An innovative 2/4-stroke secondary expansion IC engine is proposed in this paper. The expansion cylinders of this engine could switch its operation between 2 stroke and 4 stroke working mode to perform different secondary expansion modes just by varying the valve timing. For the 2-stroke secondary expansion working mode, two expansion cylinders work as one cylinder to inlet and exhaust at the same time, so the burned gas of one combustion cylinder could be exhaust into two expansion cylinders to get fully expansion. For the 4-stroke secondary expansion working mode, the burned gas of one combustion cylinder is only transferred into on expansion cylinder, and it goes through four strokes in the expansion cylinder before it is exhaust out of the engine. According to the experiment and simulation study, the following conclusion can be drawn