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Old 01-03-2009, 04:16 AM   #1 (permalink)
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Effect of ambient temperature on fuel tank volume

Hi folks,

I am about to log in my first results of a well-driven tank - I have got around 6 L/100 km, on a 1.5L 5 speed manual tranny Honda City. But when I get into the method of getting to my exact FE data, there is problem - as I think -

The SOP for finding FE is Tank up till clip off - reset trip meter - drive around till empty (of course almost empty) go back to the same pump, same orientation - tank up again till clip off - note amount of fuel pumped - note trip meter - divide fuel x 100 by trip meter reading to get L/100 km reading

But here, if there is significant temperature difference between when you tanked up earlier, and now, you will get less gas in the tank when filling up now (relatively colder ambient - shrinkage in the tank volume - less fuel to clip-off). The change in ambient temperature is because I filled my tank in Nov, and now it is almost Jan (quite a stretch in my refilling routine, because of hypermiling techniques, I should admit! )

Would that result in erroneously higher tank mileage? ( i think it will...) Should the SOP be modified? Any ways to calibrate out this error?

Wish you all a happy and fulfilling new year 2009!

PS: How can we prevent the spell check to highlight 'hypermiling' as a spelling mistake when posting? It is so embarrassing...

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Old 01-03-2009, 08:32 PM   #2 (permalink)
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We have spell check?

As to your fuel volume question, you could calculate the difference using this information, your tank volume, and the temperatures you experience:

Gasoline volume changes approximately 0.125% for every 1C change in temperature (0.058%/F) .
Source: Why have 9/10 cent gasoline pricing? - Arizona motorists deserve Automatic Temperature Compensation of gasoline at the pump!

Note that your specific results will be affected by whether or not your fuel is sold by volume (e.g. as in most of the USA) or by volume-corrected constant energy density (as in Canada).
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Old 01-03-2009, 09:39 PM   #3 (permalink)
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Originally Posted by hummingbird View Post
PS: How can we prevent the spell check to highlight 'hypermiling' as a spelling mistake when posting? It is so embarrassing...
The spell check is very likely part of your browser... all you need to do is right click the misspelled word (e.g. "hypermiler") and "Add to Dictionary"
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Old 01-04-2009, 04:32 PM   #4 (permalink)
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Volume Correction Factors For Gasoline
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Old 01-04-2009, 08:11 PM   #5 (permalink)
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Great! So now I know that there is a way to correct for the density of the fuel.

But what about the tank volume changes going through that temperature difference?

Also does anybody on EM take these factors into account while fuel logging? Or it is "recognize the facts about density and tank volume changes - bemoan winter for low FE - put in figures from the pump and topup - carry on" business that is the norm?
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Old 01-04-2009, 10:28 PM   #6 (permalink)
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If the fuel in your tank is the same temperature as the fuel you're pumping into the tank (a huge - almost certainly unwarranted assumption) - and you know that temperature (it probably isn't air temperature, then you can use the chart CobraBall provided to make a calculation.

I certainly can't speak for everyone here, but I have better things to do with my time than attempt to correct for this factor.

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