Variable Valve Timing

[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

[jacovdw]

The Italians have a flair for doing things a little differently.... 

Just look at the desmodromic valve setup that is used by Ducati in their motorcycle engines, but that is a whole other topic altogether...

[Benz Dr.]

I believe that most of their bikes use that system but some of the base models don't. Very neat bikes and even the Hardly Ableson guys don't mind them so much.

[Shvegel]

Not sure about first use but Mercedes used Desmodromic valves on the W196 300SLR straight 8 back in the mid fifties.

The BMW VANOS system uses a small hydraulic cylinder at the front of the camshaft that pushes or pulls on a screw shaped piston that is also attached to the cam shaft. The "Nut" if you will is attached to the camshaft gear so as you push or pull on the piston it adjusts the cam timing advance/retard. Besides being able to optimize cam timing for the various speed ranges it also allows under certain circumstances to retard the exhaust timing to the point that the piston begins to drop on the intake stroke while the exhaust valve is still open thereby sucking a little exhaust back into the cylinder. EGR without any pesky valves.

Of course the real trick will first be pneumatic valves followed by getting rid of the camshaft all together. Imagine being able to adjust each cylinder based on performance relative to it's mates. BMW already uses the timing between cylinders to detect misfiring but imagine how smooth an engine you would have if you could constantly balance each cylinder by changing the valve openings.

[Cees Klumper]

Honda employs VTEC that varies the timing, valve lift and duration by using two sets of lobes on the camshaft.

Now that we're on the subject, have a look at 'VTEC just kicked in Yo!' on Google ... pretty funny. I'm learning new concepts all the time, like 'old skool' and 'fat', and now this 'VTEC just kicked in Yo!' thing ...

[Raymond]

Makes me miss my old hot rod Acura.  I had a control that let me play with the VTEC.  One setting I tried made her scream like a banshee and go way faster than I should.    But 8,500 RPM was pretty hard on the little 4-banger.