A heat engine operates by taking n moles of an ideal gas through the cycle ABCDA shown in figure. The thermal efficiency of the engine is  (Take Cv = 3R/2, where R is molar gas constant)

Heat is absorbed in the process AB (isochoric) and BC (isobaric). The temperature rise ΔT in AB = (2PoVo-PoVo)/nR and heat absorbed = nCVΔT = 1.5PoVo. The temperature rise ΔT in BC = (4PoVo-2PoVo)/nR and heat absorbed = nCPΔT = 5PoVo. Total heat absorbed = 6.5PoVo. Work done is area ABCD = PoVo. Thermal efficiency  = PoVo/6.5PoVo = 0.15