THREE STAGES OF COMBUSTION
According to Ricardo, There are three stages of combustion in the SI Engine.
1. Ignition delay stage
2. Flame propagation stage
3. After the burning phase
combustion phases in SI engines1. Ignition delay stage:
There is a certain time interval between the moment of the spark and the moment when there is a noticeable increase in pressure due to combustion.
This time interval is called IGNITION LAG.
Ignition delay is the time interval in the chemical reaction process during which molecules are heated to the auto-ignition temperature, are ignited, and produce a self-propagating flame nucleus. Ignition delay is generally expressed in terms of crank angle (θ1). The ignition delay period is shown by path ab. The ignition delay is very short and is between 0.00015 and 0.0002 seconds. An ignition delay of 0.002 seconds corresponds to a crank rotation of 35 degrees when the engine is running at 3,000 RPM. The advance angle increases with speed. This is a chemical process that depends on the nature of the fuel, temperature and pressure, exhaust gas proportions and rate of oxidation or burning, exhaust gas proportions and rate of oxidation or burning.
2. Flame propagation stage:
Once the flame in “b” is formed, it must be self-sustaining and capable of propagating through the mixture. This is possible when the rate of heat generation through burning is greater than the heat lost by the flame to the environment. After point “b”, the flame spread is abnormally low at first, as the heat lost is greater than the heat generated. Therefore, the increase in pressure is also slow, as the mass of the burned mixture is small. Therefore, it is necessary to provide an advance angle of 30 to 35 degrees if the peak pressure is reached 5-10 degrees after TDC. The time required was 10 degrees after PMS. The time required for the crank to rotate through an angle θ2 is known as the combustion period, during which the flame spreads. flame propagation occurs.
3. After burning:
Combustion will not stop at point “c”, but will continue after reaching the peak pressure at point “c”. This combustion is known as afterburning. This usually happens when rich mixture is supplied to the engine.
