How to Calculate Methane Number

Environmental agencies warn against using fuels with low methane numbers.
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An engine's spark plug should ignite fuel, but additional fuel pockets sometimes combust, creating "knocking" in the engine. A fuel's methane number describes how likely it is to combust uncontrollably. Hydrogen receives a methane number of "0," and methane receives a number of "100." Other fuels lie elsewhere on this scale. Engineers measure a fuel's methane number experimentally, using an adapted engine. But when a fuel has a carbon-hydrogen ratio of at least 2.5, you can use that ratio to calculate its methane number.

    Determine your fuel's H/C ratio. You can calculate this with the chemical formula by dividing the number of hydrogen atoms by the number of carbon atoms, but heavier fuels will more likely be labeled by H/C ratio than by chemical formula. This example will use a fuel with an H/C of 3.72.

    Multiply the H/C by 508.04. So 3.72 * 508.04 = 1,889.9

    Square the H/C ratio. So 3.72 * 3.72 = 13.84

    Multiply your answer by -173.55. So 13.84 * -173.55 = -2,401.93

    Find the cube of the H/C ratio:

    3.72 * 3.72 * 3.72

    \= 51.48

    Multiply your answer by 20.17. So 51.48 * 20.17 = 1,038.35

    Add the answers from steps 2, 4 and 6. 1,889.9 + -2,401.93 + 1,038.35 = 526.32

    Subtract 406.14 from your answer. 526.32 - 406.14 = 120.18

    Multiply your answer by 1.624. So 120.18 * 1.624 = 195.17

    Subtract 119.1 from your answer. 195.17 - 119.1 = 76.07

    This answer is the methane number.

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