An admission valve is a device used in internal combustion engines to control the flow of air or fuel-air mixture into the engine's cylinders during the intake stroke. The admission valve opens and closes at precise times as the piston moves up and down in the cylinder to allow the air or fuel-air mixture to enter the cylinder, while preventing it from escaping during compression and combustion. Proper timing and operation of the admission valve are critical for the efficient operation and performance of the engine. Different types of admission valves include poppet valves, rotary valves, and reed valves, among others.