A monoatomic ideal gas is taken through a cycle ABCA as shown. The efficiency of the cycle is
Work done in the cycle = Area under PV graph =12×P×2V=PV
As process AB is isochoric QAB=nCvΔT=CvRV(2P−P)=3PV2
As process BC is isobaric QBC=nCPΔT=CpR2P(3V−V)=10PV
For process CA, ΔU decreases as temperature decreases and ΔW decreases as Volume decreases. Hence QCA decreases(heat is rejected in this process)
Total heat supplied is Q=QAB+QBC=23PV/2
Efficiency = WorkdoneQ η=WQ=2PV23PV×100=8.7%