Combustion in compression-ignition (CI) Engine
The
process of combustion in the compression-ignition engine is fundamentally
different from that in a spark-ignition engine. In the CI engine (12:1 to 22:1)
during the compression stroke raising highly its temperature and pressure. In
this highly compressed and highly heated air in the combustion chamber (well
above ignition point of fuel) one or more jets of fuel are injected in the
liquid state, compressed to a high pressure of 110 to 200bar by means of a fuel
pump. Each minute droplet as it enters the hot air (temperature 450-550ׄ° and pressure 30-40 bar) is quickly
surrounded by an envelope of its own vapour and this, in turn, and after an
appreciable interval, is inflamed at the surface of the envelope.
Stages of combustion in CI engines
1.
First stage: Ignition
delay period during which some fuel has been admitted but has not yet been
ignited. The ignition delay is counted from the start of injection to the point
where the p-θ curve separates from the pure air compression curve. The delay
period is a sort of preparatory phase.
2.
Second stage: Rapid
or uncontrolled combustion following ignition. In this second stage the
pressure rise is rapid because during the delay period the fuel droplet have
had time to spread themselves over a wide area and they have fresh air all
around them. The period of rapid or uncontrolled combustion is counted from the
end of delay period to the point of maximum pressure on the indicator diagram.
About one-third of the heat is evolved during this period.
3.
Third stage: Controlled
combustion. The second stage or uncontrolled combustion is followed by the
third stage-the controlled combustion. At the end of second stage the
temperature and pressure are so high that the fuel droplets injected during the
last stage burn almost as they enter and any further pressure rise can be
controlled by purely mechanical means i.e., by the injection rate. The period
of controlled combustion is assumed to end at the maximum cycle temperature.
The heat evolved by the end of controlled combustion is almost 70 to 80 percent
of the total heat of the fuel supplied during the cycle.
4.
Fourth stage: After
burning. Theoretically it is expected that combustion process shall end
after the third stage. However, because of poor distribution of the fuel
particles, combustion continues during part of the remainder of the expansion
stroke. This after-burning can be called the fourth stage of combustion. The
duration of the after-burning phase may correspond to 70 to 80 degrees of crank
travel from TDC and the total heat evolved by the end of entire combustion
process is 95 to 97% and 3 to 5% of heat goes as un-burnt fuel in exhaust.
