Internal Combustion Engines
Faculty - Er. Ashis Saxena
Index
Unit 1
Introduction to I.C Engines
Fuels
Unit 2
SI Engines
Unit 3
CI Engines
Unit 4
Engine Cooling
Lubrication
Supercharging
Testing and Performance
Unit 5
Compressors
Unit - 3
Chapter – 3
CI Engines
Injection System
For the compression ignition engine, it is very important to promote a means of injecting
fuel into the cylinder at the proper time in the cycle. This is so because the injection
system starts and controls the combustion process.
Difference between the Carburation and Injection

Basically, the purpose of carburetion and fuel-injection is the same viz., preparation of the
combustible charge. But in case of carburetion fuel is atomized by processes relying on the
air speed greater than fuel speed at the fuel nozzle, whereas, in fuel-injection the fuel speed
at the point of delivery is greater than the air speed to atomize the fuel.

In carburetors, air flowing through a venturi picks up fuel from a nozzle located there. The
amount of fuel drawn into the engine depends upon the air velocity in the venturi. In a fuelinjection system, the amount of fuel delivered into the air stream going to the engine is
controlled by a pump which forces the fuel under pressure.
Injection Process

When the fuel is injected into the combustion chamber towards
the end of compression stroke, it is atomized into very fine
droplets.

These droplets vaporize due to heat transfer from the compressed
air and form a fuel- air mixture.

Due to continued heat transfer from hot air to the fuel, the
Temperature reaches a value higher than its self-ignition
temperature. This causes the fuel to ignite spontaneously initiating
the combustion process.
Objectives of an Injection System
The injection system of the compression ignition engine should fulfill the following objectives
consistently and precisely:

Introduction of the fuel into the combustion chamber should take place within a precisely defined
period of the cycle.

The metering of amount of fuel injected per cycle should done very accurately.

The quantities of fuel metered should vary to meet the changing load & speed requirements.

The injection rate should be such that it results in the desired heat-release pattern.

The injected fuel must be broken into very fine droplets.

Proper spray pattern to ensure rapid mixing of fuel & air.

Beginning & end of injection should be sharp.

Timing the injection of the fuel correctly in cycle so that maximum power is obtained, ensuring
economy & clean burning.

Uniform distribution of fuel droplets throughout the combustion chamber.
Parts of an Injection System
To accomplish these objectives, a number of functional elements are required. These constitute
together, the fuel injection system of the engine. These elements are as follows.

Pumping elements to transfer the fuel from the tank to the cylinder, along with the
associate piping and hardware.

Metering elements to measure and supply the fuel at the rate as desired by the speed and
load conditions prevailing.

Metering controls to adjust the rate of the metering elements for changes in load and speed
of the engine.

Distributing elements to divide the metered fuel equally among the cylinders in a multi
cylinder engine.

Timing controls to adjust the start and stop of injection.

Mixing elements to atomize and distribute the fuel within the combustion chamber
Fuel Injection Systems
There are two main classifications for fuel-injection systems,
namely

Air injection System:

Solid (or airless) injection systems.
Air injection System

In this method fuel is forced into the cylinder by means of compressed air
to a very high pressure. The rate of fuel admission can be controlled by
varying the pressure of air.

The fuel is metered & pumped to the fuel valve by a camshaft driven fuel
pump.

The fuel valve is opened by means of a mechanical linkage operated by
the camshaft which controls the timing of injection.

The fuel valve is also connected to a high pressure air line fed by a multi
stage compressor which supplies air at a pressure of about 60 to 70 bar.
Air injection System
Advantages:

It provides better atomization & distribution of fuel.

Heavy & viscous fuels, which are cheaper can also be injected.
Disadvantages: This method is not used now a day due to following reasons:

It requires a high pressure multi stage compression.

A separate mechanical linkage is required to time the operation of fuel valve.

The fuel valve sealing requires considerable skill.

Due to the compression & the linkage the bulk of the engine increases. This also
results in reduced B.P due to power loss in operating the compression & linkage.

In case of sticking of the fuel valve, the system becomes quite dangerous due to
the presence of high pressure air.
Solid (or airless) injection systems

In this system the liquid fuel is injected directly into the combustion chamber
without, the aid of compressed air. Hence, it is also called airless injection or solid
injection system.

Solid injection systems can be classified into four types.
i.
Individual pump and nozzle system
ii. Unit injector system
iii. Common rail system
iv. Distributor system
Fuel Injection Systems
All the above systems comprise mainly of the following components.

Fuel tank,

Fuel feed pump to supply fuel from the main fuel tank to the injection system,

Injection pump to meter and pressurize the fuel for injection,

Governor to ensure that the amount of fuel injected is in accordance with
variation in load,

Injector to take the fuel from the pump and distribute it in the combustion
chamber by atomizing it into fine droplets,

Fuel filters to prevent dust and abrasive particles from entering the pump and
injectors thereby minimizing the wear and tear of the components.
A typical Solid Injection System
Pressure relief valve
A typical Solid Injection System
A typical arrangement of various components for the solid injection system used in a
Cl engine is shown in Fig.

Fuel from the fuel tank first enters the coarse filter from which is drawn into the
plunger feed pump where the pressure is raised very slightly.

Then the fuel enters the fine filter where all the dust and dirt particles are
removed. From the fine filter the fuel enters the fuel pump where it is pressurized
to about 200 bar and injected into the engine cylinder by means of the injector.

Any spill over in the injector is returned to the fine filter. A pressure relief valve is
also provided for the safety of the system. The above functions are achieved with
the components listed above.
Individual Pump and Nozzle System

In this system, each cylinder is
provided with one pump and an
injector.

In this arrangement a separate
metering and compression pump
is provided for each cylinder.

The pump may be placed close to
the cylinder as shown in Fig. (a) or,
Individual Pump and Nozzle System

They may be arranged in a cluster as
shown in Fig. (b).

The high pressure pump plunger is
actuated by a cam, and produces
the fuel pressure necessary to open
the injector valve at the correct
time.

The
amount
of
fuel
injected
depends on the effective stroke of
the plunger.
Unit Injector System

The unit injector system, is
one in which the pump and
the
injector
nozzle
are
combined in one housing.

Each cylinder is provided
with one of these unit
injectors.
Unit Injector System

Fuel is brought up to the injector by a low pressure pump,
where at the proper time, a rocker arm actuates the plunger
and thus injects the fuel into the cylinder.

The amount of fuel injected is regulated by the effective
stroke of the plunger.

The pump and the injector can be integrated in one unit.
Common Rail System

In the common rail system, a
HP pump supplies fuel, under
high pressure, to a fuel header.

High pressure in the header
forces the fuel to each of the
nozzles located in the cylinders.
Common Rail System

At
the
proper
mechanically
time,
operated
a
(by
means of a push rod and rocker
arm) valve allows the fuel to
enter
the
proper
through the nozzle.
cylinder
Common Rail System

The pressure in the fuel header must be that, for which the
injector system was designed, i.e., it must enable to penetrate
and disperse the fuel in the combustion chamber.

The amount of fuel entering the cylinder is regulated by varying
the length of the push rod stroke.

A high pressure pump is used for supplying fuel to a header,
from where the fuel is metered by injectors (assigned one per
cylinder).
Common Rail System
Distributor System

In this system the pump which pressurizes the fuel also meters and times
it.

The fuel pump after metering the required amount of fuel supplies it to a
rotating distributor at the correct time for supply to each cylinder.

The number of Injection strokes per cycle for the pump is equal to the
number of cylinders.

Since there is one metering element in each pump, a uniform distribution
is automatically ensured.

Not only that, the cost of the fuel-injection system also reduces to a value
less than two-thirds of that for individual pump system.
Distributor System
Comparison of Solid Injection Systems
Comparison of Various Fuel-injection Systems
Job
Air
injection
system
Solid injection system
Individual
Common
Distributor
pump
Metering
Pump
Pump
Timing
Fuel cam
Pump cam
Injection
rate
Atomization
Distribution
rail
Injection
valve
Fuel cam
Pump
Spray valve Pump cam
Spray valve Spray tip
Spray valve
Spray tip
Fuel cam
Spray tip
Spray valve Spray tip
Spray tip
Spray tip
Fuel cam
Fuel feed pump

Fig shows fuel feed pump which is of
spring
loaded
plunger
type.
The
plunger is actuated through a push rod
from the cam shaft.

At the minimum lift position of the cam
the spring force on the plunger creates
a suction which causes fuel flow from
the main tank into the pump.
Fuel feed pump

When the cam is turned to its
maximum
lift
position,
the
plunger is lifted upwards.

At the same time the inlet valve
is closed and the fuel is forced
through the outlet valve.
Fuel feed pump

When the operating pressure
gets released, the plunger return
spring
resulting
ceases
in
to
varying
function
of
the
pumping stroke under varying
engine loads according to the
quantity of fuel required by the
injection pump.
Injection pump

The main objectives of fuel-injection pump is to deliver
accurately metered quantity of fuel under high pressure (in the
range from 120 to 200 bar) at the correct instant to the injector
fitted on each cylinder.

Injection pumps are of two types.
i.
Jerk type pumps
ii.
Distributor type pump
Jerk Type Pump

It consists of a reciprocating plunger
inside a barrel.

The plunger is driven by a camshaft.

The working principle of jerk pump is
illustrated in Fig.
a) A sketch of a typical plunger is
shown.
a
Jerk Type Pump
b) A schematic diagram of the plunger within the
barrel is shown.
Near the port A, fuel is always available under
relatively low pressure.
While the axial movement of the plunger is
through cam shaft, its rotational movement about
its axis by means of rack D.
Port B is the orifice through which fuel is
delivered to the injector. At this stage it. is closed
by means of a spring loaded check valve.
b
Jerk Type Pump
When the plunger is below port A. the fuel
gets filled in the barrel above it.
As the plunger rises and closes the port A the
fuel will flow out through port C.
This is because it has to overcome the spring
force of the check valve in order to flow
through port B.
Hence it takes the easier way out. via port C.
Jerk Type Pump
c) At this stage rack rotates the plunger and as a result port C also closes.
The only escape route for the fuel is past the check valve through orifice B to the
injector.
This is the beginning of injection and also the effective stroke of the plunger.
c
d
Jerk Type Pump
d) The injection continues till the helical indentation on the plunger uncovers port C.
Now the fuel will take the easy way out through C and the check valve will close the orifice B.
The fuel injection stops and the effective stroke ends.
Hence the effective stroke of the plunger is the axial distance traversed between the time
port A is closed off and the time port C is uncovered.
c
d
Jerk Type Pump
e) & (f) The plunger is rotated to the position shown. The same sequence of events
occur. But in this case port C is uncovered sooner. Hence the effective stroke is
shortened.
e
f
Jerk Type Pump
It is important to remember
here that though the axial
distance
traversed
by
the
plunger is same for every stroke,
the rotation of the plunger by
the rack determining the length
of the effective stroke and thus
the quantity of fuel injected. A
typical example of this type of
pump is the Bosch fuel-injection
pump shown in Fig. below.
Diagram
illustrating an
Actual Method
of Controlling
Quantity of Fuel
Injected in a CI
engine.
Distributor Type Pump

This pump has only a single pumping
element and the fuel is distributed to
each cylinder by means of a rotor.

There is a central longitudinal passage
in the rotor and also two sets of radial
holes (each equal to the number of
engine cylinders) located at different
heights.

One set is connected to pump via
central passage whereas the second
set is connected to delivery lines
leading to injectors of the various
cylinders.
Distributor Type Pump

The fuel is drawn into the central rotor
passage from the inlet port when the
pump plunger move away from each
other.

Wherever, the radial delivery passage
in the rotor coincides with the delivery
port for any cylinder the fuel is
delivered to each cylinder in turn.

Main advantages of this type of pump
lies in its small size and its light weight.

A schematic diagram Roosa Master
distributor pump is shown in Fig.
below.
Electronic fuel injection system

Modern gasoline injection system use engine sensors, a
computer and solenoid operated fuel injectors to meter & inject
the right amount of fuel into the engine cylinder.

These system is called as electronic fuel injection system(EFL)
use electrical & electronic devices to monitor & control engine
operation.

An electronic control unit (ECU) or computer receives electrical
signals in the form of current or voltage from various sensors.
Electronic fuel injection system

It then uses the stored data to operate the injectors , ignition
system & other engine related devices.

As a result, less unburned fuel leaves the engine as emissions &
the vehicle gives better mileage.

Under full load, the ECU will sense a wide open throttle, high
intake manifold pressure,& high inlet air flow.

The ECU will then increase the injector pulse width to enrich the
mixture which will enable the engine to produce higher power.
Electronic fuel injection system

Under low load & idling conditions, the ECU will shorten the
pulse width by which the injectors are kept in the closed
position over a longer period of time.

Because of this, air-fuel mixture will become leaner & will result
in better fuel economy.

EFI system has a cold start injector too.
Electronic fuel injection system

This is an extra injector that sprays fuel into the center of the
engine intake manifold when the engine is cold.

It serves the same purpose as the carburetor choke.

The cold start injector ensures easy engine start up in very cold
weather.
Sensors used in EFI system

Exhaust gas (or) oxygen sensor.

Engine temperature sensor.

Air flow sensor.

Air inlet temperature sensor.

Throttle position sensor.

Manifold pressure sensor.

Camshaft position sensor.

Knock sensor.
Electronic fuel injection system
Advantages

Better atomization & vaporization will make the engine less knock.

Manifold wetting is eliminated due to the fuel being injected into or close to cylinder & need
not flow through the manifold.

Atomization of fuel is independent of cranking speed & therefore starting will be easier.

Distribution of fuel being independent of vaporization less volatile fuel can be used.

Position of the injection unit is not so critical & thereby the height of the engine can be less.
Disadvantages

High maintenance cost.

Difficulty in servicing.

Possibility of malfunction of some sensors.
Injection Timing

Consider a cylinder of a four cylinder engine.

The fuel is injected into the inlet manifold of each cylinder at different
timings.

The timing at which the injection of the fuel takes place inside the
inlet manifold is called injection timing.

Below is the injection timing for one cylinder of this four cylinder
engine.

In one cylinder, the piston moves up from the BDC to TDC during the
exhaust stroke.
Injection Timing

Just before the piston reached TDC during this exhaust stroke, injection of
fuel takes place into the inlet manifold of this cylinder at about 60 crank
angle before TDC.

This injected fuel mixes with the air intake chamber. Thus the air fuel
mixture is obtained.

At the beginning of the suction stroke, intake valve opens and the air fuel
mixture is sucked into the cylinder during the suction stroke.

According to the firing order, the injection of the fuel takes place inside the
inlet manifolds of the other three cylinders at various timings.
Injection Timing

In this four cylinder engine, the ECU calculates the approximate
injection timing for each cylinder and the air fuel mixture is
made available at each suction stroke.

In order to meet the operating conditions, the injection valve is
kept open for a longer time by ECU.

For ex, if the vehicle is accelerating, the injection valve will be
opened for a longer time, in order to supply additional fuel to
the engine.
Scavenging

In automotive usage, scavenging is the process of pushing exhausted gas-charge
out of the cylinder or the process of removing burnt gases, from the combustion
chamber of the engine cylinder, is known a scavenging.

The last stroke of an IC engine is the exhaust, which means the removal of burnt
gases from the engine cylinder.

It has been experienced that the burnt gases in the engine cylinder are not
completely exhausted before the suction stroke but a part of the gases still
remain inside the cylinder and mix with the fresh charge.

As a result of this mixing, the fresh charge gets diluted and its strength is
reduced.
Scavenging in 2 & 4 stroke engines

Four-stroke cycle engines: In a four stroke cycle engine, the scavenging is very
effective as the piston during the exhaust stroke pushes out the burnt gases from
the engine cylinder. It may be noted that a small quantity of burnt gases remain
in the engine cylinder in the clearance space.

Two-stroke cycle engine: In a two-stroke cycle engine, the scavenging is less
effective as the exhaust port is open for a small fraction of the crank revolution.
Moreover, as the transfer and exhaust port are open simultaneously during a
part of the crank revolution, therefore fresh charge also escapes out along with
the burnt gases. This difficulty is overcome by designing the piston crown of a
particular shape.
Types of Scavenging
1. Cross flow scavenging: In this method, the
transfer port (or inlet port for the engine
cylinder) and exhaust port are situated on the
opposite sides of the engine cylinder (as is
done in case of two-stroke cycle engines).
The piston crown is designed into a particular
shape, so that the fresh charge moves
upwards and pushes out the burnt gases in
the form of cross flow as. Shown in (a).
Types of Scavenging
2. Backflow or loop scavenging: In this
method, the inlet and outlet ports are
situated on the same side of the engine
cylinder.
The fresh charge, while entering into
the engine cylinder, forms a loop and
pushes out the burnt gases as shown in
(b).
Types of Scavenging
3. Uniflow scavenging: In this method, the
fresh charge, while entering from one side
(sometimes two sides) of the engine
cylinder pushes out the gases through the
exit valve situated on the top of the
cylinder.
In uniflow scavenging, both the fresh charge
and burnt gases move in the same upward
direction.
Combustion in CI engines Versus in SI Engine
 In
the SI engine, a homogeneous carbureted mixture of
gasoline vapor and air, in a certain proportion, is
compressed and the mixture is ignited at one place
before the end of the compression stroke by means of
an electric spark.
A
single flame front progresses through the air-fuel
mixture after ignition.
Combustion in CI engines Versus in SI Engine

In the Cl engine, only air is compressed
through a high compression ratio 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(a)].
Combustion in CI engines Versus in SI Engine

This spray envelope is created both by
the atomization and vaporization of the
fuel.

The turbulence of the air in the
combustion chamber passing across the
jet tears the fuel particles from the
core.

A mixture of air and fuel forms at some
location in the spray envelope and
oxidation starts.
Combustion in CI engines Versus in SI Engine

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 vapor and the air reach the level of the auto-ignition
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.
Combustion in CI engines Versus in SI Engine

Since the fuel droplets cannot be injected and distributed uniformly throughout
the combustion space, the fuel-air mixture is essentially heterogeneous.

If the air within the cylinder were motionless under these conditions 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 own products of
combustion [Fig. (b)].
Combustion in CI engines Versus in SI Engine

Hence, an orderly and controlled movement must be imparted to the air and
the fuel so that a continuous flow of fresh air is brought to each burning
droplet and the products of combustion are swept, away. This air motion is
called the air swirl and its effect is shown in Fig. (c).
Combustion in CI engines Versus in SI Engine

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, is an orderly
movement of the whole body of air with a particular direction of
flow and it assists the breaking up of the fuel jet.

Intermixing of the burned and unburned portions of the mixture
also takes place due to this swirl.
Combustion in CI engines Versus in SI Engine

In the SI engine, the ignition occurs at one point with a slow rise
in pressure whereas in the Cl engine, the ignition occurs at
many points 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.
Combustion in CI engines Versus in SI Engine

In an SI engine, the air-fuel ratio remains close to stoichiometric from no
load to full load. But in a Cl engine, irrespective of load, at any given speed,
an approximately constant supply of air enters the cylinder.

With change in load, the quantity of fuel injected is changed, varying the airfuel 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 Cl 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.
Combustion in CI engines Versus in SI Engine

Thermodynamic analysis of the engine cycles has clearly established that
operating an engine with a leaner air-fuel ratio 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 CI engine is always designed to operate with an excess air, of 15% to
40% depending upon the application.
Stages of combustion in CI engines
 The
combustion in a Cl engine is considered to be taking
place in four stages.
 It
is divided into
• The ignition delay period,
• The period of rapid combustion,
• The period of controlled combustion
• The period of after-burning.
Stages of combustion in CI engines : Ignition Delay
i.
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 pressuretime curve separates from the motoring curve indicated as start of combustion in
Fig.

The fuel does not ignite immediately upon injection into the combustion chamber.
There is a definite period of inactivity between the time when the first droplet of
fuel hits the hot air in the combustion chamber and the time it starts through the
actual burning phase. This period is known as the ignition delay period.
Stages of combustion in CI engines : Ignition Delay
The ignition delay period can be divided into two parts, the physical delay & the
chemical delay.

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, 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.
Stages of combustion in CI engines : Ignition 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 reactions are faster and the physical delay
becomes longer than the chemical delay.
Stages of combustion in CI engines :
Period of Rapid Combustion
ii.
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, the droplets have had time to spread over a
wide area and fresh air is always available around the droplets.

Most of the fuel admitted would have evaporated and formed a
combustible mixture with air.

By this time, the preflame reactions would have also been completed.
Stages of combustion in CI engines :
Period of Rapid Combustion

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.

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 accumulated in the cylinder during the delay period).
Stages of combustion in CI engines :
Period of Controlled Combustion
iii.
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 is already quite high.

Hence the fuel droplets injected during the second stage burn faster
with reduced ignition delay as soon as they find the necessary oxygen
and any further pressure rise is controlled by the injection rate.

The period of controlled combustion is assumed to end at maximum
cycle temperature.
Stages of combustion in CI engines :
Period of Controlled Combustion
iv.
Period of After-Burning

Combustion does not cease 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.

Usually this period starts from the point of maximum cycle temperature and
continues over a part of the expansion stroke.

Rate of after-burning depends on the velocity of diffusion and turbulent mixing
of unburnt and partially burnt fuel with the air.

The duration of the after-burning phase may correspond to 70-80 degrees of
crank travel from TDC.
Block Diagram
illustrating the
Combustion
Process in a Cl
Engine
Phenomenon of knock in CI engines

In CI engines the injection process takes place over a definite interval of time.

Consequently, as 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 be 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
pressure rise that will exert a smooth force on the piston, as shown in Fig. (a).
Phenomenon of knock in CI engines

If, on the other hand, 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 in Fig. (b),
resulting in a jamming of forces against the piston 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, as shown in Fig (c).
Phenomenon of knock in CI engines

Such a situation produces the extreme pressure differentials and violent gas
vibrations known as knocking and is evidenced by audible knock.

The phenomenon is similar to that in the SI engine. However, in the SI engine,
knocking occurs near the end of combustion whereas in the Cl engine, knocking
occurs near the beginning of combustion.

In order to decrease the tendency of knock it is necessary to start the actual
burning as early as possible after the injection begins.

In other words, it is necessary to decrease the ignition delay and thus decrease
the amount of fuel present when the actual burning of the first few droplets
start.
Combustion chamber design

The most important function of the Cl engine combustion chamber is to
provide proper mixing of fuel and air in a short time.

In order to achieve this, an organized air movement called the air swirl
is provided to produce high relative velocity between the fuel droplets
and the air.

The fuel is injected into the combustion chamber by an injector having a
single or multiple orifices.

The increase in the number of jets reduces the intensity of air swirl
needed.
Combustion chamber design

When the liquid fuel is injected into the combustion chamber, the spray cone
gets disturbed due to the air motion and turbulence inside.

The onset of combustion will cause an added turbulence that can be guided by
the shape of the combustion chamber.

CI engine combustion chambers are classified into two categories:

Direct-Injection (Dl) 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.
Combustion chamber design

Indirect-Injection (IDI) Type: In this type of combustion chambers, the
combustion space is divided into two parts, one part in the main cylinder and
the other part in the cylinder head. The fuel-injection is effected usually into
that part of the chamber located in the cylinder head.

These chambers are classified further into:
i.
Swirl chamber in which compression swirl is generated.
ii.
Pre-combustion chamber in which combustion swirl is induced!
iii. Air cell chamber in which both compression and combustion swirl are
induced.
Direct - lnjection 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.

In four-stroke engines with open combustion chambers, induction swirl is obtained
either by careful formation of the air intake passages or by masking a portion of the
circumference of the inlet valve whereas in two stroke engines it is created by suitable
form for the inlet ports. These chambers mainly consist of space formed between a flat
cylinder head and a cavity in the piston crown in different shapes. The fuel is injected
directly into this space. The injector nozzles used for this type of chamber are generally
of multihole type working at a relatively high pressure (about 200 bar).
Direct - lnjection Chambers
The main advantages of this type of chambers are:

Minimum heat loss during compression because of lower surface area to
volume ratio and hence, better efficiency.

No cold starting problems.

Fine atomization because of multihole nozzle.
The drawbacks of these combustion chambers are:

High fuel-injection pressure required and hence complex design of fuelinjection pump.

Necessity of accurate metering of fuel by the injection system particularly for
small engines.
Direct - lnjection Chambers - Types
Shallow
Depth
Chamber:
In
shallow depth chamber the depth
of the cavity provided in the
piston
is
quite
small.
This
chamber [Fig. (a)] is usually
adopted
for
large
engines
running at low speeds. Since the
cavity diameter is very large, the
squish is negligible.
Direct - lnjection Chambers - Types
Hemispherical Chamber: This
chamber [Fig. (b)] also gives
small
squish.
However,
the
depth to diameter ratio for a
cylindrical
chamber
can
be
varied to give any desired squish
to give better performance.
Direct - lnjection Chambers - Types
Cylindrical Chamber: This design
[Fig. (c)] was attempted in recent
diesel
engines.
modification
of
This
the
is
a
cylindrical
chamber in the form of a truncated
cone with base angle of 30°. The
swirl was produced by masking the
valve
for
nearly
180°
of
circumference. Squish can also be
by varying the depth.
Direct - lnjection Chambers - Types
Toroidal Chamber: The idea behind
this shape [Fig. (d)] is to provide a
powerful squish along with the air
movement, similar to that of the
familiar smoke ring, within the
toroidal chamber. Due to powerful
squish the mask needed on inlet
valve is small and there is better
utilization of oxygen. The cone angle
of spray for this type of chamber is
150° to 160°.
Indirect - lnjection 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.
Indirect - lnjection Chambers - Types
Swirl Chamber:

Swirl chamber consists of a spherical-shaped chamber separated from the
engine cylinder and located in the cylinder head.

Into this chamber, about 50% of the air is transferred during the compression
stroke.

A throat connects the chamber to the cylinder which enters the chamber in a
tangential direction so that the air coming into this chamber is given a strong
rotary movement inside the swirl chamber and after combustion, the products
rush back into the cylinder through the same throat at much higher velocity.
Indirect - lnjection Chambers - Types

This causes considerable heat loss to the walls of the passage which can be
reduced by employing a heat-insulated chamber.

However, in this type of combustion chambers even with a heat insulated
passage, the heat loss is greater than that in an open combustion chamber
which employs induction swirl.

This type of combustion chamber finds application where fuel quality is
difficult to control, where reliability under adverse conditions is more
important than fuel economy. The use of single hole of larger diameter for the
fuel spray nozzle is often important consideration for the choice of swirl
chamber engine.
Indirect - lnjection Chambers - Types
Precombustion Chamber:

A typical precombustion chamber consists of an antichamber connected to the
main chamber through a number of small holes (compared to a relatively large
passage in the swirl chamber).

The precombustion chamber is located in the cylinder head and its volume
accounts for about 40% of the total combustion space. During the compression
stroke the piston forces the air into the precombuhtion chamber.

The fuel is injected into the prechamber and the combustion is initiated.
Indirect - lnjection Chambers - Types

The resulting pressure rise forces the flaming droplets together with some air
and their combustion products to rush out into the main cylinder at high
velocity through the small holes.

Thus it creates both strong secondary turbulence and distributes the flaming
fuel droplets throughout the air in the main combustion chamber where bulk
of combustion takes place. About 80% of energy is released in main
combustion chamber.
Indirect - lnjection Chambers - Types

The rate of pressure rise and the maximum pressure is lower compared to
those of open type chamber.

The initial shock of combustion is limited to precombustion chamber only.

The precombustion chamber has multi-fuel capability without any modification
in the injection system because of the temperature of prechamber.

The variation in the optimum injection timing for petrol and diesel operations
is only 2° for this chamber compared to 8° to 10° in the other designs.
Indirect - lnjection Chambers - Types
Air-Cell Chamber:

In this chamber, the clearance volume is divided into two parts, one in the
main cylinder and the other called the energy cell. The energy cell is divided
into two parts, major and minor, which are separated from each other and
from the main chamber by narrow orifices. A pintle type of nozzle injects the
fuel across the main combustion chamber space towards the open neck of the
air cell.

During compression, the pressure in the main chamber is higher than that
Inside the energy cell due to restricted passage area between the two.
Indirect - lnjection Chambers - Types

At the TDC the difference in pressure will be high and air will be forced at high
velocity through the opening into the energy cell and this moment the fuelinjection also begins.

Combustion starts initially in the main chamber where the temperature is
comparatively higher but the rate of burning is very slow due to absence of any
air motion. In the energy cell, the fuel is well mixed with air and high pressure
is developed due to heat release & the hot burning gases blow out through the
small passage into the main chamber. This high velocity jet produces swirling
motion in the main chamber and thereby thoroughly mixes the fuel with air
resulting in complete combustion.