Why do big engines not shut down cylinders from running?

[Wafflesmk2]

I was thinking about bigger engines, crap mpg and other fuel things.

Take a standard 6 cylinder engine, say for example a 330d.

When im going down the motorway, or round country lanes, then fair enough, fire on all 6 cylinders. However in the middle of london, doing 3mph for 2 hours, why do all 6 need to be running? In really thick stop/start traffic, you could easily move the car on 2-3 cylinders couldnt you?

And then if you're just pottering around town at 25-30mph, again with stop start traffic, why doesnt the engine run on 4 cylinders. It would surly be similar to driving a 320d, and would save on emissions, fuel and all that. I can imagine a button on the dash similar to a sport button to tell the car what 'mode' you are driving in, and obviously if you need the power, you put the boot down and all 6 fire up. I cant imagine it would be hard to shut down and start up cylinders, using the ECU/fuel delivery settings.

You could apply this to a v8, v10 or whatever. I know its like stop/start technology, but for bigger engines. It's either done and i've never seen it, or there's a good reason they don't do it.

Edited by Wafflesmk2 on Saturday 19th May 18:51

[MrMoonyMan]

Some do.

http://en.wikipedia.org/wiki/Chrysler_Hemi_engine

Explained here;

http://en.wikipedia.org/wiki/Active_Fuel_Managemen...

Edited by MrMoonyMan on Saturday 19th May 18:52

[GTIR]

Yes some big V8 do.
I'd imagine you'd need balancing shafts and stuff which means extra costs, larger engine. Dunno.

[FisiP1]

It is fairly recent in mainstream cars, I have no idea if it one of those technologies that made an appearance in the past then fell out of favor or not.

Mercedes seem to have taken to it the most of the big manufacturers.

[kambites]

Some do, but the fuel savings aren't huge and you can only do it if you have a firing order that will remain reasonably smooth when only certain cylinders firing.

For example with a straight-6 which fires 1-5-3-6-2-4, you'd end up with either 1-3-2 or 5-6-4 so all the firing cylinders would be at one end of the block. I've no idea if that would cause a problem (heat gradient along the block?) but I could quite believe that it would.

[Oilchange]

I'm wondering about Stop/Start as well, whats the harm in implementing that, it's run off the battery and starter motor after all....

I expect it would help enormously

[cure]

You still have the friction and compression of the cylinders even if they're not fired, so you're not simply making it a smaller 4 cylinder engine by not firing and fueling the others.

[davepoth]

First one was a Cadillac back in the 80s. It didn't work very well then, and that put people off it for a while.

[anonymous-user]

Primerarily for cost reasons!

You can't just turn off the fuel to shut down a cylinder, (because it would still be an inefficient air pump, resulting in large losses and cruically a mean exhaust lambda leaner than 1, resulting in your catalyst not working at all !), so you need to deactivate the valve train to prevent the valves opening. This is complicated and expensive. And you still have the frictional losses from the piston rings (which are a large proportion of engine friction).

It's better to just spend the same amount of effort improving the specific output of a small capacity / cylinder count engine, and that way you get all the efficiency benefits ;-)

For products that must have some fundamental engine layout to meet their product aspirations (sports cars with V12's for example) then cylinder deac will still appear, but it will be religated to high end (expensive) niche models mostly.

[Wafflesmk2]

This is where i prove how much of an idiot i am.

I was just thinking that without fuel, the piston would just stop and the cylinder would be 'dead'. I forgot it would keep moving...

[Blown2CV]

Could you not just somehow add a clutch to the cam for each pair of cylinders? It may be hard to do but as there have been production cars with this feature then clearly it is possible!

[Crafty_]

Blown2CV said:

Could you not just somehow add a clutch to the cam for each pair of cylinders? It may be hard to do but as there have been production cars with this feature then clearly it is possible!

Still need to open the valves on deactivated cylinders (unless you find some way to stop the pistons too...)

[paulmoonraker]

Wafflesmk2 said:

This is where i prove how much of an idiot i am.

I was just thinking that without fuel, the piston would just stop and the cylinder would be 'dead'. I forgot it would keep moving...

What about two engines and a big leaver to engage the second one.

[Bobley]

Crafty_ said:

Still need to open the valves on deactivated cylinders (unless you find some way to stop the pistons too...)

When you de-acc a cylinder you disable the valve gear. This is done either by having a hydraulic plunger inside the tappet which is forced over by oil pressure from a solonoid valve or a similar plunger locking finger followers together.

The gas/air which is trapped inside the cylinder is then mashed up an down repeatedly acting as a pneumatic spring so most of the work done in compressing the gas is given back on the downstroke.

Other other way to improve the fuel consumption on a six cylinder is to partially shut down the valves, this means the engine only gets very small gulps of air and the throttle is opened up so pumping losses are reduced. Most (if not all) Porsche engines use this technology by having a collapsing tappet.

This illustration shows the tappets collapsing. The centre part of the tappet is running on soft lobe of the cam (which doesn't look like its going to open it at all) while the 2 thin lobes for full lift are just pushing down on the collapsable part.



BMW use a second eccentric shaft to adjust the amount lift from the camshaft which is applied to the valve.

Edited by Bobley on Monday 21st May 17:37