Variable valve control

When it comes to highperformance engines, timing is everything

Variable valve control has the potential to significantly reduce light load pumping losses. The simpler technology of variable valve timing (VVT) by cam phasing has been widely adopted in luxury cars in Europe and North America, and in a large fraction of all cars in Japan; in fact, almost all Toyota models in 2004 had VVT. Variable valve lift and timing (VVLT) is less common; Honda is the only manufacturer to have a two step (or three-step) valve lift control in many of its models. A continuously variable lift system was recently introduced by BMW in its luxury 7-series cars. More exotic technologies actuate the valves without camshafts, using hydraulic or electric solenoids, but it is not clear if the additional cost is worth the marginal benefit in fuel economy relative to mechanical VVLT systems. Such systems will reduce shortfall in inner-city driving, but will increase shortfall in high speed or aggressive driving

In an internal combustion engine, optimizing the airflow in and out of each combustion chamber is a key to just about every aspect of performance. Power, emissions, fuel economy and other critical parameters are all dependent upon how good the engine flows air in and out.

Clearing the Air 

Four-stroke automotive piston engines have poppet intake valves in their cylinder heads to let air in, and separate poppet exhaust valves to let exhaust gases out. These valves are opened at the appropriate time by one or more rotating camshafts. The cams are turned in precise relation to the crankshaft by chains, belts or gears—or a combination thereof. Lobes machined on the camshafts push the valves open at just the right time in the engine's operating cycle, then keep the valves open for just the right duration. There are other ways to operate the valves, including various pneumatic and electromagnetic schemes, but for production engines, the inexpensive and long wearing camshaft is king.

On the other hand, advancing the intake cam timing as needed pays significant benefits, by broadening an engine's power delivery to make it pull harder at low rpm, without running out of steam at high revs. By providing just the right amount of valve-timing lead, variable valve timing can provide more complete cylinder filling at all engine speeds.