Physical Delay:

This represents the time interval from the time of injection of fuel to its attainment of self-ignition temperature during which the fuel is atomized, vaporized and mixed with air.

Chemical Delay:

After physical delay period is over, the time interval upto the time the fuel auto-ignites and flame appears is called chemical delay. During this period preflame reactions take place. This period corresponds to ignition lag of S.I. engines.

In practice, it is very difficult to separate exactly these two delay periods since the processes involved are very complex.

Factors on which Delay Period Depends:

Compression Ratio: Increased CR increases the density, pressure and temperature of the charge. Increased temperatures and pressure reduces the delay period.

Inlet pressure (supercharging): Increased inlet pressures increases the pressures in the compression stroke and reduces the delay period.

Intake Temperature: Higher intake temperatures will result into high temperatures at the time of fuel injection, therefore, it will reduce the delay period.

Jacket water temperature: Increased jacket water temperature increases the air temperature in the cylinder, hence, reduces the delay period.

Load on engine: Increased loads on the engine reduces delay period. Since the air-fuel ratio decreases with the increase in operating temperatures.

Injection pressure: Increased injection pressure will give better atomization of fuel. It generally tends to reduce the delay period slightly.

Engine size: It has no effect on delay period in terms of time. However, large engines operate at lesser speed, therefore, delay period in terms of crank angle is smaller. Hence, less fuel enters the cylinder and the engine will run smooth.

Fuels: Higher the self-ignition temperature of the fuel, higher will be the delay period.

Engine Speed: Increased speed will increase the delay period in terms of degree of crank rotation, since the fuel pump is driven by the engine through gears.