[AlanHo]

I have already made my views known about high mileage claims on this and other forums, but I would like to introduce another thought for us to toy with.

I normally wait until the low fuel light comes on before refuelling - I then brim the car, taking a few minutes each time until the fuel is visible and stationary in the filler neck. What I have noticed is that if I refuel at my local Shell station - I normally get more fuel into the car than when I use the local Jet station or supermarket where the fuel is sometimes much cheaper. This is not a casual observation - I have looked back through my spreadsheet where I record every fill and there is a positive indication of this.

I have also noticed that I seem to get better mileage out of the Shell fuel compared with supermarket.

So - my question is - does supermarket fuel differ in its foaming characteristics such that the churning by the pump causes a small volumetric expansion before it is metered and you are buying foamy fuel by the litre rather than pure liquid. 

Alternatively - if the foaming actually occurs in the car's tank rather than in the fuel pump - does the foam in supermarket or other cheap fuel last much longer deep down in the tank giving the impression that the tank is brimmed.

Either way - your economy calculation would be exaggerated.


Just a thought for our experts to toy with....................

[Phil №❶]

I thought you were the expert Alan, having an engineering background. 

My thoughts are that as the feed pipe from the tank is under the fuel surface there should not be any foaming, but a knowledge of fluid dynamics & cavitation would be required, which I don't have.

I have noticed that at the outlet of the fuel nozzle, the fuel appears to be a creamy white colour. This is definitely bubbles & occurs before entering the tank. Just where this happens, I don't know but I hope we are not paying for foam instead of liquid. 

[Ugly Mongrel]

Interesting observations & comments, Alan. I hope someone can give some ideas because I have absolutely none.

[AlanHo]

Interesting observations & comments, Alan. I hope someone can give some ideas because I have absolutely none.

Looking at your reversion to your old avatar - it looks as though you have brimmed both tanks.................

I remain convinced that some of the extreme economy claims by some drivers - who I accept are sincerely believed by them - are the result of some abberation they are unaware of.

For example - one of my nephews -  who is a bit of a petrol head, changed his rims and tyres to something more trendy and lowered his Focus car. He immediately claimed a significant improvement in economy until I worked out the change in rolling radius for the new tyres. His odometer was now way off and his mileage calculations adrift.

[eye30]

In the UK fuel pumps are able to deliver up to 10ml extra per litre of fuel and is only allowed to deliver 5ml less per litre of fuel - and bear in mind that a teaspoon is approximately 5ml.

Link:

http://www.leics.gov.uk/index/business/tradingstandards/ts_consumers/recent_consumer_issues/petrol_pump_questions_answered.htm

Here is an interesting post on Snopes:
http://www.snopes.com/politics/gasoline/cheating.asp

[beerman]

To answer this question I can draw on my long held hobby of home brewing to assist.

I have kegs that hold beer, which is then taken to the glass via tubing. I have observed that the fluid is amber and full of beerly goodness. The foaming only occurs with the combination of beer under pressure hitting the glass.

[Ugly Mongrel]

To answer this question I can draw on my long held hobby of home brewing to assist.

I have kegs that hold beer, which is then taken to the glass via tubing. I have observed that the fluid is amber and full of beerly goodness. The foaming only occurs with the combination of beer under pressure hitting the glass.

Tbsp, beerman. At last, a response that makes sense.

[asathorny]

Interesting observations & comments, Alan. I hope someone can give some ideas because I have absolutely none.

Looking at your reversion to your old avatar - it looks as though you have brimmed both tanks.................

I remain convinced that some of the extreme economy claims by some drivers - who I accept are sincerely believed by them - are the result of some abberation they are unaware of.

For example - one of my nephews -  who is a bit of a petrol head, changed his rims and tyres to something more trendy and lowered his Focus car. He immediately claimed a significant improvement in economy until I worked out the change in rolling radius for the new tyres. His odometer was now way off and his mileage calculations adrift.

Now then, I have no knowledge of foaming or cavitation (which I thought reffered to the problem with ship propellers?)....
HOwever I feel the need to put my twopennyworth in...

In my simplistic world I would just start thinking that the two bowsers were dispencing different amounts of fuel, despite the little man from weights and measures popping round with is 'control weight' in a little leather bag....    ??????????????   No!!!!!!!!!!!!!

[AlanHo]

Much of the diesel now sold includes some bio diesel which is made from waste vegetable oil and is much more environmentally friendly than ordinary diesel. However, it can cause the filters in the pumps to clog up, which in turn restricts the flow of fuel through the pump. If the flow of diesel is too slow, the cut-off mechanism on the nozzle will not work. If this happens to you report it to the garage as it is likely that the pump will require maintenance. You can however be reassured that you will not be receiving short measure as the pump will only register the fuel which passes through the meter regardless of the speed of delivery.

If it bungs up the service station filters - what is it doing to our cars?

[Mad Dog]

All petrol comes from the same refineries and basically starts off just the same regardless of whether Shell or ASDA buy it. There can be differences with additives if chosen or with Octane ratings. If you choose the same rating with no additives then it's the same petrol with a different logo.

When I worked in a garage any foaming occurred on the surface and cleared in seconds.

Do you think the oil companies wouldn't be ramming it down our throats if their petrol did more MPG than ASDA?

[AlanHo]

Copied from a 2 year old report on UK fuel distribution

There are a handfull of refineries around the country owned by different companies (ie most of the fuel supplied to humberside/south yorks/lincs will come from Lindsey oil refinery at Grimsby owned by Total). They supply base Petroleum spirit (and other fuel oils) to terminals (again, any terminal could be owned by any oil company). Every local man and his tanker will fill up at that terminal. The majors usually have additive dispensers on the gantries so they can add their additive to the base Petroleum Spirit or Diesel at the terminal.
 
Some companies have their own storage elsewhere, then it all depends on the best price they can get. Sometimes they may send tankers to the nearest terminal, then other days use from their own storage. All the oil companies and oil distributors in Scotland, for example, will be using that same base stock from Grangemouth oil refinery. The same principle applies everywhere else, the nearest refinery will normally supply the majority of the stock, but in some parts of the country it could be coming from 2 refineries.
 
Hence the difference in the product at the pump is basically related to the additives that are put in at the storage terminal by each end usage supplier.

[eye30]

Hence the difference in the product at the pump is basically related to the additives that are put in at the storage terminal by each end usage supplier.

I remember a fuel branded JET had a terrible smell yet the station just up the road , I think Shell, had a sweeter smell.

One day I watched as the same named tanker distributors visited both stations at the same time and they both came from the same refinery, phone number on tanker. 

So yes I believe it is what they add that gives fuel its particular "edge"

From what I know, frothing is the lighter fractions of the fuel turning from liquid to vapor, this will happen when the pressure drops. The fuel is pumped from the tank up to the nozzle under pressure, your hand opens a valve in the handle when you require fuel. So the major pressure drop in the line occurs at the hand piece/nozzle. (Even when the trigger is pulled there will be a relatively significant pressure drop over the restriction in the hand piece. (where the "now open" valve is) So although foaming may occur at this point I don't believe it will impact on the metering accuracy which happens prior to the nozzle and hose. I believe weights and measures would be totally unbiased, their guy's would perform a similar check following the same procedure at every service station pump.

So if you manage to get less fuel in the tank at one station than another then I can only think of small entrained vapor bubbles captured among the mass of liquid fuel "padding" out the volume normally filled by the liquid fuel. Maybe due to different anti foaming additives.

Alan, maybe next time you top off you could then pay for the fuel (to waste a few minutes) then re-check the level, maybe the level continues to drop at a much reduced rate.