As previously discussed, there are at least two cylinders to from a working unit for the 4-stroke secondary expansion IC engine, but the 2-stroke secondary expansion IC engine needs at least three cylinders to form a whole working unit. To apply these two secondary expansion modes on one engine, a 4-cylinder engine working process is proposed as shown in รูป(3) to solve the problem of cylinder number difference.There are two combustion cylinders and two expansion expansion cylinders for the 2/4-stroke switchable secondary expansion engine. Both combustion cylinders work on the traditional 4-stroke cycle, and both expansion cylinders can switch its operation between 2-stroke and 4-stroke by just varying the valve timing. The exhaust valve of both combustion cylinders and the intake valves of both expansion cylinders are connected by a chamber to regulate the pressure during the burned gas transferring process