VVT-i Control

· The VVT-i improves the engine's overall performance based on the engine rpm and the extent to which the throttle valve is opened.

· When greater power is needed, it changes the timing to that which enhances torque, and when less power is needed, it changes the timing to that which enhances fuel economy and cleaner exhaust gases.

· Let us confirm how the VVT-i achieves "high power output", "low fuel consumption", and "clean exhaust gases" by using the various driving condition as the VVT-i control example.

· Idling

o Because the intake lag that occurs during idle minimal, it is not necessary to open the intake valve early.

o However, on a conventional engine in which the valve timing is fixed, the valves open early because they are designed with the intake lag in mind, which takes place during high-rpm operation.

o When the air-fuel mixture is contaminated in this manner, it does not produce reliable combustion and results in an unstable engine rpm.

o For these reasons, in conventional engines that avoid valve overlap, it becomes necessary to raise the rpm at idle to a certain extent.

o During idle, the VVT-I delays the timing at which the intake valve opens to eliminate the valve overlap, thus preventing the exhaust gases from flowing back into the intake port.

· Light and medium load driving

o During normal driving, the VVT-i advances the valve timing to achieve a large valve overlap.

o During normal driving, when the throttle valve is not opened too widely, a vacuum is created in the intake port. Similar to when the engine is idling, when the valve overlap is large, this vacuum causes the exha ust gases to flow back into the intake side.

o Why does the VVT-i increase the valve overlap, which could the state of the air-fuel mixture to worsen?

o As the exhaust gases are drawn in, the vacuum in the intake port becomes lower and reduces the resistance (the force that tries to pull the piston upward) during the descent of the piston during the intake stroke.

o The engine's power loss is reduced and fuel economy is increased.

o The unburned fuel that is contained in the exhaust gases is re-introduced into combustion, resulting in a decreased amount of HC emissions and cleaner exhaust gases.

o The mixing of the inactive gases into the air-fuel mixture lowers the combustion temperature, resulting in a decreased amount of NOx emissions and cleaner exhaust gases.

· Heavy-load with low & medium rpm

o When the driver is about to depress the accelerator pedal all the way to start accelerating, the engine rpm is low and the piston is operating at a low speed, so the intake lag of the air-fuel mixture is short.

o Under such conditions, when the piston turns around at its bottom-dead-center, the pressure in the cylinder and the pressure in the intake port soon reach their equilibrium.

o Therefore, unless the intake valve is closed before the piston ascends, the air-fuel mixture gets pushed back.

o By advancing the timing of closing the intake valve, and enable the valve to close when the piston is near the bottom dead- center, a sufficient amount of air-fuel mixture can be drawn in.

o However, closing the intake valve early also means that the intake valve opens early, causing the valve overlap to increase.

o During idle, if the valve overlap is large, the exhaust gases flows back due to the vacuum that is present in the intake port.

o However, during sudden acceleration, in the intake port is low, and there is practically no back flow of the exhaust gases.

· Heavy-load with high rpm

o When the accelerator pedal continues to be depresses, the engine rpm increases, resulting in a longer intake lag.

o Using sensors to monitor the engine rpm, the VVT-I gradually delays the timing to close the valve so that the maximum amount of air-fuel mixture can be drawn in according to the increase in the engine rpm.