What is VVA? How it improves vehicles performance?
First commercialized by Hitachi in mid 1980s Variable Valve Actuation is used for improving the fuel economy and reducing the exhaust emissions. The result of the VVA is the increased engine output with stable combustion and reduced level of CO2 emission.
A VVA system can be based on the camshaft or it can cam less. While cam based systems are most widely used cam-less systems offer more flexibility in the valve lift and timing control than camshaft based system as the valve event must occur within the confines of the cam profile.
In a conventionally throttled engine, the pressure inside the engine cylinder is less than atmospheric during intake stroke. During exhaust blow-down, the engine pumps out the burnt gas at a pressure which is above atmospheric. The net result is the pumping of the sub-atmopsheric intake to the higher pressure exhaust. This operation results in the loss of the output power available at the engine crankshaft.
By early intake valve closing (EIVC) the desired amount of charge is trapped, instead of throttling the incoming charge by means of a throttle plate. The solution is then to control the intake valve closing time in such a way that the pumping loss is minimized.
Another advantage of the EIVC is improvement in combustion process within the engine at low speeds. At low speeds a reduction in the valve lift can speed up the combustion process relative to the throttled operation with normal valve lift. The reason for improvement is an increase in the turbulence at the top dead center that increases the turbulent burning velocity. This improvement can be put use to extend the lean limit of an engine.
The engine performance is further improvement by using EIVC which is based on the RPM of the crankshaft. Thus the idea behind the use of VVA systems is to limit the pumping loss and improve combustion efficiency.
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