According to the the camshaft number of how many, can be divided into single overhead camshaft (SOHC) and double overhead camshaft (DOHC), two kinds. The single overhead camshaft is only one camshaft, double overhead camshaft is two, this is too straightforward explanation.
    The single overhead camshaft with a camshaft in the cylinder head, direct drive into the exhaust valve, it has a simple structure, suitable for high-speed engine. Generally used in the past side camshaft, the camshaft in the cylinder side, is driven directly by a timing gear. The valve lifter to the rotation of the camshaft is converted into reciprocating motion of the valve must be used to transfer power. Thus, more parts of the reciprocating motion, the inertial mass, is not conducive to high-speed movement of the engine. Moreover, the slender tappet has a certain degree of flexibility, prone to vibration, accelerated component wear, even the valve control is lost.
    DOHC cylinder head equipped with two camshafts, one is used to drive the intake valve, the other for driving the exhaust valve. Double overhead camshaft camshaft and valve spring design less demanding, especially for the hemispherical combustion chamber of the valve V-shaped configuration, but also facilitate and used in conjunction with four-valve gas distribution agencies.