# A fuel theory; may be a modern myth

Discussion in 'The Perfect Line and Other Riding Myths' started by Frostback, May 28, 2013.

1. ### FrostbackFrostback

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I like to watch my gas mileage indicator (2009 R1200 GS) while on trips. The bike always seems to get better mileage during the first 100 miles. I was wondering if the fuel put in the tank cold (about 10 degrees C) expands in the tank as it sits over over a warm engine and behind an oil radiator.

If it does expand, the fuel volume indicator called a fuel gauge would indicate less fuel use than is true, and hence, better mileage, right?

any engineers out there know what the volumetric expansion of 4 gallons of fuel would be like in moving from 10 C to say 25 C?

Lee
2. ### Jayrod1318Poster

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3. ### markjennLong timer

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I always thought the computer uses injector pulse width (which can be converted to fuel flow) to compute fuel mileage, not the relatively inaccurate level gauge in the tank.

- Mark
4. ### AlpineRAMPartsChaser

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Yes, it uses the pulse width x fuel pressure x count per distance to calculate the mileage.
But with colder fuel the necessary volume to get the mixture just right is a bit lower because you have a higher fuel density.
5. ### Paebr332Good news everyone!

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Most fuel gages are linear or nearly linear, while the tank volume varies in a non-linear fashion from top to bottom.

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7. ### ttpeteRectum Non Bustibus

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The only time fuel density would be considered would be if the system ran in closed loop using a wide band lambda sensor.

Air density is far more important than the very small change in fuel density due to temperature changes, as anyone who has raced two-strokes can tell you.
8. ### High Country HerbAdventure Connoiseur

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Good explanation. You must be a calculus teacher, or a good student of the subject.
9. ### randyoLong timer

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back in the day, vacuum gauges were calibrated to show fuel economy

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MPG indicators don't go off fuel level sensors, they calculate based on load/MAP/INJ PW/Fuel Pressure/Speed sensor. The computer is simply dividing the mileage you cover by the calculated volume of fuel used for whatever given time frame it has ("instantaneous" update where it is averaged for so many seconds, recorded for display, then next average cycle begins a new calculation, or "total average" which is just what it sounds like until manually reset.

I don't know of any current bike or auto maker that gives a MPG readout from the fuel level sensor. Many have a dampened calculation from the level sensor for remaining mileage.
11. ### FrostbackFrostback

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It just seems odd to me that a MPG calculation would NOT avail itself of the calculated fuel loss as the level goes down. I wonder if someone whose in-tank fuel strip has gone wonky (not uncommon I am told) could chime in and let us know if the MPG calculations simultaneously go off the rails.

Lee
12. ### trc.rhubarbZoomSplat!

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My 07 R1200GSA has a crazy fuel strip. It works most of the time.
DTE is always wacky but MPG is about the same all the time, even if DTE and Fuel Level says I used 8 gallons in 100 miles.

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Tank level is not linear and fuel is sloshing all the time. People ride up and down hills, they accelerate and brake and whip through corners. It might not be enough to throw the fuel gauge loopy (although it will if you've ever seen an undampened/real time fuel gauge, it isn't accurate unless you're stopped or driving on hockey rink) but it can't read high enough resolution to measure a mpg. If you've got a MPG indicator, go unhook your fuel level sensor and see what happens

It is so much easier from an engineering standpoint (especially with fixed fuel pressure) to say that size X injector flows Y volume of fuel/ms at P pressure. Now if your pump is regulated to, let's say 45 PSI constant, then you simply have a Y=volume / Distance from vehicle speed sensor. One simple division, at whatever update rate they choose, sent to display. It isn't exact, but usually when it is off, it is off a consistent amount, therefore useful.
14. ### Mambo DaveI cannot abide.

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Keep in mind, too, that radiant heat builds up at the carbs or throttle bodies and heats up the air, there, too.

This could make later air create less HP, thus reducing fuel mileage.

I'm not sure if my DR650 has that issue as much, but everyone I've personally met with a DR-variant (350's, 650's) has noticed that the idle speed gets much higher after everything is really warmed up. So much so that there is no real happy medium - either it isn't going to idle well when cooler (without the choke), or the idle will sound too high to even cagers sitting beside you in traffic once warmed up. All this tells me is that some changes of substance happen when my system warms up.
15. ### FrostbackFrostback

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Ahhh. . . It took a little while for the engineers to show up in force but a convincing case now appears for the MPG metering to be coming from fuel flow and not tank volume. thanks,

Lee
16. ### bwalshLong timer

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Fixed.

Fuel gauges aren't precision measuring devises. Just a close estimate of how much fuel is in the tank. For example, I've never had a vehicle that had exactly half a tank of gas when the gauge was at half.
My truck has a rectangular fuel tank. No weird angles. It's fuel gauge doesn't move at all until I have been 150 miles then the mileage varies between each 1/4 mark. I can get close to 550 miles between fill ups. The mileage between each quarter after the needle starts to move is different too.
In other words, you can't judge how much gas you've used accurately by glancing at the fuel gauge. You would need to fill up from each 1/4 mark to narrow it down to what the OP is looking for.
17. ### vortexauOutside the Pod-bay

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Yes, I've noticed this on a restored Triumph Bonneville that was built when 100 octane gasoline was commonly available.

I worked with a guy that had water-injection on his hot-rodded truck. The idea is that it allowed for higher compression ratios by cooling the combustion chambers.

https://en.wikipedia.org/wiki/Water_injection_%28engines%29
19. ### ttpeteRectum Non Bustibus

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Water injection was used in WWII aircraft to retard detonation at high boost pressures. It was 50/50 Methanol/water. Straight water doesn't work as well.
20. ### k-moeLong timer

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Straight water freezes at altitude. It gets freaking cold above 10,000 feet.