Diesel cycle is a thermodynamic cycle upon which a compression ignition engine works. Compression Ignition (or CI) engine uses diesel as fuel. Diesel cycle was invented by Rudolf Diesel in 1890s.
Compression ignition engine is a type of internal combustion engines.
Below are P-V and T-S Diagrams of the Diesel Cycle.
Diesel Cycle is comprised of four processes (apart from the intake and exhaust process)
Process 1-2
This process comes after intake process. In case of CI engines, intake is only air (unlike spark ignition engines where it is a mixture of air and petrol). In this process air is compressed in isentropic manner.
Process 2-3
In this process fuel is injected in the cylinder. Due to high pressure and compressed air, fuel gets instantly burned. This process is a constant pressure heat addition process.
Process 3-4
After the combustion process, the exhaust gases expend. This process is an isentropic expansion process.
Process 4-1
After the expansion process, exhaust valve opens. This results into sudden drop of pressure inside engine cylinder. This process is a constant volume heat rejection process.
Note: Process 0-1 is the intake process and process 1-0 is the exhaust process.
Below is the table which shows heat and work interactions of the engine, along with the change in the internal energy.
| Process | Change in Internal Energy | Work Interaction | Heat Interaction |
| Process 1-2 | CV(T2-T1) | CV(T1-T2) | 0 |
| Process 2-3 | CV(T3-T2) | P2(V3-V2) | CP(T3-T2) |
| Process 3-4 | CV(T4-T3) | CV(T3-T4) | 0 |
| Process 4-1 | CV(T1-T4) | 0 | CV(T1-T4) |
Note: Negative value of heat interaction indicates heat rejected by the system and positive value of heat interaction indicates heat added to the system. Positive value of work interaction indicates work done by the system and negative value of work interaction indicates work done on the system.
Efficiency of Diesel Cycle
Efficiency of the Diesel Cycle is the ratio of work output to the heat input.
Work output = [CP(T3-T2)] – [CV(T4-T1)]
Heat Input = CP(T3-T2)
Efficiency = Work Output/Heat Input
After putting values of heat input and work output in the above formula, we get
η = 1 – [CV(T4-T1)/CP(T3-T2)]