0.2 kmol of oxygen in a piston-cylinder assembly undergoes a cycle of three processes in series as follows: Process 12, Constant pressure at 5 bar from V1 = 2 m³ to V2 = 1 m³ ; Process 23, Constant volume to p3 = 8 bar; Process 31, Polytropic process with Vpn = const. Assuming ideal gas behavior and neglecting kinetic and potential energy effects, determine the thermal efficiency of this cycle. Note that the index n for process 31 needs to be determined from the given conditions.