Variable Valve Timing explain

Variable valve timing (VVT) is a process of altering the timing of a valve lift event in internal combustion engines. It is used to improve performance, fuel economy or emissions. VVT allows different overlap periods to be used at different engine speeds. VVT systems were developed in the 1960s to improve volumetric efficiency, decrease NOx emissions, and decrease pumping losses. There are two major types of VVT: cam phasing and cam changing.