An engine using 1 mol of an ideal gas initially at 22.3 L and 454 K performs a cycle consisting of four steps:
1) an isothermal expansion at 454 K from 22.3 L to 40 L ;
2) cooling at constant volume to 270 K ;
3) an isothermal compression to its original volume of 22.3 L; and 4) heating at constant volume to its original temperature of 454 K
Find its efficiency. Assume that the heat capacity is 21 J/K and the universal gas constant is 0.08206 L · atm/mol/K = 8.314 J/mol/K.