The engine valve is a very important part of the engine. Its principle is to realize the normal operation of the engine by controlling the air entering and exiting the cylinder. The working principle of the engine valve includes three situations:

1. Cylinder retraction: When there is a rod cavity for air intake and no rod cavity for exhaust, the piston rod retracts. If there is a rod cavity and a rod-less cavity to alternately intake and exhaust, the piston realizes reciprocating linear motion.

2. Cylinder extension: When compressed air is input from the rod-less cavity, the rod cavity is exhausted, and the pressure difference between the two cavities of the cylinder acts on the piston to push the piston to move and make the piston rod extend.

3. The intake and exhaust valves are closed at the same time: the piston moves from bottom dead center to top dead center, and the crankshaft rotates 180 °. At this time, the pressure and temperature of the mixed gas in the cylinder are continuously increased after being compressed. When the compression end is reached, the pressure can reach 800~2 000kPa and the temperature can reach 600~750K. On the indicator diagram, the compression stroke is curve a ~ c.

In general, the working principle of the engine valve is to realize the normal operation of the engine by controlling the airflow in the cylinder.