A kind of technology employed in internal combustion engines, variable valve timing (VVT) allows for the modulation of various aspects of intake or exhaust valves — including lift, duration, and timing — while the engine is running. The purpose of variable valve timing is to alter engine characteristics as needed, to optimize performance and economy under changing driving conditions. It has increased in popularity among car manufacturers as a way to meet tighter emissions standards, while continuing to meet customer demand for power.
In an internal combustion engine, air and vaporized fuel is supplied to the combustion chamber by parts known as the cams. These are located along the camshaft, and open the valves for a certain amount of time during each intake and exhaust cycle. The height that the valves are opened is known as lift, the length they are opened is known as duration, and the synchronization between intake and exhaust valves opening is known as timing.
In traditional non-VVT engine configurations, the position and shape, collectively known as the profile, of the cams and camshaft is static, and set for a certain revolutions-per-minute (RPM). This is typically a balance between low-RPM torque and high-RPM power. Variable valve timing allows cam profiles to change dynamically to optimize torque at low RPMs and horsepower at high RPMs. Most VVT systems work by advancing or retarding the timing of the intake or exhaust valves.
Variable valve timing has existed as a viable technology in automobiles since the 1960s, when patents for VVT were first filed. It was not embraced on a large scale, however, until the 1980s, when Japanese automakers began adopting variable valve timing in production cars. The first production vehicle to feature a VVT engine was the 1987 Nissan 300ZR.
The most prolific implementation of VVT is Honda's Variable Valve Timing and Lift Electronic Control (VTEC), which was finalized and put into use beginning in 1989. VTEC® was initially capable of modulating between a low engine speed setting for fuel economy, and a performance setting that engaged at high engine speeds to improve horsepower. Honda's updated VVT technology, known as i-VTEC®, can alter intake valve timing continuously and is not limited to two specific profiles. Since the introduction of VTEC by Honda, nearly all major car manufacturers have developed and now produce variable valve timing engines.