1. COMBUSTION PHENOMENON
IN C.I ENGINE
Y.AJAY KUMAR
GITAM UNIVERSITY
3/14/2014 1

2. HOW COMBUSTION TAKES PLACE IN
C.I ENGINE?????
• In C.I engine combustion occurs by the high
temperature produced by the compression of
air.
• Minimum compression ratio of 12 is required
for combustion process to takes place.
• The normal compression ratios are in the
range of 14 to 17 but maximum up to 23.
• The air-fuel ratio lies between 18 and 25.
3/14/2014 2

3. Intake:
The intake valve
opens, and fresh
air (containing no
fuel), is drawn into
the cylinder.
3/14/2014 3

4. Compression:
As the piston rises,
the air is compressed,
causing its
temperature to rise.
At the end of the
compression stroke,
the air is hot enough
to ignite fuel.
3/14/2014 4

5. Injection:
Near the top of the
compression stroke,
the fuel injector
drives fuel into the
cylinder. The fuel
immediately ignites
upon contact with
the hot compressed
air.3/14/2014 5

6. Power:
As the fuel burns, the
gas in the cylinder
heats and expands,
driving the piston.
3/14/2014 6

7. Exhaust:
The exhaust valve
opens, and the
exhaust is driven out
of the cylinder.
3/14/2014 7

8. 3/14/2014 8

9. HOW FUEL INJECTOR WORKS IN
C.I ENGINE
3/14/2014 9

10. 3/14/2014 10

11. 3/14/2014 11

12. IGNITION DELAY PERIOD
• The ignition delay in a diesel engine is defined as
the time interval between the start of injection
and the start of combustion. This delay period
consists of
• (a) physical delay, wherein atomization,
vaporization and mixing of air fuel occur and
• (b) of chemical delay attributed to pre-
combustion reactions.
• Physical and chemical delays occur
simultaneously.
3/14/2014 12

13. • Due to the delay period the pressure reached
during second stage will depend up on the
duration of delay period.
• The longer delay will cause rough running and
may cause diesel knock.
• Delay period should be as short as possible
both for the sake of smooth running and in
order to maintain control over the pressure
changes.
• But , some delay period should be necessary
other wise the droplets would not disappear
in the air for complete combustion.
3/14/2014 13

14. PERIOD OF RAPID OR UNCONTROLLED
COMBUSTION
• It is the second stage of combustion in C.I engine.
• This period is counted from end of the delay
period to the point of maximum pressure on the
indicator diagram.
• The rise of pressure is rapid because during the
delay period the droplets of fuel have had time to
spread themselves over a wide area and they
have fresh air all around them.
• About 1/3 of heat is evolved during this period.
3/14/2014 14

15. PERIOD OF CONTROLLED
COMBUSTION
• At the end of the second stage of combustion ,
the temperature and pressure are so high that
the fuel droplets injected in third stage burn
almost as they enter and any further pressure
rise can be controlled by injection rate .
• The heat evolved at the end of the
compression is about 70 to 80 percent.
3/14/2014 15

16. AFTER BURNING
• The combustion continues even after the fuel
injection is over , because of poor distribution
of particles
• This burning may be continue in the expansion
stroke up to 70 to 80(deg) of crank revolution
from TDC.
• The total heat evolved by the entire
combustion process is 95 to 97%; 3 to 5% of
heat goes as un burnt fuel in exhaust.
3/14/2014 16

17. ABNORMAL COMBUSTION IN
C.I ENGINES
• The only abnormality in C.I engine is “diesel
knock”.
• This occurs when the delay period is exclusively
long so that there is a large amount of fuel in
cylinder for the simultaneous explosion phase.
• Knock is thus a function of fuel chosen and may
be avoided by choosing a fuel with characteristics
that do not give too long a delay period.
3/14/2014 17

18. 3/14/2014 18