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Variable Valve Timing abbreviated as VVT is a method that can be used on internal combustion engines to
achieve the most output and efficiency during low and high RPMs (revs per minute). At 6000rpm the valves will open and close at 50 times per second. At that speed there isn't enough time for as much air/fuel to enter the cylinder as compared to 2000rpm where the valves open and close at approximately 16.6 times per second. Most engines are effective at lower RPMs because this is better for everyday use. Some engines use racing camshafts that are configured to work best at high revs but the problem with this method is that it doesn't allow the engine to idle smoothly and the it will be hard to start. By using variable valve timing adjustments can be made so camshafts can be used to utilize the benefits of both high and low revs. The goal is to allow the intake valve to open earlier at high revs to facilitate more air/fuel mixture entering the cylinder. The camshaft can have two lobes (one larger, one regular sized) for each valve or longer than average lobes (part of the lobe larger, part regular sized). Shifting between the two sizes changes the timing of valves. During high revs oil pressure can be used to shift the camshaft so the larger lobe now opens the valve or an ECU/engine management system can send a signal to move the camshaft. Some VVT systems only alter the opening of the intake valves and other systems alter both the intake and exhaust valves. Although the two sets of valves ie (intake and exhaust) are open together for a period of time a vacuum force is maintained to send the exhaust gases out and pull the intake in and the higher speed of motion also prevents the intake and exhaust gases from mixing because the inward and outward gases move at relatively the same speed and by the time the intake gases start to follow the vacuum force to go out, the exhaust valves are closed. Various marques have their own way of identifying the variable valve timing feature for their brand. Some of the most common are as follows Honda - VTEC & iVTEC, Mitsubishi - MIVEC, BMW - Valvetronic and Hyundai - CVVT. |
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