Petrol Cost Cutting System Article Discussion Area

Former_MSE_Archna

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Charles_Brown_2

Hi

My first message to you after the article in the Guardian

I regret to see you apparently have not read your own article!

Step 3 says the biggest saving is cutting fuel costs.

Yet further down you report the RAC says 30% savings are possible by better driving. This equates to around 30p / litre, much more than the price savings you quote!

These savings are possible, not just with less braking and acceleartion, but with lower top speeds which drink the fuel.

Regards

Charles Brown

MSE_Martin

Thanks Charles, your post made me smile. Those tips were from the RAC and are calculated from the very worst to the very best. It actually says "can increase fuel efficiency by up to 30%", however that doesn't mean it directly equates to savings. Most people don't drive that way and as such the savings are much smaller for most people.

As for not having read my own article, confused as to how that could be, but i get your point

praven

I drive a diesel and I have a hypothesis that some diesels give better economy than others. I fill up with the regular diesel at either Sainsbury or Tesco and I am sure that I get significantly more miles per litre with the Tesco fuel. At this stage this is just based on a quick mental caluclation each time I fill up, and of course it could be influenced by the type of driving I have been doing.

I wonder whether there is some property of the fuel that the company has to publish that makes it possible to get some kind of measure of the economy to be had from the fuel.

I'm talking about as much as 5%.

Any thoughts?

Paul

tomstickland

Just some general thoughts on fuel consumption...

Firstly internal combustion engines are around 25-30% efficient. ie: energy delivered to the wheels is about 25% of the chemical energy in the fuel. The losses are due to mechanical friction, burning efficiency and fundamental thermodynamics (Carnot cycle) which limits the efficiency of a heat engine.

At a steady speed then the power required to overcome drag and rolling resistnace scales with the speed cubed, but the time spent travelling at that speed to cover a distance scales with 1/speed. So mpg scales with speed squared. Hence going from 70 to 80mph would increase fuel consumtion by 30%. This argument falls apart at very low speeds because the engine itself has an efficient operating range.

In "internal combustion engine fundamentals" there's a graph of bsfc (brake specific fuel consumption" versus rpm and load. Roughly speaking, peak efficiency is around 2000-3000rpm and 50% load (from memory). High rpm running increases frictional losses in the engine. Low rpm running is inefficient for reasons I can't remember; probably the filling efficiency (volumetric losses) and other gas flow processes.

Drag forces are proportional to frontal area of vehicle. So tall and wide vehicles will increase fuel consumption. Hence why sports cars are generally low and small.

Acceleration requires a lot of energy. Braking turns this energy into heat. Stop-start driving is inefficient. Vehicle mass is very important. The work done accelerating the vehicle is proportional to the mass of the vehicle.

So, for best mpg you want:
-small car
-light car
-drive 2000-3000rpm
-keep peak speeds down
-smooth driving, avoiding heavy braking

One area that is open to debate, is the question of whether a short burst of hard accceleration is different from a long gradual acceleration.

wronskian

It is definitely the case that different brands of petrol/diesel will suit different cars differently!

The keenest MoneySavers will want to try careful comparisons, by brimming the tank with one particular brand of fuel, then driving and recording fuel consumption, before trying the experiment again with a different make of fuel, but as similar driving conditions as possible.

Personally, I find I get anything up to 5mpg less from supermarket petrol (Tesco and Sainsbury's are the only ones we have locally) than the brandname alternative (Shell or Esso in this case). Supermarket petrol also lacks beneficial additives, which over a long period of time will cost you in car maintenance. No, this is not old wives' tales: it's simple fact. Go and discuss it on the excellent Honest John forum if you don't believe me! (https://www.honestjohn.co.uk - the Daily Telegraph's motoring Q&A columnist.)

So do your homework, do the maths and check that the cheaper petrol really is saving you money.

tomstickland

There must be a reason for that. Either in terms of Octane number or fuel calorific value. Before comparing different brands I suggest that you test the repeatability of your experiments by testing 10 tanks of identical fuel and seeing what sort of scatter you get in the results.

I can agree that you might see a small difference between brands on some cars for the following reason:
some engines will make better use of higher Octane rating by advancing the timing, so you will see an improvement in mpg. The calorific value of some high Octane fuels is lower than for the normal petrol, so could give lower mpg if the engine can't utilised the higher Octane rating.

Years ago I had rather rubbish 1.1 Escort and that was very sensitive to fuel brand. Big names worked better than supermarket fuel in terms of engine smoothness and response.

Stickman_2

I've spent considerable time looking into fuel saving as I do a lot of driving - just a few notes tho Or I'd be writing you all essays of stuff!

There's a good article I found recently in January's Evo Magazine which has a quite careful study of the major brands to answer the questions of whether different fuels are better... (available from stores, £3.95, or cheaper by subscription if you want repeat issues) - i suggest you sneak into tesco's an have a sneaky read if it's just that article you want

Also, there used to be much information on the now closed AA petrolbusters website about efficient driving techniques: one that I remember is, it's not only aircon that uses more fuel (only a few percent more actually, not a HUGE amount) but also driving with a window down, as that greatly increases drag slowing the vehicle down.

Also, If you're a true penny-pincher, don't use high power items like the rear screen heaters (usually 30amp) or full-speed air blowers - if it's getting warm you want, having them blow at low speed can often seem to blow warmer air onto your hands than at high speed, as the air spends more time in the heater matrix getting warm. (better yet just wear gloves for the first couple of minutes until your engine is warm) But it's mainly the driving that makes the difference - uses more power than any accessories.

The old petrolbusters site also mentioned that for most cars the peak efficiency was at around 55mph - more for some with longer ratio gearboxes designed for motorway cruising, like the Vauxhall Vectra.

Staying a safe distance behind a large/high vehicle (like a big lorry) can help a little by cutting drag, as it's wake is "pulling" you slightly - so being stuck behind a lorry does have it's upside - don't be tempted to get too close tho - as the repair bill or driving fine will easilly outweigh any savings you make

I have one point to note about what is written in the "money saving tips" email - that about re-filling at a quarter tank before empty - although it's best not to let it get to the bottom of the tank, a quarter remaining is unnecessary, even on sporty cars a quarter tank will be 60 miles remaining, and fuel that's left in the tank can lose potency (octance rating drops) which some people call "going stale." - leaving that last quarter never used, although diluted with fresher petrol - is not ideal.

kcandteresa

Everyone keeps banging on about how aircon uses lots of energy, but can anyone give me some estimate about just HOW bad it is? I drive a deisel 406, use aircon almost continually, yet still get 40 - 45 in mixed driving.

tomstickland

I read somewhere that it made a few mpg difference.
It's typical of modern cars - too much power sapping tat.

carl310166

If there is ANYTHING you need to know about diesels and fuel economy,read here,http://forums.tdiclub.com/forumdisplay.php?f=24

These guys are fanatics,one point to remember though,their US gallon is different.