Here's how variable valve timing works on your car's engine. In this video a variable timing gear, actuator and engine head is taken apart to demonstrate how the system works.
The variable valve timing, or VVT, allows for a phase shift in the timing of the valve opening and closing. It does this by offsetting the camshaft at a relative angle to the timing chain.
The VVT gear is a fixed gear when there is no timing advance or retard. However when the computer sees it necessary to change the timing, the gear fills with engine oil.
Passages inside the engine head allow oil to flow from the pump to the variable valve timing solenoid. The ECU controls this solenoid to allow oil to enter the VVT gear. The gear will unlock its fixed pin and then become two independently rotating pieces. The pockets inside the gear fill up with oil causing an angular offset between the input (chain) and output (camshaft).
The system is controlled by the ECU using the cam position sensor for closed loop control. The angle of the offset is infinitely variable between zero and 15 degrees or so.
Please note, VVT is different from VTEC or VVT-L. Those systems use a separate cam profile that varies the lift amount, duration and phase. However since the cam profile is fixed, it is not infinitely variable like VVT is.
Varying the timing of the valve's opening and closing will result in cleaner combustion, generating either more power, or less harmful emissions.
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