$\mathrm{CdT}=\mathrm{C}_{\mathrm{V}} \mathrm{dT}+\mathrm{PdV}$ $....(1)$
$\therefore \quad \mathrm{PV}^2=\mathrm{RT}$
$\quad \mathrm{P}=\text { constant }$
$\quad \mathrm{P}(2 \mathrm{VdV})=\mathrm{RdT}$
$\quad \mathrm{PdV}=\frac{\mathrm{RdT}}{2 \mathrm{~V}}$
Put in equation $(1)$
$\mathrm{C}=\mathrm{C}_{\mathrm{V}}+\frac{\mathrm{R}}{2 \mathrm{~V}}$
$1.$ efficiency more than $27 \%$
$2.$ efficiency less than the efficiency a Carnot engine operating between the same two temperatures.
$3.$ efficiency equal to $27 \%$
$4.$ efficiency less than $27 \%$
