An air-standard Diesel cycle consists of the following processes:

$1$-$2$: Air is compressed isentropically.

$2$-$3$: Heat is added at constant pressure.

$3$-$4$: Air expands isentropically to the original volume.

$4$-$1$: Heat is rejected at constant volume.

If $\gamma$ and $T$ denote the specific heat ratio and temperature, respectively, the efficiency of the cycle is

- $1-\displaystyle{\frac{T_4-T_1}{T_3-T_2}}$
- $1-\displaystyle{\frac{T_4-T_1}{\gamma (T_3-T_2)}}$
- $1-\displaystyle{\frac{\gamma (T_4-T_1)}{T_3-T_2}}$
- $1-\displaystyle{\frac{T_4-T_1}{(\gamma -1)(T_3-T_2)}}$