon for an automobile engine to continue on for thousands
of kilometers with mild detonation, pre-ignition can destroy an engine in just a few
strokes of the piston.
Causes of Pre-ignition
1. Carbon deposits can form a heat insulation causing pre-ignition
2. Overheated spark plug
3. Glowing carbon deposits on exhaust valves indicates that the valve is
running too hot due to poor seating, weak valve spring, or insufficient valve
lash.
4. An engine that is running hotter than normal due to a cooling system
problem.
5. Excessive amount of oxygen in the combustion chamber.
6. A lean fuel mixture and insufficient oil in the engine.
7. Sharp edges in the combustion chamber and pistons
8. Sharp edges on valves
Prevention of Pre-ignition
1. Appropriate heat-range spark plug
2. Removal of asperities
3. Reduced metal edges in the engine
4. Well-cooled exhaust valves
5. Removal of deposits
Figures:a) Difference between Normal and Pre-ignition, b) Correct Timing of
Normal Ignition
5.6 Combustion Chamber Requirements
Combustion is a chemical reaction in which certain elements of the fuel like
hydrogen and carbon combine with oxygen liberating heat energy and causing an
increase in temperature of the gases.
A combustion chamber is an enclosed space inside of a combustion engine in
which a fuel and air mixture is burned. Burning fuel releases a gas that increases in
temperature and volume.
Combustion Chamber Design
The design of the combustion chamber for an SI engine has an important influence
on the engine performance and its knocking tendencies.
The design involves
• the shape of the combustion chamber,
• the location of spark plug and
• the location of inlet and exhaust valves.
The important requirements of an SI engine combustion chamber are
• High power output.
• High thermal efficiency
• low specific fuel consumption
• Smooth engine operation
• Reduced exhaust pollutants.
I. Smooth engine operation
The aim of any engine design is to have a smooth operation and a good
economy.
These can be achieved by the following:
a. Moderate Rate of Pressure Rise
Limiting the rate of pressure rise as well as the position of the peak pressure
with respect to TDC affect smooth engine operation.
b. Reducing the Possibility of Knocking
Reduction in the possibility of knocking in an engine can be achieved by,
• Reducing the distance of the flame travel by centrally locating the spark plug
and also by avoiding pockets of stagnant charge.
• Satisfactory cooling of the spark plug and of exhaust valve area which are
the source of hot spots in the majority of the combustion chambers.
• Reducing the temperature of the last portion of the charge, through
application of a high surface to volume ratio in that part where the last
portion of the charge burns.
II. High Power Output and Thermal Efficiency
This can be achieved by considering the following factors:
a. A high degree of turbulence is needed to achieve a high flame front velocity.
• Turbulence is induced by inlet flow configuration or squish
• Squish is the rapid radial movement of the gas trapped in between the piston
and the cylinder head into the bowl or the dome.
• Squish can be induced in spark-ignition engines by having a bowl in piston
or with a dome shaped cylinder head.
b. High Volumetric Efficiency
• More charge during the suction stroke, results in an increased power output.
• This can be achieved by providing ample clearance around the valve heads,
• large diameter valves and straight passages with minimum pressure drop.
c. Improved anti-knock characteristics
Improved anti-knock characteristics permits the use of a higher compression
ratio resulting in increased output and efficiency.
d. A Compact Combustion Chamber
Reduces heat loss during combustion and increases the thermal efficiency.
Types of Combustion Chambers
Different types combustion chambers have been developed over a period of time
Some of them are shown in Fig.
• T-Head Type
• L-Head Type
• I-Head Type or Overhead Valve
• F-Head Type
T-Head Type
This was first introduced by Ford Motor Corporation in 1908. The T-head
combustion chambers were used in the early stage of engine development. These
are very prone to detonation. There was violent detonation even at a compression
ratio of 4.Since the distance across the combustion chamber is very long, knocking
tendency is high in this type of engines. This is because the average octane number
in 1908 was about 40 -50. This configuration provides two valves on either side of
the cylinder, requiring two camshafts. They require two cam shafts (for actuating
the in-let valve and exhaust valve separately) by two cams mounted on the two
cam shafts.From the manufacturing point of view, providing two camshafts is a
disadvantage.
L-Head Type
A modification of the T-head type of combustion chamber is the L-head type. This
was first introduced by Ford motor in 1910-30 and was quite popular for some
time. It provides the two values on the same side of the cylinder, and the valves are
operated through tappet by a single camshaft.
The main objectives of the Ricardo's turbulent head design, axle to obtain fast
flame speed and reduced knock.
I Head Type or Overhead Valve:
Over head valve or I head combustion chamber :- Since 1950 or so mostly
overhead valve combustion chambers are used. This type of combustion chamber
has both the inlet valve and the exhaust valve located in the cylinder head. An
overhead engine is superior to side valve engine at high compression ratios.
Some of the important characteristics of this type of valve arrangement are:
• less surface to volume ratio and therefore less heat loss
• less flame travel length and hence greater freedom from knock
• higher volumetric efficiency from larger valves or valve lifts.
• Lower pumping losses and higher volumetric efficiency from better
breathing of the engine from larger valves or valve lifts and more direct
passageways.
• Less distance for the flame to travel and therefore greater freedom from
knock, or in other words, lower octane requirements.
• Less force on the head bolts and therefore less possibility of leakage (of
compression gases or jacket water).
•
Removal of the hot exhaust valve from the block to the head, thus confining
heat failures to the head. Absence of exhaust valve from block also results in
more uniform cooling of cylinder and piston.
• Lower surface-volume ratio and, therefore, less heat loss and less air
pollution.
• Easier to cast and hence lower casting cost.
F-Head Type
The F-head type of valve arrangement is a compromise between L-head and I-head
types. Combustion chambers in which one valve is in the cylinder head and the
other in the cylinder block are known as F-head combustion chambers. Modern Fhead engines have exhaust valve in the head and inlet valve in the cylinder block.
The main disadvantage of this type is that the inlet valve and the exhaust valve are
separately actuated by two cams mounted on to camshafts driven by the crankshaft
through gears.One of the most F head engines (wedge type) is the one used by the
Rover Company for several years. And another successful design of this type of
chamber is that used in Willeys jeeps.
5.7 Turbocharging and Supercharging
Supercharging and turbocharging are similar in process and differ in operation, it
means, both are used for same purpose i.e. to increase engine power, efficiency,
torque by compressing the air in multistage for increasing quantity of air, pressure
and temperature.
Fig: - Turbocharger vs. Supercharger
Source: -https://blob-s-docs.googlegroups.com
As the air is compressed in multistage, its temperature could exceed the maximum
safe temperature, if it exceeds the maximum temperature, the cylinder parts will be
affected like wear due to excess temperature. To avoid this incident, inter-cooler is
used between supercharger/Turbocharger and engine cylinder. In inter-cooler the
compressed air is cooled to reduce the temperature due to compression while
keeping the same pressure.
Turbocharging
•
In turbocharging, the exhaust gases
from the engine cylinder is used to
drive the turbine. The turbine and
compressor are mounted on the same
shaft. When the exhaust gases are
passed through turbine, the turbine
rotates as the gases import heat
energy, hence the turbine produces
mechanical energy i.e. rotation of
shaft for driving compressor. Now the compressor also rotates to compress
the inlet air to the cylinder. The inlet air is compressed before reaching
engine cylinder.
Fig: - Turbocharger
Source: -https://www.procharger.com
Supercharging
•
In supercharging, the rotation of crank
shaft is used to drive the turbine
through gears and chains or pulleys
and belts. The turbine and compressor
are mounted on the same shaft. When
the crank shaft rotates, the shaft of the
turbine also rotates since both are
connected mechanically through gear
and
chain
or
pulley
and
belt
arrangements. Hence the turbine produces mechanical energy i.e. rotation of
shaft for driving compressor.
Fig: - Supercharger
Source: -https://www.procharger.com
Need of turbocharger and supercharger
For ground installations, it is used to produce a gain in the power output of the
engine.
For aircraft installations, in addition to produce a gain in power output at sea level,
it also enables the engine to maintain a higher power output as altitude is increased.
Disadvantages of turbocharger and supercharger
 Cost and complexity
 Detonation
 Parasitic losses
 Space
 Turbo lag
5.9 Engine Emission and Emission Control
Engines are operated by burning the fuels like natural gas, gasoline, petrol,
diesel, coal etc. And as a result of this combustion engine emits gas called exhaust
gas. Exhaust gas are the major components of motor vehicle emissions.
Various emission gases are:
•
Hydrocarbons (HC) - A class of burned or partially burned fuel, hydrocarbons
are toxins. Hydrocarbons are a major contributor to smog, which can be a major
problem in urban areas. Prolonged exposure to hydrocarbons contributes to
asthma, liver disease, lung disease, and cancer. Regulations governing
hydrocarbons vary according to type of engine and jurisdiction; in some cases,
"non-methane hydrocarbons" are regulated, while in other cases, "total
hydrocarbons" are regulated. Technology for one application (to meet a nonmethane hydrocarbon standard) may not be suitable for use in an application
that has to meet a total hydrocarbon standard. Methane is not directly toxic, but
is more difficult to break down in fuel vent lines and a charcoal canister is
meant to collect and contain fuel vapors and route them either back to the fuel
tank or, after the engine is started and warmed up, into the air intake to be
burned in the engine.
•
Carbon monoxide (CO) - A product of incomplete combustion, inhaled carbon
monoxide reduces the blood's ability to carry oxygen; overexposure (carbon
monoxide poisoning) may be fatal. (Carbon monoxide persistently binds to
hemoglobin, the oxygen-carrying chemical in red blood cells, where oxygen
(O2) would temporarily bind; the bonding of CO excludes O2 and also reduces
the ability of the hemoglobin to release already-bound oxygen, on both counts
rendering the red blood cells ineffective. Recovery is by the slow release of
bound CO and the body's production of new hemoglobin—a healing process—
so full recovery from moderate to severe [but nonfatal] CO poisoning takes
hours or days. Removing a person from a CO-poisoned atmosphere to fresh air
stops the injury but does not yield prompt recovery, unlike the case where a
person is removing from an asphyxiating atmosphere [i.e. one deficient in
oxygen]. Toxic effects delayed by days are also common.)
•
NOx- Generated when nitrogen in the air reacts with oxygen at the high
temperature and pressure inside the engine. NOx is a precursor to smog and acid
rain. NOx is the sum of NO and NO2.[1] NO2 is extremely reactive. NOx
production is increased when an engine runs at its most efficient (i.e. hottest)
operating point, so there tends to be a natural tradeoff between efficiency and
control of NOx emissions.
•
Particulate matter – Soot or smoke made up of particles in the micrometre size
range: Particulate matter causes negative health effects, including but not
limited to respiratory disease and cancer. Very fine particulate matter has been
linked to cardiovascular disease.
•
Sulfur oxide (SOx) - A general term for oxides of sulfur, which are emitted
from motor vehicles burning fuel containing sulfur. Reducing the level of fuel
sulfur reduces the level of Sulfur oxide emitted from the tailpipe.
•
Volatile organic compounds (VOCs) - Organic compounds which typically
have a boiling point less than or equal to 250 °C; for example
chlorofluorocarbons (CFCs) and formaldehyde. Volatile organic compounds are
a subsection of Hydrocarbons that are mentioned separately because of their
dangers to public health.
Effects of engine emissions
Exhaust gases are contributors of air pollution. These are major ingredient in the
creation of smog. According to the study of MIT every year 53,000 early deaths
occur because of vehicle emissions.
The major effects of engine emissions are :
1. Pollutants and Air Quality
2. Ozone & Photochemical Smog
3. Global Warming
4. Acid Rain
5. Reduced Atmospheric Visibility
Emission in SI and CI engines
Methods of emission control
1. Designing the geometry.
2. Chemical method.
3. Ammonia injection method.
4. Exhaust gas recycle.
Designing The Geometry
 Carbon soot particulate generation can be controlled by modifying with new
technology.
 The fuel injector and combustion chambers are changed in there geometry
to reduce emissions.
Chemical Method
 Cyanuric acid is a chemical substance to reduce NOx emissions.
 Cyanuric acid is a low-cost solid material that sublimes in the exhaust flow.
 The gas dissociates, producing isocyanide that reacts with NOx to form N2,
H20, and CO2.
 Up to 95% NOx reduction has been achieved with no loss of engine
performance.
Ammonia Injection Method
 Large ship engines and some stationary engines reduce NOx emissions with
an injection system.

NH3 is sprayed into the exhaust flow.
 4 NH3 + 4 NO + 02 ~ 4 N2 + 6 H20
 6N02 + 8NH3 ~ 7 N2 + 12H20
 Careful control must be taken, as NH3 itself is an undesirable emission.
Exhaust Gas Recycle
 It is most effective way of reducing NOx emissions.
 In this method, combustion chamber temperature is lowered down.
 The simplest practical method of reducing maximum flame temperature is to
dilute the air-fuel mixture with a non-reacting parasite gas.
 This gas absorbs energy during combustion without contributing any energy
input. The net result is a lower flame temperature.
Some of the more advanced techniques include:
Advances in engine and vehicle technology continually reduce the toxicity of
exhaust leaving the engine, but these alone have generally been proved insufficient
to meet emissions goals. Therefore, technologies to detoxify the exhaust are an
essential part of emissions control.
Air injection
One of the first-developed exhaust emission control systems is secondary air
injection. Originally, this system was used to inject air into the engine's exhaust
ports to provide oxygen so unburned and partially burned hydrocarbons in the
exhaust would finish burning. Air injection is now used to support the catalytic
converter's oxidation reaction, and to reduce emissions when an engine is started
from cold. After a cold start, an engine needs an air-fuel mixture richer than what it
needs at operating temperature, and the catalytic converter does not function
efficiently until it has reached its own operating temperature. The air injected
upstream of the converter supports combustion in the exhaust headpipe, which
speeds catalyst warmup and reduces the amount of unburned hydrocarbon emitted
from the tailpipe.
Exhaust gas recirculation
In the United States and Canada, many engines in 1973 and newer vehicles (1972
and newer in California) have a system that routes a metered amount of exhaust
into the intake tract under particular operating conditions. Exhaust neither burns
nor supports combustion, so it dilutes the air/fuel charge to reduce peak
combustion chamber temperatures. This, in turn, reduces the formation of NOx.
Catalytic converter
The catalytic converter is a device placed in the exhaust pipe, which converts
hydrocarbons, carbon monoxide, and NOx into less harmful gases by using a
combination of platinum, palladium and rhodium as catalysts.
There are two types of catalytic converter, a two-way and a three-way converter.
Two-way converters were common until the 1980s, when three-way converters
replaced them on most automobile engines. See the catalytic converter article for
further details.
References
1. "solutions for pre-ignition ("mega-knock"), misfire, extinction, flame
propagation and conventional "knock" . cmcl innovations, UK.
2. Jack Erjavec (2005). Automotive technology: a systems approach.
Cengage Learning.
3. Engine Basics: Detonation and Pre-Ignition, Allen W. Cline.
4. Charles Fayette Taylor, Internal Combustion Engine In Theory And
Practice, Second Edition, Revised, Volume 2
5. https://www.quora.com
6. https://www.energy.gov
7. http://ssmengg.edu.in
8. https://www.slideshare.net
9. http://marineengineeringonline.com
CHAPTER 6 : Engine lubrication system
Submitted by :
072BME 634,635,636,637,638,639
Submitted to:
Engine lubrication system
The lubricating system of an engine is an arrangement of mechanism and devices which
maintains supply of lubricating oil to the rubbing surface of an engine at correct pressure. The
Engine lubrication system is considered to give a flow to the clean oil at the accurate
temperature, with a appropriate pressure to each part of the engine. The oil is sucked out into the
pump from the sump, as a heart of the system, than forced between the oil filter and pressure is
fed to the main bearings and also to the oil pressure gauge. The oil passes through the main
bearings feed- holes into the drilled passages which is in the crankshaft and on to the bearings of
the connecting rod. The bearings of the piston-pin and cylinder walls get lubricated oil which
dispersed by the rotating crankshaft. By the lower ring in the piston the excess being scraped.
Each camshaft bearing is fed by the main supply passage from a branch or tributary. And there is
another branch which supplies the gears or timing chain on the drive of camshaft. The oil which
is excesses then drains back to the sump, where the heat is being transferred to the surrounding
air.
Functions of lubrication:
1. It helps to control the friction between the moving surfaces.
2. It reduces abrasive wear
3. It protects the surfaces from corrosive substances
4. It absorbs and transfers heat and thus controls the temperature
5. It transport the particle and other contminents to filters/ seperators.
Major lubrication system.
1. Splash
2. Wet sump Lubrication
3. Dry sump Lubrication
4. Mist
1. Splash
The lubricating oil is charged into the bottom of the crankcase and maintained at predetermined
level. The oil is drawn by a pump and delivered through a distributing pipe in to the splash
troughs. A dipper is provided under each connecting rod.
Splash lubrication is commonly used in smaller engines. More specifically, this technique is used
in lawnmower and outboard boat engines or motors that have sufficient amounts of oil in the
trough to fully lubricate the machine.
Due to the benefits of splash lubrication, many of our lines of positive displacement blowers and
vacuum pumps also utilize this process including our DuroFlow and Sutorbilt blowers. These PD
blowers are manufactured with an oil slinger that dips into a reservoir filled with oil. As the gears
rotate in the blower, the oil is then splashed upon them, hence the term, splash lubrication.
2.Wet Sump Lubrication
A wet sump is a lubricating oil management design for piston engines which uses
the crankcase as a built-in reservoir for oil, as opposed to an external or secondary reservoir used
in a dry sump design.
The oil is forced to all the main bearings of crankshaft. Pressure relief valve is fitted to
maintain the predictable pressure values. Oil hole is drilled from the center of each crankpin to
the center of an adjacent main journal through which oil can pass from the main bearing to the
crankpin.
3.Dry sump Lubrication
A dry-sump system is a method to manage the lubricating motor oil in four-stroke and
large two-stroke piston driven internal combustion engines. The dry-sump system uses two or
more oil pumps and a separate oil reservoir, as opposed to a conventional wet-sump system,
which uses only the main sump (U.S.: oil pan) below the engine and a single pump. A dry-sump
engine requires a pressure relief valve to regulate negative pressure inside the engine, so internal
seals are not inverted.
In this system the oil is carried in an external tank. An oil pump draws oil from the supply tank
and circulates it under pressure to the various bearings of the engine. Oil dripping from the
cylinders and bearings in to the sump is removed by a scavenging pump which in turn the oil is
passed through a filter and fed back to the supply tank The capacity of scavenging pump is
always greater than the oil pump. A separate oil cooler provided to remove heat from the oil.
4.Mist
Oil mist lubrication oils are applied to rolling element (antifriction) bearings as an oil mist.
Neither oil rings nor constant level lubricators are used in pumps and drivers connected to plant-
wide oil mist systems. Oil mist is an atomized amount of oil carried or suspended in a volume
of pressurized dry air.
This system is used where crankcase lubrication is not suitable In 2-stroke engine as the charge is
compressed in the crankcase, it is not possible to have the lubricating oil in the sump. In such
engines the lubricating oil is mixed with the fuel, the usual ratio being 3% to 6%. The oil and
fuel in the form of mist goes via the crankcase in to the cylinder.
Theory of Hydrodynamic Lubrication
A theoretical analysis of hydrodynamic lubrication was carried out by Osborne Reynolds.
The following assumptions were made by Reynolds in the analysis:
▪
The lubricating fluid is Newtonian - the flow is laminar and the shear stress between the flow
layers is proportional to the velocity gradient in the direction perpendicular to the flow
(Newton’s law of viscosity):
Where:
η–
dynamic
viscosity
of
oil,
v–
linear
velocity
of
the
laminar
layer,
y - the axis perpendicular to the flow direction.
▪ The inertia forces resulted from from the accelerated movement of the flowing lubricant are
neglected.
▪ The lubricating fluid is incompressible.
▪ The pressure of the fluid p is constant in the direction perpendicular to the laminar
flow: dp/dy=0 (assumption of thin lubrication film).
▪ The viscosity of the fluid is constant throughout the lubrication film.
Consider
the
equilibrium
of
a
unit
volume
in
the
lubricant
film.
The pressure forces act on the right and the left faces of the unit volume.
The shear forces resulting from the relative motion of the laminar layers act along the upper and
the
lower
faces
of
the
unit
volume.
Assuming that there is no flow in z direction the equation of the equilibrium of the forces in the
direction
of
the
flow
is
as
follows:
Substitution
of τ from
Newton’s
law
of
viscosity
(1)
results
in:
The velocity function is obtained by integrating the equation (2) with respect to y:
The constants of integration C1 and C2 may be determined from the boundary conditions:
v=U when y=0,then C2=U
v=0 when y=h,then:
Substituting C1 and C2 in(3)we
The
total
obtain:
flow
Substituting v from
of
the
(4)
lubricant
we
is:
get:
According to the assumption about incompressibility of the lubricant the flow Q does not change
in x direction:
Differentiating
the
equation
(5)
with
respect x results
in:
This
is
Reynolds
equation
for
one
dimensional
flow.
It can be used with the assumption of no flow in z direction (bearings with infinite length).
If the flow in z direction is taken into account (bearings with side leakage of the lubricating
fluid) then the analysis results in Reynolds equation for two dimensional flow:
Where:
h – local
oil
film thickness,
η –dynamic viscosity
of
oil,
p –local
oil
film pressure,
U –linear
velocity
of
journal,
x –circumferential
direction.
z - longitudinal direction.
Properties of lubricants
Liquid lubricants, generally referred to as oils, share the properties of all liquids, are able to flow,
and take the shape of their containers. Lubricating oils, like other types of lubricants, are tested
for many different properties that determine how they will function over a range of conditions
and environments.
Some of the most important properties of Liquid Lubricants are:
Lubricity – Some lubricants are said to have high lubricity, or oiliness. This property comes
from the chemical compositions of the oils, which reduce wear and friction even in extreme
conditions.
Viscosity – Viscosity is a measurement of a fluid’s thickness, or resistance to flow. The higher a
lubricant’s viscosity, the thicker it will be and the more energy it will take to move an object
through the oil. One common scale used to describe viscosity in lubricating oils is the numerical
grading given by The Society of Automotive Engineers, or SAE.
Viscosity Index – The viscosity index, or VI, of a lubricant describes how the oil’s viscosity
changes as its temperature changes. As temperatures increase, viscosities decrease, and vice
versa. For example, a piece of machinery that operates over a wide range of temperatures will
require a lubricant with a high VI, meaning that the oil will retain its lubricating characteristics
whether it is starting up cold or running at full speed and peak temperature.
It is the rate of change of viscosity of an oil with respect to change in temperature. An oil with
low viscosity index has greater change of viscosity with change in temperature. An oil with high
viscosity index has very little change of viscosity with change in temperature, which is a
desirable property for lubricating oil. For crankcase oil, viscosity index is 75 to 85. For cylinder
oil, viscosity index is 85. Hydraulic oils should have high viscosity index for faster response of
the system. It is usually around 110.
Cloud Point – Petroleum-based lube oils contain dissolved wax. At a low enough temperature,
referred to as the cloud point, this wax will separate from the oil and form wax crystals. These
crystals can clog filters and small openings, deposit on surfaces such as heat exchangers, and
increase the viscosity of the oil.
Pour Point – The pour point of a lubricant is the lowest temperature at which the oil will flow
from its container. At low temperatures, the viscosity of the oil will be very high, causing the oil
to resist flow. This is important in equipment that operates in a cold environment or handles cold
fluids.
It is the lowest temperature below which an oil will stop flow. Below this temperature the oil
becomes plastic, so it does not produce hydrodynamic lubrication and therefore cannot be used
below this temperature. Pour point indicates that oil is suitable for cold weather or not. Pour
point of engine crankcase should be -18°C.
Oxidation and Corrosion – When lubricants are exposed to oxygen and certain metals or
compounds at temperatures above 160 degrees Fahrenheit, they can be prone to oxidation.
Oxidation of lubricants can lead to several undesirable consequences, such as increased oil
viscosity, formation of corrosive acids, and sludge buildup. Preferred lubricants are those that
have a high resistance to oxidation and inhibit corrosion by protecting components from water,
oxygen, and chemical attacks.
High boiling point and low freezing point (in order to stay liquid within a wide range of
temperature)
Flash point
Another important property of motor oil is its flash point, the lowest temperature at which the
oil gives off vapors which can ignite. It is dangerous for the oil in a motor to ignite and burn,
so a high flash point is desirable. Close flash point for crankcase lubricating oil is around
220°C.
Thermal Stability
Fluid temperature stability is essential to the success of mechanical systems. All hydraulic and
lubricating fluids have practical limits on the acceptable operating temperature range - both high
and low levels.
The machine loses stability and experiences conditional failure whenever the system’s fluid
temperature violates these limits. If left unabated, the conditional failure ultimately results in
both material and performance degradation of machine components.
Hydraulic Stability
In lubricants, water is the second most destructive contaminant behind particles. It causes issues
such as rust and decreased load-carrying capacity (film strength) in oil and also leads to
permanent degradation of the lubricant. Similar to oxidation, hydrolysis is the degradation of the
base oil’s molecules as a result of water. Not only can a base oil fall prey to this process, but
additives are susceptible as well.
As a lubricant is contaminated with water, the question then becomes how stable the fluid is, in
relation to the water. The ability of a lubricant and its additives to resist chemical decomposition
in the presence of water is known as the lubricant’s hydrolytic stability.
Total Base Number (TBN) and TAN
Another manipulated property of motor oil is its Total base number(TBN), which is a
measurement of the reserve alkalinity of an oil, meaning its ability to neutralize acids. The
resulting quantity is determined as mg KOH/ (gram of lubricant). Analogously, Total acid
number(TAN) is the measure of a lubricant's acidity. TBN for an oil used for cross head type
diesel engine crankcase is 8mg KOH/gram of oil. TBN for an oil used for trunk type engine
using heavy oil is 30mg KOH/gram of oil.
Demulsibility
Demulsibility is the ability to release water. Because oil is hygroscopic, meaning that it
absorbs water, it seems that water is destined to get soaked into the oil. Water enters through
thermal breathing (hydraulic systems and mechanical systems that heat and cool and through
any number of ways. Thus it must be removed.
Corrosion prevention
A lubricant should not corrode the working parts.
1. High resistance to Oxidation
Lubricating oils may break down at high tempera-ture due to oxidation producing hard
carbon and varnish, which deposits on the engine parts. Therefore, lubricants must resist
oxidation.
Types of Lubricants and Additives
A lubricant can be of mainly three types which are as follows:
1. Liquid Lubricants
Liquid lubricants are also called as lubricating oils. The Base ingredients that are mostly used in
lubricating oils are hydrocarbon components made from crude oil.
Liquid lubricants are further classified on the basis of crude base as:
1. Vegetable (Castor, Rapeseed) oils :
- Less stable (rapid oxidation) than mineral oils at high temp
- Contain more natural boundary lubricants than mineral oils.
2. Animal fats
–
–
–
–
These are fatty substances extracted from animals, and fish.
They are composed of fatty acids and alcohols.
They are called fixed oils because they do not volatilize unless they decompose
Common examples of these lubricants are tallow, castor oil and fish oil
3. Mineral Oil
–
Mineral oil consists of hydrocarbons (Composed of 83-87% carbon and 11-14% hydrogen by
wt) with approximately 30 carbon atoms in each molecule (composed of straight & cyclic
carbon chains bonded together). Also contain Sulphur, oxygen, nitrogen
–
Mineral oils are classified as paraffin, naphthenic and aromatic
3.1 Paraffinic Oil
These oils have good natural resistance to oxidation. But on oxidation it forms acids, which
means when burnt, leaves a hard carbonaceous deposit.
• Good thermal stability :
- Low volatility.
- High viscosity index (VI=90-115)
- High flash point.
- Pour point higher than naphthenic or aromatic.
3.2 Naphthenic Oils :
• Lower VI (15-75)
• Less resistant to oxidation.
• Lower flash points than paraffinic.
• Lower pour point than paraffinic therefore good for low temperature applications.
• When burnt soft deposits are formed, therefore abrasive wear is lower.
• Oxidation leads to undesirable sludge type deposits.
4. Synthetic Oil
–
Synthetic oils are engineered specifically in uniformly shaped molecules with shorter carbon
chains which are much more resistant to heat and stress.
–
–
–
–
Viscosity does not vary as much with temperature as in mineral oil.
Rate of oxidation is much slower.
Expensive cost
Can be applied where mineral oils are inadequate
Common synthetic oils are :
4.1 Polyglycols (Polyalkylene glycol):
- Originally used as brake fluids VI = 200, absorb water.
- Distinct advantages as lubricants for systems operating at high temperatures such as furnace conveyor
belts, where the polyglycol burns without leaving a carbonaceous deposit. Used in textile industry.
4.2 Esters:
- Better (in reducing friction, resisting oxidation, prolong draining period, volatility) than mineral oils.
- Costs only a little more than mineral oils.
4.3 Silicon:
- VI 300, chemically inert, poor boundary lubricant, low solubility, space application, high production
cost.
- Perfluoropolyalkylether: Good oxidation & thermal stability VI= 200. In vacuum used for thin film
lubrication.
4.4 Perfluoropolyethers :
- High oxidation (3200C) & thermal (3700C) stability.
- Low surface tension & chemically inert
2. Semisolid Lubricants (Grease)
Grease is a black or yellow sticky mass used in the bearings for lubrication purpose.
Lubricating greases consist of lubricating oils, often of quite low viscosity, which have
been thickened by means of finely dispersed solids called thickeners. It consist of base
oils (75 to 95%), additives (0 to 5%) and minute thickener fibers (5 to 20%).
Grease is useful under heavy load conditions and in inaccessible parts where the supply
of Lubricant cannot easily be renewed. Grease is applied by packing enclosed parts with
it or by pressing it into moving parts
1.
2.
3.
4.
5.
6.
7.
8.
Advantages of Semisolid Lubricants
Remains at application point & adhere to surface
Less-frequent application needed.
Good for inclined/vertical shafts.
Seal out contaminants & less expensive seals needed.
Water resistant & reduce oil vapor problems.
Prolong the life of worn parts by filing irregularities
Provide better mechanical lubrication cushion for extreme conditions such as shock
loading, reversing operations, low speeds & high loads.
Reduce noise and vibration
Disadvantages of Semisolid Lubricants
1. Because of semi-solid nature of greases, it does not perform the cooling, so poor
dissipation of heat.
2.
Once dust or dirt enters the grease, it cannot be easily removed and would act as
deterrent in performance.
3. No filtration.. So contaminants/wear-debris cannot be separated.
3. Solid Lubricants
A solid lubricant is basically any solid material which can be placed between two bearing
surfaces and which will shear more easily under a given load than the bearing materials
themselves. Solid lubricants are especially useful at high and low temp.,in high vacuums and in
other applications where oil is not suitable common solid lubricants are Graphite and
Molybdenum Disulphide.
Two primary property of solid lubricants are:
1. Material must be able to support applied load without significant distortion, deformation
or loss in strength.
2. Coefficient of friction and the rate of wear must be acceptably low.
1.
2.
3.
4.
5.
1.
2.
3.
Advantages of Solid Lubricants
More effective than liquid lubricant at high load
High resistance in deterioration in storage.
High stable in extreme temperature, radiation and reactive environment
Permit equipment to be lighter and simpler
Superior cleanliness
Disadvantages of Solid Lubricants
Higher coefficient of friction than liquid lubricants
A broken solid film tends to shorten the usefullife of lubricant
In case of polymers,it has low thermal conductivity resulting poor heat dissipation
Additives
Additives are substances formulated for improvement of the anti-friction, chemical and physical
properties of base oils (mineral, synthetic, vegetable or animal), which results in enhancing the
lubricant performance and extending the equipment life.
Combination of different additives and their quantities are determined by the lubricant type
(Engine oils, Gear oils, Hydraulic oils, cutting fluids, Way lubricants, compressor oils etc.) and
the specific operating conditions (temperature, loads, machine parts materials,
environment).Amount of additives may reach 30%
A large number of additives are used to impart performance characteristics to the lubricants. The
main families of additives are:
Friction modifiers
– Friction modifiers reduce coefficient of friction, resulting in less fuel consumption.
– Crystal structure of most of friction modifiers consists of molecular platelets (layers),
which may easily slide over each other.
The following Solid lubricants are used as friction modifiers:
Graphite;
Molybdenum disulfide;
Boron nitride (BN);
Tungsten disulfide (WS2);
Polytetrafluoroethylene (PTFE).
Anti-wear additives
– Anti-wear additives prevent direct metal-to-metal contact between the machine parts
when the oil film is broken down.
– Use of anti-wear additives results in longer machine life due to higher wear and score
resistance of the components.
– The mechanism of anti-wear additives: the additive reacts with the metal on the part
surface and forms a film, which may slide over the friction surface.
The following materials are used as anti-wear additives:
Zinc dithiophosphate (ZDP);
Zinc dialkyldithiophosphate (ZDDP);
Tricresylphosphate (TCP).
Extreme pressure (EP) additives
– Extreme pressure (EP) additives prevent seizure conditions caused by direct metal-tometal contact between the parts under high loads.
– The mechanism of EP additives is similar to that of anti-wear additive: the additive
substance form a coating on the part surface. This coating protects the part surface from a
direct contact with other part, decreasing wear and scoring.
The following materials are used as extra pressure (EP) additives:
Chlorinated paraffins;
Sulphurized fats;
Esters;
Zinc dialkyldithiophosphate (ZDDP);
Molybdenum disulfide;
Rust and corrosion inhibitors
– Rust and Corrosion inhibitors, which form a barrier film on the substrate surface reducing
the corrosion rate.
– The inhibitors also absorb on the metal surface forming a film protecting the part from
the attack of oxygen, water and other chemically active substances.
The following materials are used as rust and corrosion inhibitors:
Alkaline compounds;
Organic acids;
Esters;
Amino-acid derivatives.
Anti-oxidants
– Mineral oils react with oxygen of air forming organic acids. The oxidation reaction
products cause increase of the oil viscosity, formation of sludge and varnish, corrosion of
metallic parts and foaming.
– Anti-oxidants inhibit the oxidation process of oils.
– Most of lubricants contain anti-oxidants.
The following materials are used as anti-oxidants:
Zinc dithiophosphate (ZDP);
Alkyl sulfides;
Aromatic sulfides;
Aromatic amines;
Hindered phenols.
Detergents
– Detergents neutralize strong acids present in the lubricant (for example sulfuric and nitric
acid produced in internal combustion engines as a result of combustion process) and
remove the neutralization products from the metal surface.
– Detergents also form a film on the part surface preventing high temperature deposition of
sludge and varnish.
– Detergents are commonly added to Engine oils.
Phenolates, sulphonates and phosphonates of alkaline and alkaline-earth elements, such as
calcium(Ca), magnesium (Mg), sodium (Na) or Ba (barium), are used as detergents in lubricants
Grading of Lubricating oils.
Lubricating oils are generally rated using a viscosity scale established by the Society of
Automotive Engineerng (SAE).
ζ = Ц x (dU/dy)
ζ= Shear force per unit area
Ц= dynamic viscosity
(dU/dy)= velocity gradient
Common viscosity grades used in engine are:
SAE 5
SAE 30
SAE 10
SAE 40
SAE 20
SAE 45
SAE 50
The oils with lower numbers are less viscous and are used in cold weather operation. Those with
higher numbers are more viscous and are used in modern high- temperature, high-speed closetolerance engines.
If oil viscosity is too high, more work is required to pump it and to shear it between
moving parts. This results in greater friction work and reduced brake work and power output.
Fuel consumption can be increased by as much as 15%
Starting a cold engine lubricated high viscosity oil is very difficult.
Multigrade oil was developed so that viscosity would be more constant over the operating
temperature range of an engine. When certain polymers are added to an oil, The temperature
dependency of the oil viscosity is reduced. These oils have low-number viscosity values when
they are cold and higher numbers when they are hot.A value such that
SAE 10W-30 means that the oil has perporties of 10 viscocity when it is cold (W= winter) and
30 viscosity when it is hot.
(source:Wikipedia)
How is Viscosity Graded?
Viscosity is notated with the common "XW-XX." The number preceding the "W" rates the oil's
flow at 0 degrees Fahrenheit (-17.8 degrees Celsius). The "W" stands for winter, not weight as
many people think. The lower the number here, the less it thickens in the cold. So 5W-30
viscosity engine oil thickens less in the cold than a 10W-30, but more than a 0W-30. An engine
in a colder climate, where motor oil tends to thicken because of lower temperatures, would
benefit from 0W or 5W viscosity. A car in Death Valley would need a higher number to keep the
oil from thinning out too much.
The second number after the "W" indicates the oil's viscosity measured at 212 degrees
Fahrenheit (100 degrees Celsius). This number represents the oil's resistance to thinning at high
temperatures. For example, 10W-30 oil will thin out at higher temperatures faster than 10W-40
will.
Monograde oils such as SAE 30, 40 or 50 are no longer used in latest automotive engines, but
may be required for use in some vintage and antique engines. Straight SAE 30 oil is often
specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and
gas-powered chain saws.
Viscosity is notated with the common "XW-XX." The number preceding the "W" rates the oil's
flow at 0 degrees Fahrenheit (-17.8 degrees Celsius). The "W" stands for winter, not weight as
many people think. The lower the number here, the less it thickens in the cold. So 5W-30
viscosity engine oil thickens less in the cold than a 10W-30, but more than a 0W-30. An engine
in a colder climate, where motor oil tends to thicken because of lower temperatures, would
benefit from 0W or 5W viscosity. A car in Death Valley would need a higher number to keep the
oil from thinning out too much.
The second number after the "W" indicates the oil's viscosity measured at 212 degrees
Fahrenheit (100 degrees Celsius). This number represents the oil's resistance to thinning at high
temperatures. For example, 10W-30 oil will thin out at higher temperatures faster than 10W-40
will.
Monograde oils such as SAE 30, 40 or 50 are no longer used in latest automotive engines, but
may be required for use in some vintage and antique engines. Straight SAE 30 oil is often
specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and
gas-powered chain saws.
CHAPTER 7: AIR AND WATER COOLING SYSTEM
(072BME 640,642,643,644,645,646)
Cooling of IC Engines
•
An automobile's cooling system is the collection of parts and substances (coolants)
that work together to maintain the engine's temperature at optimal levels. Comprising
many different components such as water pump, coolant, a thermostat etc. the system
enables smooth and efficient functioning of the engine at the same time protecting it
from damage
•
An automotive cooling system must perform several functions
a. Remove excess heat from the engine
b. Maintain a constant engine temperature
Need of Cooling System
• Get the engine up to optimum operating temperature as quickly as possible and
maintains it at that temperature.
• Controls the heat produced in combustion chamber, so that the engine parts are not
damaged & the oil does not break down.
• The temperature of component must be maintained within certain limit in order to obtain
maximum performance of engine. Adequate cooling is then a fundamental requirement
associated with reciprocating I.C.engine
Types of cooling system
In order to cool the engine a cooling medium is required. On the basis of medium, in general
use for cooling I.C. engine, types of cooling system are:I. Air or direct cooling system.
II. Liquid or indirect cooling system.
Air Cooled System
• In air cooled system a current of air made to flow past the outside of the cylinder barrel,
outer surface area which has been considerably increased by providing cooling fins.
•
The amount of heat dissipated to air depends upon :
a. Amount of air flowing through the fins.
b. Fin surface area.
c. Thermal conductivity of metal used for fins.
Cooling Fins
In the study of heat transfer, a fin is a surface that extends from an object to increase the rate of
heat transfer to or from the environment by increasing convection. The amount
of conduction, convection, or radiation of an object determines the amount of heat it transfers.
Increasing the temperature difference between the object and the environment, increasing the
convection heat transfer coefficient, or increasing the surface area of the object increases the heat
transfer.
Baffles:
The rate of heat transfer from the cylinder walls can be substantially increased by using
baffles which force the air through the space between the fins.
Advantages and Disadvantages of Air cooling system
Advantages:• Radiator/pump is absent hence the system is light.
• In case of water cooling system there are leakages, but in this case here are no
leakages.
• Coolant and antifreeze solutions are not required.
• This system can be used in cold climates, where if water is used it may freeze
Disadvantages:•
Comparatively it is less efficient.
•
It is used in aero planes and motorcycle engines where the engines are exposed to
air directly.
WATER COOLING SYSTEM
Water cooling system is of following two functions:
1. It takes the excessive heat from engine.
2. It keeps the engine temperature normal to increase engine’s efficiency.
There are four types of water cooling system.
1. Direct or non-return system
2. Thermo-Syphone system
3. Hopper system and
4. Pump/forced circulation system.
In modern days only forced circulation system is used.
Pump/forced circulation system.
Parts of water cooling systems:
Radiator
Radiator is a honeycomb heat exchanger with hot coolant flowing from top to bottom
exchanging energy with cooler flowing from front to back.
There are two types of radiators:
(1) Cross flow:- The cross-flow radiator is normally shorter than a down flow radiator allowing
for shorter hood line
(2) Down flow:- A down-flow radiator is used on larger vehicles that require more cooling
capacity.
Radiator cap
Radiator caps in modern days are placed to hold on pressure.
The defective caps must be replaced.
Thermostat valve
Thermostat checks the water to flow back to radiator,
It holds water until engine is back to its working temperature.
Water pumps
Impeller type pump is used to circulate water. The shaft of pump is driven by the engine itself.
Water jackets
There is cooling water jackets around cylinder heads valves.
After absorbing heat the water is again cooled in radiator.
Cooling fan
Maintains adequate air flow in radiator matrix. It is driven by the belt through crankshaft. The
major problem with fan is that it consumes power from engine and makes noise.
Antifreeze
During winter the coolants in pipes may freeze and the pipes may burst .for that prevention
antifreeze is used.
Some commonly used anti freezers are:
Ethylene glycol, isopropyl alcohol methanol,
ethanol propylene glycol.
Construction details of air cooling system:
The cylinder head and cylinders to use the motorcycle term, of an air cooled engine are cast with
fins. These fins spread the heat of the engine over a wide area. If a barrel is made without fins
and is 15 cm long, all its heat will be spread over the length. If the barrel is manufactured with
ten fins each five cm deep, the same amount of heat will be depressed over 100cm. This will
lower the overall barrel temperature and permit the air greater excess to surface that most require
cooling. The engine driven fan directs a blast of cool air on to the fins. The fan is necessary
because an air cooled engine requires a very large air flow; 4000 times more air than water, by
volume, is needed to cool an engine, so the flow of air created by the car cannot be relied upon.
Water Cooling System
Water cooling system is the configuration of any automobile to cool down the heated engine
parts for proper operation of the engine at favorable temperature conditions to give desired
efficiencies. Huge amount of heat energy from engine is wasted which heats up the engine
components. At operating conditions, engine heats at around 300-650oC or even more. This
temperature must be controlled to a favorable conditions. If we do not maintain favorable
temperature to the system, engine will not sustain for long period and will have wear and tear.
So, to maintain the proper operation of engine and to protect it for damage, it must be cooled
either by air cooling system, water cooling system or any other process. Within an engine, it is
possible to cool certain parts of an engine by oil. Such process is possible for inner portion of an
engine. But, for cooling the outer system water cooling system is used basically for heavy
vehicles (except two wheelers).
Cooling an Internal Combustion Engines
The engine block of a water cooled engine is surrounded with a water jacket through which
coolant liquid flows. Water being a good heat transfer fluid and everywhere available can be
used as a coolant. But it has some drawbacks such as freezing point of 0oC, so not acceptable for
northern winter climates. Its boiling temperature even under pressurized conditions has low
boiling temperature. Aside physical properties limitations, it also promotes rust and corrosion in
many materials which can damage the engine components. Resolving all such limitations water
is mixed with ethylene glycol to improve heat transfer advantages and also improves other
physical properties. Ethylene glycol not only acts as antifreeze but also acts as a corrosion
inhibitor and a lubricant for the water pump. This can be achieved by mixing small amount of
ethylene glycol to about 70%. At high concentrations the heat transfer properties lags as desired.
Ethylene glycol is water soluble and has a boiling temperature of 197oC and a freezing
temperature of -11oC in pure form at atmospheric pressure. When ethylene glycol is mixed with
water to use as a coolant, the concentration with water is usually determined by the coldest
weather temperature to be experienced by the engine. The environment temperature must be
taken into consideration to prevent the coolant freeze in the tubes which may block the
circulation of fluid flow. At low temperature the coolant is more serious when its temperature
lowers and expands on cooling and possibly cracks the walls and damages the cooling system.
To prevent the coolant temperature drop below certain temperature, a thermostat is installed
which acts as a GO/NO GO valve. If temperature falls below this minimum temperature then
thermostat valve is closed and does not allow to cool further. But in case the temperature is
above desired temperature it opens up and flows throughout the system. The greater the
temperature, the greater the valve openings resulting higher coolant flow.
The hot coolant that has cooled the engine flows through the system to release their heat energies
at the radiator. A radiator is provided with an inlet and outlet hose pipe through which coolant
flows to cool down at normal ambient temperature. A radiator is finned pipes through which
water flows and is cooled by a fan attached behind the radiator. A radiator is topped with a
radiator cap which acts as a safety valve. If an engine overheats the fluid boils up and vaporizes
and increases the pressure in the radiator. If such conditions occur the radiator cap releases the
high pressure by releasing the vapour into an overflow container. This prevents engine from
failing and perform properly with greater efficiencies.
Variation of gas temperature
 Only a part of total fuel energy supplied to the IC engine is converted into useful work.
Rest of fuel energy is rejected as follows:
 Heat lost from engine boundries due to radiation, convection and
conduction (to small extent)

As enthalpy of exhaust gas

Heat rejected to coolant
 General heat balance(for a diesel engine)
 About 35% of total chemical energy that enters the engine in fuel is converted to useful
crankshaft work.
 About 30% of fuel energy is carried away from the engine in the exhaust flow in the form
of enthalpy and chemical energy.
 This leaves about one third of the total energy that must be dissipated to the surrounding
by some mode of heat transfer.
 This is achieved by cooling systems; either air-cooling or water cooling.
 Gas temperature varies over the cycle differently for different types of engine.
 The gas temperature is min. during suction stroke and max. near TDC
Temperature values at steady state conditions:
Hottest spots:
 Around spark plug
 Exhaust valve and port
 Face of the piston
Also, they are difficult places to cool.
Variation of temperatures in engine cooling systems: