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. The main factor preventing this technology from wide use in production automobiles is the ability to produce a cost effective means of controlling the valve timing under the conditions internal to an engine. An engine operating at 3000 revolutions per minute will rotate the camshaft 25 times per second, so the valve timing events have to occur at precise times to offer performance benefits. Electromagnetic and pneumatic camless valve actuators offer the greatest control of precise valve timing, but, in 2014, are not cost effective for production vehicles.
Early and late exhaust valve closing can also reduce emissions. Traditionally, the exhaust valve opens, and exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it travels upward. By manipulating the timing of the exhaust valve, engineers can control how much exhaust gas is left in the cylinder. By holding the exhaust valve open slightly longer, the cylinder is emptied more and ready to be filled with a bigger air/fuel charge on the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions.Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include: AVCS (Subaru), AVLS (Subaru), CPS (Proton, Volvo), CVTCS (Nissan, Infiniti), CVVT (Alfa Romeo, Citroën, Geely, Hyundai, Iran Khodro, Kia, Peugeot, Renault, Volvo), DCVCP - dual continuous variable cam phasing (General Motors), DVVT (Daihatsu), MIVEC (Mitsubishi), N-VCT (Nissan), S-VT (Mazda), VANOS (BMW) ,VarioCam (Porsche), VCT (Ford, Yamaha), i-VTEC (Honda), VVC (MG Rover), VVL (Nissan), Valvelift (Audi), VVEL (Nissan, Infiniti), VVT (Chrysler, General Motors, Proton, Suzuki, Volkswagen Group), VVT-i (Toyota, Lexus), VTVT (Hyundai. Kia)
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 the valve timing. Therefore the timing can be optimized to suit all engine speeds and conditionsThe simplest form of VVT is cam-phasing, where the angle of a camshaft is rotated forwards or backwards (relative to the crankshaft). Thus the valves open and close earlier or later; however, the camshaft lift and duration cannot be altered with a cam-phasing system. Achieving variable duration on a VVT system requires a more complex system, such as multiple cam profiles or oscillating cams.