Combustion in CI Engine
Combustion in a CI engine is quite different
from that of an SI engine. While combustion in
an SI engine is essentially a flame front moving
through a homogeneous mixture, combustion
in a CI engine is an unsteady process
occurring simultaneously in many spots in a
very non-homogeneous mixture controlled by
fuel injection.
Air intake into the engine is unthrottled, with
engine torque and power output controlled
by the amount of fuel injected per cycle.
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Only air is contained in the cylinder during
compression stroke, and a much higher
compression ratios (12 to 24) are used in CI
engines.
In addition to swirl and turbulence of the
air, a high injection velocity is needed to
spread the fuel throughout the cylinder and
cause it to mix with the air.
Fuel is injected into the cylinders late in
the compression stroke by one or more
injectors located in each cylinders.
Injection time is usually about 200 of
crankshaft rotation (150 bTDC and 50 aTDC).
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Cylinder pressure as a function
of crank angle for a CI engine.
A : point of fuel injection
B : point of ignition AB : delay period
C : end of fuel injection
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Combustion in CI Engine
In a CI engine the fuel is sprayed directly into the
cylinder and the fuel-air mixture ignites spontaneously.
These photos are taken in a CI engine conditions
with swirl air flow
1 cm
0.4 ms after ignition 3.2 ms after ignition
3.2 ms after ignition Late in combustion process
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Four Stages of Combustion in CI Engines
Start of End of
injection injection
-20 -10 TC 10 20 30
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Combustion in CI Engine
The combustion process proceeds by the following stages:
Ignition delay (ab) - fuel is injected directly into
the cylinder towards the end of the compression
stroke. The liquid fuel atomizes into small drops
and penetrates into the combustion chamber.
The fuel vaporizes and mixes with the high-
temperature high-pressure air.
Premixed combustion phase (bc) –
combustion of the fuel which has mixed with
the air to within the flammability limits (air at
high-temperature and high-pressure) during
the ignition delay period occurs rapidly in a
few crank angles.
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Combustion in CI Engine – contd.
Mixing controlled combustion
phase (cd) – after premixed
gas consumed, the burning
rate is controlled by the rate
at which mixture becomes
available for burning. The
rate of burning is controlled in
this phase primarily by the
fuel-air mixing process.
Late combustion phase (de) – heat release may
proceed at a lower rate well into the expansion
stroke (no additional fuel injected during this phase).
Combustion of any unburned liquid fuel and soot is
responsible for this.
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CI Engine Types
Two basic categories of CI engines:
i) Direct-injection – have a single open
combustion chamber into which fuel is
injected directly
ii) Indirect-injection – chamber is divided into
two regions and the fuel is injected into the
“pre-chamber” which is connected to the
main chamber via a nozzle, or one or more
orifices.
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CI Engine Types – contd.
• For very-large engines (stationary power
generation) which operate at low engine speeds
the time available for mixing is long so a direct
injection quiescent chamber type is used (open or
shallow bowl in piston).
• As engine size decreases and engine speed
increases, increasing amounts of swirl are used to
achieve fuel-air mixing (deep bowl in piston).
• For small high-speed engines used in automobiles
chamber swirl is not sufficient, indirect injection is
used where high swirl or turbulence is generated in
the pre-chamber during compression and
products/fuel blowdown and mix with main
chamber air.
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Types of CI Engines
Glow plug
Orifice
-plate
Direct injection: Direct injection:
quiescent chamber swirl in chamber Indirect injection: turbulent
and swirl pre-chamber 12
Direct Injection Direct Injection Direct Injection Indirect injection
quiescent chamber multi-hole nozzle single-hole nozzle swirl pre-chamber
swirl in chamber swirl in chamber
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Combustion Characteristics
Combustion occurs
throughout the chamber
over a range of
equivalence ratios
dictated by the fuel-air
mixing before and
during the combustion
phase.
In general most of the
combustion occurs under
very rich conditions
within the head of the jet,
this produces a
considerable amount of
solid carbon (soot).
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Ignition Delay
Ignition delay is defined as the time (or crank angle
interval) from when the fuel injection starts to the onset
of combustion.
Both physical and chemical processes must take place
before a significant fraction of the chemical energy of
the injected liquid is released.
Physical processes are fuel spray atomization,
evaporation and mixing of fuel vapour with cylinder air.
Good atomization requires high fuel-injection pressure, small
injector hole diameter, optimum fuel viscosity, high cylinder
pressure (large divergence angle).
Rate of vaporization of the fuel droplets depends on droplet
diameter, velocity, fuel volatility, pressure and temperature of
the air.
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Ignition Delay
Physical processes are fuel spray atomization,
evaporation and mixing of fuel vapour with
cylinder air.
Chemical processes similar to that described
for auto-ignition phenomenon in premixed fuel-
air, only more complex since heterogeneous
reactions (reactions occurring on the liquid fuel
drop surface) also occur.
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Fuel Ignition Quality
The ignition characteristics of the fuel
affect the ignition delay.
The ignition quality of a fuel is defined
by its cetane number CN.
For low cetane fuels the ignition delay is
long and most of the fuel is injected
before autoignition and rapidly burns,
under extreme cases this produces an
audible knocking sound referred to as
“diesel knock”.
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Fuel Ignition Quality
For high cetane fuels the ignition delay
is short and very little fuel is injected
before auto-ignition, the heat release
rate is controlled by the rate of fuel
injection and fuel-air mixing – smoother
engine operation.
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Factors Affecting Ignition Delay
Injection timing – At normal engine conditions the
minimum delay occurs with the start of injection
at about 10-15 BTC.
The increase in the delay time with earlier or later
injection timing occurs because of the air
temperature and pressure during the delay
period.
Injection quantity – For a CI engine the air is not
throttled so the load is varied by changing the
amount of fuel injected.
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Factors Affecting Ignition Delay – contd.
Increasing the load (bmep) increases the
residual gas and wall temperature which results
in a higher charge temperature at injection
which translates to a decrease in the ignition
delay.
Intake air temperature and pressure – an
increase in ether will result in a decrease in the
ignition delay, an increase in the compression
ratio has the same effect.
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Factors Affecting
Ignition Delay
(gauge)
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Factors Affecting Delay Period (DP)
1. Compression Ratio: DP decreases with
increase of CR.
2. Engine Speed: DP decreases with increase
of engine speed.
3. Power Output: DP decreases with increase
of power output.
4. Fuel Atomization: DP decreases with fineness
of atomization.
5. Fuel Quality: DP decreases with higher
cetane number.
6. Intake Temp. & Pressure: DP decreases with
increase of Temperature and pressure.
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Effect of
Ignition
Delay
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Knock in SI and CI Engines
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