The exhaust strokes of the combustion cylinders are separated by 360องศา CA. If the expansion ylinders work as the 2-stroke working mode, The valves of these two cylinders open and close simultaneously. So the burned gas can be exhaust into these two expansion cylinders at the same time, as shown in รูป(3a.) และ รูป(4). In this way,Two expansion cylinders work as one cylinder to realize the 2-stroke secondary expansion mode, The intake strokes would be also separated by 360องศา CA.When the cylinder#1 exhausts, Cylinder #2 is right on the intake stroke, and when the cylinder #4 exhausts, cylinder #3 is right on the intake stroke,