Variable Valve Timing
If you have learned how engine works,you know about the valves that let air into the engine and let exhaust out of the engine.You also know the camshaft that controls the valves.
The camshaft uses rotating lobes that push against the valves to open and close them. Since an engine's valves play a major role in how air gets in an out of the combustion chamber,it makes sense to focus on them when looking to increase horsepower and efficience.
When you increase the rpm,however,this configuration for the camshaft does not work well. If the engine is running at 4000 r/min,the valves are opening and closing 2000 times every minute. When the intake valve opens at the top of the intake stroke, it turns out that the piston has a lot of trouble getting the air moving into the cylinder in the short time available. Therefore, at a higher rmo ranges you want the intake valve to open prior to the intake stroke-actually back in the exhaust stroke-so that by the time the piston starts movind downward in the intake stroke,,the valve is open and air moves freely into the cylinder during the entire intake stroke. For maximum engine performance at low engine speeds,the valves need to open and close differently than they do at higher engine speeds.If you put in a good low-speed camshaft,it hurtsthe engine’s performance at high speeds,and if you put in a good high-speed camshaft, it hurts the engine’s performance at low speeds.
VTEC(which stands for Variable Valve Timing and Lift Electronic Control0 is an electronic and mechanical system in some Honda engines that allows the engine to effectively have multiple camshafts.As the engine moves into diferent rpm ranges,the engine’a computer can activate alternate lobes on the camshaft and change the cam’s timing.In this way, the engine gets the best features of low-apeed and high-speed camshafts in the same engine
MIVEC (Mitsubishi Innovation Valve timing and lift Electronic control ) which is used on the Mitsubishi engines.
VVT-I (Variable Valve Timing with Intelligence) and now VVTL-I ((Variable Valve Timing with Intelligence AND Lift with Intelligence) which is being used on the current Toyota and some Lexus engines.
VVL(Variable Valve Lift )which is used on the Nissan engines, The cam VVL, Nissan’s new variable lift system that looks identical to Honda's VTEC system.In fact, it looks the same.But it appears that the only significant difference between the VTEC and VVL is the fact that the VVL switch the intake and exhaust at slightly different engine speeds in an effort to smooth the transmission between cam profiles.
VANOS(Variable Nockenwellen Steuerung) which is used in the BMW engines and alsi the Dpuble VANOS system on the new 3 Series and they are many more similar system used by manufacturers such sa Ford, Lamborghini and evev Ferrari.
auther:candywww.sinodiesel.com
