Previously I've expressed disappointment in these e-pages about the fuel consumption of my 07/56 320d (ie without the latest fuel-saving tweaks): I'd never been able to crack 50 mpg on the computer. On Monday I hot-footed it to central Manchester - about 175 miles on the M40/M42/M6, then A556/M56 etc(I think). Anyway, I pressed on and seemed to be doing 80/85 when I could. The computer showed 52.3 mpg!
Took it a bit easier on the way back, and overall for the return trip got 50.4 - maybe truly around 48/49 mpg. So I take it the engine is loosening up after 19,000 miles, and of course currently economy is helped by the hot weather - quicker warm-up from cold. Does my reasoning make sense?
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There's a guy on one of the Merc forums (with a Merc, obviously - E320CDi auto) who swears blind his car is regularly more economical if he drives at 80ish rather than 70ish.
All I can say is, mine (C270CDi auto) isn't!
Edited by Bill Payer on 30/07/2008 at 11:42
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As you say, economy is helped by hotter weather, but also my old Passat 130 was about 10% more economical at an 85 cruise rather than 75.
On a 150 mile motorway journey the computer would typically show 53 - 55mpg cruising at 85, but 47 - 48mpg cruising at 75.
However, above 85 economy would also fall quite sharply.
In part I reckon it's because at 80 - 85, you're going quickly enough so you're not getting baulked by trucks and slower drivers, which means your throttle position remains constant for longer (better for economy).
But also every car has a "sweet spot" where the specific characteristics of the engine's fuelling and torque curves, its rolling resistance and aerodynamic drag will combine to give an optimum balance between speed and economy.
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There's a guy (...) who swears blind his car is regularly more economical if he drives at 80ish rather than 70ish.
He must be thinking of the perpetual motion machine he's made at the bottom of his garden.
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He must be thinking of the perpetual motion machine he's made at the bottom of his garden.
I note also Craig's comments above. The only thing I can think of is that perhaps at 85 you're tending to drive more in a gas on / gas off mode and spending more time on the over-run (which is the preferred method people who compete in those extreme MPG competitions) than if doing a constant 75.
The other possibility is that, certainly in Merc autos, the box locks up proportionately, so there's less slip the faster you go, although the differences between 75 and 85 should be miniscule, and waaaay outweighed by extra wind resistance.
Edited by Bill Payer on 30/07/2008 at 13:47
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I tend to find I'm more gas on / off at 75 than at 85, because doing 75 means you're more involved with trucks & vans ...
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Hot weather - lower charge density - less fuel.
You can't buck the laws of Physics.
659.
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There is an arguement that judicious use of the 'go' pedal with diesels clears soot from the exhaust etc. and allows the car to run more efficiently than one that is molly-coddled all the time and becomes sooted up. I subscribe to this and rarely get any smoke on full-ish throttle, whereas gently run diesels tend to smoke more when given the berries.
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Hot weather - lower charge density - less fuel.
Interesting. I thought that cold weather = higher charge density = more oxygen available for combustion = more efficiency = higher mpg. Where have I gone wrong?!
Boxsterboy, i tend to have a periodic clear-out, which is why, when I accelerated out of the M6 Hilton services I could see a nicely extensive grey smog behing me...
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Max, "wrong" is a strong word, but diesels have moved on a bit.
Without writing a text book, the max fuelling limit for any diesel these days is governed by smoke - quite rightly dictated by legislation. So, to get the most power out of a given engine, you have to fuel it up to the acceptable smoke limit but not beyond.
This limit can be increased by further developments to the fuel system such as pilot injection, the use of very high injection pressures (PD or common rail) and careful combustion chamber design. The trick here is to achieve the best surface area to volume ratio whilst giving a high degree of turbulence in order to achieve a charge optimised for flame propagation. The objective is the same in all cases - most fuel for least smoke.
Modern diesels have transducers associated with their fuelling systems to achieve this - mass air flow meter, charge pressure measurement, charge temperature measurement, fuel temperature measurement and jacket temperature measurement - there are a few others used in some special applications. This means that the max fuel limit is a variable one which depends on these measured parameters.
In the example given, we have a turbocharged and intercooled diesel which will exhibit a lower charge density due to the high prevailing ambient temperature. The engine controller will therefore correspondingly reduce the max fuel in order to stay within the allowable smoke limit.
659.
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@ 659 -- do you find your Superb has a sweet spot? On my Passat with the AVF motor, it was at 2650rpm in 6th, which corresponded to 85mph.
Once at this rpm, it very rarely needed extra throttle input to climb a typical m/way gradient, the car would just take it in its stride.
At this pace on motorways, the instantaneous consumption readout would settle between 50 and 60mpg, depending on gradient / wind etc. On a typical 150 mile journey I could rely on 52 - 55mpg on the trip computer, which was reliably optimistic by 5%.
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Craig, My Superb has very slightly taller gearing than your old Passat - 33 mph/1000 rpm in top - probably due to bigger wheels. It's a bit heavier and longer and I suspect as with ships, lengthening the body reduces the drag. If your old Passat was an estate, this would generally make it less favourable than a saloon in terms of drag.
I really can't claim to have found a "sweet spot" with the AWX engine - I tend to use the car for all sorts of jobs including a lot of towing so it's difficult to establish an optimum. Having said that, I'm very well pleased with its performance and economy; with about 35k miles covered, it's probably now at its best.
Completely generally, most (admittedly much bigger) diesels I've tested in the Lab have an optimum SFC (specific fuel consumption) at around max torque. The torque band of the PD engines is very wide - especially the 2 valve units - but I did do one experiment based on this supposition.
2000 rpm is probably a shade over the speed corresponding to max torque for the AWX but gives a convenient 66 mph in my car. On a recent journey from London to Nottingham on a clearish M40/42 it returned 58 mpg genuine with the cruise set at about 67 mph. I was not in a hurry and avoided wasteful driving techniques. To get there and back on comfortably less than half a tank of fuel pleased me. It was also a very relaxing drive.
I agree with your comments concerning that silly trip "computer" - mine can be up to 10% optimistic. I wish they had fitted a good accurate clearly visible to all analogue car clock instead.
659.
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If yours goes anything like mine, it should stay at its best for a long while yet.
Although the motor in mine got smoother and quieter after 30K miles, it was every bit as quick in-gear at 60K miles as it was at 1500 miles.
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I've also seen claims like this where I ended up feeling cheated because my car unfortunately adheres to the law of physics, so ends up needing more energy to make it go faster and maintain the higher speed against frictional forces.
However, I drove my colleague's E320CDI and found that at 100km/h it appeared to still be in 5th gear. It didn't engage 6th until 110km/h and 7th around 120km/h. The gearbox also changes down earlier than an economy minded driver would. So, in the above example, it is possible that at 70mph the car is still in 5th gear and at 80mph in 7th.
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