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I have been using Asda Petrol in my Ford Focus for around 4/5 months solid now. However, I filled up with Morrsions petrol time before last and my MPG went down alarmingly, from close to 32 mpg to just under 28. Has anyone else noticed this with Morrisions petrol? A few others I know have mentioned this too, a colleague regularly gets 19-20 out of a 2.0 petrol Rav-4. Also, what is the best value petrol brand - standard and premium -, in terms MPG? Forgive me if this is not the spirit of Autocar Forums

It's as relevant to the forum as any other question.

Generally supermarket petrol is of a lower grade to that of the major manufacturers, even though the octane ratings are the same.

To raise the octane rating of cheap petrol, the suppliers will "butanise" the fuel by literally blowing Butane through it.  This raises the octane rating and allows it to be sold according to the British Standards.

Depending on the supplier / supermarket further additives are added to the mixture when it is loaded on to the distribution lorries.  You may remember a while back Morrisions had a problem with Silicon contamination in its fuel.  This would have occurred at this point.

It isn't just the supermarkets that use additives.  Most of the manufacturers will have some sort of "cleaning detergent", or "performance increasing" chemical in them. 

To muddy the water further, the quality also depends on the distribution depot in the area you are in.  For instance fuel supplied from the Thurrock depot will be slightly different to that of the Liverpool depot.

Personally, of all the supermarket brands I have found Asda and Morrisions petrol to be the worst for performance an fuel consumption.  Tesco's however seems to be the best.

 

I haven't used Supermarket fuel since early last year when there was that contamination problem. We had a Grande Punto at the time and it suffered badly from the faulty fuel, needing a new sensor and two trips to the dealer to put right. That fuel came from Tesco and I have refused to use them since then. It's just not worth it.

My work involves Upstream/Downstream petroleum sectors.  Teg is right to an extent in that additives are the main differentiator between supermarket and other fuels.  I would dispute the butane issue though.  However, one issue with Morrisons is that their fuels are all bio, regardless of the grade. This is not true of many other brands, although some are retailing bio fuels without even knowing it, and certainly without advertising it.  The other point that was well made is the refinery or storage depot where the fuel is delivered from.  However, the reason for this being an issue is that of temperature - 'hot fuel'.  To paraphrase someone else who can explain it far better than I can (taken from a US site): 

"The phrase “hot fuel” refers to expanded diesel fuel or gasoline that is sold at retail pumps at temperatures higher than the century-old government standard of 60 degrees. That is the temperature/volume used in the petro-chemical industry to measure all petroleum liquids.

At the 60-degree standard, a gallon of fuel delivers a certain amount of measurable energy, or BTU. But when expanded by higher temperatures, that same amount of fuel actually delivers less energy.

The warmer the fuel, the less BTU and fewer miles to the gallons a vehicle will receive. Consequently, if a vehicle averages six miles per gallon, 200 gallons of 98-degree fuel is going to take a truck 36 fewer miles than 60-degree fuel.

Fuel experiences significant expansion and contraction with temperature changes:

 

kerrecoe:

I haven't used Supermarket fuel since early last year when there was that contamination problem. We had a Grande Punto at the time and it suffered badly from the faulty fuel, needing a new sensor and two trips to the dealer to put right. That fuel came from Tesco and I have refused to use them since then. It's just not worth it.

Tesco might have sold it, but it was not just their fuel.  In that, other retailers who are not Tesco might (might) have been selling the same stuff but not tarred with the 'contaminated fuel' brush.  I only by from independent retailers anyway, but for different reasons (they need the money!).

julianphillips:

I would dispute the butane issue though

I will bow to your judgement here, you have far more technical knowlede on the subject than I do.  As always I am impressed. 

My comment are very generalised and based on information that may be out of date.  Certainly I know they used to butanise the fuel at a depot near where I am (Vopac).  It is possible this has now changed.

Actually I'm not an expert on this and you are not completely wrong, but it is a complicated issue.  The addition of butane to fuel give it a higher octane - and it makes it more volatile /or ignitable.  To use an inaccurate but descriptive term, you could say that the more Butane there is, the more effervescent the fuel is - and like a can of Coke, when you open it (especially if it has been shaken), you will lose some of the fuel.  In this case, it would be lost in the form of vapour release when the fuel leaves the refinery facility and is loaded.

However, the Fuel Composition and Content Regulations, a statutory instrument, sets a minimum standard that the base fuel has to meet.  This base standard applies to supermarkets just as it applies to a franchise site.  It is true that in order to meet the standard, this could partly be achieved by adding more butane, but their are limits.  Alternative to lead, which is easier. 

Theoretically - through higher volatility - highly butanised fuel could lose a high element of vapour content, so giving a vehicle less MPG.  But this would also be very much affected by temperature.  There would be no difference in performance or damage to the engine or anything like that.

The key reason for butanising is, I think, because it is harder toa chieve the higher octanes since we moved away from leaded fuels.

Greenergy supplies many of the supermarkets with BIOfuels and also butanizes at Vopak.

This somewhat contradicts my previous post but this is a complicated issue and butane is just one element of a hige number of processes involved in the production of motor fuels.  Lecture over!

 

julianphillips:

Tesco might have sold it, but it was not just their fuel.

 

Well, you know how it is Julian, 'once bitten, twice shy' and all that. I don't buy from any of the supermarkets now- it's not just Tesco that I don't trust.

I put used chip fat in my Veyron and never get any problems.

 I also never buy from supermarkets. 

i prefer my local corner shop, great choice of P*rn mags as well

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