Flywheel, a disk-shaped part with large moment of inertia, acts as an energy storage. For a four stroke engine, work is done once every four piston strokes, that is, work is done only by the stroke, while work is consumed by the three strokes of exhaust, intake and compression. Therefore, the output torque of crankshaft changes periodically, and the crankshaft speed is unstable. In order to improve this situation, a flywheel is installed at the rear end of the crankshaft. At the power output end of crankshaft, that is, the side where the transmission and the connecting power equipment are connected. The main function of flywheel is to store the energy and inertia outside the engine working stroke. Only one stroke of four stroke engine is used to absorb, compress and exhaust energy from the energy stored by flywheel. The balance is corrected. The balance of the engine is mainly based on the balancing block on the axle. The single cylinder engine has the balance shaft. Flywheel has a large moment of inertia. Because the work of each cylinder of the engine is discontinuous, the engine speed is also changing. When the engine speed increases, the kinetic energy of flywheel increases and the energy is stored; When the engine speed decreases, the kinetic energy of flywheel is reduced and the energy is released. Flywheel can be used to reduce the speed fluctuation during engine operation.