Combustion is the process of burning something. It’s a chemical reaction involves the combination of a substance with oxygen, involving the production of heat and light. Combustion on SI engines is a flame front moving through a homogenous mixture whereas Combustion on CI engines occurs on the fuel injection which is an unsteady process simultaneously on many spots of a non- homogenous mixture. Air taken into the engine is un-throttled with engine torque and power output controlled by the amount of fuel injected per cycle. On the second phase of the stroke (i.e. the compression stroke) , only the air is contained in the cylinder. Compression ratio on CI engine is much higher than the SI engine which is 12 to 24 and 8 to 12 respectively. In …show more content…
The ignition delay is consisted of physical delay and the chemical delay where the physical delay is of atomization of the liquid fuel jet, vaporization of the fuel droplets and the mixing of air fuel occurrence and the chemical delay is of pre-combustion reaction of the fuel, air and residual gas mixture which leads to ignition. Both the delays occur simultaneously. Ignition delay is because of strong function of ambient temperature and pressure said by P.A.Lakshminarayanan et. …show more content…
It is usually expressed in crank angle degrees (CAD) relative to TDC(Top Dead Center) of the compression stroke. The minimum delay occurs at the start of the injection which is about 10-12 BTC , under normal conditions.
Injection quantity: As the load increases, the residual gas and the wall temperature also increases which results in the higher charge temperature at injection which causes ignition delay. The cetane number (CN) of the fuel is the important parameter which is responsible for the delay period. The other properties of fuel also affects the delay period which are volatility, latent heat, viscosity and surface tension.

Compression ratio: Increase in the density of the compressed air decreases the minimum auto ignition temperature of a fuel which results in a closer contact between the molecules of fuel and oxygen reducing the time of reaction. The increase in the density of the compressed air as well as the decrease in the auto ignition temperature decreases the delay