Diesel cycle

The Diesel cycle is a combustion process of a reciprocating internal combustion engine.
Diesel cycle is similar to Otto cycle except in the fact that it has one constant pressure process instead of a constant volume process.

                                                                                   Picture References wikipedia.org

Process 1 to 2 is isentropic compression of the fluid
Process 2 to 3 is reversible constant pressure heating
Process 3 to 4 is isentropic expansion
Process 4 to 1 is reversible constant volume cooling


In process 1 to 2, the piston moves from Bottom Dead Centre to Top Dead Centre position. Air is compressed inside the cylinder. Pressure of air increases, temperature increases, and volume decreases.
Work is done on the system in this process.


In process 2 to 3, heat is added at constant pressure from an external heat source. Volume increases, temperature increases.

In process 3 to 4 the compressed and heated air is expanded isentropically inside the cylinder. The piston is forced from TDC to BDC in the cylinder. Pressure of air decreases, temperature decreases, and volume increases.

In process 4 to 1, heat is rejected at constant volume. Pressure decreases,temperature decreases and entropy decreases.