Friday 9th May 2003

Displacement on Demand

There's no substitute for cubic inches - when you need them

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[RichardR]

Bit disconcerting when your V8 rumble keeps turning into a 4 cylinder whimper!

[IPAddis]

When loads are light, the control system automatically closes both intake and exhaust valves for half of the cylinders, cutting off their air and fuel supply.

Surely closing both valves causes drag as the cylinder copes with pressurising and de-pressurising the cylinder every stroke. I would have thought it would be better to close the intake valve but leave the exhaust valve open.

Ian A.

[replication]

I think the theory is that a lot of the compression energy is recovered after the piston goes over the top. So all you really need to do is cut of the fuel to some of the injectors. The computer can handle this, and there would not have to be any secondary systems (to hold valves open for example) to create mechanical problems.

[GreenV8S]

The article suggests they do have some sort of mechanical system to stop the valves opening.

[mad dawg]

I think the main reason for holding the valves shut would be for emissions. Without combustion in the chamber to burn off blow-by gases the hydrocarbon emissions would go through the roof...

[ATG]

Pretty sure one of the Vauxhall range did this years ago. I know nothing at all about vauxhalls so maybe talking out of my hat, but did one of the Senators not run a sort-of-V6 that shut down 3 cylinders until there was a demand for power?

[Fatboy]

Thought the senators all had straight 6s?

[Mr2Mike]

GreenV8S said: The article suggests they do have some sort of mechanical system to stop the valves opening.

Which makes sense as putting the effort into opening a redundant set of valves wouldn't help with economy.