09-10-2012, 04:40 PM
Combustion in CI engine
combustion_in_CI_engine.ppt (Size: 622 KB / Downloads: 517)
Delay Period
The time required to start the actual combustion after starting the fuel injection is known as “Delay period” and the crank angle required for this is known as “ Delay period angle”
The time and place where the ignition will start is not fixed by anything in CI engine as in SI engines.
The air-fuel mixture in the combustion chamber before starting the combustion is very heterogeneous and concentration of fuel may vary from 0 to 100%. The first ignition (flame) generally occurs in the region of chemically correct A:F mixture because it requires minimum reaction time.
Once the ignition takes place, the flame formed propagates through the mixture of air and vaporized fuel and ignites the adjacent part of charge or it may initiate the auto-ignition in the part of A:F mixture away from the flame front by transferring the heat by radiation.
Period of Controlled combustion
The fuel injected after the process of uncontrolled combustion burns at the same rate at which it is injected because, the vaporization of fuel, mixing with the air and burning takes place at almost instantaneously as the fuel leaves the nozzle.
This is because , the temperature and pressure inside the cylinder are sufficiently high and sufficient turbulence is created due to previous burning, thus the delay period for the fuel injected after the process of uncontrolled combustion is almost zero. This period of combustion is known as “Controlled combustion”.
Afterburning
The Thermal decomposition of the part of fuel takes place during uncontrolled and controlled combustion.
The decomposed fuel molecules contain enough number of hydrocarbons and carbon particles which have lower reaction rate.
Some carbon and hydrocarbons, decomposed from fuel are left in the combustion products because the rate of decomposition during uncontrolled and controlled combustion I more than the rate of reaction of these molecules during that period.
These unburned hydrocarbons and carbon generally burn after stopping the fuel injection or during the expansion stroke.This process of combustion of decomposed carbon atoms is known as “After Burning”.