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Variable Valve Timing

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DaveB:
Prompted by Olazz's query about compression ratio I read that cam grind affects compression ratio.
Is that because the inlet valve remains open for some time after the piston begins to rise on the power stroke, so some of the air/fuel mix is pushed back past the valve? If that's what happens, it seems counter productive but I'm imagining there may be a point higher in the rev range where the mixture is still rushing in while the piston rises, so the cylinder gets a full charge plus a bit more. Is this what variable valve timing in modern cars is about - optimising that relationship over the rev range? How is vvt achieved? How can the relationship between crank and cam be slipped like that?
I think my understanding is probably wrong so I would appreciate some enlightenment!

Benz Dr.:
My basic understanding of variable valve timing is that the cam shaft can also move sideways as well as rotating against a follwer. The cam lobe is ground in such a way that it changes the point at which the valve opens and closes.

DaveB:
That's right. I looked it up, it's quite ingenious. the cam slides forward or backward depending on revs, and the lobes are ground wider at one end. I don't know the mechanism of the forward/backward motion though.

jacovdw:
The exact mechanism to achieve variable valve timing depends on the way the manufacturer implements the concept.

BMW uses VANOS that changes the the timing of the inlet valve by rotating the camshaft in relation to the cam gear (double VANOS does the same with the exhaust valve).
Honda employs VTEC that varies the timing, valve lift and duration by using two sets of lobes on the camshaft.

In the case of the VTEC system the shift is effected by means of oil pressure after a solenoid is activated (under control from the ECU). When the solenoid is activated, oil under pressure activates a locking pin that binds the low rpm cam follower and the high rpm cam follower together and thus the valve opens and closes according to the high rpm cam profile.

With VANOS, oil under pressure is used (under control of the ECU) to effect the advancement/retardation of the camshaft timing at the cam gear.

In both systems, the camshaft itself does not move forward or rearward in relation to the engine.

DaveB:
Thanks jacovdw,
Several ways to skin the cat. The forward/backward cam mechanism I read was apparently by Ferrari.
http://www.howstuffworks.com/camshaft2.htm

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