In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event. It is often used to improve performance, fuel economy, or emissions. It is increasingly being used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and camless systems. Increasingly strict emissions regulations are causing many automotive manufacturers to use VVT systems.
Two-stroke engines use a power valve system to get similar results to VVT.