Variable Valve Timing (VVT)
Download as PPTX, PDF26 likes14,500 views
Variable valve timing allows the valve opening and closing points to be adjusted based on engine speed and load conditions. This improves engine performance and fuel economy over a fixed valve timing system. Variable valve timing is achieved through advance/retard systems that adjust the cam shaft timing using a movable tensioner controlled by the engine control module. Another method uses multiple cam profiles activated by oil pressure to vary the valve lift and duration. Variable valve timing provides benefits like better fuel efficiency, torque, and emissions but at a higher cost and complexity over a standard cam shaft setup.
Variable Valve Timing (VVT)
- 2. How the valves are opened and closed? • Consists of a central cam shaft • When cam shaft rotates the lobes push against the valves to open them • The cam is driven by taking input from the crank shaft. • A single cam shaft can be used to open both the (i.e. intake and exhaust ) valves (SOHC) or two cams can be used one for intake and one for exhaust (DOHC).
- 3. What is variable valve timing? • As the rpm of the engine changes the “ideal” valve opening and closing points change • Without variable valve timing , the valve timing must be the same for all engine speeds and conditions which decreases the engine performance and fuel economy • Variable valve timing (VVT) allows valve opening and/or closing points to be changed at specific engine speeds
- 4. How variable valve timing is achived? Advance/Retard Systems • Cam shaft is connected to crankshaft by means of a chain or a timing belt. It is kept in tensioner by a tensioner, • A movable tensioner moves the chain “slack” from one side to the other thereby advancing or retarding the camshaft • This movable tensioner is either controlled by a centrifugal governor or by ECM. • This technology is used by Porche and Lexus
- 5. Multiple Cam Profiles • The most common technology under this is Variable Timing And Lift Electronic Timing System (VTEC) • Uses three rocker arms per two intake valves • At low rpm the valves are actuated by a short duration cam lobe • An oil line passes through the cam shaft, • 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. Solenoid Operated Valves • Depending on RPM and engine load the ECM would directly control all valve timings. • There will be no cam ( Camless Valves) • Valve timing would be infinitely controllable • Valve timing will be controlled electrically by Electronic Control Module (ECM) • It is a future technology and currently not in production.
- 7. Advantages: • Better fuel efficiency • High initial torque • Lover emission • Better performance Disadvantages: • Higher cost of engine • More complex mechanisms • Lower overall engine life because of increased wear.