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.

发动机是将化学能转化为机械能的机器，它的转化过程实际上就是工作循环的过程，简单来说就是是通过燃烧气缸内的燃料，产生动能，驱动发动机气缸内的活塞往复的运动，由此带动连在活塞上的连杆和与连杆相连的曲柄，围绕曲轴中心作往复的圆周运动，而输出动力的。我们分析一下这个过程： 　一个工作循环包括有四个活塞行程（所谓活塞行程就是指活塞由上止点到下止点之间的距离的过程）：进气行程、压缩行程、膨胀行程（作功行程）和排气行程。