The problem with fixed valve timing that the valve train is set by the automaker for peak efficiency running at a specific point in the engine's operating range. When the vehicle is moving slower or faster than this ideal operating point, the engine's combustion cycle fails to properly burn the air/fuel mixture leading to considerably compromised engine performance and wastes fuel. Variable Valve Timing (VVT) is a solution developed to overcome this engine deficiency, dynamically altering the valve's opening and closing for optimal performance at any speed.
First used as a technology through experimental aircraft of the 1940s, It wasn't until 1982 when the first automotive use of VVT was found inside the engine of an Alfa Romeo Spider. Becoming the launch pad for the technology, Honda introduced the VTEC system in Japan and Europe in 1989. It was the 1990 Acura NSX that brought variable valve control to the United States, eventually sparking a flourishing interest in VVT to the mainstream auto market of today.
Advantages with VVT Engine
Variable valve timing is not a horsepower booster like a turbocharger. Instead, VVT is designed to make use of the engine's existing power in the most effective way possible. Assisting right from engine start up, timing would configure the valves for a shorter opening period aiding the initial warming up of the engine. When the engine reaches higher performance, valve staging is adjusted for a longer opening time allowing more air/fuel to flow into the combustion chambers. Ultimately, engine efficiency is greatly improved which also means less fuel is needed to run a VVT engine. Vehicles including VVT engines would expect fuel saving benefits of 5-10%.
VVT systems are also an effective component for emission control. Since the 1970s, automobile engines would reuse exhaust gases through the next combustion cycle to reduce NOx, an major smog-creating pollutants. For this, an EGR (Exhaust Gas Recirculation) valve was developed. VVT technology uses a process called internal EGR where a small amount of exhaust gas leaks back into the combustion chamber.
How Valve Timing is Changed
Still a fairly new innovation, variable valve timing itself has evolved greatly. Early versions of variable valve timing would have adjustments occur at after a fixed threshold is reached. VVT setups now use engine management to set valve timing at any RPM point. This is known as continuously variable valve timing.
Best exhibiting VVT technology is Honda's VTEC engines. Using a four-valve per cylinder engine, the valves are controlled by camshafts machined with three lobes. One larger lobe rides sandwiched between two lobes trimmed to a smaller profile. At lower engine speeds, the smaller cam lobes drive the cylinder's valves along the cam's specially-designed rocker arm assembly. When the engine revs higher, a synchronizing pin is driven by hydraulic pressure locking into the larger cam lobe. This cam lobe will then takes over, opening the valves longer. One camshaft controls the intake valves while the other regulates the exhaust valves. Newer i-VTEC engines work similarly but is designed with only a single camshaft for intake and exhaust. The function of Honda's VTEC system is for the most part imitated on VVT engines produced by other automakers. Toyota's VVT-i, Nissan's VVL, and the Mitsubishi MIVEC system also use two differing cam lobes for variable valve timing. However, BMW and Porsche have applied some aggressive attributes to VVT systems. BMW's sophisticated Valvetronic system uses a eccentric shaft while Porsche's Variocam Plus uses a stressed chain in reorienting the camshaft to provide valve timing. Uniquely, Variocam Plus is designed to run at three stages of valve timing for greater engine control.
Variable valve timing is also accomplished through cam phasers (hydraulic pressure-actuated solenoids) rather than through mechanical interaction. This system is utilized by General Motors, combining with Active Fuel Management.
Well into the future, valve timing may be accomplished more easily as engine's camshaft may be completely discarded and replaced with electrical solenoids controlling each valve.