$$
\begin{aligned}
&Q_{1}=\frac{C_{p}}{R} p_{1}\left(V_{2}-V_{1}\right), Q_{2}^{\prime}=\frac{C_{p}}{R} p_{2}\left(V_{3}-V_{4}\right) \\
&\text { So } \quad \eta=1-\frac{p_{2}\left(V_{3}-V_{4}\right)}{p_{1}\left(V_{2}-V_{1}\right)} \\
&\text { Now } p_{1}=n p_{2}, p_{1} V_{2}^{\gamma} \text { or } V_{3}=n \frac{1}{\gamma} V_{2} \\
&p_{2} V_{4}^{\gamma}=p_{1} V_{1}^{\gamma} \text { or } V_{4}=n^{\frac{1}{\gamma}} V_{1} \\
&\text { so } \eta=1-\frac{1}{n} \cdot n^{\frac{1}{y}}=1-n^{\frac{1}{\gamma}-1}
\end{aligned}
$$