INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTERJagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
COMBUSTION PROCESS IN CI ENGINES
In SI engine, uniform A: F mixture is supplied, but in CI engine A: F 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 generated 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. To burn the liquid fuel is more difficult as it is to be
evaporated; it is to be elevated to ignition temperature and then burn.
STAGES OF COMBUSTION IN CI ENGINE (JAN 2007/JULY2006)
The combustion in CI engine is considered to be taking place in four phases:
• Ignition Delay period /Pre-flame combustion
• Uncontrolled combustion
• Controlled combustion
• After burning
Ignition Delay period /Pre-flame combustion
The fuel does not ignite immediately upon injection into the combustion chamber. There
is a definite period of inactivity between the time of injection and the actual burning this
period is known as the ignition delay period.
Fig1.Stages of combustion Fig 2. Pressure Time diagram illustrating Ignition delay
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
In Figure 2. the delay period is shown on pressure crank angle (or time) diagram
between points a and b. Point “a” represents the time of injection and point “b”
represents the time of combustion. The ignition delay period can be divided into two
parts, the physical delay and the chemical delay.
The delay period in the CI engine exerts a very great influence on both engine design
and performance. It is of extreme importance because of its effect on both the
combustion rate and knocking and also its influence on engine starting ability and the
presence of smoke in the exhaust.
2 Period of Rapid Combustion
The period of rapid combustion also called the uncontrolled combustion, is that phase in
which the pressure rise is rapid. During the delay period, a considerable amount of fuel
is accumulated in combustion chamber, these accumulated fuel droplets burns very
rapidly causing a steep rise in pressure. The period of rapid combustion is counted
from end of delay period or the beginning of the combustion to the point of maximum
pressure on the indicator diagram. The rate of heat-release is maximum during this
period. This is also known as uncontrolled combustion phase, because it is difficult to
control the amount of burning / injection during the process of burning.
It may be noted that the pressure reached during the period of rapid combustion will
depend on the duration of the delay period (the longer the delay the more rapid and
higher is the pressure rise since more fuel would have been present in the cylinder
before the rate of burning comes under control).
3 Period of Controlled Combustion
The rapid combustion period is followed by the third stage, the controlled combustion.
The temperature and pressure in the second stage are so high that fuel droplets
injected burn almost as they enter and find the necessary oxygen and any further
pressure rise can be controlled by injection rate. The period of controlled combustion is
assumed to end at maximum cycle temperature.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
4 Period of After-Burning
Combustion does not stop with the completion of the injection process. The unburnt and
partially burnt fuel particles left in the combustion chamber start burning as soon as they
come into contact with the oxygen. This process continues for a certain duration called
the after-burning period. This burning may continue in expansion stroke up to 70 to 80%
of crank travel from TDC.
The sequence of the events in the entire combustion process in a CI engine including
the delay period is shown in Figure 3 by means of a block diagram.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
Ignition Delay or Ignition Lag ( VTU Feb 2006)
The delay period is the time between the start of
injection and start of combustion. The delay period
extends for about 13 deg movement of crank. This
delay time decreases with increase in speed. If
there is no delay, the fuel would burn at injector and
there would be oxygen deficiency around the
injector, which results in incomplete combustion. If
the delay period is too long, amount of fuel
availability for simultaneous explosion , is too great , which results in rapid pressure rise.
The delay period should be as short as possible since long delay period gives more
rapid rise in pressure and thus causes knocking.
Component of Ignition Delay or Ignition Lag ( VTU Feb 2006)
Ignition delay can be divided into two parts:
Physical Delay: The physical delay is the time between the beginning of injection and
the attainment of chemical reaction conditions. During this period, the fuel is atomized,
vaporized, mixed with air and raised to its self-ignition temperature. This physical delay
depends on the type of fuel, i.e., for light fuel the physical delay is small while for heavy
viscous fuels the physical delay is high. The physical delay is greatly reduced by using
high injection pressures and high turbulence to facilitate breakup of the jet and
improving evaporation.
Chemical Delay: During the chemical delay reactions start slowly and then accelerate
until inflammation or ignition takes place. Generally, the chemical delay is larger than
the physical delay. However, it depends on the temperature of the surroundings and at
high temperatures, the chemical reactions are faster and the physical delay
Total delay period = Physical delay + Chemical delay
t p c t = t + t ,
In CI engine p t >> c t ,
In SI engine p t » 0
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
Combustion phenomenon in CI engine V/s combustion in SI engine. ( VTU July 2006)
SL
NO
COMUSTION IN SI ENGINE COMBUSTION IN CI ENGINE
1 Homogeneous mixture of petrol vapour and air is
compressed ( CR 6:1 to 11:1) at the end of
compression stroke and is ignited at one place by
spark plug.
Air alone is compressed through large
Compression ratio (12:1 to 22:1)and fuel is
injected at high pressure of 110 to 200 bar using
fuel injector pump.
2 Single definite flame front progresses through air
fuel mixture and entire mixture will be in
combustible range
Fuel is not injected at once, but spread over a
period of time. Initial droplets meet air whose
temperature is above self ignition temperature
and ignite after ignition delay.
3 For effective combustion, turbulence is required.
Turbulence which is required in SI engine implies
disordered air motion with no general direction of
flow to break up the surface of flame front and to
distribute the shreds of flame thought-out in
externally prepared homogeneous combustible
mixture.
For effective combustion, swirl is required. Swirl
which is required in CI engine implies an orderly
movement of whole body of air with a particular
direction of flow, to bring a continuous supply of
fresh air to each burning droplets and sweep
away the products of combustion which
otherwise suffocate it.
4 In SI Engine ignition occurs at one point with a
slow rise in pressure
In the CI engine, the ignition occurs at many
points simultaneously with consequent rapid rise
in pressure. There is no definite flame front.
5 In SI engine physical delay is almost zero and
chemical delay controls combustion
In CI engine physical delay controls
combustion.
6 In SI engine , A/F ratio remains close to
stoichiometric value from no load to full load
In CI engine , irrespective of load, at any speed,
an approximately constant supply of air enters
the cylinder. With change in load, quantity of fuel
is changed to vary A/F ratio. The overall A/F can
Range from 18:1 to 80:1.
5 Delay period must be as long as possible. High
octane fuel(low cetane) is required.
Delay period must be as short as possible. High
cetane (low octane) fuel is required
Home work : Good SI engine fuel is bad CI engine fuel – Justify this statement
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
EFFECT OF VARIOUS FACTORS ON DELAY PERIOD IN CI ENGINE ( VTU July 06/July07)
Many design and operating factors affect the delay period. The important ones are:
 compression ratio
 engine speed
 output
 injection timing
 quality of the fuel
 intake temperature
 intake pressure
1.Compression Ratio. The increase in the compression
temperature of the air with increase in compression
ratio evaluated at the end of the compression stroke is
shown in Fig. It is also seen from the same figure that
the minimum auto ignition temperature of a fuel
decreases due to increased density of the compressed
air. This results in a closer contact between the
molecules of fuel and oxygen reducing the time of
reaction. The increase in the compression
temperature as well as the decrease in the
minimum auto ignition temperature decrease the
delay period. The maximum peak pressure during
the combustion process is only marginally affected
by the compression ratio (because delay period is
shorter with higher compression ratio and hence the pressure rise is lower).
Then why we do not use very high compression ratio in CI?
One of the practical disadvantages of using a very high compression ratio is that the
mechanical efficiency tends to decrease due to increase in weight of the reciprocating
parts. Therefore, engine designers always try to use a lower compression ratio which
helps in easy cold starting and light load running at high speeds.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
2.Engine Speed:
The delay period could be given either in terms of absolute time (in milliseconds) or in
terms of crank angle degrees
With increase in engine speed, the
loss of heat during compression
decreases, resulting in the rise of both
the temperature and pressure of the
compressed air thus reducing the
delay period in milliseconds. However,
in degrees of crank travel the delay period increases as the engine operates at a higher
rpm. The fuel pump is geared to the engine, and hence the amount of fuel injected
during the delay period depends on crank degrees and not on absolute time. Hence, at
high speeds, there will be more fuel present in the cylinder to take part in the second
stage of uncontrolled combustion resulting in high rate of pressure rise.
3 Outputs
With an increase in engine output the air-fuel ratio decreases, operating temperatures
increase and hence delay period decreases. The rate of pressure rise is unaffected but
the peak pressure reached may be high.
4. Injection timing:
The effect of injection advance on the
pressure variation is shown in Fig. for three
injection advance timings of 90°, 18°, and 27°
before TDC. The injected quantity of fuel per
cycle is constant. As the pressure and
temperature at the beginning of injection are
lower for higher ignition advance, the delay
period increases with increase in injection
advance. The optimum angle of injection advance depends on many factors but
generally it is about 20°bTDC.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
5. Quality of Fuel used:
The physical and chemical properties of fuel play very important role in delay period.
The most important property of fuel which is responsible for chemical delay is its selfignition
temperature. Lower the self-ignition temperature, lower the delay period.
The cetane number (CN) of the fuel is another important parameter which is responsible
for the delay period. A fuel of higher cetane number
gives lower delay period and provides smoother
engine operation.
The effect of cetane number on the indicator diagram
when injection timing is same is shown in adjacent
figure.
The delay period for a fuel having CN = 50 is lowest
and pressure rise is also smooth and maximum
pressure rise is least as most of the fuel burns during
controlled combustion.
The other properties of fuel which affects the physical delay period are volatility, latent
heat, viscosity and surface tension. The viscosity and surface tension are responsible
for the better atomization whereas latent heat and viscosity are responsible for the rapid
evaporation of fuel.
6. Intake Temperature
The delay period is reduced either with increased
temperature. However, preheating of charge for this
purpose is not desirable because it reduces the density
of charge and volumetric efficiency and power output.
7. Intake pressure
Increase in intake pressure or supercharging reduces the
auto ignition temperature and hence reduces the delay
period. The peak pressure will be higher since the
compression pressure will increase with intake pressure.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
The following table gives the summary of the factors which influence the delay
period in CI engine.
EFFECT OF VARIABLE ON DELAY PERIOD – SUMMARY
SL
No
Increase in variables Effect on Delay period Reason
1 Cetane Number of fuel Reduce Reduces the self ignition
temperature
2 Injection pressure Reduce Reduces the physical delay due
to greater surface to volume ratio
3 Injection timing advance Increase Reduces the pressure and
temperature when the injection
begins
4 Compression ratio Reduce Increases air temperature and
pressure and reduces auto
ignition temperature
5 Intake temperature Reduce Increase air temperature
6 Jacket water temperature Reduce Increase wall and hence air
temperature
7 Fuel temperature Reduce Increases chemical reaction due
to better vaporization
8 Intake pressure Reduce Increases the density and also
reduces the auto ignition
temperature
9 Speed Increase in terms of crank
angle but reduces in
terms of milliseconds.
Reduce loss of heat
10 Load ( Fuel/air ratio) Decrease Increase the operating
temperature
11 Engine size Increase in terms of crank
angle but little effect in
terms of milliseconds.
Larger engines operate at
normally slow speeds.
12 Type of combustion
chamber
Lower for engines with
pre-combustion chamber
Due to compactness of the
chamber.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
PHENOMENON OF DIESEL KNOCK ( VTU Feb 2006)
Knocking is violet gas vibration and audible sound produced by extreme pressure
differentials leading to the very rapid rise during the early part of uncontrolled second
phase of combustion.
In C.I. engines the injection process takes place
over a definite interval of time. Consequently, as
the first few droplets injected are passing
through the ignition lag period, additional
droplets are being injected into the chamber. If
the ignition delay is longer, the actual burning
of the first few droplets is delayed and a greater
quantity of fuel droplets gets accumulated in
the chamber. When the actual burning
commences, the additional fuel can cause too
rapid a rate of pressure rise, as shown on
pressure crank angle diagram above, resulting
in Jamming of forces against the piston (as if
struck by a hammer) and rough engine
operation. If the ignition delay is quite long, so
much fuel can accumulate that the rate of
pressure rise is almost instantaneous. Such, a
situation produces extreme pressure
differentials and violent gas vibration known as
knocking (diesel knock), and is evidenced by
audible knock. The phenomenon is similar to
that in the SI engine. However, in SI Engine knocking occurs near the end of
combustion whereas in CI engine, knocking the occurs near the beginning of
combustion.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
Delay period is directly related to Knocking in CI engine. An extensive delay period
can be due to following factors:
 A low compression ratio permitting only a marginal self ignition temperature to be
reached.
 A low combustion pressure due to worn out piston, rings and bad valves
 Low cetane number of fuel
 Poorly atomized fuel spray preventing early combustion
 Coarse droplet formation due to malfunctioning of injector parts like spring
 Low intake temperature and pressure of air
METHODS OF CONTROLING DIESEL KNOCK ( VTU Feb 2006)
We have discussed the factors which are responsible for the detonation in the previous
sections. If these factors are controlled, then the detonation can be avoided.
 Using a better fuel. Higher CN fuel has lower delay period and reduces knocking
tendency.
 Controlling the Rate of Fuel Supply. By injecting less fuel in the beginning and
then more fuel amount in the combustion chamber detonation can be controlled
to a certain extent. Cam shape of suitable profile can be designed for this
purpose.
 Knock reducing fuel injector : This type of injector avoid the sudden increase in
pressure inside the combustion chamber because of accumulated fuel. This can
be done by arranging the injector so that only small amount of fuel is injected first.
This can be achieved by using two or more injectors arranging in out of phase.
 By using Ignition accelerators : C N number can be increased by adding
chemical called dopes. The two chemical dopes are used are ethyl-nitrate and
amyle –nitrate in concentration of 8.8 gm/Litre and 7.7 gm/Litre. But these two
increase the NOx emissions
 Increasing Swirl : Knocking can be greatly reduced by increasing swirl ( or
reducing turbulence). Swirl helps in knock free combustion.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
COMPARISON OF KNOCK IN SI AND C ENGINES
It may be interesting to note that knocking in spark-ignition engines and compressionignition
engines is fundamentally due to the auto ignition of the fuel-air mixture. In both
the cases, the knocking depends on the auto ignition lag of the fuel-air mixture. But
careful examination of knocking phenomenon in SI and CI engines reveals the following
differences:
1.In spark ignition engines, auto ignition of end gas away from the spark plug, most
likely near the end of combustion causes knocking. But in compression engines the
auto ignition of charge causing knocking is at the start of combustion.
2.In order to avoid knocking in SI engine, it is necessary to prevent auto ignition of the
end gas to take place at all. In CI engine, the earliest auto –ignition is necessary to
avoid knocking
3.The knocking in SI engine takes place in homogeneous mixture, therefore , the rate of
pressure rise and maximum pressure is considerably high. In case of CI engine, the
mixture is not homogenous and hence the rate of pressure is lower than in SI engine.
4.In CI engine only air is compressed, therefore there is no question of Pre-ignition in CI
engines as in SI engines.
5.It is lot more easy to distinguish between knocking and non-knocking condition in SI
engines as human ear easily finds the difference. However in CI engines, normal
ignition itself is by auto-ignition and rate of pressure rise under the normal conditions is
considerably high (10 bar against 2.5 bar for SI engine) and causes high noise. The
noise level becomes excessive under detonation condition. Therefore there is no
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
definite distinction between normal and knocking combustion.
6.SI fuels should have long delay period to avoid knocking. CI fuels should have short
delay period to avoid knocking.
The following table gives a comparative statement of various characteristics that reduce
knocking in SI and CI engines
Knock rating of CI fuels ( CETANE NUMBER) ( VTU July 2007/ Jan 07.)
The cetane number is a numerical measure of the influence the diesel fuel has in
determining the ignition delay. Higher the cetane rating of the fuel lesser is the
propensity for diesel knock. The cetane number of a diesel fuel is a measure of its
ignition quality.
The cetane number of a fuel is the percentage by volume of cetane in a mixture of
cetane [C16H34] and a -methylnapthalane [C10H7 CH3] that has same performance in the
standard test engine as that of the fuel. Cetane is arbitrarily assigned a number 100 and
originally a -methylnapthalane was given a number 0 but now reference fuels is
heptamethylnonane (HMN) which is given a value of 15. HMN is used because it is
more stable compound and has slightly better ignition
quality.
The relation between the cetane number and delay period
is shown in adjacent figure
Cetane number 40 means a mixture containing 40 %
cetane and 60 % of heptamethylnonane (HMN) by volume
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
which gives same ignition delay as tested fuel. For high sped engine, cetane number of
50 is required, for medium speed engine about 30.
High octane number implies low cetane number . In other words good CI engine fuel is
bad CI engine fuel. An approximate relationship between Cetane (CN) and octane (ON)
number is given by
The following graph shows relationship of other properties of fuel with CN
DIESEL INDEX ( DI) (VTU Jan 2007)
Diesel index is a cheap method of predicting ignition quality. This scale is made
possible because ignition quality is quite sensitive to hydrocarbons compositions. That
is paraffin have high ignition quality and aromatic compounds have low ignition quality.
Thus the diesel index gives an indication of ignition quality obtained from certain
physical characteristics of fuel as opposed to an actual determination in the test engine.
The index is derived from knowledge of aniline point and American petroleum Institute
(API) gravity.
Aniline point of fuel is the temperature at which equal parts of fuel and pure aniline
dissolve each other. It therefore gives an indication of chemical composition of fuel
since the more “parafinnic” the fuel the higher solution temperature. Likewise, a higher
API gravity reflects a low specific gravity and indicates a high paraffinic content, which
corresponds to a good ignition quality.
100 gravity
API
F )x
point (
Aniline
DI
o
=
2 ON
60
CN
= −
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
Good SI engine fuel is a bad CI engine fuel
To reduce knocking Diesel oil should have low self ignition temperature and short time
lag, whereas petrol should have high self ignition temperature and a long ignition lag.
In SI engine knocking occurs near the end of combustion, where as in CI engine this
occurs in the beginning of combustion. Because of this dissimilarity in the time of
starting of knock in SI and CI engines . The conditions which reduce the knock
tendency in SI engine will increase the knocking tendency in CI engine.
Diesel has a high cetane number (40-60) and low octane number(30) and petrol has
high Octane number (80-90) ad low cetane number(20).
Figure shows typical indicator diagram of a diesel engine with sharp pressure oscillating
during the combustion caused by shock waves when using petrol
Weak mixture gives better efficiency in CI engine- (July 2007)
As the mixture is made lean (less fuel) the temperature rise due to combustion will be
lowered as a result of reduced energy input per
unit mass of mixture. This will result in lower
specific heat. Further, it will lower the losses due
to dissociation and variation in specific heat. The
efficiency is therefore, higher and, in fact,
approaches the air-cycle efficiency as the fuel-air
ratio is reduced as shown in adjacent figure.
INTERNAL COMBUSTION ENGINES (ELECTIVE) (ME667) SIXTH SEMESTER
Jagadeesha T, Assistant Professor, Department of Mechanical Engineering, Adichunchanagiri Institute of Technology, Chikmagalur
Thermodynamic analysis of the engine cycles has clearly established that operating an
engine with a leaner air-fuel ratio always gives a better thermal efficiency but the mean
effective pressure and the power output reduce. Therefore, the engine size becomes
bigger for a given output if it is operated near the stoichiometric conditions, the A/F ratio
in certain regions within the chamber is likely to be so rich that some of the fuel
molecules will not be able to find the necessary oxygen for combustion and thus
produce a noticeably black smoke. Hence the CT engine is always designed to operate
with an excess air, of 15 to 40% depending upon the application. The power output
curve for a typical CI engine operating at constant speed is shown in Fig. given below.
The approximate region of A/F ratios in which visible black smoke occurs is indicated by
the shaded area.