Is using less than all cylinders damaging?

[RSmith]

If you had, say a nice big V8, and you wanted better mpg... could you (with a bit of tinkering and remapping) get the engine to only use half its cylinders on partial throttle? So the pretence is good fuel economy when cruising, and excellent power when flooring it.

Would it be damaging to the crank or pistons? I know Caddilac managed it with the V16 in their Sixteen concept model, but is it possible with a regular engine?

 

[obi_waynne]

In theory this would be nice but in practice you have the extra drag of the pistons in the unused chamber and the engine would not run smoothly either.

If you had 2 engines arranged in a V then you could run a double crank and turn off the unused bank when not required.

 

[RSmith]

If you had 2 engines arranged in a V then you could run a double crank and turn off the unused bank when not required.

Wouldn't that make it sort of like a W engine?

Also, you say the ride would be jerky... what about a V12 which the driver could use as a V12 or switch half of it off and have a cruising V6 using a dashboard control?

 

[obi_waynne]

I was thinking of 2 straight 4's as a V (Probably more of U really) but that the W principle still holds true here.

 

[lutonmatt]

i am sure some of the new bigger v8's do cut half out to save mpg
but cant say what ones sure chevy do it and if they do it then dodge will do it

is not hard to do as all you have to do it cut the injectors and spark power and they will not fire and as everything is coil per plug now its easyer to do

and as long as its 4 or 2 pots not firing it sill still be balanced

 

[claymore]

i am sure some of the new bigger v8's do cut half out to save mpg
but cant say what ones sure chevy do it and if they do it then dodge will do it

is not hard to do as all you have to do it cut the injectors and spark power and they will not fire and as everything is coil per plug now its easyer to do

and as long as its 4 or 2 pots not firing it sill still be balanced

Would you not need some way of "de-compressing" the un used cylinders to stop the drag?

 

[HDi fun]

The compression / expansion bit is not so bad as the compresed air will help push the pistons back down as well. There will still be some build up of heat - this is called hysterisis. Venting the cylinders not in use will not reduce the friction, it will simply quell the heat build up.

 

[obi_waynne]

Didn't someone experiment with an engine that changed it's cylinder volume depending on load? I seem to think it might have been SAAB!

 

[RSmith]

Venting the cylinders not in use will not reduce the friction, it will simply quell the heat build up.

Probably as far as engineering simplicity goes... would it be easier to program the ecu to cut out spark and injector activity on certain cylinders (eg. every other cylinder in the firing order) and just keep the cam lifting the valves and ventilating the unused cylinders? My biggest worry is that prolonged use of this 'cruising mode' could cause crankshaft damage?

 

[HDi fun]

I think that's probably the easiest system. I can't see there being crank damage since all will be firing once extra torque is summoned with the right pedal.

I can see a few snags:

The engine would have to have an even number of cylinders;

The catalysts would become burnt out with the excess are being bled into the exhaust from the non running cylinders on each stroke. (This would not be an issue with, say, a V8 with independent exhaust systems and with a whole bank being shut down).

Turbocharging would be very difficult indeed.

 

[Country Bumpkin]

You would need to cut fuel from the unused cylinders or it would just induce bore wash.
As for turbocharging, there's simple ways round that. We already write different maps for using different boost pressures on the same engine, the same theory would apply. The technicalities of turbo lag could be overcome by using sequential turbos, the smaller one to spool up the larger one and when controlled by boost pressure/waste gate action they operate according to which engine configuration you are running at the time.
You could use additional injector control to enhance the fueling at the cross/switch over period so it smoothes the engine running.
There's plenty to think about here and i'm sure that these routes have all been trialled but it's good to discuss ideas like this.

 

[HDi fun]

Cross bank tubocharging would be impossible with one whole bank shut down.

Also, the back pressure from the turbo would force exhaust gases back into the 'off' cylinders.

I suspect that turbocharging is a better way to produce large torque and power outputs from small engines than shutting down individual or banks of cylinders is on large engines.

Perhaps that's why turbocharging is ever increasing in popularity and switchable engines aren't?

If we could devise an engine that burns fuel so thoroughly that cats are redundant then the lean burn technology could be re-introduced and developed further.

The simplest option at the moment seems to be hybrid drive. It still has a way to go but let's watch and see.

 

[RSmith]

YWe already write different maps for using different boost pressures on the same engine, the same theory would apply. The technicalities of turbo lag could be overcome by using sequential turbos, the smaller one to spool up the larger one and when controlled by boost pressure/waste gate action they operate according to which engine configuration you are running at the time.

OR!!! - I've had an idea. What if you had (and I know this sounds a bit OTT but bear with me) an individual filter, turbo & inlet manifold system for each cylinder on something like a V8 and an individual exhaust pipe to solve backpressure problems. That way you wouldn't have to worry about half the cylinders cutting out at the wrong time and excessive pressure being forced into the remaining cylinders. You'd also have complete control ECU wise and could probably gain a lot of extra HP. Would that work?

By the way, thanks for all your responses guys... don't know what I'd do without you!

 

[obi_waynne]

The sheer weight of all those turbos and the piping would be a non starter before you even start looking at wastegate and blow off control.

This would probably also mean a separate exhaust from each cylinder to a turbo.

You would also need to feed the turbo with the exhaust from another firing cylinder, or have a bank of turbos in the main exhaust and this will induce a lot of extra drag in the exhaust stream.

 

[nig]

Have been thinking the same about 2 x 4cy engines side by side

 

[HDi fun]

Or Pug1Off's twin V6 Peugeot 306. Not exactly an eco-statement but the principles of operation are identical.

 

[davevm]

I think, in fuel injection, you can try two DC solenoid valves before fuel injection
and shut off alternate cylinders, it can work. This you cannot do for long, as engine temp gradients may cause leakages, for one. Also check the firing sequence, shuting off cyl fire would give you more torsional vibrations.