variable valve timing
The process of altering the timing of a valve lift event in an internal combustion engine, and is often used to improve performance, fuel economy or emissions.
variable valve timing: synchronized valve opening for engine efficiency
Variable valve timing (VVT) is a system that adjusts when the intake and exhaust valves open and close relative to the position of the crankshaft. In a fixed-timing engine, valve events occur at the same crankshaft angles regardless of engine speed or load; VVT systems shift these events by a few degrees, typically between 20 and 50 degrees of crankshaft rotation, to match what the engine actually needs at any given moment.
The mechanism works through a helical gear or cam phaser mounted on the camshaft sprocket or directly on the camshaft itself. Oil pressure, controlled by a solenoid valve receiving signals from the engine control unit, acts on the phaser to rotate it relative to the crankshaft timing reference. This allows continuous or stepped adjustment of valve overlap, the period when both intake and exhaust valves are open simultaneously. Most production systems adjust only the intake camshaft; some performance engines adjust both intake and exhaust timing independently.
Early VVT systems, introduced in the late 1980s, used discrete positions controlled by oil passages and check balls. Modern systems employ continuously variable phasers with proportional solenoids, allowing smooth adjustment across the entire operating range. The response time is typically under 100 milliseconds, fast enough to react to throttle changes and transient conditions.
Trade-offs in practice
At idle and low speeds, advancing intake valve opening improves cylinder scavenging and combustion quality, reducing emissions and rough running. At higher engine speeds, retarding intake timing reduces overlap and prevents the exhaust scavenging effect that can pull fresh charge out through the exhaust valve. This tuning improves both power and efficiency across the rev range. However, excessive overlap causes unburned hydrocarbons and increases cold-start emissions until the system warms up.
Durability issues center on the phaser itself: wear in the helical grooves or cam followers creates lash that degrades response precision. Sludged or degraded oil reduces solenoid responsiveness and can cause the phaser to stick. The solenoid windings and seals are exposed to engine heat cycling and can fail after 150,000 to 200,000 miles in some designs. Worn phaser symptoms include rough idle, poor fuel economy, and illuminated check-engine lights. Repair typically requires camshaft removal and phaser replacement rather than adjustment.