I recently used Shell V-Power in my 2005 MINI Cooper S and 2007 BMW 320d to see if it gave better fuel consumption over 'cooking' petrol/diesel. Total mileage of test approx 3,000. Result: no meaninful - if any - improvement. Conclusion (for me and my cars): premium fuel = waste of money.
Of late I have taken to accelerating harder up to the speed I'm aiming for, and I THINK this improves consumption a bit. On the BM the 'swingometer' fuel consumption needle stays on the 50 mark for much longer, which is always nice to see...
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Accelerating hard and then holding a speed is good for economy;your engine is most efficient when operating at full throttle.These super-economy specials 3,000+ mpg operate this way-accelerate hard and then freewheel.
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3000mpg specials will have a tiny engine that is probably designed to run efficiently at one throttle setting. The actual rate of acceleration achieved is likely to be low.
Normal cars will use a lot more fuel if used at full throttle to accelerate. All the guides to driving for fuel economy say use light throttle.
Minimising braking and keeping an even speed wherever possible makes a huge difference to economy - especially with modern cars that are very heavy because they're full of safety kit like door bars, air bags and thick door pillars. All that weight takes a lot of energy to accelerate. Every time you brakle and accelerate again all that energy gets wasted. The less you brake and accelerate, the more energy stays in the fuel tank.
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I did the same on my Volvo, and was expecting great things being a turbocharged engine, and with the manual stating that 98 RON was recommended.
Result? A 3 mpg improvement which does not even get close to offseting the extra cost, and possibly an improvement in part throttle response, but certainly no big performance gain.
Will use up this tank and switch back to standard Shell 95 which has served various cars of mine well over the years.
I agree that swift acceleration up to the desired cruise speed does improve economy. I believe this has been confirmed in various tests too.
Cheers
DP
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>swift acceleration up to the desired cruise speed does improve economy
I would be interested if you had any links to any robust papers on this.
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It's not exactly robust evidence but I remember an older edition of Top Gear (when they still had Tiff & Quentin) doing an economy comparison, and the conclusion was that accelerating briskly to the target speed was more economical than gentle acceleration.
At a basic level is does stand to reason: even though harder acceleration uses more fuel, the work is being done over a shorter period of time so it probably uses less juice overall.
Especially in more modern drive-by-wire cars, which should only deliver the exact amount of fuel that can be burned in any given circumstance.
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Conclusion (for me and my cars): premium fuel = waste of money.
This is in agreement with my Focus handbook which says ......... "Higher octane unleaded fuels may be used but offer no significant advantage."
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You need to go back to the basics of combustion and particularly "specific fuel consumption" which in my days was measured in "lbs/hp/hr".Full throttle gave much better economy than part throttle.
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I have measured the SFC (specific fuel consumption) of many engines when I worked in this field and I'm old enough to have used lb/hp.h (not lb/hp/hr) - this is dimensionally incorrect.
It's very hard to generalise and many petrol engines will use transient mixture enrichment which significantly worsens the SFC over short bursts of acceleration. My main experience is with diesel engines, where a good rule can be generally applied. Operate the engine at the speed at which maximum torque is developed (see maker's curves or rating) and apply as little fuel with the right foot as will adequately do the job.
Turbocharger lag will slightly cloud (sorry) the issue as transient overfuelling will occur until the required boost pressure is developed. Remember with a diesel, your right foot controls the fuel tap, so however much fuel goes in is largely up to you.
659.
Edited by 659FBE on 24/04/2008 at 12:35
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If you let a cruise control accelerate the car up to the desired speed, I reckon you'll get a fair idea of the degree of acceleration appropriate for best fuel economy. I'd describe it as making firm and smooth progress relatively swiftly, rather than flooring it.
I agree with what has been said above: the greatest impact you can make on fuel consumption is to drive as smoothly as possible, minimizing harsh acceleration and harsh braking. I was reading one of HJ's old books of motoring answers (compilations from his Saturday column) the other evening and noted his mention of driving many long-distance miles without touching the brakes. That's what I try to do, and if you allow enough space around you then it's not that difficult. Watch the amount of braking in front of you and stay well back from it -- not only more economical (fuel and brake pads/disks) but also safer.
Edited by Roger Jones on 24/04/2008 at 13:28
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I seem to recall an article in the press a year or so ago saying that accelerating at full throttle then coasting in gear gave the best economy for petrol engined fuel injected cars.
The reason given, IIRC, is that the engine is most efficient at full throttle and a modern fuel injection system won't inject any more fuel than is needed for the given load, regardless of the throttle opening.
It's not the smoothest way to drive but they did claim some remarkable improvements in mpg.
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Handy comments here, including the well chosen word "briskly", which doesn't mean "flooring it":
tinyurl.com/5qhnjm
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The shortage of engine braking in my car makes slowing down by taking your foot off the accelerator (rather than by braking) more difficult to judge. I think there was more engine braking when cars generally had 2 valves per cylinder instead of the current trend for having 4 per cylinder.
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I seem to recall an article in the press a year or so ago saying that accelerating at full throttle then coasting in gear gave the best economy for petrol engined fuel injected cars.
I did experiment with "pulse and glide" in a petrol focus and my mpg was worse by about 2mpg
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Interesting Roger. I don't think (?) that CC is renowned for improving MPG age. My experience is that it is very thirsty.
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CC is certainly a factor in yielding 27-28 mpg from my MB E320 Coupé, and was similarly helpful in the previous Coupé and in getting 31+ out of the preceding Audi 100. It is widely recommended as a fuel-economy aid, but not in hilly areas and other circumstances where it can't be sensibly used. Given the nanosecond monitoring that is involved, I bet its a far more sensitive and effective device than my right foot.
Edited by Roger Jones on 24/04/2008 at 18:19
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