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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 design and theory
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
variable duration lifters
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
advance retard systems
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)
multiple cam profiles
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
solenoid operated valves
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)