19.3mpg on the first half tank of Optimax, 18.1 on a tank of Esso 97RON...
Just refilled with Optimax and the idle has smoothed out again and throttle response is crisper - not a huge diff, but there.
I'll be buying Optimax from now.
Consume1 is still showing 23.7 so that would seem to be what it'll do when not driven in ShinyToy mode. =8)
I've just spent £120 on Single Malt Petrol going nowhere - worth every penny. :)
A
V8 diesels are a lot of fun..
V8 petrols are a lot more fun.
more fun at the filling station too eh?
I accept myself that there is a power output which I would call sufficient, I know that technology allows a diesel and petrol engine to both reach this output, the diesel will be cheaper to run and have more torque, so it must be better....
Not found one myself, it's certainly >340bhp...
You need to compare like with like - forced induction is not the same as atmospheric.
*charge a petrol motor to the same extent as a diseasel and you might start to approach "sufficient" power :) A
the *sufficient power is determined by myself, it doesnt matter how they reach this output, as long as they are the same
Like for a like, a Petrol will have as much torque (i.e. both forced induction or NA) over a much greater powerband.
in news: snipped-for-privacy@individual.net, "DanTXD" slurred :
Eh? For a specified max power, the diesel will have gob loads more torque, albeit over a narrower band. I'd definitely take the diesel if I was limited specifically by the power figure.
In fairness though, that tends to be because you are comparing a blown diesel, with an NA petrol (you mentioned specified max power).
However, if you truly compare like to like - ie NA diesel with NA petrol, and turbo'd diesel with turbo'd petrol, I'm not fully convinced that torque is always likely to favour DERV.
in news:42415875$0$10950$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :
Yes it will. The derv still won't rev as well as the petrol, and will make it's peak power at lower revs, so by definition it must have more torque.
Diesels need *double* the torque to reach the same power, cos they only rev half as high !
Remember, Power is Torque * RPM.
If you're halving the RPM, then you need to double the torque to keep the same power !
OK.
How does it follow that it _must_ have more torque?
What characteristics of the engine are you asserting make the difference? Compression? Stroke length?
in news:42418af9$0$10940$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :
Good :-)
I'm not, 'cos that's irrelevant. power(W) = torque(nm)*[rpm*60]*2*Pi Ignore all the constants-> power=torque*rpm.
To get a given power, if you reduce the rpm, the torque must be higher. So and engine producing XXBHp @ 3500rpm produces 2x the torque of an engine producing XXBHP @ 7000rpm.
A turbo petrol will almost certainly produce peak power at higher revs than a diesel, so if they produce the same max power, the diesel will produce more torque. That's all I was saying.
Much easier to remember two pies 'n' tea. Since in the UK most still quote in BHP and lb.ft
2 x Pi(3.14) x RPM x torque(lb.ft) BHP = --------------------------- 33,000
Smashing ;-)
Really? The proposition that somehow or other, diesel engines by their very characteristics must generate more torque - like for like - than a petrol engine?
Whilst the physics seems OK, torque is more an area-under-the-graph thing, than a snapshot at a rev point.
Also, the point I was making, being that truly comparing like for like - capacity to capacity, and induction to induction, I don't think is likely to show much difference in the torque capabilities.
What normally happens, is that people compare a blown diesel, with an NA petrol. What you did was compare peak power output, and true enough, for that scenario - at a _snapshot_ you can likely claim higher peak torque output. However, the torque is only available for a fairly narrow range (compared with a petrol engine).
But _overall_ available torque is less, because there's _less_ revs available.
Skewing it in favour of one metric, ignores the big picture. I'm far from convinced that on a like-for-like, engine capacity and induction basis, there's much in favour of diesel engines where torque is concerned. Comparing peak power is deceptive, because it's highly likely that you'll be comparing a turbo'd diesel with an NA petrol.
As a peak figure, perhaps. But as it has much bigger rev range at which it will be outputting torque, _overall_ torque available will be higher (think area under the curve). Peak torque isn't of as much relevance as the range it's available for.
And I get the purist physics perspective - but it's flawed - it either ignores the salient aspect of torque, or compares engines with different induction systems in order to compare a flawed angle.
in news:4241aae4$0$10951$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :
Splendid :-P
Within the useable rev-range, given typical petrol & TD characterisitcs and a specified max power, then yes.
Um. No it isn't. Tractability/flexibility may be.
Ah. Er. There are far too many variables there for me to deal with. I wasn't attempting to compare aspiration types or anything like that.
Not really. A petrol has a wider rev range and a flatter torque curve, but the diesel torque curve is higher from where the turbo starts blowing up to the sudden high-rpm drop off. Good modern diesels are pulling hard at 1400 rpm and tailing off at nearly 5000rpm, i.e. a dynamic range of about 3.7, which is comparable with a lot of petrol engines, and better than quite a few turbo petrols! (c.f. my old TD, dynamic range ~2.3)
Area under-curve wise, the diesel is better for a given max power because of the big mid-range torque peak!
Yes, but so what? My argument didn't depend on whether the petrol was the same capacity, or turbo'd or anything. Because petrols have a flat torque curve and rev higher, a diesel of same max power will be quicker at anything other than peak-power rpm!
I find your concept of 'overall torque' difficult to put a physical interpretation on. The flexibility of an engine could be defined as being the ratio of maximum useful revs to minimum useful revs, in which case modern diesels compare fairly well with petrols.
I agree that peak power figures are not particularly important in normal driving, and that having a nice fat torque curve is, but it is also undeniably the case that diesels do have more torque, and it's available over a pretty wide range in the modern lumps.
The induction systems are essentially irrelevant! This has strayed quite far from my initial quite conservative argument, but even so, I'd have the diesel if I had to choose based purely on the max-power!
The only way you can usable compare max power though, is to compare a TD with an NA petrol. If you compare a turbo'd diesel with a turbo'd petrol of the same capacity, you'd likely see a different picture. The diesel won't have anything like the same peak power, on account of not being able to rev as high, and I expect the torque _available_ will still favour the petrol engine.
It is from a driving perspective.
Peak torque isn't of huge importance if it's only for a brief range of revs.
But they are like for like.
The area under the curve is likely smaller, though, on account of the smaller usable range of revs.
It only ever favours the diesel engine if you compare based on peak power output.
Only if your comparison basis is peak power output.
For the same displacement and induction, I doubt it's quite as favourable.
That's the generalisation I was objecting to.
How so?
How does a diesel engine somehow magically get this higher torque output?
But it's rare that diesels and petrols are compared so - because it's only that way that diesels sound favourable.
On a capacity and induction based comparison, it's not as likely to be as favourable - and that's more representative.
in news: snipped-for-privacy@davenoise.co.uk, "Dave Plowman (News)" slurred :
Bah. SI units all the way - I'm not using mad olde worlde units. Like miles, or hours. Um. :-)
in news:4241b419$0$10944$ snipped-for-privacy@reading.news.pipex.net, "Douglas Hall" slurred :
Thats absolutely true, but it's also beside the point - we were talking about a specific case where peak power was the contstrained variable. You can have a TD, a NA petrol of roughly the same capacity or a forced induction petrol of somewhat smaller capacity - the argument holds.
No it isn't :-)
But since I'm a big nerd with too much spare time, I've made the comparison anyway, using Dyno plots from the superchips.co.uk website, comparing the VAG 2.0 FSI (150BHP) petrol engine with the VAG 1.9 TDI (150BHP). I couldn't think of any small capacity turbo'd engines with
150 BHp to compare.The area under the torque curve for the petrol (arb units) = 1845 Area under torque curve for TD (same scaling) = 2085
A more meaningful test in my opinion is to stretch the rpm-scales, such that the max-useable revs match, and scale the torque values accordingly (i.e. power is still correct at each point) In effect I've just included a gearbox - so we are looking at wheel torque and powers.
The area-under curve ratios remain the same, but the results are quite interesting. Rpm range for both is approx from 1000-5000rpm. Below 1800rpm the petrol gives more power at the wheels. From 1800 -
4200rpm, the diesel is better, and from 4200-5000 the petrol is better.So, basically, if you primarily drive in the bottom 36% of the rev range, or the top 16%, then the petrol is a better option. If you stay between
36% & 84%, then the diesel is better. If you mainly use the top 30% then both are almost exactly equal..I reckon that, for me, in day to day driving, the diesel is by far my better option, and in press-on driving they are almost exactly equal, which is a bit spooky.
My final discaimer is that all this is for the artificial case of a fixed max power. If I was to make a choice without that limit, I'd go for the turbo petrol every time.
Because Power = Torque * RPM
If the Diesel is making the same power as the Petrol, at less RPM, then it
*MUST* be making more torque ! Otherwise, it would be making less power !MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.