Variable valve timing

Variable valve timing, or VVT, is a generic term for an automobile piston engine technology. VVT allows the lift, duration, and timing (or all three) of the intake or exhaust valves (or both) to be changed while the engine is in operation.

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 profile of these cams is optimized for a certain engine speed, 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 speed. Still others can alter duration and lift continuously.

VVT Implementations

• BMW VANOS - Varies intake and exhaust timing and lift by moving the fulcrum of the camshaft
• Ford Variable Cam Timing - Varies valve timing by rotating the camshaft
• GM Variable Valve Timing - Varies timing (phase) with hydraulic pressure
• 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
• Mazda S-VT - Varies timing by rotating the camshaft
• Mitsubishi MIVEC - Varies valve timing and 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
• Rover VVC - Varies timing and lift with an eccentric disc
• 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