Diesel Cycle - Thermodynamics:

Diesel cycle is also called as the constant pressure cycle as all addition of heat takes place at constant pressure. The cycle of operation is shown in the figure given below on P-V and T-S diagrams.

The sequence of operations is stated below:

1. The air is compressed isentropically from the condition '1' to the condition '2'.

2. Heat is supplied to compressed air from external source at the constant pressure that is represented by the process 2-3.

3. The air expands isentropically till it reaches the condition '4'.

4. The heat is rejected by air to the external sink at constant volume until it reaches the condition T and the cycle is repeated.
The air standard efficiency of the cycle is determined as follows:

 
 
is also > 1,thus for the same compression ratio the efficiency of diesel cycle is less than that of otto cycle.