Steam engine is an engine whose power is generated from vapor. In the early stage, it was broadly used in industrial factories and transportation. It also played an important role in the era of Industrial Revolution. The shown horizontal steam engine model has a pipe on its left for connecting the boiler to introduce the vapor into its cylinder. Then the introduced vapor can force the piston back and forth. The piston is linked with a linkage that is used to transmit piston power to the flywheel (or wheel) on its right. Besides, there also exists a linkage that is used to control the intake valve inside the engine. A bar of the linkage is designated for controlling the open and close of the intake valve. When the bar is at right, the valve is opened from which the vapor can enter into the engine; when the bar is at left, the valve is closed from which the vapor is stopped outside the engine. While the vapor is permitted for entrance, the valve will move right and left alternately according to the piston timing to open the right and left pipes, respectively. Thus it can promote a periodically reciprocal push force on the piston, that causes the forward and the backward stroke are both of the power strokes.