In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and 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. Background theory The valves within an internal combustion engine are used to control the flow of the intake and exhaust gases into and out of the combustion chamber. The timing, duration and lift of these valve events has a significant impact on engine performance. Without variable valve timing or variable valve lift, the valve timing is the same for all engine speeds and conditions, therefore compromises are necessary. An engine equipped with a variable valve timing actuation system is freed from this constraint, allowing performance to be improved over the engine operating range. Piston engines normally use valves which are driven by camshafts. The cams open (lift) the valves for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing, relative to the position of the crankshaft, is important. The camshaft is driven by the crankshaft through timing belts, gears or chains. An engine requires large amounts of air when operating at high speeds. However, the intake valves may close before enough air has entered each combustion chamber, reducing performance. On the other hand, if the camshaft keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. Opening the intake valve while the exhaust valve is still open may cause unburnt fuel to exit the engine, leading to lower engine performance and increased emissions. According to engineer David Vizard's book "Building Horsepower", when both intake & exhaust are open simultaneously, the much-higher-pressure exhaust pushes the intake-charge back, out from the cylinder, polluting the intake-manifold with exhaust, in worst cases. Continuous versus discrete Early variable valve timing systems used discrete (stepped) adjustment. For example, one timing would be used below 3500 rpm and another used above 3500 rpm. More advanced "continuous variable valve timing" systems offer continuous (infinite) adjustment of