Variable Valve Timing
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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.
Overview
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, like 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 cams at a certain engine RPM. Still others can alter duration and lift continuously, which is called Continuous variable valve timing or CVVT.
History
General Motors patented the first variable valve timing and lift or smart valve system in September of 1975. 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.Fiat had employed the first functional variable valve timing system, including variable lift. Developed by Giovanni Torazza in the 1970s, the system used hydraulic pressure to vary the fulcrum of the cam followers. The hydraulic pressure changed according to engine speed and intake pressure. The typical opening variation was 37%.
The next big 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 profile is comparable to the profile of a race cam and comes into operation at high engine speeds to provide a large 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/Civic Hatchback available in Japan and Europe. In 1991 the Acura/Honda NSX became the first VTEC equipped vehicle available in the US.
In another development in 1991, Clemson University researchers had developed and patented the Clemson Camshaft which improved fuel economy. American Society of Mechanical Engineers (1991-12-01), Clemson Camshaft improves auto economy, Mechanical Engineering-CIME The Clemson Camshaft system provided a more continuously variable system. One cam shaft rotates inside the other creating infinitely more settings. Similarly, General Motors, the maker of Cadillac employed an advanced technology to develop a continuously variable system for the Cadillac Northstar System, VVT (Variable Valve Timing). The NorthStar VVT provides continuously variable system throughout the RPM range. The overhead cam (OHC) Cadillac NorthStar, already a fuel efficient V-8 engine, became even more efficient with VVT. GM engines use the double overhead cam varying both intake and exhause for better performance. In 2005, General Motors offered the first Variable Valve timing system for overhead valve V6 engines, LZE and LZ4. The 2007 Cadillac Escalade with Varable Valve Timing has 403 horsepower, making it not only the most powerful, but also the most fuel efficient vehicle in its class.
In 1992, BMW introduced VANOS, their version of a variable valve timing system, on the BMW M50 engine used in the 3 Series. VANOS significantly enhances emission management, increases output and torque, and offers better idling quality and fuel economy. The latest version of VANOS is double-VANOS, used in the new M3. Double-VANOS adds an adjustment of the intake and exhaust camshafts.
Ford became the first manufacturer to use variable valve timing in a pickup-truck, with the top-selling Ford F-series in the 2004 model year. The engine used was the 5.4L 3-valve Triton.
VVT Implementations
- Alfa Romeo Twin Spark - TS stands for "Twinspark" engine, it is equipped with Variable Valve Timing technology.
- BMW VANOS - Varies intake and exhaust timing by rotating the camshaft in relation to the gear.
- Ford Variable Cam Timing - Varies valve timing by rotating the camshaft
- GM DCVCP (Double Continuous Variable Cam Phasing) - Varies timing with hydraulic vane type phaser (see also Ecotec LE5).
- Honda VTEC - Varies intake, duration, and lift by using two different sets of cam lobes
- Honda i-VTEC - Adds cam phasing (timing) to traditional VTEC
- Hyundai MPI CVVT Varies power, torque, exhaust system, and engine response
- Mazda S-VT - Varies timing by rotating the camshaft
- Mitsubishi MIVEC - Varies valve timing and lift
- Nissan N-VCT - Varies the rotation of the cam(s) only, does not alter lift or duration of the valves.
- Nissan VVL - Varies intake, duration, and lift by using two different sets of cam lobes
- Porsche VarioCam - Varies intake timing by adjusting tension of a cam chain
- Porsche VarioCam Plus - Varies intake timing by adjusting tension of a cam chain as well as valve lift by different cam profiles
- Proton Campro - Still under development, said to be based on Lotus technology which developed Porsche's VarioCam
- PSA Peugeot Citroën CVVT - Continuous variable valve timing
- Rover VVC - Varies timing with an eccentric disc
- Suzuki VVT
- Subaru AVCS - Varies timing (phase) with hydraulic pressure
- Toyota VVT-i - Varies intake timing by advancing the cam chain
- Toyota VVTL-i - Varies timing by advancing the cam chain and switching between two sets of cam lobes
- Volkswagen VVT
- Volvo VVT
See also
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