Discrete variable valve timing and continues variable valve timing
The early type of VVT used stepped or discrete adjustment. For example “Cam switching”, one valve timing used below 3000rpm and another valve timing used above 3000rpm. The modern advance VVT uses “continuous variable valve timing" adjustment. Continuous VVT offer a continues or infinite number of adjustment of valve timing in the operating speed range of engine, and therefore they can be optimized to use all engine condition and speed. For example “Oscillating cam”.
Cam phasing VVT and variable duration VVT
Cam phasing is the simplest form of VVT, in which the phase angle of camshaft rotated forward or backward relative to the crankshaft to get lead and lag in valve movement. This arrangement changes the opening and closing angle (timing) of valves, but do not alter the duration of valve operations. Variable duration VVT is achieved by using more complex system, such as oscillating cams, multiple cam profiles etc.