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Variable Valve Timing

Variable Valve Timing. Variable Valve Timing Design and Theory. As the rpm of the engine changes the “ideal” valve opening and closing points change Mechanical cam drive systems require that a compromise be made between low-rpm drivability and high-rpm power production

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Variable Valve Timing

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  1. Variable Valve Timing

  2. Variable Valve Timing Design and Theory • As the rpm of the engine changes the “ideal” valve opening and closing points change • Mechanical cam drive systems require that a compromise be made between low-rpm drivability and high-rpm power production • Variable valve timing (VVT) allows valve opening and/or closing points to be changed at specific engine speeds

  3. Variable Duration Lifters • Simplest VVT system • At low RPM the oil is able to slowly leak past the hydraulic lifter’s plunger • By the time the cam is beginning to close the lifter has bled down to the point that the valve will close approx 10 degrees earlier than normal • This makes the cam act as though it has a shorter duration, less overlap and as though it was advanced • At high rpm the lifter doesn’t have time to bleed down and therefore the valve is held open for the entire duration period

  4. Advance/Retard Systems • Used by Porsche and Lexus • A movable tensioner moves the chain “slack” from one side to the other thereby advancing or retarding the camshaft • Does not affect lift, duration, overlap, or lobe separation (except on DOHC)

  5. Multiple Cam Profiles • Honda VTEC and Jaguar • VTEC • Uses three rocker arms per two intake valves • At low rpm the valves are actuated by a short duration cam lobe • At high rpm the ECM opens an oil control valve which hydraulically engages the third rocker arm which runs on a cam lobe with more duration and lift

  6. VTEC

  7. Solenoid Operated Valves • Not in current production use • Valve timing would be infinitely controllable • Depending on RPM and engine load the ECM would directly control all “camshaft” characteristics (would not use a camshaft)

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