Exploring the Diesel Cycle to know efficiency of Diesel Cycle

The Diesel cycle is a theoretical cycle used for slow speed Compression Ignition or diesel engines. Unlike the Otto cycle, which has heat addition at constant volume, the Diesel cycle has heat addition at constant pressure. The efficiency of Diesel cycle is more than the Otto cycle under different operating conditions. The diesel cycle is also known as the constant pressure cycle.

 However, it is important to avoid using the term “constant pressure cycle” as it can be confused with the Joule cycle and note that in the diesel cycle at the end of the expansion process, there is a constant volume pressure drop or blowdown. 

Processes Involving in the Diesel Cycle

Diesel cycle consists of following four processes

Figure shows Diesel cycle consists of following four processes Adiabatic compression, Constant pressure heat addition, Adiabatic expansion and constant volume heat rejection.

Diesel cycle PV diagram and TS Diagram

Diesel cycle PV diagram
Diesel cycle PV diagram
TS diagram of Diesel Cycle
TS diagram of Diesel Cycle

Understanding How the Diesel Cycle Operates

Piston Cylinder Diesel Engine

Piston Cylinder Diesel Engine to illustrate Diesel cycle operation.
TDC=Top dead centre; BDC=bottom dead centre; IV=Inlet valve; EV= Exhaust valve;FI= Fuel Injector; CR= connecting Rod; C= Crank; EC = Engine cylinder

Assumptions made in the Diesel cycle

  • Compression and expansion processes are adiabatic and reversible.
  • During the constant pressure heat addition process, combustion is complete with 100% combustion efficiency.
  • Valves open and close at the piston’s top and bottom dead centre position inside the cylinder.
  • Suction and exhaust take place at constant pressure.

This cycle can be seen in Figures on both p-V and T-s diagrams.

Operation of the Diesel Cycle

To analyze the Diesel cycle as a closed cycle, induction and exhaust processes are ignored, which are represented by 0-1 and 1-0, respectively on Diesel cycle PV diagram.

The compression process, represented by 1-2, is a reversible adiabatic compression of air at a compression ratio of r = V1/V2. 

Then, heat is added at constant pressure, represented by 2-3, causing the air to expand and do work from volume v2 to v3.

In an actual diesel engine, fuel is injected and self-ignites due to the high temperature caused by the high compression ratio, burning at constant pressure. 

At point 3, called the cutoff point, the fuel supply is stopped. The cutoff ratio, denoted by ⍴, is the volume ratio of v3 to v2. 

Next, there is a reversible adiabatic expansion from 3 to 4. After expansion, heat is rejected in the constant volume process 4-1, completing the cycle.

Note that, unlike the Otto cycle, the compression ratio and expansion ratio are not equal in the Diesel cycle.

The Efficiency of Diesel Cycle

With respect to the Diesel cycle PV diagram and TS diagram

A simple derivation of the efficiency of Diesel Cycle

Thermal efficiency of Diesel cycle in terms of Temperature 

Thermal efficiency in terms of compression ratio “r” and the cutoff ratio “⍴”

It is worth noting that the efficiency of the Diesel cycle differs from that of the Otto cycle only by the bracketed term, which is always greater than unity, except in cases where there is no heat addition and ⍴=1

The simplest way of solving problems related to the Diesel cycle efficiency is instead of applying the formula, calculate temperatures T2 , T3 and T4 and then use the equation 𝜂=1-(Q2/Q1)

How to improve the efficiency of Diesel cycle?

Efficiency Vs various Compression Ratios "r" and various Cutoff ratios"⍴" for Diesel cycle
Efficiency Vs various Compression Ratios “r” and various Cutoff ratios”⍴”
Efficiency Vs various Cutoff Ratios "⍴" and various Compression Ratios "r"  for Diesel cycle
Efficiency Vs various Cutoff Ratios “⍴” and various Compression Ratios “r”

From the graph, it is clear that the efficiency of the Diesel cycle increases with a decrease in the cutoff ratio and an increase in the compression ratio.

In Diesel engines the cutoff ratio “⍴” depends on load, being maximum for maximum load. Hence the air standard efficiency of the Diesel cycle depends on the load and increases as the load is decreased

The maximum efficiency it can achieve is equal to that of Otto cycle efficiency and this is possible only when there is no load on the engine.


We have seen that for a given Compression ratio Diesel cycle is not efficient compared to the Otto cycle. However, one should carefully consider the practical implications of the difference in efficiency between the Otto and Diesel cycles

In practice, Diesel engines (compression-ignition engines) operate at much higher compression ratios than spark-ignition engines (petrol and gas engines) based on the Otto cycle.

As a result, despite the lower efficiency of the theoretical Diesel cycle for the same compression ratio, the actual efficiency of diesel engines is typically higher than that of petrol engines.

If you want to see the comparison of the theoretical Otto cycle and the Theoretical diesel cycle under different conditions then click/tap here


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