1. Variable Valve Timing

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. Advance/Retard Systems
How variable valve timing is achieved?
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: Disadvantages: 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.