How to calculate methane number

Written by ryan menezes
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How to calculate methane number
Environmental agencies warn against using fuels with low methane numbers. (gas pump image by jedphoto from Fotolia.com)

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.

Skill level:
Easy

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Instructions

  1. 1

    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 labelled by H/C ratio than by chemical formula. This example will use a fuel with an H/C of 3.72.

  2. 2

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

  3. 3

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

  4. 4

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

  5. 5

    Find the cube of the H/C ratio:

    3.72 * 3.72 * 3.72

    = 51.48

  6. 6

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

  7. 7

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

  8. 8

    Subtract 406.14 from your answer. 526.32 - 406.14 = 120.18

  9. 9

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

  10. 10

    Subtract 119.1 from your answer. 195.17 - 119.1 = 76.07

    This answer is the methane number.

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