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What Is Diesel Cycle? What Are The Four Processes Of Diesel Cycle

Diesel cycle is also called as constant pressure cycle. The diesel engine operates on this cycle. This cycle also contains four processes, out of which two processes are adiabatic, the third one is constant pressure process and forth process is constant volume process.Diesel cycle is an air-standard cycle (a combustion process), which is used to design mostly compression ignition engines. Generally, these engines are heavier than petrol engines.

Processes In Diesel Cycle-

1–2= Adiabatic compression
2–3= Heat addition at constant pressure
3–4= Adiabatic expansion
4–1= Heat rejected at constant volume

Following is the pressure vs volume graph for a diesel cycle.
Isentropic Compression (Process 1–2)

This process is called isentropic as there is no heat transferred (adiabatic) to or from the system and it is a reversible process.
The gas inside the cylinder is compressed isentropically from a volume V1 to V2.
The ratio of V1 and V2 is referred to as the compression ratio.
Work is done by the piston on gases (negative work Win), which means external work has to be done to compress the gases.
This process is characterized by the compression stroke of the 4-stroke cycle.

Isobaric Heat Addition (Process 2–3)-

Isobaric means that the process carried out at constant pressure.
With the pressure being constant, heat is added externally until volume V3 is reached.
The ratio of V3 and V2 is referred to as the cut-off ratio.
Heat is added to the system (positive heat Qin), by combusting the air-fuel mixture.
This process is characterized by the initial part of the power stroke of the 4-stroke cycle, until volume has expanded to V3.

Isentropic Expansion (Process 3–4)

This process is also isentropic.
The gas inside the cylinder expands from V3 to V4 which is equal to V1.
The ratio of V4 (or V1) and V3 is known as the expansion ratio.
Work is done by the gases on the piston (positive work Wout), thus powering the engine by pushing the piston down.
This process is characterized by the final part of the power stroke of the 4-stroke cycle, until volume has expanded to V4.

Isochoric Expansion (Process 4–1)

Isobaric means that the process carried out at constant volume.
With the volume being constant, heat is removed until pressure comes down to p1.
Heat is removed from the system (negative heat Qout), by flushing out the combusted gases.
This process is characterized by the exhaust and intake stroke of the 4-stroke cycle.