2. ď 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. ď What is variable valve timing?
⢠As the rpm of the engine changes the âidealâ 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. ď 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
âslackâ 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. ďAdvantages:
⢠Better fuel efficiency
⢠High initial torque
⢠Lover emission
⢠Better performance
ďDisadvantages:
⢠Higher cost of engine
⢠More complex mechanisms
⢠Lower overall engine life because of increased wear.