RE: Driven: Audi S8
Discussion
kambites said:
That doesn't answer the question - why would a bigger engine need more fuel to generate the same amount of power? The fuel contains a set amount of energy per unit volume so to generate a given amount of power you need a certain amount of fuel per unit time, irrespective of the swept combustion volume. The only advantage cylinder deactivation gives you is more efficient burning of the fuel - ie a higher proportion of the fuel injected into the cylinder goes towards pushing the piston down.
Explaining why cylinder deactivation helps at all, is quite complicated.
The way I see it is thus:Explaining why cylinder deactivation helps at all, is quite complicated.
Edited by kambites on Thursday 13th October 08:59
Fuel may contain a set amount of energy but not all of that is harnessed - it depends on the efficiency of the burn. Petrol burns best under load and the difference in load between the two engines is the key.
Take for example two identical cars one with a 2.0 petrol and the other with a 1.6 petrol. If both are in the same gear cruising at 60mph the 2.0 will be at a slightly less throttle opening and so less efficient burn. So the 2.0l is operating less efficiently so needs slightly more petrol than the 1.6 relatively speaking.
It doesn't always work like this - if the 1.6 was running flat out and the 2.0 was matching it the chances are that the 1.6 is operating above it's peak efficiency level and so would be less efficient than the 2.0.
It's one reason exhaust gas recirculation is used to dilute the incoming air when at low throttle - if you reduce the combustible mixture you need more of it so you can run at a higher throttle making things more efficient. Deactivating half the cylinders has a similar effect.
It'd never be a perfect setup as you'd still get friction and lossed from the other cylinders moving about and IIRC the Audi system closes the valves on the deactivated cylinders so the engine loses some power from compressing air trapped inside but then gains some by it springing back on the downstroke.
minimatt1967 said:
alexpa said:
Ronin chase is quality. S8 is a sledge hammer. Doesnt say, but it MUST be 4WD?
Erm, the Ronin chase involving the S8 is, Audi S8 V8 vs Peugeot 306 police car, probably diesel, not the hi-light of the film really.Still a cool car though.
Roop said:
thewheelman said:
Roop said:
Riggers said:
thewheelman said:
I think you'll find the lowest drag coeffiient in this class is the Lexus LS600h, which is 0.27. If it's a luxury, powerful & fuel efficiet barge you want, you'd be mad to not consider the big Lexus. A combined output rating of 439bhp & 30+mpg is pretty impressive.
When Audi says 'in its class' it means XJ supersport and merc S63. And possibly BMW 760iL. it doesn't count the LS600h as a rival for the S8. Edited by thewheelman on Wednesday 12th October 15:10
(But FWIW I think you're right
)ads_green said:
The way I see it is thus:
Fuel may contain a set amount of energy but not all of that is harnessed - it depends on the efficiency of the burn. Petrol burns best under load and the difference in load between the two engines is the key.
Take for example two identical cars one with a 2.0 petrol and the other with a 1.6 petrol. If both are in the same gear cruising at 60mph the 2.0 will be at a slightly less throttle opening and so less efficient burn. So the 2.0l is operating less efficiently so needs slightly more petrol than the 1.6 relatively speaking.
It doesn't always work like this - if the 1.6 was running flat out and the 2.0 was matching it the chances are that the 1.6 is operating above it's peak efficiency level and so would be less efficient than the 2.0.
It's one reason exhaust gas recirculation is used to dilute the incoming air when at low throttle - if you reduce the combustible mixture you need more of it so you can run at a higher throttle making things more efficient. Deactivating half the cylinders has a similar effect.
It'd never be a perfect setup as you'd still get friction and lossed from the other cylinders moving about and IIRC the Audi system closes the valves on the deactivated cylinders so the engine loses some power from compressing air trapped inside but then gains some by it springing back on the downstroke.
Indeed, So cylinder deactivation starts from the default of the same economy and makes small incremental improvements via more efficient burning of fuel. It's never going to be even of the order of magnitude of twice as efficient and there's no reason it would be. 10% sounds entirely believable to me. Fuel may contain a set amount of energy but not all of that is harnessed - it depends on the efficiency of the burn. Petrol burns best under load and the difference in load between the two engines is the key.
Take for example two identical cars one with a 2.0 petrol and the other with a 1.6 petrol. If both are in the same gear cruising at 60mph the 2.0 will be at a slightly less throttle opening and so less efficient burn. So the 2.0l is operating less efficiently so needs slightly more petrol than the 1.6 relatively speaking.
It doesn't always work like this - if the 1.6 was running flat out and the 2.0 was matching it the chances are that the 1.6 is operating above it's peak efficiency level and so would be less efficient than the 2.0.
It's one reason exhaust gas recirculation is used to dilute the incoming air when at low throttle - if you reduce the combustible mixture you need more of it so you can run at a higher throttle making things more efficient. Deactivating half the cylinders has a similar effect.
It'd never be a perfect setup as you'd still get friction and lossed from the other cylinders moving about and IIRC the Audi system closes the valves on the deactivated cylinders so the engine loses some power from compressing air trapped inside but then gains some by it springing back on the downstroke.
Well, I'm lucky enough to have the V8 Diesel (4.1 and a bit... so 4.2 in Audi's eyes) and it is a stonking engine. I will, none the less, have a go on an S8 when I get the chance and you never know. 4.2s to 60 and 4wd me likey (see username for clue)
To address the "nay sayers" with my two penneth...
The A8 is all about subtlety. Yes, it looks like a big A4 but maybe that's the point. I'm a freelance consultant, so like my big luxo barge but can lurk in a car park quite easily without everyone in my clients offices tutting about my daily rate.
The interior is far superior to the BM, Lexus & Jag in my opinion and nowhere near as "bling" as the S class merc (I did try them all before I plumped for the 8) The ergonomics suit me and the seats are "best in class" according to my arse.
The others in class (yes.. inlcuding the Lexus) are all good. You don't spend this kind of money on a car without expecting it to be pretty special, however to me Audi's all have a certian something that the other don't (Stop shouting "Understeer" at the back.. the D4 really is a lot better!)
I for one am glad Audi have made the S8 as it means you don't have to by the AMG!!!
To address the "nay sayers" with my two penneth...
The A8 is all about subtlety. Yes, it looks like a big A4 but maybe that's the point. I'm a freelance consultant, so like my big luxo barge but can lurk in a car park quite easily without everyone in my clients offices tutting about my daily rate.
The interior is far superior to the BM, Lexus & Jag in my opinion and nowhere near as "bling" as the S class merc (I did try them all before I plumped for the 8) The ergonomics suit me and the seats are "best in class" according to my arse.
The others in class (yes.. inlcuding the Lexus) are all good. You don't spend this kind of money on a car without expecting it to be pretty special, however to me Audi's all have a certian something that the other don't (Stop shouting "Understeer" at the back.. the D4 really is a lot better!)
I for one am glad Audi have made the S8 as it means you don't have to by the AMG!!!
thewheelman said:
I think you'll find the lowest drag coeffiient in this class is the Lexus LS600h, which is 0.27. If it's a luxury, powerful & fuel efficiet barge you want, you'd be mad to not consider the big Lexus. A combined output rating of 439bhp & 30+mpg is pretty impressive.
and bugger all room in the boot so useless!Edited by thewheelman on Wednesday 12th October 15:10
ads_green said:
The way I see it is thus:
Fuel may contain a set amount of energy but not all of that is harnessed - it depends on the efficiency of the burn. Petrol burns best under load and the difference in load between the two engines is the key.
Take for example two identical cars one with a 2.0 petrol and the other with a 1.6 petrol. If both are in the same gear cruising at 60mph the 2.0 will be at a slightly less throttle opening and so less efficient burn. So the 2.0l is operating less efficiently so needs slightly more petrol than the 1.6 relatively speaking.
It doesn't always work like this - if the 1.6 was running flat out and the 2.0 was matching it the chances are that the 1.6 is operating above it's peak efficiency level and so would be less efficient than the 2.0.
It's one reason exhaust gas recirculation is used to dilute the incoming air when at low throttle - if you reduce the combustible mixture you need more of it so you can run at a higher throttle making things more efficient. Deactivating half the cylinders has a similar effect.
It'd never be a perfect setup as you'd still get friction and lossed from the other cylinders moving about and IIRC the Audi system closes the valves on the deactivated cylinders so the engine loses some power from compressing air trapped inside but then gains some by it springing back on the downstroke.
What about this, is an engine not most efficient at peak torque? Should it therefore not be most efficient if held at that speed?Fuel may contain a set amount of energy but not all of that is harnessed - it depends on the efficiency of the burn. Petrol burns best under load and the difference in load between the two engines is the key.
Take for example two identical cars one with a 2.0 petrol and the other with a 1.6 petrol. If both are in the same gear cruising at 60mph the 2.0 will be at a slightly less throttle opening and so less efficient burn. So the 2.0l is operating less efficiently so needs slightly more petrol than the 1.6 relatively speaking.
It doesn't always work like this - if the 1.6 was running flat out and the 2.0 was matching it the chances are that the 1.6 is operating above it's peak efficiency level and so would be less efficient than the 2.0.
It's one reason exhaust gas recirculation is used to dilute the incoming air when at low throttle - if you reduce the combustible mixture you need more of it so you can run at a higher throttle making things more efficient. Deactivating half the cylinders has a similar effect.
It'd never be a perfect setup as you'd still get friction and lossed from the other cylinders moving about and IIRC the Audi system closes the valves on the deactivated cylinders so the engine loses some power from compressing air trapped inside but then gains some by it springing back on the downstroke.
Stu R said:
Lexus for me all day long, the LS600h is incredible, does everything a car in that class should do, perfectly.
I know I'm going to get flamed for this but who cares.....Come on!!! A Lexus, no matter how hard it tries will not be in "the class" in the same way a GT-R will never be in the same class as a Fezza or a Lambo.
I drove the 600 for half a day, and not only was it the "deadest" car to drive, it felt plastic, the interior was designed by a 7 year old accountant wo thinks tons of buttons are cool and what's more, it smells like a prius.
Call my car a munter all day long if that's what does it for you, but come on man, a blinged up Toyota?.. gimme a break.
The best in class is without doubt the Merc, the Audi and BM are pretty much neck and neck with the jag maybe creaping ahead on the kudos factor, but I'm 37.. and the A8 is the only one without a hefty lump of "old man car" image.
http://world.honda.com/automobile-technology/VCM/d...
http://world.honda.com/automobile-technology/VCM/
A bit of an explanation lives up there! It is old info though.
http://world.honda.com/automobile-technology/VCM/
A bit of an explanation lives up there! It is old info though.
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