VW quantifies MPG gain of 4- to 2-cylinder deactivation at 31 mph = +16 mpg

[pete c]

The unused cylinders will have reduced wear. I wonder if it would be worth the added expense of making all 4 cylinders deactivatable?

Didn't think my spell check would like that word.

Doing so would extend the life of the engine, assuming it spent a good deal of time in 2 cylinder mode. The ECM could keep track of hours run on inner/outer cylinders and balance it out.

[jmcc]

Don't think it would make a huge difference, and it's not usually wear of the valvetrain that kills an engine.

[shovel]

I don't see any big scary red flag on this, engine building is a pretty well established science and I trust VW's done their homework. They do a 10/100 powertrain warranty on the touareg, which would probably be appropriate for this too.

I'm curious though, wouldn't vacuum pull valves open a bit even if the cam wasn't pushing them? Especially with the help of manifold pressure on the other side of the exhaust valves..

[Spencer Fitch]

If it pulled in air it would/should only be 1 time because it's way easier for air to get in than to get out..

[Frank Lee]

Originally Posted by pete

The unused cylinders will have reduced wear. I wonder if it would be worth the added expense of making all 4 cylinders deactivatable?

All the pistons are still going up and down- that's a good deal of it right there. Today's engines have better cylinder materials than the play-doh that's been used in the past. I've seen cylinders in a Ford 3.0 with 150,000 miles on 'em that looked like the day they left the factory- no ridge, vivid crosshatching, etc. Contrast that with the cylinders of a small block Chevy from anytime pre-'80s or '90s- 100,000 miles on 'em and they probably need to be bored .030 to clean 'em up.

Originally Posted by shovel

I'm curious though, wouldn't vacuum pull valves open a bit even if the cam wasn't pushing them? Especially with the help of manifold pressure on the other side of the exhaust valves..

Ever tried to compress a valve spring?

[NeilBlanchard]

I wonder if anyone has tried a hydraulic coupling that splits the crankshaft and lets the idled pistons sit still?

[Frank Lee]

I've pondered such a thing and if you could get it to index to one position when they are coupled that would be great. If not, I'm thinking it would have bad, bad, BAD, bad vibrations (it's giving me bad vibrations).

[jmcc]

I think a big issue is that you deactivate cylinders on the same throw (for a 4cylinder, either the middle pair or the end pair) you can't split half way as you would get uneven firing. If you rearranged so you had it splittable in the middle you'd have terrible dynamic balance when running 4 cylinders, and even worse once split.

[chrisoverson]

Hi guys, new ecomodder here

Just as a thought, would shutting off the intake and exhaust ports on all cylinders under overrun (gliding at high rpm) conditions decrease pumping losses and allow you to travel further without dropping into neutral? I suppose the gains would be marginal though.

[jime57]

Originally Posted by Ecky

Leaving the valves closed basically uses the air as a spring. The piston uses energy to compress it, but the air pushes the piston back down on the other side of the stroke. Leaving the valves open would waste the compression energy.

I think this may be the key to the arrangement. Since they chose cylinders 2 & 3, then not only is there an air "spring" there are two air springs which are 360 degrees of chank rotation apart. The spring effect is doubled, symetrical and complimentary.