Combustion in CI engines

Combustion in CI engines

1. Lecture-18 Prepared under
QIP-CD Cell Project

Internal Combustion Engines
Conbastin sa CT Eggs

Ujjwal K Saha, Ph.D.

Department of Mechanical Engineering

Indian Institute of Technology Guwahati
1

2. Combustion in Cl Engine

A Combustion in a Cl engine is quite different
from that of an Sl engine. While combustion in
an S engine is essentially a flame front moving
through a homogeneous mixture, combustion
in a Cl engine is an unsteady process
occurring simultaneously in many spots in a

very non-homogeneous mixture controlled by
fuel injection.

O Air intake into the engine is unthrottled, with
engine torque and power output controlled
by the amount of fuel injected percycle.

3. Q Only air is contained in the cylinder during
compression stroke, and a much higher
compression ratios (12 to 24) are used in Cl

engines.

Q 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.

O 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 20° of
Crankshaft rotation (15% bTDC and 5% aTDC).

4. Cylinder pressure as a function
of crank angle for a Cl engine.

e
2
$
a
2
=
Só

\
Lx je = i L
640 660 680 700 TDC 20

Crank Angle, @ (degrees)

: point of fuel injection
point of ignition : delay period
end of fuel injection

5. Q In a Cl engine the fuel is sprayed directly into the
cylinder and the fuel-air mixture ignites spontaneously.
These photos are taken in a RCM under Cl engine
conditions with swirl air flow

3.2 ms after ignition Late in combustion process

6. In Cylinder Measurements

This graph shows the fuel injection flow rate, net heat release
rate and cylinder pressure for a direct injection Cl engine.

Pressure, MPa

Y

—40 -204 TC 20 40 60 80
Start of injection 1 Crank angle, deg
Start of combustion —

End of injection

7. 3
2
E
3
2
=
©
3

Four Stages of Combustion in Cl Engines

+ delay
period

Start of
injectio

Ly |

Ignition

Premixed combustion phase

Mixing-controlled combustion phase

Late
End of —++— combustion

n injecction

i 1

-20 4

TC
Crank angle, deg

8. Combustion in Cl Engine

The combustion process proceeds by the following stages:

( 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.

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.

9. Combustion in Cl 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.

10. Cl Engine Types
Two basic categories of Cl 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.

11. 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.

12. Types of Cl Engines

Fuel jets
y L\

Direct injection: Direct injection:
quiescent chamber swirl in chamber

Glow plug

Orifice

Indirect injection: turbulent
and swirl pre-chamber ¡>

13. Direct Injection
quiescent chamber

Direct Injection
multi-hole nozzle
swirl in chamber

Direct Injection
single-hole nozzle
swirl in chamber

Indirect injection
swirl pre-chamber

13

14. Combustion Characteristics

Q Combustion occurs
throughout the chamber
over a range of
equivalence ratios
dictated by the fuel-air
mixing before and
during the combustion
phase.

Q 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).

15. 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.

16. 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.

17. Fuel Ignition Quality

a The ignition characteristics of the fuel
affect the ignition delay.

Q The ignition quality of a fuel is defined
by its cetane number CN.

a 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”.

18. Fuel Ignition Quality

Q 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.

19. Cetane Number

+ The method used to determine the ignition
quality in terms of CN is analogous to that
used for determining the antiknock quality
using the ON.

+ The cetane number scale is defined by
blends of two pure hydrocarbon reference
fuels.

«+ By definition, isocetane (heptamethylnonane,
HMN) has a cetane number of 15 and cetane
(n-hexadecane, C,,H,,) has a value of 100.

20. Cetane Number

+In the original procedures a-
methylnaphtalene (C,,Hi9) with a
cetane number of zero represented the
bottom ofthe scale. This has since been
replaced by HMN which is a more stable
compound.

+ The higher the CN the better the
ignition quality, i.e., shorter ignition
delay.

21. Cetane Number Measurement

The method developed to measure CN usesa
standardized single-cylinder engine with
variable compression ratio

The operating condition is:

Inlet temperature (°C)
Speed (rpm)

Spark advance (°BIC)
Coolant temperature (°C)
Injection pressure (MPa)

22. Cetane Number Measurement —conid.

aWith the engine running at these
conditions on the test fuel, the
compression ratio is varied until
combustion starts at TC, ignition delay
period of 13°.

QO The above procedure is repeated using
blends of cetane and HMN. The blend that
gives a 13° ignition delay with the same
compression ratio is used to calculate the
test fuel cetane number.

23. Cetane vs Octane Number

The octane number and cetane number
of a fuel are inversely correlated.

El

i
vos
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3
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354
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seat

24. Factors Affecting Ignition Delay

Injection timing — At normal engine conditions the
minimum delay occurs with the start of injection
at about 10-15 BIC.

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 Cl engine the air is not
throttled so the load is varied by changing the
amount of fuel injected.

25. 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 hasthe same effect.

26. tin delay, ms

Ignition delay, ms

Factors Affecting
Ignition Delay

(huge)

Pi (gauge)
O Naturally

aspirated
+ 103 kPa

Ignition delay, ms

N y
300
bmp, kPa

Y
1200

1
1600

27. 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. 8 Pressure: DP decreases with
increase of lemperature and pressure.

28. Effect of
Ignition
Delay

29. A Knock in Sl and Cl engines are fundamentally
similar. In S eng it occurs near the end of
PR uston: € OCCU

a Knock in Cl engines is related to

. When DP is longer, there will be more
and more accumulation of fuel droplets in
combustion chamber. This leads to a too rapid
a pressure rise due to ignition, resulting in
jamming of forces against the piston and rough
engine operation. When the DP is too long, the
rate of pressure rise is almost instantaneous
with more accumulation of fuel.

30. Knock in S and Cl Engines

Start of combustion

Ss
aa
9
2
et
0
E
3
an

Pressure
Ignition
Pressure

SI Engine CI Engine

31. References

Crouse WH, and Anglin DL, (1985), Tata McGraw Hill.
Eastop TD, and McConkey A, (1993),

, Addison Wisley.
Fergusan CR, and Kirkpatrick AT, (2001), John
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Taylor CF, (1985), Vol.1 & 2,
The MIT Press, Cambridge, Massachusetts.

31

32. Web Resources

http://www.mne.psu.edu/simpson/courses

http://me.q' nsu.ca/courses

http://www.eng.fsu.edu

http: // www.personal.utulsa.edu

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http://online.physics.uiuc.edu/courses/phys140

http: //widget.ecn.purdue.edu/~yanchen /ME200/ME200-8.ppt -
http://www.bae.uky.edu