According to Wikipedia, variable valve timing (VVT) is “the process of altering the timing of a valve lift event, and is used to improve performance, fuel economy or emissions”.
A combustion engine takes in air to the cylinder chamber and gives off exhaust gases by means of valves which open and close the intake and exhaust passages. Variable valve timing changes the manner in which the intake and exhaust valves work at different engine speeds – helping to improve the engine’s performance and fuel economy and emissions by enabling the optimisation of engine performance under different loads and operations. As an example, a low engine speed valve timing can be advanced to help throttle response and engine torque, whilst under load. Valve timing can be retarded in order to reduce exhaust emissions and increase power at higher RPM, where valve opening times are greater.
Although VVT was first patented by Fiat in the late 1960’s, the first production vehicle to be made available to the market with a VVT system was the Alfa Romeo Spider 2000, released in 1980. Since then, many motor manufacturers have adopted the technology, with implementation variations and different names – for example Toyota has VVT-I or Variable Valve Timing with Intelligence and Honda has VTEC (Variable Valve Timing and Lift Electronic Control – but they are all based on the same underlying technology.
Engines without VVT have a non-adjustable camshaft, with fixed valve movement duration and timing.
A rotating camshaft sits above the valves and controls the opening and closing of the valves. The technology controls the timing or points in the piston’s movement at which the valves open and close, how long the valves stay open and the size of the opening. Sensors measuring airflow and camshaft position feed information to the engine control unit (ECU), which controls the valve movement.
Using the camshaft to control the opening and closing of the valves does limit the variability of the valve. Better engine performance would be possible if we were able to directly control each valve – and this is where the future lies for this technology.