Variable displacement engines uneven cylinder sleeve wear?

[mrspaghettipete]

I've been tasked with a college assignment of talking about variable displacement engines, specifically if certain components such as cylinder sleeves or piston heads wear quicker on one side of the engine then they do on the other side/bank? this seems an obvious problem so i'm pretty sure there has been some kind of counter towards it? just cant seem to find anything about it?

[Jimmyarm]

Surely the simplest solution would be to have the ecu not always shut down the same cyclinders but alternate the balanced ones after say 1000 miles ?

Over the life of the engine that should minimise any wear differences ?

[anonymous-user]

All engines that do cylinder de-ac use a "round robin" strategy to maintain as even a thermal and lubrication distribution as possible, especially when it comes to ring / bore wear

[NNH]

Max_Torque said:

All engines that do cylinder de-ac use a "round robin" strategy to maintain as even a thermal and lubrication distribution as possible, especially when it comes to ring / bore wear

The VW 1.4 ACT engine shuts down the middle two cylinders, apparently for optimum heat distribution.

[AER]

engines will only have valve de-activation hardware on some of the cylinders for cost reasons, so deactivated cylinders will always be the same ones. Having said that, they are usually de-activated by barring the exhaust valve first, trapping a bunch of combustion gas in the cylinder. This is done to keep the ring pack working and maintain oil control.

Wear isn't a big problem because cylinder de-ac only happens at light loads where the fuel consumption gain is worthwhile and wear at light loads is negligible, even on the running cylinders. Also the torque amplitude is much lower making the loss of cylinders much less obvious.

[GavinPearson]

Remember that deactivation occurs when the engine output requirement is low relative to the engine's output. Materials for the engine are selected to cope with peak output. So pistons / rings / liners will generally be unaffected by deactivation.

[227bhp]

AER said:

they are usually de-activated by barring the exhaust valve first, trapping a bunch of combustion gas in the cylinder. This is done to keep the ring pack working and maintain oil control.

I've heard of a bunch of flowers before, but never a bunch of gas, regardless of which precisely how is that working? It sounds very interesting.

[AER]

your girlfriend says she's heard of them too, but never seen any...

[227bhp]

I'll take that as a 'Actually I have no idea' then.

[NNH]

227bhp said:

I'll take that as a 'Actually I have no idea' then.

The inlet and exhaust valves stay shut, so a "bunch"* of gas will stay in the cylinder and act as a spring.

• "bunch" is a technical term used by experts like is in the variable displacement engine world...

[AER]

Sorry, I didn't spot your question after being taken to task about my choice of words. NNH has it.

[227bhp]

NNH said:

The inlet and exhaust valves stay shut, so a "bunch"* of gas will stay in the cylinder and act as a spring.

• "bunch" is a technical term used by experts like is in the variable displacement engine world...

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

[AW111]

227bhp said:

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

Most of the energy used to compress the gas is returned to the crank as the piston moves down after TDC. If the valves stay open you waste energy pushg gas past the valve on compression then sucking gas back in past the valve on the down stroke - aka "pumping loss")..

[AER]

Indeed.

In politics you follow the money. In engineering you follow the energy. Where did it go and to what form was it transformed? Can you get it back again?

In the case of a piston, cylinder and a bunch of trapped gas, it is a pretty frictionless spring. Check it out with your bicycle pump if you like.

[227bhp]

AW111 said:

227bhp said:

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

Most of the energy used to compress the gas is returned to the crank as the piston moves down after TDC. If the valves stay open you waste energy pushg gas past the valve on compression then sucking gas back in past the valve on the down stroke - aka "pumping loss")..

Which valves are we talking about here, in or ex?

[NNH]

227bhp said:

AW111 said:

227bhp said:

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

Most of the energy used to compress the gas is returned to the crank as the piston moves down after TDC. If the valves stay open you waste energy pushg gas past the valve on compression then sucking gas back in past the valve on the down stroke - aka "pumping loss")..

Which valves are we talking about here, in or ex?

Both sets. The cylinder is virtually sealed, with the piston acting on the gases within just like an air spring. If the valves were open then there'd be pumping losses, which is a big part of what sloes down a conventional engine on the overrun.

[227bhp]

NNH said:

227bhp said:

AW111 said:

227bhp said:

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

Most of the energy used to compress the gas is returned to the crank as the piston moves down after TDC. If the valves stay open you waste energy pushg gas past the valve on compression then sucking gas back in past the valve on the down stroke - aka "pumping loss")..

Which valves are we talking about here, in or ex?

Both sets. The cylinder is virtually sealed, with the piston acting on the gases within just like an air spring. If the valves were open then there'd be pumping losses, which is a big part of what sloes down a conventional engine on the overrun.

So the piston goes downwards with both valves closed?

[NNH]

227bhp said:

NNH said:

227bhp said:

AW111 said:

227bhp said:

I'm not really seeing how that is helping anything though, if anything it seems counterproductive as it takes effort or power to compress the bunches. The valves stay shut together once every four stokes anyhow, if anything you would expect them to be left open....

Most of the energy used to compress the gas is returned to the crank as the piston moves down after TDC. If the valves stay open you waste energy pushg gas past the valve on compression then sucking gas back in past the valve on the down stroke - aka "pumping loss")..

Which valves are we talking about here, in or ex?

Both sets. The cylinder is virtually sealed, with the piston acting on the gases within just like an air spring. If the valves were open then there'd be pumping losses, which is a big part of what slows down a conventional engine on the overrun.

So the piston goes downwards with both valves closed?

And upwards too. The gas inside the cylinder expands and contracts, with energy going in to compress it, and then (almost) the same amount of energy is returned when it expands again.

When the VW 1.4 ACT is running on two cylinders, you're halving the pumping losses (gas being forced through intake, exhaust, valves, etc) in the engine, but you've still got the same amount of friction losses on all the bearing surfaces. I'd guess that the engine is tuned so that combustion is more efficient on two cylinders at 40% power instead of four cylinders at 20% power.

Edited by NNH on Wednesday 16th March 17:36

[227bhp]

NNH said:

And upwards too. The gas inside the cylinder expands and contracts, with energy going in to compress it, and then (almost) the same amount of energy is returned when it expands again.

When the VW 1.4 ACT is running on two cylinders, you're halving the pumping losses (gas being forced through intake, exhaust, valves, etc) in the engine, but you've still got the same amount of friction losses on all the bearing surfaces. I'd guess that the engine is tuned so that combustion is more efficient on two cylinders at 40% power instead of four cylinders at 20% power.

Edited by NNH on Wednesday 16th March 17:36

The motion of the piston going downwards will create so much vac it will pull the valves open and take a lot of power to do it. Not only that, whilst it's doing it, it will try to pull the air up from the crankcase and pull any oil which is around the piston with it.
It can't work like that.

[NNH]

227bhp said:

The motion of the piston going downwards will create so much vac it will pull the valves open and take a lot of power to do it. Not only that, whilst it's doing it, it will try to pull the air up from the crankcase and pull any oil which is around the piston with it.
It can't work like that.

I'm presuming that the valves shut off after intake so it starts at atmospheric pressure at BDC, and compresses to ~10atm at TDC, which would put the valves and piston rings under less strain than a normal combustion stroke.