Delay period is the time interval (measured in milliseconds) between the commencement of fuel injection and the beginning of ignition and combustion. The start of combustion is indicated by the deviation of point of the pressure curve above the normal compression pressure. In practice, this actual time is as low as 0.006 seconds. The delay period consists of the following:
1. Physical delay period.
2. Chemical delay period.
Physical delay period:
Physical delay period or the mixing period is the time elapses between the beginning of fuel injection and the beginning of pre flame reactions. During this period, heating, and vaporization of the fuel drop and diffusion of air into the vapour layer takes place. This results in the formation of suitable mixture of fuel vapour and air ready for chemical reaction.
Chemical delay period:
Chemical delay period or integration period is the time that elapses between the beginning of chemical and beginning of ignition. During this period, the fuel vapour is being oxidized at an ever increasing rate until ignition occurs. In other words, this is the period taken up by the chemical reactions to attain the point of burning or self-ignition. In addition to the above, the delay period is also influenced by the degree of atomization i.e.disintegration of fuel jet injected and characteristics of combustion chamber. The delay is the more or less constant in time units. In high speed diesel engines, ignition delay is in the order of 0.0012 to 0.0018 seconds.
Factors influencing delay period:
1. Characteristics of fuel (self-ignition temperature, volatility and viscosity).
2. Temperature and pressure of compressed air.
3. Degree of atomization of injected fuel.
4. Air motion / turbulence present in the combustion space.
5. Engine speed.
6. Injection timing.
7. Characteristics of the combustion chamber.