How VVT-I Engine Works

How VVT-i Works

In ordinary engines the relationship between inlet and exhaust valve opening and closing is non-adjustable because it is dependent upon their direct connection to the camshafts.

Engine designers have known for a long time that they could get better performance out of an engine under certain circumstances by allowing the intake valve to open slightly before the exhaust valve closes. This increases the time for the fuel/air mixture to enter the cylinder during the intake stroke. In this condition the exhaust and intake valves are open at the same time; this is called 'valve overlap'. In conventional engines 'valve overlap' timing is fixed.

Fixed valve overlap allows the engine to perform well within a certain rev range, however there are three main undesirable side effects.

1.Fuel is wasted – the fuel/air mixture is not always efficiently burned and this causes unburnt fuel to pass through the engine.


2.Higher levels of undesirable exhaust emissions are produced.


3.Power output potential is not fully realised.

Variable valve timing allows the relationship between the separate inlet and exhaust camshafts to vary the valve timing overlap. In doing so it overcomes the side effects described above by using a computer to continuously vary the intake valve timing and overlap. The valve timing and overlap are adjusted through a series of simple mechanisms to ensure the optimum conditions apply across all the working rev range. The advantages are lower fuel consumption, lower exhaust emissions and higher power output. Because the system is continuously variable, an 'i' for 'intelligent' has been added to the acronym.