Variable Valve Timing, or VVT, is a generic term for an automobile piston engine technology. VVT allows the lift or duration or timing (some or all) of the intake or exhaust valves (or both) to be changed while the engine is in operation. Two-stroke engines use a Power valve system to get similar results to VVT.
Piston engines normally use poppet valves for intake and exhaust. These are driven (directly or indirectly) by cams on a camshaft. The cams open the valves (lift) for a certain amount of time (duration) during each intake and exhaust cycle. The timing of the valve opening and closing is also important. The camshaft is driven by the crankshaft through timing belts, gears or chains.
The profile, or position and shape of the cam lobes on the shaft, is optimized for a certain engine rpm, and this tradeoff normally limits low-end torque or high-end power. VVT allows the cam profile to change, which results in greater efficiency and power.
At high engine speeds, an engine requires large amounts of air. However, the intake valves may close before all the air has been given a chance to flow in, reducing performance.
On the other hand, if the cam keeps the valves open for longer periods of time, as with a racing cam, problems start to occur at the lower engine speeds. This will cause unburnt fuel to exit the engine since the valves are still open. This leads to lower engine performance and increased emissions.
Pressure to meet environmental goals and fuel efficiency standards is forcing car manufacturers to turn to VVT as a solution. Most simple VVT systems (like Mazda's S-VT) advance or retard the timing of the intake or exhaust valves. Others (like Honda's VTEC) switch between two sets of cam lobes at a certain engine RPM. Still others (like BMW's Valvetronic) can alter timing and lift continuously, which is called Continuous variable valve timing or CVVT.
Fiat was the first auto manufacturer to patent a functional variable valve timing system which included variable lift. Developed by Giovanni Torazza in the late 1960s, the system used hydraulic pressure to vary the fulcrum of the cam followers (US Patent 3,641,988). The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.
In September 1975, General Motors patented a system intended to vary valve lift. GM was interested in throttling the intake valves in order to reduce emissions. This was done by minimizing the amount of lift at low load to keep the intake velocity higher, thereby atomizing the intake charge. GM encountered problems running at very low lift, and abandoned the project.
Alfa Romeo was the first manufacturer to use a variable valve timing system in production cars (US Patent 4,231,330). The 1980 Alfa Romeo Spider 2.0 L had a mechanical VVT system in Spica fuel injected cars sold in the USA. Later this was also used in the 1983 Alfetta 2.0 Quadrifoglio Oro models as well as other cars.
In 1986, Nissan developed their own form of VVT with the VG30DE(TT) engine for their Mid-4 Concept. Nissan chose to focus their NVCS (Nissan Valve-Timing Control System) mainly at low and medium speed torque production because the vast majority of the time, engine RPMs will not be at extremely high speeds. The NVCS system can produce both a smooth idle, and high amounts of low and medium speed torque. Although it can help a little at the top-end also, the main focus of the system is low and medium range torque production. The VG30DE engine was first used in the 300ZX (Z31) 300ZR model in 1987, this was the first production car to use electronically controlled VVT technology.
Honda's VTEC motorcycle engine employed on the Japanese market-only Honda CBR400F in 1983 provided as a technology basis for VTEC. The next step was taken in 1989 by Honda with the VTEC system. Honda had started production of a system that gives an engine the ability to operate on two completely different cam profiles, eliminating a major compromise in engine design. One profile designed to operate the valves at low engine speeds provides good road manners, low fuel consumption and low emissions output. The second is a high lift, long duration profile and comes into operation at high engine speeds to provide an increase in power output. The VTEC system was also further developed to provide other functions in engines designed primarily for low fuel consumption. The first VTEC engine Honda produced was the B16A which was installed in the Integra, CRX, and Civic hatchback available in Japan and Europe. In 1991 the Acura/Honda NSX powered by the C30A became the first VTEC equipped vehicle available in the US. VTEC can be considered the first "cam switching" system and is also one of only a few currently in production.
In 1991, Clemson University researchers patented the Clemson Camshaft which was designed to provide continuously variable valve timing independently for both the intake and exhaust valves on a single camshaft assembly. This ability makes it suitable for both pushrod and overhead cam engine applications.<ref>American Society of Mechanical Engineers (1991-12-01), Clemson Camshaft improves auto economy, Mechanical Engineering-CIME </ref>
In 1992 BMW introduced the VANOS system. Like the Nissan NVCS system it could provide timing variation for the intake cam in steps (or phases), the VANOS system differed in that it could provide one additional step for a total of three. Then in 1998 the Double Vanos system was introduced which significantly enhances emission management, increases output and torque, and offers better idling quality and fuel economy. Double Vanos was the first system which could provide electronically controlled, continuous timing variation for both the intake and exhaust valves. In 2001 BMW introduced the Valvetronic system. The Valvetronic system is unique in that it can continuously vary intake valve lift, in addition to timing for both the intake and exhaust valves. The precise control the system has over the intake valves allows for the intake charge to be controlled entirely by the intake valves, eliminating the need for a throttle valve and greatly reducing pumping loss. The reduction of pumping loss accounts for more than a 10% increase in power output and fuel economy.
In 2007 DaimlerChrysler became the first manufacturer to produce a cam-in-block engine with independent control of exhaust cam timing relative to the intake. The 2008 Dodge Viper uses Mechadyne's concentric camshaft assembly to help boost power output to 600 Bhp.
- Aftermarket Modifications - Conventional hydraulic tappet can be engineered to rapidly bleed-down for variable reduction of valve opening and duration.
- Alfa Romeo Twin Spark - TS stands for "Twinspark" engine, it is equipped with Variable Valve Timing technology.
- BMW Valvetronic - Provides continuously variable lift for the intake valves; used in conjunction with Double VANOS.
- BMW VANOS - Varies intake timing by rotating the camshaft in relation to the gear.
- BMW Double VANOS - Continuously varies the timing of the intake and exhaust valves.
- Ford Variable Cam Timing - Varies valve timing by rotating the camshaft.
- DaimlerChrysler - Varies valve timing thought the use of concentric camshafts developed by Mechadyne enabling dual-independent inlet/exhaust valve adjustment on the 2008 Dodge Viper.
- GM VVT - Varies valve timing continuously throughout the RPM range for both intake and exhaust for improved performance in both overhead valve and overhead cam engine applications.(See also Northstar System).
- GM DCVCP (Double Continuous Variable Cam Phasing) - Varies timing with hydraulic vane type phaser (see also Ecotec LE5).
- Holden Alloytec - Continuously variable camshaft phasing for inlet cams. Continuously variable camshaft phasing for inlet cams and exhaust cams (High Output Alloytec).
- Honda VTEC - Varies duration, timing and lift by switching between two different sets of cam lobes.
- Honda i-VTEC - In high-output DOHC 4 cylinder engines the i-VTEC system adds continuous intake cam phasing (timing) to traditional VTEC. In economy oriented SOHC and DOHC 4 cylinder engines the i-VTEC system increases engine efficiency by delaying the closure of the intake valves under certain conditions and by using an electronically controlled throttle valve to reduce pumping loss. In SOHC V6 engines the i-VTEC system is used to provide Variable Cylinder Management which deactivates one bank of 3 cylinders during low demand operation.
- Honda VTEC-E - Unlike most VTEC systems VTEC-E is not a cam switching system, instead it uses the VTEC mechanism to allow for a lean intake charge to be used by closing one intake valve under certain conditions.
- Hyundai MPI CVVT - Varies power, torque, exhaust system, and engine response.
- Kawasaki - Varies position of cam by changing oil pressure thereby advancing and retarding the valve timing, 2008 Concours 14.
- Lexus VVT-iE - Continuously varies the intake camshaft timing using an electric actuator.
- Mazda S-VT - Varies timing by rotating the camshaft.
- Mitsubishi MIVEC - Continuously varies valve timing, duration and lift by switching between two different sets of cam lobes. The 4B1 engine series uses a different variant of MIVEC which varies timing (phase) of both intake and exhaust camshafts continuously.
- Nissan N-VCT - Varies the rotation of the cam(s) only, does not alter lift or duration of the valves.
- Nissan VVL - Varies timing, duration, and lift of the intake and exhaust valves by using two different sets of cam lobes.
- Nissan VVT introduced with the HR15DE HR16DE MR18DE MR20DE new engines in September 2004 on the Nissan Tiida and north american version named Nissan Versa (in 2007); and finally the Nissan Sentra (in 2007).
- Porsche VarioCam - Varies intake timing by adjusting tension of a cam chain.
- Porsche VarioCam Plus - Varies intake valve timing by rotating the cam in relation to the cam sprocket as well as duration, timing and lift of the intake and exhaust valves by switching between two different sets of cam lobes.
- Proton Campro CPS - said to be based on Lotus technology which developed Porsche's VarioCam. Varies intake valve timing and lift by switching between 2 sets of cam lobes without using rocker arms as in most variable valve timing systems. Debuted in the 2008 Proton Gen-2 CPS, 2008 Proton Waja CPS and 2009 Proton Exora.
- PSA Peugeot Citroën CVVT - Continuous variable valve timing.
- Renault Clio 182, Clio Cup and Clio V6 Mk2 VVT - variable valve timing.
- Rover VVC - Varies timing with an eccentric disc.
- Suzuki - VVT - Suzuki M engine
- Subaru AVCS - Varies timing (phase) with hydraulic pressure, used on turbocharged and six-cylinder Subaru engines.
- Subaru AVLS - Varies duration, timing and lift by switching between two different sets of cam lobes (similar to Honda VTEC). Used by non-turbocharged Subaru engines.
- Toyota VVT - Toyota 4A-GE 20-Valve engine introduced VVT in the 1992 Corolla GT-versions.
- Toyota VVT-i - Continuously varies the timing of the intake camshaft, or both the intake and exhaust camshafts (depending on application).
- Toyota VVTL-i - Continuously varies the timing of the intake valves. Varies duration, timing and lift of the intake and exhaust valves by switching between two different sets of cam lobes.
- Volkswagen - VVT introduced with the 1.8T engine. The intake timing intentionally runs advanced and a retard point is calculated by the ECU. A hydraulic tensioner retards the intake timing.
- Volvo - VVT
- Yamaha - VCT (Variable Cam Timing) Varies position of cam thereby advancing and retarding the valve timing.