Technical Question About On/Off Cylinders

[Ross]

OK, I know that's not the correct term, but I'm talking about the engines where 4 cylinders fire all the time, and the other 4 only kick in at high throttle. If 4 cylinders are firing all the time, won't those 4 be subject to more wear than the ones which just fire occassionally? Wouldn't this lead to the engine suffering more wear (and more uneven wear) than if all 8 cylinders fired all the time?
I know the purpose of this design is not to increase engine life, but to increase mileage and use less gas. Still, it just seems like the engine would wear out more quickly with just half the cylinders working all the time.

[RF Overlord]

Ross:

In the above scenario, you would be right, although I'm not familiar with any passenger car motors that operate exactly as you describe.

In the early 80's, Cadillac had the infamous V8-6-4 engine that normally ran on all 8 cylinders, but selectively shut down cylinders during light throttle/low power demand, such as highway cruising, to allegedly improve economy. It was a good idea on paper, but a disaster in practice...

[RCSignals]

GM is doing that again, as is Daimler-chrysler.

[TripleTransAm]

In the old 8-6-4, it was a matter of some mechanical method of releasing the rocker arm from the rocker shaft such that the fulcrum of the rocker arm became the valve stem itself. So once the rocker arm was pivoting about the valve stem, the valve spring would keep that end locked solid, while the pushrod action would simply lift the rocker off the rocker shaft. To actuate the cylinder again, the rocker has to lock down again such that the fulcrum is in the center region once again, and any pushrod action would result in the other side pushing down on the valve stem, opening the valve.

Yeah, it would seem that the always-on cylinders would see more wear. However, with the likely-electronic nature of this new attempt at selective displacement, I could see the PCM alternating which cylinders were shut down, in an attempt to even out engine wear. But that's just a guess. I'm guessing the shut-down method isn't rocker arm related, but rather some other element in the valvetrain chain of parts.

One thing I'm not sure the old Cadillac engine did was leave a valve open (seems unlikely since that would mean something constantly pushing against the valve spring pressure, and the technology wasn't that advanced back then). So this means that even though the cylinder was effectively sealed from the outside world, that was the problem... it was sealed from the outside world, meaning it kept compressing and decompressing whatever was left inside it at the time of the cylinder shut-down. That means some sort of pumping loss in that cylinder, which actually sapped useable power and negated some of the benefit of the displacement decrease (ie. the engine effectively became 'heavier' to rotate). I think I read something about the modern attempts leaving the exhaust valve(s) open, so the piston simply pumps air/exhaust in and out of the exhaust valve, with only the restriction of the exhaust port to sap power.

[RCSignals]

I can understand leaving a valve open to relieve pressure, but wouldn't leaving an exhaust valve open cause some exhaust gasses to be drawn into the cylinder from the manifold? couldn't that have a fouling effect especially since there would presumably be no ignition burn cycle in that cylinder?