Four stages of Combustion in CI engines are:
1. Ignition
Delay Period,
2. Period of
Rapid Ignition,
3. Period of
Controlled combustion,
4. Period after
burning.
1. Ignition
Delay period:
The fuel does not ignite
immediately upon injection into the combustion chamber.
There is a definite period
of inactivity between the time of injection and the actual burning this period
is known as the ignition delay period.
The ignition delay period
can be divided into two parts, the physical delay and the chemical delay.
When the fuel is injected
into the combustion chamber, the fuel mixes with air. The time spend in this
process is known as physical delay.
Certain pre flame reactions
starts and after some time fuel burns automatically and the time consuming
during this process is known as chemical delay.
2. Period
of Rapid Combustion:
During total delay period
more fuel droplet come from the injector into the combustion chamber.
This group of fuel droplets
burn together and this produces uncontrolled combustion is known as knocking
and this occurs at the end of delay period or the beginning of combustion.
The rate of heat-release is
maximum during this period.
This is also known as
uncontrolled combustion phase, because it is difficult to control the amount of
burning/ injection during the process of burning.
3. 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 are so high that fuel droplets injected burn
almost as they enter and any further pressure rise can be controlled by
injection rate.
The period of controlled
combustion is assumed to end at maximum cycle temperature.
4. Period
of After-Burning:
Combustion
does not stop 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.
This
burning may continue in expansion stroke up to 70 to 80 degrees of crank travel
from TDC to BDC.
This fourth
stage may not be present in all cases but due to poor distribution of fuel
particles combustion may continue in the expansion stroke.
#Combustion stages in Combustion Ignition Engines, Combustion stages in C.I Engines