Variable Valve Timing

Variable valve timing, commonly abbreviated VVT, is a technology that allows an engine to alter when its valves open and close relative to the position of the pistons, rather than having that timing fixed for all conditions. Because the ideal moment to open and close the intake and exhaust valves differs greatly between idling, cruising and high-rpm operation, a fixed compromise inevitably leaves performance, economy or emissions short in some part of the range. VVT resolves this by adjusting the timing dynamically as conditions change.

The most common implementation acts on the camshaft through a device called a cam phaser, fitted to the end of the camshaft. Controlled by the engine management system, the phaser uses oil pressure, directed by a solenoid valve, to rotate the camshaft a few degrees forwards or backwards relative to the crankshaft drive. Advancing or retarding the cam in this way shifts the whole valve-opening event earlier or later. On engines with separate intake and exhaust camshafts, each can often be phased independently, giving the control unit fine authority over the crucial period of valve overlap when intake and exhaust valves are open together.

The value of this control is that it optimises several competing goals at once across the rev range. At low speeds, suitable timing improves idle stability and smoothness; in the mid-range it maximises torque and flexibility; at high revs it favours peak power by keeping valves open longer to fill and empty the cylinders; and throughout, well-managed overlap can improve fuel economy and reduce emissions, sometimes by recirculating a controlled amount of exhaust internally. A single engine can therefore feel tractable in town, economical on a cruise and energetic when pressed, behaviour impossible with fixed timing.

More advanced systems go beyond merely phasing the camshaft and also vary the valve lift or the cam profile itself. Honda's celebrated VTEC, for example, switches between separate cam lobes to give a dramatically different valve action at high rpm, while systems such as BMW's Valvetronic vary lift continuously to control engine load without a conventional throttle, reducing pumping losses. These approaches combine timing changes with changes in how far and how long the valves open, extracting still greater gains in efficiency and output.

The technology is so beneficial that it has become near-ubiquitous, and manufacturers market it under a profusion of brand names — Toyota's VVT-i, Honda's VTEC and i-VTEC, BMW's VANOS, and many others — that all describe variants of the same underlying idea. Reliance on oil pressure and condition means VVT systems are sensitive to oil quality and level, and worn phasers or sticking solenoids are recognised faults on higher-mileage engines. Built directly upon the basic valve and camshaft, and frequently paired with multi-valve and double-overhead-camshaft layouts, variable valve timing is one of the most influential refinements in the development of the modern internal-combustion engine.