Thermodynamic-Fuel and efficiency

Why is the maximum theoretical efficiency of Ar engine higher than that of Nitrogen engine? On a molecular level.


For an Otto Cycle


* $\mathrm{Eff} \le\left(1-r^{1-\gamma}\right)$
* $\gamma=\dfrac{C\_p}{C\_v}$
* $r=\dfrac{v\_1}{v\_2}$


Sys. Fluid 


* Nitrogen, $\gamma=1.4,\ r=10,\ \mathrm{eff(max)}=60.2%$
* Argon, $\gamma=1.67,\ r=10,\ \mathrm{eff(max)}=78.5%$


