Diesel cycle is a gas power cycle invented by Rudolph Diesel in the year 1897. It is widely used in diesel engines.
Diesel cycle is similar to Otto cycle except in the fact that it has one constant pressure process instead of a constant volume process (in Otto cycle).
Diesel cycle can be understood well if you refer its pV and Ts diagrams.
pV and Ts Diagrams of Diesel Cycle:
pV Diagram  Ts Diagram 

Processes in Diesel Cycle:
Diesel cycle has four processes. They are:
 Process 12: Isentropic (Reversible adiabatic) Compression
 Process 23: Constant Pressure (Isobaric) Heat Addition
 Process 34: Isentropic Expansion
 Process 41: Constant Volume (Isochoric) Heat Rejection
Process 12: Isentropic Compression
In this process, the piston moves from Bottom Dead Centre (BDC) to Top Dead Centre (TDC) position. Air is compressed isentropically inside the cylinder. Pressure of air increases from p_{1} to p_{2}, temperature increases from T_{1} to T_{2}, and volume decreases from V_{1} to V_{2}. Entropy remains constant (i.e., s_{1} = s_{2}). Work is done on the system in this process (denoted by W_{in} in the diagrams above).
Process 23: Constant Pressure Heat Addition
In this process, heat is added at constant pressure from an external heat source. Volume increases from V_{2} to V_{3}, temperature increases from T_{2} to T_{3} and entropy increases from s_{2} to s_{3}.
Heat added in process 23 is given by
Q_{in} = mC_{p}(T_{3} − T_{2}) kJ ………… (i)
where,
m → Mass of air in kg
C_{p} → Specific heat at constant pressure in kJ/kgK
T_{2} → Temperature at point 2 in K
T_{3} → Temperature at point 3 in K
Process 34: Isentropic Expansion
Here 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 from p_{3} to p_{4}, temperature decreases from T_{3} to T_{4}, and volume increases from V_{3} to V_{4}. Entropy remains constant (i.e., s_{3} = s_{4}). Work is done by the system in this process (denoted by W_{out} in the pV and Ts diagrams above).
Process 41: Constant Volume Heat Rejection
In this process, heat is rejected at constant volume (V_{4} = V_{1}). Pressure decreases from P_{4} to P_{1}, temperature decreases from T_{4} to T_{1} and entropy decreases from s_{4} to s_{1}.
Heat rejected in process 41 is given by
Q_{out} = mC_{v}(T_{4} − T_{1}) kJ ………… (ii)
where,
m → Mass of air in kg
C_{v} → Specific heat at constant volume in kJ/kgK
T_{2} → Temperature at point 2 in K
T_{3} → Temperature at point 3 in K
For a good understanding of every process, refer the pV and Ts diagrams above
Airstandard Efficiency of Diesel Cycle:
Airstandard efficiency (or thermal efficiency) of diesel cycle is given by:
\(\mathrm{\large{\eta\,_{Th}=\eta\,_{Diesel}=}\Large{\frac{Heat\; Added\; \; Heat\; Rejected}{Heat\; Added}} \large{ \; \times \; 100 \; \%}}\)
\(\mathrm{\large{\eta\, _{Diesel}=}\Large{\frac{Q_{in}\; \; Q_{out}}{Q_{in}}}\large{ \; \times \; 100 \; \%}}\)
From equations (i) and (ii)
\({\mathrm{\large{\eta\, _{Diesel}=}\Large{\frac{mC_{p}\;(T_{3} \; – \; T_{2})\; – \;mC_{V}\;(T_{4}\; – \; T_{1})}{mC_{p}\;(T_{3} \; – \;T_{2})}}} \large {\; \times \; 100 \; \%}}\)
\(\mathrm{\large{\eta\, _{Diesel}=}\LARGE{(}\large{ 1\; \;} \Large{\frac{mC_{V}\;(T_{4}\; \;T_{1})}{mC_{p}\;(T_{3}\; \; T_{2})}}\LARGE{)} \large {\; \times \; 100 \; \%}}\)
\(\mathrm{\large{\eta\, _{Diesel}=}\LARGE{(}\large{ 1\; \;} \Large{\frac{C_{V}\;(T_{4}\; \;T_{1})}{C_{p}\;(T_{3}\; \; T_{2})}}\LARGE{)} \large {\; \times \; 100 \; \%}}\)
\(\mathrm{\large{\eta\, _{Diesel}=}\LARGE{(}\large{ 1\; \;}\Large{\frac{1}{\gamma}}\Large{\frac{(T_{4}\; \;T_{1})}{ (T_{3}\; \;T_{2})}}\LARGE{)} \large{ \; \times \; 100 \; \% \; \; \; \; \; ( Since,} \Large{\frac{C_{p}}{C_{V}}}=\large{\gamma }\large{\;\Rightarrow\;} \Large{\frac{C_{V}}{C_{p}}=\frac{1}{\gamma}})}\)
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