Poppet valves and spool valves

A poppet valve is a valve typically used to control the timing and quantity of gas or vapor flow into an engine. It consists of a hole or open-ended chamber, usually round or oval in cross-section, and a plug, usually a disk shape on the end of a shaft known as a valve stem. A pressure differential on either side of the valve can assist or impair its performance. In exhaust applications higher pressure against the valve helps to seal it, and in intake applications lower pressure helps open it.
Spool valves are normally designed so that the leakage in the spool is small compared to the hydraulic system flow rate. Spool valves regulate the flow of fluid in hydraulic systems. Spool valves would slide backward and forward so that the fluid flow can be either in one direction or another around a circuit of pipes. Spool valves have two basic components, a cylindrical barrel in which slides a plunger or spool.