The valve train consists of lifters, push rods, rocker arms, valves and camshaft(s). The opening /closing and duration of the valve train, as well as its geometry, control the amount of air and fuel entering the combustion chamber at any instant. Timing for opening /closing or duration is controlled by the camshaft which is synchronized to the crankshaft via gear, belt or chain.
Valve trains are built in many configurations and each of it varies slightly in layout yet they still perform the task of opening and closing the valves at the time required for proper operation of the engine. These layouts can be distinguished by the location of the camshaft within the engine like the overhead camshaft where the camshaft (or camshafts which depends on the design employed) is located above the valves inside the cylinder head, and it operates either directly or indirectly on the valves. Secondly, we have Cam – In – Block in which the camshaft is located within the engine block which operates directly on the valves, or indirectly by pushrods and rocker arms and as they often require the pushrods they are quite often referred to as pushrod engines. Lastly, there is Camless layout which uses no camshafts at all. Technologies such as solenoids are generally used to individually actuate the valves.
As valves are usually of the poppet type many other types have been developed such as rotary, slide and sleeve valves. These Poppet valves generally requires small coil springs, named correctly valve springs, so as to keep them shut when not actuated by the camshaft. They are joined to the valve stem ends, adjacent to spring retainers. There are other mechanisms which can be used instead of valve springs to keep the valves shut: like Formula 1 engines employ pneumatic valve springs in which pneumatic pressure closes the valves whereas motorcycle manufacturer Ducati uses desmodromic: a system used to mechanically close the valves.