In this paper, a 2/4-stroke switchable secondary expansion gasoline IC engine with two combustion cylinders and two expansion cylinders is proposed.The combustion cylinders work on the traditional 4-stroke working cycle,and the expansion cylinders work without combustion,whose operation could be switched between 2-stroke and 4-stroke just by varying the valve timing to perform different secondary expansion modes.For the 2/4-stroke switching has already been realized in the previous studies,this paper will not discuss the stroke switching method.