Ignition Delay in Compression Ignition Engine

The time period elapse between the start of injection of first droplet of fuel in combustion chamber and start of ignition of injected fuel is called ignition delay period in compression ignition (CI) engine. The ignition delay period can also be named as preparatory phase where injected fuel prepares itself to get ignited for producing energy. Delay period has great effect on engine performance as both combustion rate and knocking are influenced by delay period. In addition to that, starting ability of engine and presence of smoke in exhaust is also affected by delay period. 

In CI engine delay period comprises of two parts i.e. physical delay and chemical delay. During physical delay, the fuel present in the combustion chamber prepare itself to attain chemical reaction condition after being injected which comprises of customization of fuel, its vaporization and attainment of its self-ignition temperature. Chemical delay comprises of time duration between start of pre-flame reaction and actual ignition. It has been seen that, chemical delay duration is generally greater than physical delay duration. But again, it is not mandatory always, as their relative time depend upon the temperature of air at the end of compression stroke. At high temperature, physical delay is longer whereas at relatively low temperature chemical delay is longer.