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  1. #1
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    335 Fuel Sensor Charactorization

    Every since reading about the benefits of using E85 and finding a station 3 miles from my house, I have wanted to experiment with ethanol fuel blends above the 10% you get normally with 93 AKI gas. The trouble is how much do you need to put into your tank to maintain the desired concentration? I decided to collect some data to figure out.

    Every time I filled up with gas over the course of about a year, I recorded the temperature, the gas gauge reading, and the amount of fuel the car took. I would use the same fueling method every time of pumping until the pump shuts off automatically, then waiting ten seconds or so and pumping again until an auto shutoff. After collecting and analyzing the data I can say the empty volume in my fuel tank can be found using the following formula:

    G = (107 - P) / 6.94

    where G is the volume in gallons and P is the percent expressed as a whole number, i.e. not in its normal decimal form. For instance, if your gauge was reading half full, P would equal 50 not 0.5.

    What is most remarkable is the fuel gauge reading is completely independent of the temperature as far as I can tell. Also interesting is the fuel tank in my car ('07 E90) is the same as in an '07 E92 which is the same as in an LCI E92.

    Therefore, with your help, I would like to verify this formula using any and all 335s out there. Fill up your car with the method described in paragraph 2 and report back here with your results. I have been able to achieve at least 0.1 gallon accuracy which is far beyond my expectations. Accuracy of the formula will be dependent on an accurate reading of the gas gauge. Each tick on the gauge represents 5%. It should be easy to read the gauge accurately to at least half of that.

    If you decide to participate, please include the following

    Year and chassis code
    Fuel gauge reading
    Estimated empty volume in tank using formula
    Actual empty volume read from the gas pump after fueling

    For example:

    07 E90
    50%
    8.213 gallons
    8.234 gallons

    Attached please find my data:
    fuel sensor charactorization.pdf.zip
    Eppur si muove.

  2. #2
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    So no one really cares about being able to accurately estimate the amount of fuel the car will take? This of course allows accurate fuel blending at the pump provided you know the ethanol content of the fuels. I gassed up my car last night and the formula said I needed 13.977 gallons. The car took 14.040 gallons. That's only a 0.45% difference or eight fluid ounces. I'm just curious if it is as accurate on other cars as they all appear to use the same tank.
    Eppur si muove.

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    Click here to enlarge Originally Posted by ajm8127 Click here to enlarge
    So no one really cares about being able to accurately estimate the amount of fuel the car will take? This of course allows accurate fuel blending at the pump provided you know the ethanol content of the fuels. I gassed up my car last night and the formula said I needed 13.977 gallons. The car took 14.040 gallons. That's only a 0.45% difference or eight fluid ounces. I'm just curious if it is as accurate on other cars as they all appear to use the same tank.
    I think that's a lot of math for something that you still cannot accurately calculate (assuming the point is to get your ethanol content to a specific percentage). You are missing the X factor, which is the ethanol content of the 93 octane fuel. E10 basically means up to 10% ethanol; that content is not guaranteed. I'm impressed you are able to get the volumetric accuracy that close, though.

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    You will never be able to get it 100% perfect, but close is what counts. Give yourself a little more E85 for your map than you think you need to give yourself some headroom. The DME will remove some timing if it needs to (timing corrections). For this example lets assume you are shooting for E30, and already have E30 in the tank.

    The E content of E10 will be under 10, but close to 10 because it keeps the price per gallon as low as possible. Let say 9.

    E85 will have to be measured periodically, but you know when it changes using the table in the back of the Ethanol Handbook and you can buy a reusable plastic tester for like 15 bucks. Lets say it's 85 for the sake of this example.

    Look at your gas gauge and plug the numbers into the calculator on your smart phone. Lets say it's at 25%:

    107 - 25%= 82
    82 / 6.94 = 11.816g

    Use a calculator like this : http://dbtest.net16.net/ethanol-01.html

    We find 8.551g of E10 (E09 actually) and 3.265g of E85.

    You would not want to fuel up like this completely open loop. Every so often you would need to check the actual E content of your tank, or just run pure E10 for a tank or so to reset yourself to a known state.

    If you are starting with only E10 in the tank, try to run it down as far as practical, and use a little extra E85 to compensate for the E10 originally in the tank. Or you can estimate the remaining fuel if you subtract the total tank volume from the estimated empty volume. I see the tank is about 16 gallons. Even if there is 1 gallon of E10 left in the tank, and you fill it with E30, you end up with E28.8. So that's still pretty close.
    Eppur si muove.

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