combustion in ci engine

1. INTRODUCTION TO
COMBUSTION IN CI ENGINE

2. • CI engine was develop by Dr Rudolf Diesel he
got a patent of his engine in 1892.

3. • It is very important prime movers now a days
and is finding wide application in
• Locomotives
• Small and medium electric generation
• Marine propulsion

4. Following points are worth noting in CI
engine
 its thermal efficiency is higher than SI engine
CI engine are less expensive than SI engine
CI engine have higher specific gravity than
petrol engine
Since fuel is sold on volume basis and not on
mass basis more fuel per litres are obtained in
purchasing CI engine fuel.
Running cost of CI engine is less than SI engine

5. A CI engine is not much favoured in
passenger cars due to the following
reason
Heavier weight
Noise and vibration
Smoke
Odour
In view of the utilization of the heavier compression
ratios(12:1 to 22:1) compare to 6:1 to 11:1 of SI
engine) the heavy force acts on the parts of the engine
and therefore heavy parts are required,
Because of heterogeneous mixture, lean mixture is
used.

6. • The incomplete combustion of heterogeneous
mixture and droplet combustion result in the
smoke and odour.
CI engine are manufactured in the following
range of speed
particulars range
1 piston diameter 50mm to 900mm
speeds 100 rpm to 4400rpm
Power output 2 B.P to 40000 B.P

7. combustion phenomenon in CI engine
In CI engine combustion occurs by the high
temperature produced by the compression of the air .
It is auto ignition.
A minimum ratio of 12 is required
Efficiency increased with higher value of compression
ratio. But maximum pressure reached in the cylinder
also increases so it require heavier construction
Normal compression ratio are between 14 to 17 and up
to 23
Air fuel ratio is about 18 and 25 against about 14 in SI
engine hence CI engine is heavier for the same power
than SI engine

8. In CI engine the intake is air alone and the fuel
is injected at high pressure in the form of fine
droplet near the end of compression. This lead
to delay period in the CI engine.

9. The exact phenomenon of combustion
in CI engine
 Each minute droplet of fuel as it enter the highly heated air
of engine cylinder is quickly surrounded by an envelop of its
own vapor and this in turn at an appreciable interval is
inflamed at the surface of the envelop. The vapor will be
burning as long as it can find fresh oxygen this means it
depend upon the rate at which the fuel is moving through
the air or the air moving past it.
 The fuel is not fed all at once but is spread over a definite
period. The first arrival meet air whose temperature is only
above their self ignition temperature, and the delay is more
or less prolonged. The later arrival find air to a higher
temperature and therefore light more quickly. But the
progress is lessened as there is less oxygen to find.

10. Three phase of CI engine combustion
• 1. ignition delay period
• 2. period of rapid or uncontrolled combustion
• 3. period of controlled combustion

11. Ignition delay period
The ignition delay period is counted from the
start of ignition to the point where the
combustion starts. If there is no delay period
the fuel would burn at the injector and there
would be an oxygen deficiency around the
injector resulting in incomplete combustion.
If the delay is too long the amount of fuel
available for simultaneous explosion is too
great resulting pressure rise is too rapid.

12. Factors on which delay period depends
1. temperature and pressure in the cylinder at
the time of injection
2. nature of the fuel mixture strength
3. relative velocity between the fuel injection
and air turbulence
4. presence of residual gasses
5. rate of fuel injection
6. fineness of the fuel spray.

13. Delay period is subdivided into
1. physical delay
2. chemical delay

14. Physical delay
It is the time between the time of injection
and the attainment of chemical reaction
condition
Here the fuel is atomized, vaporized, mixed
with air, and raised with temperature

15. Chemical delay
Period reaction start slowly and then
accelerate until inflammation or ignition take
place

16. If the air inside the cylinder were motionless
only a small portion of fuel will find oxygen, it
is impossible to distribute the droplets
uniformly through out the combustion space.
Therefore some air movement is necessary.
Air swirl the rotational motion of air within
the cylinder.

17. Delay period can be due to following reason
 a low design compression ratio permitting
only a marginal self ignition temperature to be
reached
Low combustion pressure due to worn piston
rings or badly seating valves
Poor fuel ignition quality, low cetane number
A poorly atomize fuel spray preventing early
ignition to be reached.
A very low intake temperature in cold weather
and cold starting.

18. • The longer the delay the more rapid and higher
the pressure rise since more fuel will be present
in the cylinder before the rate of burning comes
under control. This causes rough running and
may cause diesel knock.
• We must keep the aim to keep ignition delay as
short as possible to maintain smooth running and
pressure change. Some ignition delay is necessary
some droplets would not be dispersed in the air
for complete combustion

19. • 2. period of rapid or uncontrolled combustion
After ignition delay the second stage is rapid
combustion.
It is counted at end of delay period to the
maximum pressure on the indicator diagram.
The rise of pressure is rapid as during the delay
period the droplet of fuel have time to spread
themselves over a wide area and have fresh
air all around them.

20. • NOTE: the rate of pressure rise depend on the
amount of fuel present at the end of delay
period, degree of turbulence, fineness of
atomization and spray pattern

21. 3. Period of controlled combustion
 At the end of second stage the temp and
pressure are so high that the fuel injected in
third stage burn almost as they enter and any
further rise can be controlled by purely
mechanical means by injection rate this period
is assumed to end at maximum cycle temp.

22. 4 after burning
• The combustion continues even after fuel
injection is over, because of poor distribution
of fuel particles. The burning continues in
expansion stroke up to 70 -80 % o crank travel
from TDC. It is fourth stage of combustion.
• Total heat at end of combustion is 95-97% the
rest 3 to 5% goes as unburned fuel.

23. Fundamentals of the combustion
process in CI engines
Effect of compression ratio and engine speed on
cylinder pressure and temperature
1. The power output of a diesel engine is
controlled by varying the amount of fuel spray
injected into the cylinder filled with compressed
and heated air.
2. the pressure and temperature reached at the
compression stroke will depend primarily upon
the compression ratio, intake temperature and
speed of the engine

24. It has been noted that ignition usually
commence 15o to 20o before TDC when temp
and pressure are much lower.

25.  Diesel engine heterogeneous charge mixing
The mixing of the localised spray of fuel droplets
in the hot air charges stoichiometric (14.7:1 by
weight) the average air fuel mixture ratio range
may vary from rich, a full load 20:1 to a weak no
load 100:1 air fuel ratio
Most engine operate with 20% excess air due to
difficulty of introducing sufficient exposed oxygen
to the fuel vapor in the given time available so
that the process can be completed before the
exhaust valve open. If oxygen is partially
prevented from getting to the fuel vapor early
during power stroke then incomplete combustion
polluted exhaust gas and dark smoke will result.

26. Diesel engine injected spray
combustion process
 the fuel spray entered the hot combustion chamber does
not ignite immediately it breaks up into very small droplets,
and once this liquid droplets are formed, their outer
surface will immediately start to evaporate so there will be
a liquid core surrounded with a layer of vapor burning
hydrocarbon in air is purely oxidation process. The
temperature rises which in turn speed up the oxidation
process further increasing the heat release until a flame
site or sites re established this is known as ignition and the
temperature t which I occurs is called the self- ignition
temperature.
 The heat require for further evaporation of the fuel droplet
will thus be provided from heat released by the oxidation
process

27. Diesel knock
 it is the sound produced by the very rapid
rate of pressure rise during the early rate of
the uncontrolled rate of combustion caused
due to prolonged delay period.

28. • Detonation is define as the sudden
combustion that occurs due to the burning of
the remaining gasses which produces high
pressure waves and sound inside the cylinder
• At the end of the compression stroke
explosive combustion occurs due to auto
ignition of the unburnt air-furl mixture which
produces high vibration and noise
• Due to detonation a high intensity pinging
sound is produced inside the engine which is
like a knock which is known as knocking.

29. Method to control detonation
 high charge temperature
High fuel temperature
Good turbulence
A fuel with a short induction period.

30. Primary consideration in the design of
combustion chamber
• 1. High thermal efficiency
• 2. ability to use less expensive fuel (multi fuel)
• 3. ease of starting
• 4. ability to handle variations in speed
• 5. smoothness of operation, low diesel knock
• 6. low exhaust emission
• 7. nozzle design
• 8. high volumetric efficiency
• 9. high brake mean effective pressure

31. Basic method of generating air swirl
 by directing the flow of air during its entry
during the cylinder, known as induction swirl, this
method is used in open combustion chamber.
By forcing the air through a tangential passage
into a separate swirl chamber known as
compression swirl. This method is used in swirl
chamber
By use of initial pressure rise due to partial
combustion to create swirl turbulence known as
combustion induced swirl. This method is used in
pre combustion chamber.

32. Induction swirl
 in a four cylinder engine a induction swirl can
be obtained either by careful formation of air
intake passage or masking of shrouding a
portion of circumference of inlet valve . The
angle of musk is from 90o to 140o of the
circumference
 induction swirl generated by air intake is
very weak. Therefore, we have to use a
multiple orifice injector.

34.  induction swirl can be obtained by two
method
1. by bending the inlet manifold
2. by providing the mask on inlet valve
Swirl can be increased by squish, squish is the
secondary air movement created by piston
crown.

35. advantages
 in open combustion chamber the intensity of
swirl is low the heat loss to the chamber is low
resulting in easier cold starting
No additional work is needed to produce swirl

36. disadvantages
Multi orifice nozzle with high injection
pressure required as swirl is weak
As size of nozzle orifice is less it deliver less
quantity of fuel
Use of mask on inlet valve reduce efficiency
Weak swirl means low air

37. Compression swirl
It is known as swirl chamber. It is a divided
chamber
Combustion space is provided in two or more
compartment, between which there are
restriction or throat small enough so that
considerable pressure difference occur
between them during combustion process.
This swirl is maximum at about 15o before
TDC. Close to the time of injection.

39. • CI engine combustion chamber
Open chamber
Or non turbulence chamber
Divided chamber or
Turbulence chamber
Swirl chamber
Compression swirl
Pre-combustion
chamber
Energy cell

40. Open chamber or non turbulent
chamber
 the fuel is injected directly into the upper
portion of the cylinder. The heat loss to the
combustion chamber is relatively low, and easier
starting results, high injection and multi force
nozzles is required,, this necessitates small nozzle
opening and results in more frequent clogging or
diversion of the fuel spray by accumulated carbon
particles with higher maintenance cost.
Used on low speed engines

42. Divided chamber or turbulent
chamber
 the upward moving piston forces all the air
70-80% at a greater velocity into a small
antechamber. As the fuel is injected into the
rotating air it is partially mixed with this air
and commences to burning. Pressure in
antechamber expands and it forces burning
gasses and un burnt fuel and air mixture back
into the main chamber imparting high
turbulence and further assisting combustion.

44. Advantages
The hot running combustion chamber
shortens the delay period and limits the rate
of pressure rise, results in smoother running
The turbulence is responsible for rapid mixing
and burning of fuel.
Suitable for high speed

45. disadvantages
Cold starting is difficult since air loose heat to
combustion chamber wall during the
compression stroke.

46. Pre combustion chamber
 the combustion chamber is divided into two chamber. The
smaller one of the chamber occupy about 30% of the total
combustion . space. The communication between them is a
narrow restricted passage or number of small holes.
 The air is forced into the pre combustion chamber by piston
during the compression stroke. Fuel is injected into the pre
combustion chamber.
 The chamber is designed to run hot and result in shortening
the delay period
 The product from this chamber rush to main combustion
space and no delay period since temp is already high due to
combustion in pre combustion chamber combustion in
main chamber is rapid and compete.

48. Advantages
No delay period, tendency to knock is minimum
Combustion in third stage is rapid
DISADVANTAGES
Velocity of burning mixture is too high during the
passage from pre chamber so the heat loss is very
high, reduction in thermal efficiency.
Cold starting will be difficult as the air looses heat
to chamber wall during compression,

49. Energy cell
 The energy cell is more complex than pre combustion chamber as the
piston moves up the compression stroke some of the air is forced into the
major and minor chamber of the energy cell when the fuel is injected
through the pintle type nozzle part of the fuel passes across the main
combustion chamber and enters the minor cell where it is mixed with the
entering air combustion first commences in the main combustion
chamber Where the temp is higher but the rate of burning is slower due
to insufficient mixing of fuel and air
 the burning of the minor cell is slower at the start, but due to better
mixing progression at a more rapid rate. The pressure built up in the minor
cell therefore force the burning gasses into the main combustion
chamber, thereby creating added turbulence and producing better
combustion in this chamber. In the mean time pressure is built up in the
major cell which then prolongs the action of the jet stream entering the
main chamber thus continuing to induce turbulence in the main stream

51. Cold starting from CI engine
Cold starting may become difficult under the
following condition
1. when the cylinder liner is heavily worn
2. when the valves are leaky
3. extreme cold climate

52. Several method have been used in the
past to achieve easy cold starting
 preheating the engine cylinder by warm
water
Injection of small quantity of lubricating oil or
fuel oil. This method temporarily raise the
compression ratio, and seals the piston rings
and valves
Provision of cartridge
Modifying valve timing for starting
By providing auxiliary chamber

53. Modern starting aids of high speed
engine
 electric glow plugs
Manifold heaters
Injection of ether