The delay period in CI engine is the time period between the start of fuel injection and the starting of uncontrolled combustion. It is also known as the ignition delay.
The delay in the CI engine is mainly classified into two types, physical delay, and chemical delay. It is generally difficult to separate these two delay periods.
This delay period starts with the injection of the fuel and ends when the vapors of fuel reach self-ignition temperature. The physical delay basically consists of the following phenomenon:-
- Injection of fuel as spray
- Atomization of fuel in small droplets
- Vapourization of the fuel droplets and mixing of these fuel vapurs with air molecules.
- Fuei molecules attains the self igniton temperature.
The chemical delay period starts when the fuel molecules reach self-ignition temperature and it ends with the appearance of the flame. This chemical delay period consists of pre-flame reactions, the formation of ignition nucleus, and flame formation.
Delay period on pressure vs time diagram:
The ignition delay is shown on the pressure-time plot.
As shown in the above figure, the physical delay is shorter in comparison with the chemical delay.
Factors affecting delay period in CI engine:
The ignition delay period in the case of CI engine depends on the following factors:-
1] Pressure of inlet air:- Increase in pressure of inlet air increases the temperature of air that helps to attain the self-ignition temperature earlier and reduces the delay period.
2] Inlet temperature:- The increases in inlet temperature helps to reduce the ignition delay.
3] Compression ratio:- As the compression ratio increases, the pressure and the temperature of the air also increases, and thus the ignition delay decreases.
4] Fuel injection pressure:- The higher pressure of fuel injection helps for the better atomization of the fuel that helps to reduce the physical delay period.
5] Fuel:- As the cetane number of the fuel increases the delay period decreases.
6] Cooling jacket temperature:- The delay period reduces with the increase in cooling jacket temperature.
7] Engine size:- For the larger size of the engine, the delay period is smaller.
8] Engine speed:- As the engine speed increases, the delay period decreases.
9] Air-fuel ratio:- For the higher air-fuel ratio the delay period is lesser.