An engine using 1 mol of an ideal gas initially at 18.4 L and 250 K performs a cycle consisting of four steps:
1) an isothermal expansion at 250 K from 18.4 L to 40 L
2) cooling and a constant volume to 140 K
3) an isothermal compression to its original volume of 18.4 L
4) heating at constant volume to its original temperature of 250 K
Find its efficiency
