The Diesel cycle idealizes a compression-ignition reciprocating engine using an ideal gas.

Its four internally reversible processes are:

1. isentropic compression;
2. constant-pressure heat addition;
3. isentropic expansion;
4. constant-volume heat rejection.

In addition to compression ratio $r=v_1/v_2$, the cycle uses the cutoff ratio

$$r_c=\frac{v_3}{v_2},$$

which measures how far the gas expands while heat is added at constant pressure.

With constant specific heats,

$$\eta_{Diesel}=1-\frac{1}{r^{k-1}}\frac{r_c^k-1}{k(r_c-1)}.$$

The formula shows that efficiency depends on both compression and heat-addition geometry.

The air-standard Diesel cycle does not model fuel injection or combustion chemistry explicitly. It replaces those complex processes with idealized heat addition so that the influence of compression ratio, expansion and heat-rejection structure can be studied thermodynamically.