(and equal to that of each lobe alone) and when at the extreme extent of their misalignment the duration is at a maximum. The basic limitation of the scheme is that only a duration variation equal to that of the lobe nose true radius (in camshaft degrees or double this value in crankshaft degrees) is possible. In practice this type of variable cam has a maximum range of duration variation of about forty crankshaft degrees. This is the principle behind what seems to be the very first variable cam suggestion appearing in the USPTO patent files in 1925 (1527456). The "Clemson camshaft" is of this type. Helical camshaft Also known as "combined two shaft coaxial combined profile with helical movement", this system is not known to be used in any production engines. It has a similar principle to the previous type, and can use the same base duration lobe profile. However instead of rotation in a single plane, the adjustment is both axial and rotational giving a helical or three-dimensional aspect to its movement. This movement overcomes the restricted duration range in the previous type. The duration range is theoretically unlimited but typically would be of the order of one hundred crankshaft degrees, which is sufficient to cover most situations. The cam is reportedly difficult and expensive to produce, requiring very accurate helical machining and careful assembly. Camless engines Engine designs which do not rely on a camshaft to operate the valves have greater flexibility in achieving variable valve timing and variable valve lift. The only production car that uses the camless design so far is the Koenigsegg Gemera. Hydraulic system This system utilizes the engine lube oil to control the closure of inlet valve. The intake valve opening mechanism incorporates a valve tappet and a piston inside a chamber. There is a solenoid valve controlled by the engine control system which gets energized and supplies oil through a non-return valve during the time of cam lift and the oil gets filled in the chamber and the return channel to the sump is blocked by the valve tappet. During the downward movement of the cam, at a particular instant, the return passage opens and the oil pressure gets released to the engine sump. References External links Honda Technology Picture Book, VTEC Delphi Variable Cam Phasers (VCP) Volvo CVVT article MG Rover VVC article MG Rover VVC article (from sandmuseum.com) Mechadyne