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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