Air-fuel ratio requirements – stages of combustion –
normal and abnormal combustion – factors
affecting knock – fuel injection systems – mono
point, multipoint and direct injection combustion
chambers – effects of compression ratio –
introduction to thermodynamic analysis of
3. COMBUSTION IN COMPRESSION-
In the CI engine, only air is compressed through a high
compression ratio (16:1 to 20:1) raising its temperature and
pressure to a high value. Fuel is injected through one or more
jets into this highly compressed air in the combustion chamber.
Here, the fuel jet disintegrates into a core of fuel surrounded by
a spray envelope of air and fuel particles Fig.
4. • The liquid fuel droplets evaporate by absorbing the latent heat
of vaporization from the surrounding air which reduces the
temperature of a thin layer of air surrounding the droplet and
some time elapses before this temperature can be raised again
by absorbing heat from the bulk of air.
• As soon as this vapour and the air reach the level of the
autoignition temperature and if the local A/F ratio is within the
combustible range, ignition takes place. Thus, it is obvious that
at first there is a certain delay period before ignition takes place.
5. • Since the fuel droplets cannot be injected and distributed
uniformly throughout the combustion space, the fuel-air mixture
is essentially heterogenius.
• If the air within the cylinder were motionless under these
condition there will not be enough oxygen in the burning zone
and burning of the fuel would be either slow or totally fail as it
would be surrounded by its movement must be imparted to the
air and the fuel so that a continuous products of combustion Fig.
• In an SI engine, the turbulence is a disorderly air motion with no
general direction of flow. However, the swirl which is required in CI
engines orderly movement of the wide body of air with a particle
direction of flow and it assists the breaking up of the fuel jet
intermixing the burned and unburned portions of the mixture also
takes place due to swirl.
• In the SI engine, the ignition occurs at one point with a safe in
pressure whereas in the CI engine, the ignition occurs at many int
simultaneously with consequent rapid rise in pressure. In contrast to
the process of combustion in SI engines, there is no definite flame
front in CI engines.
8. AIR-FUEL RATIO IN C.I ENGINE
• In an SI engine, the air-fuel ratio remains close to semantic mine
from no load to full load. But in a CI engine, irrespective of load, at
any given speed, an approximately constant supply of sit enters the
cylinder with change in load, the quantity of fuel injected is changed,
varying the ratio.
• The overall air-fuel ratio thus varies from about 18:1 at full load
to about 80:1 at no load. It is the main aim of the CI engine designer
that the A/F ratio should be as close to stoichiometric as possible
while operating at full load since the mean effective pressure and
power output are maximum at that condition.
• Hence the CI 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
10. STAGES OF COMBUSTION IN CI ENGINES
• The combustion in a CI engine is considered to be taking place in four
stages Figure. It is divided into the ignition delay period, the period of
rapid combustion, the period of controlled combustion and the period
of after-burning. The details are explained below.
First Stage (Ignition Delay Period)
Second Stage (Rapid or Uncontrolled Combustion)
Third Stage (Controlled combustion)
Fourth Stage (After burning)
12. IGNITION DELAY PERIOD
•The ignition delay period is also called the preparatory phase during
which some fuel has already been admitted but has not yet ignited.
This period is counted from the start of injection to the point where
the pressure-time curve separates from the motoring curve indicated
as start of combustion in Fig.
•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.
13. 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, higher combustion chamber temperatures and high
turbulence to facilitate breakup of the jet and improving evaporation.
14. Chemical Delay
•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 mentions are faster and the physical
delay becomes longer than the chemical delay.
•It is clear that, the ignition lag in the SI engine is essentially
equivalent to the chemical delay for the CI engine. In most CI
engines the ignition lag is shorter than the duration of injection.
15. FACTORS AFFECTING THE DELAY PERIOD
Many design and operating factors affect the delay period. The
important ones are:
(i) Compression ratio
(ii) Engine speed Output
(iv) Atomization of fuel and duration of injection
(v) Injection timing
(vi) Quality of the fuel
(vii) Intake temperature
(viii) Intake pressure
16. PHENOMENON OF KNOCK IN CI ENGINES
•In Cl engines the injection process takes place over a definite
interval of time. Consequently, as the first few droplets to be injected
are passing through the ignition delay period, additional droplets are
being injected into the chamber.
•If the ignition delay of the fuel being injected is short the first few
droplets will commence the actual burning phase in a relatively short
time after injection and a relatively small amount of fuel will
accumulated in the chamber when actual burning commences.
• As a result the mass rate of mixture burned will be such as to
produce a rate of pre rise that will exert a smooth force on the piston,
as shown in Fig.
18. COMPARISON OF KNOCK IN SI AND CI ENGINES
•It may be interesting to note that knocking in spark-ignition engines
and compression-ignition 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 the knocking phenomenon in spark-
ignition and the compression-ignition engines reveals the following
differences. A comparison of the knocking process in SI and CI
engines is shown on the pressure-time diagrams of Fig.
20. COMBUSTION CHAMBERS FOR CI ENGINES
•Direct-Injection (DI) Type: This type of combustion chamber is
also called an open combustion chamber. In this type the entire
volume of the combustion chamber is located in the main cylinder
and the fuel is injected into this volume.
•Indirect Injection (IDI) Type: In this type of combustion chambers
the combustion space is divided into two parts, one part in the
cylinder and the other part in the cylinder head. The fuel-injection
affected usually into that part of the chamber located in the cylinder
21. Direct-Injection Chambers
•An open combustion chamber is defined as one in which the
combustion space is essentially a single cavity with little
restriction from one part of the chamber to the other and hence
with no large difference in pressure between parts of the
chamber during the combustion process. There are many
designs of open chamber some of which are shown in Fig.
23. Indirect-Injection Chambers
A divided combustion chamber is defined as one in which the
combustion space is divided into two or more distinct compartments
connected by restricted passages. This creates considerable pressure
differences between them during the combustion process.
Swirl chamber consists of a spherical-shaped chamber
separated from the engine cylinder and located in the cylinder head
Fig. Into this chamber, about 50% of the air is transferred during the
27. SPRAY FORMATION AND BEHAVIOUR
•In Cl engines, the fuel is forced through the nozzle hole 310 under
high pressure. Then fuel gets disintegrated into fine duplets due to
aerodynamic resistance created inside the combustion chamber. At
the time of fuel injection, the combustion chamber pressure is nearly
35 bar and density of 14 times than surrounding air.
•The disintegration of the fuel into a fine droplet is purely depends on
the relative velocity of fuel and air. Also depends on the physical
characteristics of both air and fuel. The spray angle depends on the
density of the medium in which the fuel is sprayed.
28. SPRAY STRUCTURE
In diesel engine, the stage of the fuel spray is very difficult to
predict. It is due to the air is highly turbulent a combustion occurs
before injection is completed. By the various case studies the
following structure and properties are observed.
30. SPRAY PENETRATION
•Penetration of fuel into the compressed air charge from the nozzle
tip requires proper distribution of fuel. The fuel particles are to be
prevented from impinging of fuel droplet on the hot combustion
chamber. Because if impinges on the surface of the combustion
chamber, it cannot ignite or burn.
•The main factor which determines the penetration of spray are the
momentum of the fuel droplet (diameter x velocity) and the density
of air in the combustion chamber. If higher the momentum, greater
penetration will be occurred.
31. The various factors which are deciding the fuel are as follows:
penetration are as follows:
1. Diameter of the orifice/nozzle
2. Fuel injection pressure
3. Length of diameter ratio of the orifice/nozzle
4. Density of air in the combustion chamber
5. Viscosity of fuel
32. SPRAY DIRECTION
•The fuel spray relative direction with air movement is very
important. When the first drop coming into the combustion chamber,
it takes heat from air and start burn at the end of ignition delay. If the
direction of spray is same that of air, the product of first part of
combustion will swept away as later part of injection process.
•If the fuel is injected upstream of air, the velocity between air and
fuel atomization will be good and delay period get reduced. But
newly arrived droplet gets insufficient oxygen for burning. So, it
gives higher smoking exhaust with poor efficiency.
33. AIR MOTION IN C.I ENGINE
•The important task in diesel engine is to intimate mixing of air and
fuel inside the combustion chamber. The air motion influences the
performance of diesel engines. The air-fuel mixing is directly
depends on the influences of combustion, performance and emission
level of the engine.
•The movement of air inside the cylinder which depends on manifold
design, inlet and exhaust value profile and combustion chamber
•The shape of the piston bowl and intake system, control the air
motion by the turbulence level and mixing methods of the direct
injection diesel engine.
34. Effects of Air Motion
1. Atomizes the injected fuel into droplets of different sizes.
2. Distributes the fuel droplets uniformly in the air inside the
3. Mixes injected fuel droplets with the air mass.
4. It improves the combustion of fuel droplets.
5. It removes the combustion product from the surface the burning
drops when they are consumed.
6. Supplies fresh air to the interior portion of the f drop and thereby
ensures complete combustion fuel.
7. Reduces after burning of fuel.
8. Reduces delay period
35. Direction and Speed of Air Motion
•The air movement in the diesel engines should be very smooth and
in order at right angles to the direction of the fuel jet. The fuel
particles split into very smaller particles and move along with jet of
36. Types of Air Motion
•The air motion in diesel engines are differentiated into three types is
39. Diesel Cycle
•In actual spark-ignition engines, the upper limit of the compression
ratio is limited by the self-ignition temperature of the fuel. This
limitation on the compression ratio can be circumvented if air and
fuel are compressed separately and brought together at the time of
•In such an arrangement fuel can be injected into the cylinder which
contains compressed air at a higher temperature than the self-ignition
temperature of the fuel. Hence the fuel ignites on its own accord and
requires no special device like an ignition system in a spark-ignition
40. •Such engines work on heavy liquid fuels. These engines are called
compression-ignition engines and they work on an ideal cycle known
as Diesel cycle. The difference between Otto and Diesel cycles is in
the process of heat addition.
•In Otto cycle the heat addition takes place at constant volume
whereas in the Diesel cycle it is at constant pressure. For this reason,
the Diesel cycle is often referred to as the constant-pressure cycle.
•It is better to avoid this term as it creates confusion with Joules
cycle. The Diesel cycle is shown on p-V and T-s diagrams in Fig.