Engine is a machine that converts chemical energy into mechanical energy. Its conversion process is actually a process of working cycle. Simply speaking, it generates kinetic energy by burning fuel in the cylinder, which drives the reciprocating motion of piston in the cylinder of engine, thereby driving the connecting rod connected to the piston and the crank connected to the connecting rod. Circumferential motion around the center of the crankshaft and output power. Let's analyze this process: a working cycle consists of four piston strokes (the so-called piston stroke refers to the distance between the piston from the top dead point to the bottom dead point): intake stroke, compression stroke, expansion stroke (work stroke) and exhaust stroke.