A valve generally, is used to control the timing and quantity of gas or vapor flowing into an engine. It consists of a hole which is normally round or oval and a tapered plug which is usually of disk shape on the end of a shaft and is also termed valve stem. The part of the hole where the plug meets with it is known as the 'seat' or 'valve seat'. The shaft guides the plug portion by sliding through a valve guide. In exhaust applications, to seal the valve pressure differential is used and also, in intake valves a pressure differential is used to open it. The valves are in use since 1770s, James Watt used them in his steam engines.
The valves used in the automobiles are generally poppet valves. This Poppet Valve is fundamentally different from a slide or an oscillating valve as instead of sliding or rocking over a seat to uncover a port this valve lifts from the seat with a perpendicular movement to the port. The main advantage of using the Poppet Valve is that it has no movement on the seat, thereby, no need for lubrication. The operating principle of Poppet Valves was described in the online article "How Poppet Valves Work". In many cases, it is better to have a "balanced poppet" in a direct – acting valve. Little force is required to move the poppet as all the forces on the poppet are cancelled by equal and opposite forces. The solenoid coil only has to counteract the spring force.
Valves are used in most of the piston engines for opening and closing the intake and also, to exhaust ports in the cylinder head. The valve is generally a flat disk of metal with a long rod referred to as the valve stem attached to its one side.