Cylinder on demand

[JAMES]

I have read up and watched the videos etc but still can't get my head to understand how this works. Yes it runs on 4 cylinders and yes the saves fuel but surely with the 4 cylinders valves closed the cylinders are still compressing the air in the cylinders which surely means the other 4 cylinders have to work harder. Or have I missed something?

[MikeFish]

I don't know, but I am pretty sure I read that there was a light on the dashboard that comes on when COD activates. I have yet to see this happen. Not sure if I have disabled it somehow but if it didn't come on when I was running the engine in then it will never come on (I was properly driving like miss daisy for the 1st 600 miles).

[JAMES]

The economy bar showing the mpg in the information display goes green and says 4 cylinder mode but you need to have the right display showing in between the clocks

[JAMES]

We're you advised 600 miles to run in?

[MikeFish]

Oh, I'm never on that screen. Maybe I'll try it tomorrow just to see if it comes on and when. From memory of when I read the manual I thought it went green when you were not pressing the throttle and the battery goes green to show charging (regen?). Or is it more than just the battery going green? Don't worry answering that, I'm going to read the manual now.

[MikeFish]

We're you advised 600 miles to run in?

The manual says to take it easy for the first 1000km, then slowly build up the load / revs up to 1500km (so basically 600 miles and 900 miles). But then it goes on to say, don't really drive it hard ever.

[JAMES]

OK I get another a8 diesel then. Thanks

[ImmortanJon]

Paragraph 5 of this article http://www.topgear.com/car-reviews/audi ... /road-test gives a bit more COD.

With regards run-in, my dealer told me there was none, but then reading the manual it does indeed state a run-in period. I can't remember but think it stated something between 500-1000 miles. I tired to keep it light during that period - torture - but could not resist the odd blast.

[JAMES]

And yes you need to have the long mpg bar displaying. Not sure if it shows anywhere else but not that I noticed In The s8

[Markp]

You can sometimes feel it it kick in and out - me running over a very small bump in the road.

[wildbore]

I have read up and watched the videos etc but still can't get my head to understand how this works. Yes it runs on 4 cylinders and yes the saves fuel but surely with the 4 cylinders valves closed the cylinders are still compressing the air in the cylinders which surely means the other 4 cylinders have to work harder. Or have I missed something?

Under light loads, a four cylinder engine is quite capable of propelling a 2 tonne car along and of turning over an attached non-running 4 cylinder engine (which takes very little effort compared to pushing a big car through the air at 80 mph).

Imagine the V8 as two separate 4 cylinder engines, both connected to the drivetrain. The first is driving the car along. The second one is on a free wheeling clutch so that it is able to run and not put any power into the drivtrain but, if revved, can add to the power being put into the drivetrain once its rotational speed exceeds that of the drivetrain side of the clutch. If you fire up the second engine and it is just idling, it contributes nothing (because the clutch is slipping) but it's still burning fuel, just like an engine idling whilst parked up. Now increase that engine's revs up to the same speed as the main engine. Still the car is being driven by the main engine and now both engines are turning at the same speed but still the second engine is not contributing to driving the car forward but now its consuming a lot of fuel. Now rev it some more and it starts to put power into the drivetrain because the clutch engages and so the engine helps the car accelerate (the main engine being also speeded up in sync).

Now mate those two engines on one crankshaft... and you have COD.

Did that explain it? Or make it less clear?

[Bit_evl]

I have seen it come on when sitting in the usual nose-to-tail M4 (literally for some people).
As mentioned you need the central (speedo/tacho) display on the economy segment.
I am going to say that you need the car in default comfort mode as well as I have not seen it in Dynamic or Individual (exhaust on sport for me)
Below the horitzontal bar a line of text comes up - 4 cylinder mode enabled or some such
I would say you do have to be feathering the throttle to get it on. I don't notice anything when it kicks in or out.

[JAMES]

Yes I understand that 4 cylinders are disengaged but the 8 are rotating and then 4 are firing so effectively it will not be as economical as a v4 or if a clutch did couple 2 v4 engines as all 8 cylinders still have compression. So it obviously saves fuel that would of entered the cylinders but effectively the 4firing will surely use slightly more fuel to rotate the non firing cylinders? I would of thought it would make sense to have a way of releasing the pressure in the 4 unused cylinders so as they would rotate freely when not in use. Anyone who has tried to turn an engine over manually would know it's not easy to compress

[MikeFish]

I switched to the economy bar display today and I saw it switch to 4 cylinder mode a few times whilst i was still waiting for the engine to warm up. And I must say it was seamless, no wonder I didn't notice it before.
I was in Dynamic mode at the time, but start-up dynamic, not true dynamic (so left in dynamic from previous use and in D). Once the engine was warm I cycled round to dynamic mode which has the effect of making putting the gearbox into S. Once in went into S mode the revs were too high for it to activate the 4 cylinder mode.

[Bit_evl]

Yes I understand that 4 cylinders are disengaged but the 8 are rotating and then 4 are firing so effectively it will not be as economical as a v4 or if a clutch did couple 2 v4 engines as all 8 cylinders still have compression. So it obviously saves fuel that would of entered the cylinders but effectively the 4firing will surely use slightly more fuel to rotate the non firing cylinders? I would of thought it would make sense to have a way of releasing the pressure in the 4 unused cylinders so as they would rotate freely when not in use. Anyone who has tried to turn an engine over manually would know it's not easy to compress

Guessing - since the TFSI is direct petrol injection at the cyl-piston so the other valves/pistons are likely to remain running with standard air pull through and no "bang" otherwise it would stall the air flow in the intake and exhaust which would be counter productive.