Science Fair Project Encyclopedia
Variable valve timing
Variable valve timing, or VVT, is a generic term for an automobile piston engine technology. VVT allows the lift or duration and timing (or all) of the intake or exhaust valves (or both) to be changed while the engine is in operation.
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 profile of these cams 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.
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.
The first experimentation with variable valve timing and lift was performed by General Motors. GM was actually 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.
The first functional variable valve timing system, including variable lift, was developed at Fiat. Developed by Giovanni Torazza in the 1970's, 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 by Honda in the 1980s and 90s, where Honda began by experimenting with variable valve lift. Pleased with the results, engineers took the knowledge and applied it to the B16A engine, fitted to the 1989 EF9 Honda Civic. From there it has been used in a variety of applications, from sport to utility, by many different auto makers.
- BMW VANOS - Varies intake and exhaust timing by moving the fulcrum of the camshaft
- 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
- 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/Kia CVTT
- Mazda S-VT - Varies timing by rotating the camshaft
- Mitsubishi MIVEC - Varies valve lift
- 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
- 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
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