I.C. ENGINES
LECTURE NO: 06
(10 Mar, 2014)
QUESTIONS
•
•
•
•
•
•
What are the types of fuels?
Define Calorific Value?
What are major characteristic of fuel?
What are the qualities of S.I. fuels?
What are the qualities of C.I. fuels?
Explain volatility, crankcase dilution, vapour
lock, viscosity?
• How common Hydrocarbon fuels are
grouped?
QUESTIONS
• What are the types of alternate fuels?
• Explain overall chemical equation for the
complete combustion of one mole of propane
(C3H8) with oxygen?
Combustion Stoichiometry
If sufficient oxygen is available, a hydrocarbon fuel can be
completely oxidized, the carbon is converted to carbon dioxide
(CO2) and the hydrogen is converted to water (H2O).
The overall chemical equation for the complete combustion of one mole of
propane (C3H8) with oxygen is:
C3 H8  aO2  bCO2  cH 2O
# of moles
species
Elements cannot be created or destroyed, so
C balance:
H balance:
O balance:
3=b
→ b= 3
8 = 2c
→ c= 4
2a = 2b + c → a= 5
Thus the stoichiometric reaction is:
C3 H8  5O2  3CO2  4H 2O
Combustion Stoichiometry
Air contains molecular nitrogen N2, if products are at a “low”
temperature the nitrogen is not significantly affected by the
reaction, it is considered inert.
The complete reaction of a general hydrocarbon CH with air is:
C H   a (O2  3.76 N 2 )  bCO 2  cH 2O  dN 2
C balance:
H balance:
O balance:
N balance:
+ /4)
=b →b=
 = 2c → c = /2
2a = 2b + c → a = b + c/2 → a =  + /4
2(3.76)a = 2d → d = 3.76a/2 → d = 3.76(





C H      (O2  3.76 N 2 )  CO2  H 2O  3.76    N 2
4
2
4


Example: The stoichiometric reaction of octane (C8H18) = 8 and = 18
C8 H18  12.5(O2  3.76N 2 )  8CO2  9 H 2O  47N 2
Combustion Stoichiometry
The stoichiometric mass based air/fuel ratio for CH fuel is:
 A / F s
 ni M i air
mair


m fuel  ni M i  fuel






M

3
.
76



 O

M N
4
4


M C   M H
2
2
Substituting the respective molecular weights and dividing top and bottom by
 one gets the following expression that only depends on the ratio of the
number of hydrogen atoms to hydrogen atoms (/) in the fuel.
 A / F s
    
1 
(32  3.76  28)
1
4 


( F / A) s
12     1
Note above equation only applies to stoichiometric mixture
Example: For octane (C8H18), / = 2.25 → (A/F)s = 15.1
(gasoline (A/F)s≈ 14.6
Fuel Lean Mixture
Fuel-air mixtures with more than stoichiometric air (excess air) can burn
With excess air you have fuel lean combustion
At low combustion temperatures, the extra air appears in the products as
O2 and N2:


C H   g (  )(O2  3.76 N 2 )  CO2  H 2O  dN2  eO2
4
2
a fuel lean mixture has excess air, so g > 1
Above reaction equation has two unknowns (d, e) and
we have two atom balance equations (O, N) so can
solve for the unknowns
Fuel
Rich
Mixture
Fuel-air mixtures with less than stoichiometric air (excess fuel)
can burn. With less than stoichiometric air you have fuel rich
combustion, there is insufficient oxygen to oxidize all the C and H
in the fuel to CO2 and H2O.
Get incomplete combustion where carbon monoxide (CO) and molecular
hydrogen (H2) also appear in the products.


C H   g (  )(O2  3.76 N 2 )  CO2  H 2O  dN 2  eCO  fH 2
4
2
a fuel rich mixture has insufficient air → g < 1
Above reaction equation has three unknowns (d, e, f)
and we only have two atom balance equations (O, N) so
cannot solve for the unknowns unless additional
information about the products is given.
Off-Stoichiometric Mixtures
The equivalence ratio, f, is commonly used to indicate if a mixture is
stoichiometric, fuel lean, or fuel rich.
 A / F s
F / Amixture
f

 A / F mixture
F / As
stoichiometric f = 1
fuel lean
f<1
fuel rich
f>1
Stoichiometric mixture:


C H      (O2  3.76 N 2 )  Products
4

Off-stoichiometric mixture:
1

C H      (O2  3.76N 2 )  Products
f
4
g 
1
f
Off-stoichiometric Conditions
Other terminology used to describe how much air is used in
combustion:
110% stoichiometric air = 110% theoretical air = 10% excess
air C3 H 8  g (   )(O2  3.76N 2 )
g  1.1 → mixture is fuel lean
4
Example: Consider a reaction of octane with 10% excess air, what is f?
Stoichiometric : C8 H18  12.5(O2  3.76N2 )  8CO2  9H 2O  47N2
10% excess air is: C8 H18  1.1(12.5)(O2  3.76N2 )  8CO2  9H 2O  aO2  bN2
O balance: 1.1(12.5)(2) = 16 + 9 + 2a → a = 1.25,
N balance: 1.1(12.5)(3.76)(2) = 2b → b = 51.7
f
 A / F s
12.5(4.76) / 1

 0.91
 A / F mixture 1.1(12.5)(4.76) / 1
1
f

1
 1 .1
.91
FUEL PROPERTIES COMPARISON
Gasoline
Diesel
Biodiesel
Propane
(LPG)
Compressed
Natural Gas
(CNG)
Chemical
Structure
C4 to C12 C8 to C25
Methyl esters C3H8
of C12 to C22
(majority)
fatty acids
and C4H10
(minority)
CH4
(83‐99%),
2H6
(1‐13%)
Fuel
Material
(feed
stocks)
Crude Oil
Crude Oil
Fats and oils
from sources
such as soy
beans, waste
cooking oil,
animal fats,
and rapeseed
A by‐product
of petroleum
refining or
natural gas
processing
Underground
reserves
1 gallon of
diesel has
113% of the
energy of one
gallon of
gasoline.
B100 has 103% of the
energy in one gallon of
gasoline or 93% of the
energy of one gallon of
diesel. B20 has 109%
of the energy of one
gallon of gasoline or
99% of the energy of
one gallon of diesel.
1 gallon of
propane has
73% of the
energy of one
gallon of
gasoline.
5.66 pounds
or 126.67 cu.
ft. of CNG has
100% of the
energy of one
gallon of
gasoline
100%
Gasoline
Gallon
Equivalent
CETANE NUMBER
• A measure of the ignition quality of a diesel
fuel, as determined in a standard single
cylinder test engine, which measures ignition
delay compared to primary reference fuels.
The higher the CETANE NUMBER, the easier a
high speed , direct injection will start and the
less white smoking and diesel knock after
start.
CETANE NUMBER
• Cetane number or CN is a measurement
of the combustion quality of diesel
fuel during compression ignition. It is a
significant expression of the quality of a
diesel fuel. A number of other
measurements determine overall diesel
fuel quality - these other measures of
diesel fuel quality include density, lubricity,
cold-flow properties, and sulfur content.
CETANE NUMBER
• Cetane number or CN is a measurement of
the combustion quality of diesel fuel during
compression ignition. It is a significant expression
of the quality of a diesel fuel. A number of other
measurements determine overall diesel fuel
quality - these other measures of diesel fuel
quality include density, lubricity, cold-flow
properties, and sulfur content. Generally, diesel
engines operate well with a CN from 40 to 55. Fuels
with higher cetane number have shorter ignition
delays, providing more time for the fuel combustion
process to be completed. Hence, higher speed diesel
engines operate more effectively with higher cetane
number fuels.
OCTANE NUMBER
• A measure of a fuel’s ability to prevent
detonation in a S.I. engine. Measured in a
standard single cylinder test engine, variable
compression ratio engine by comparison with
primary reference fuel. Under mild conditions,
the engine measures research octane number
(R.O.N.), under severe conditions Motor Octane
number (M.O.N). The arithmetic average of
(R+M)/2. It approximates the Road Octane
Number, which is the measure of how an average
car responds to the fuel.
OCTANE NUMBER
• Measure of the ignition quality of gas (gasoline or petrol).
Higher this number, the less susceptible is the gas to
'knocking' (explosion caused by its premature burning in the
combustion chamber) when burnt in a standard (sparkignition internal combustion) engine. Octane number denotes
the percentage (by volume) of iso-octane in a combustible
mixture whose 'anti-knocking' characteristic match those of
the gas being tested. In the older vehicles, high
octane number were achieved by adding lead tetraethyl to the
gas (the 'leaded gas'), a pollutant that contributes to lead
poisoning. In the newer vehicles, the same results is achieved
by the engine design that increases turbulence in the
combustion
chamber,
and/or
by
adding
aromatic
hydrocarbons
and
oxygenates
(oxygencontaining compounds such as alcohols) to the gas (the
'unleaded
gas.
Also
called
Octane
rating.
OCTANE NUMBER
• Measure of the ignition quality of gas (gasoline or petrol).
Higher this number, the less susceptible is the gas to
'knocking' (explosion caused by its premature burning in the
combustion chamber) when burnt in a standard (sparkignition internal combustion) engine. Octane number denotes
the percentage (by volume) of iso-octane in a combustible
mixture whose 'anti-knocking' characteristic match those of
the gas being tested. In the older vehicles, high
octane number were achieved by adding lead tetraethyl to the
gas (the 'leaded gas'), a pollutant that contributes to lead
poisoning. In the newer vehicles, the same results is achieved
by the engine design that increases turbulence in the
combustion
chamber,
and/or
by
adding
aromatic
hydrocarbons
and
oxygenates
(oxygencontaining compounds such as alcohols) to the gas (the
'unleaded
gas.
Also
called
Octane
rating.
PRINCIPLES OF CARBURETION
COMPOSITION OF AIR
• Air is composed of various gases, mostly nitrogen
and oxygen (78 percent nitrogen and 21 percent
oxygen by volume). These gases, as are all
substances, are made of tiny particles called
molecules. In the air surrounding the earth, as In
all gases, the molecules are able to move quite
freely in relation to each other. The molecules of
air are attracted to the earth by gravity, creating
the atmosphere . The weight of the air molecules
creates atmospheric pressure
EVAPORATION
• Evaporation is the changing of a liquid to a
vapor. The molecules of the liquid, not being
closely tied together, are constantly moving
about among themselves. Any molecule that
moves upward with sufficient speed will jump
out of the liquid and into the air. This process
will cause the liquid to evaporate over a
period of time.
RATE OF EVAPORATION
• Evaporation is the changing of a liquid to a
vapor. The molecules of the liquid, not being
closely tied together, are constantly moving
about among themselves. Any molecule that
moves upward with sufficient speed will jump
out of the liquid and into the air. This process
will cause the liquid to evaporate over a
period of time.
EVAPORATION DEPENDS UPON
• Temperature
– The rate of movement of the molecules Increases
with temperature. Because of this, the amount of
molecules leaving the liquid for a given time will
increase as the temperature Increases.
• Atmospheric Pressure
– As atmospheric pressure Increases, the amount of air
molecules present over the liquid also increases. The
Increased presence of air molecules will slow the rate
of evaporation. This is because the molecules of liquid
will have more air molecules to collide with. In many
cases, they will fall back into the liquid after collision.
EVAPORATION DEPENDS UPON
• Closed Chamber
– As evaporation takes place in a closed container, the space
above the liquid will reach a point of saturation. When this
happens, every molecule of liquid that enters the air will
cause another airborne molecule of liquid to fall back.
• Volatility
– The term volatility refers to how fast a liquid vaporizes.
Some liquids vaporize easily at room temperature. Alcohol,
for instance, vaporizes more easily than water. A highly
volatile liquid is one that is considered to evaporate easily.
• Atomization
– Atomization is the process of breaking up a liquid into tiny
globules or droplets. When a liquid is atomized, the
droplets are all exposed individually to the air. For this
reason, atomization greatly increases evaporation by
increasing the exposed surface area of the liquid.
ATOMIZATION
VENTURI EFFECT
• Venturi effect is used by the carburetor to mix gasoline
with air. The basic carburetor has an hourglass-shaped
tube called a throat. The most constricted part of the
throat is called the venturi. A tube called a discharge
nozzle Is positioned in the venturi. The discharge
nozzle is connected to a reservoir of gasoline called the
float bowl. The negative pressure that exists in the
combustion chamber because of the downward intake
stroke of the piston causes atmospheric pressure to
create an airflow through the carburetor throat. This
airflow must Increase temporarily in speed as it passes
through the venturi, due to its decreased size.
VENTURI
VENTURI
THE BASIC CARBURETOR
BASIC CARBURETOR PARTS
•
•
•
•
•
•
Carburetor Body
Air Horn
Venture
Throttle Valve
Float Circuit
Venting
CARBURETOR BODY
AIR HORN
• The air horn is also called the throat or barrel.
The parts which often fasten 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.
VENTURE
• The venture produces sufficient suction to pull
fuel out of the main discharge tube
THROTTLE VALVE
• The throttle valve is used to regulate the speed and
power output of the engine.
• It is controlled by the accelerator pedal, and usually
consists of a flat, round plate that tilts with the throttle
shaft. As the accelerator pedal is fully depressed, the
throttle valve is moved from a position of completely
restricting the throat to being completely open.
• The idle stop screw is used to keep the throttle valve
open slightly so that the engine may run at a regulated
idle speed with no foot pressure on the accelerator.
This screw may be turned in or out to regulate engine
idle speed.
THROTTLE VALVE
FLOAT CIRCUIT
• Purpose
– The float circuit maintains a steady working supply
of gasoline at a constant level in the carburetor.
– This is very critical to proper engine performance.
– An excessively high float level will cause fuel to
flow too freely from the discharge tube, causing
an overly rich mixture.
– Whereas an excessively low float level will cause
an overly lean mixture.
FLOAT CIRCUIT
• Operation
– Fuel pump delivers gasoline to the fuel pump under
pressure to the carburetor. The following events occur
as the gasoline enters the carburetor through the fuel
inlet:
• The gasoline begins to fill the float bowl.
• The float rises with the level of the gasoline.
• The needle valve is closed by the rising float as the
fuel reaches the desired level in the float bowl.
• As the engine uses the gasoline from the float
bowl, the level will drop. This will cause the float to
drop, which will open the needle valve to let in
more
VENTING
• The pressure in the float bowl must be regulated to
assure the proper delivery of fuel and purging of vapors.
The following systems and devices are added to the float
circuit system to provide for these needs.
– Balance Tube
• Due to the restriction imposed by the air filter and
changing air velocities because of varying engine
speeds, the air pressure in the air horn is usually
lower than atmospheric pressure. The pressure in
the float bowl must equal that of the air horn in
order for the carburetor to provide fuel delivery. A
tube called a balance tube is run between the air
horn and the float bowl to accomplish this task.
VENTING
– Idle Vent
• Because gasoline Is highly volatile, it can create
overly rich mixtures during long periods of engine
idle. This is because the fuel begins to evaporate
in the float bowl and the vapors get into the air
horn through the balance tube. The solution to this
problem is to have an outside vent for the float
bowl that is opened whenever the engine is idling.
The idle vent is activated by linkage from the
throttle valve. The idle vent system on later
vehicles may be part of the emission control
system