Valvetrains

  The valvetrain consists of valves, rocker arms，pushrods，lifters，and camshaft(s).Valvetrain opening/closing and duration, as well as the geometry of the valvetrain, controlthe amount of air and fuel entering the combustion chamber at any given point in time.Timing for open/close/duration is controlled by the camshaft that is synchronized to thecrankshaft by a chain, belt or gear.

    Valvetrains are built in several configurations，each of which varies slightly in layoutbut still performs the task of opening and closing the valves at the time necessary forproper operation of the engine. These layouts are differentiated by the location of thecamshaft within the engine:

    ．Overhead camshafts：The camshaft（or camshafts，depending on the designemployed) is located above the valves within the cylinder head，and operates eitherindirectly or directly on the valves.

    ．Cam-in-block：The camshaft is located within the engine block，and operatesdirectly on the valves，or indirectly via pushrods and rocker arms. Because theyoften require pushrods they are often called pushrod engines.

    ．Camless: This layout uses no camshafts at all. Technologies such as solenoids areused to individually actuate the valves.