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).
Its advisable to read and understand the four stroke cycle and
camshafts before reading this webpage.
The rate for the opening and closing of valves for an engine to properly function at a low RPM will not
be fully effective at a higher RPM.
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 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.