Why don't big engines run on less cylinders?

[fjord]

Here's a thought, why dont big engines become more enonomy driven by shutting down cylinders when they dont need them.

Similar to start/stop, but on the move.

As an example, does an M5 need all 8 cylinders firing when it's crawling along in traffic at 20 mph? Why cant it shut down 4 of them (or maybe even more) when you're crawling along, then fire them up again when you start moving fast.

Could you apply this to any speed aswell? Cruising at 70mph doesnt need all 8 cylinders, why not just run it on 5 or 6? And then the car only engages all 8 when you give it the beans.

Im sure there's a logical reason this technology isnt used though... :/

[The Nur]

There are some V8 engines that can run on one bank of cylinders when not being pressed. I can't think which though the new Bentley V8 springs to mind, but I may be wrong.

[edc]

This has already been thought and is in use already?!

[rfn]

Pretty sure the Hemi in the 300C (and probably a number of other cars) shuts down the 2nd bank of four at times.

[eldar]

The Nur said:

There are some V8 engines that can run on one bank of cylinders when not being pressed. I can't think which though the new Bentley V8 springs to mind, but I may be wrong.

Still a lot of friction spinning the idle bits around. Fewer smaller cyls and more turbos appears to be the future.

[Jon999]

[Morningside]

I *think* the V12 BMW does this already.

[briang9]

http://www.audi.co.uk/new-cars/a8/s8/efficiency.ht...

[James B]

Yeah it's one of the features of my S7. No sensation that it has happened and all of a sudden the car is doing. 30+ mpg.

Not bad for a beast!!!

[ez64]

Lincolns had this in the 70's for the fuel crisis on V8's but it was highly prone to expensive failure and got a very bad rep. Pretty common on high end stuff these days I think.

[fatboy b]

Jon999 said:

That should be a Jag advert. Doesn't work as an Audi one.

[aspirated]

Audi call it 'Cylinder On Demand'. It's even available on the 1.4 TSI 170PS turbocharged/supercharged 4 pot.

[Debaser]

fjord said:

Here's a thought, why dont big engines become more enonomy driven by shutting down cylinders when they dont need them.

Car companies have already thought of that one.

[MitchT]

Ferrari 355s are known to do this sometimes ... though I'm not sure the manufacturer intended it

[Buff Mchugelarge]

Chrysler started doing this ages ago.
I believe they tested it on their all singing supercar that they never actually made.
Can't remember the name, but it was going to have 1000bhp v16 that would run on 8 cylinders when needed.

[johnfm]

MitchT said:

Ferrari 355s are known to do this sometimes ... though I'm not sure the manufacturer intended it

[xRIEx]

I'm fairly sure the V8 in the SLK55 has cylinder deactivation as well.

This idea has been around for a few years now (apparently mainstream since 2005, and first tried in 1981 according to http://en.wikipedia.org/wiki/Variable_displacement).

[750turbo]

fatboy b said:

Jon999 said:

That should be a Jag advert. Doesn't work as an Audi one.

Given that there was a Cat in the Pic, I had to take a second look!

Bloody Germans stealing our Animals

[Ozzie Osmond]

Yes, some engines (especially V8s) use cylinder shut-down.

I suspect it's a dead-end technology to be superceded by hybrids which can give extra electric power when needed (5% of the time?)rather than keep a huge engine turning when all its power isn't needed (95% of the time?)

[ViperDave]

The Americans, yes those of backwards engineering and leaf springs, have been doing it for years, Had one for a rental a few years ago, our booked suv wasn't suitable for the conditions we were driving in so hertz gave us a choice of a Tahoe or journey (very big vs not big enough for a 4 week ski trip) We took the Tahoe with trepidation on the fuel it was going to cost us to run (4000mi trip) Darn thing turned out to be one of the most efficient SUV's we have had for a few years.