The methane number is used for quantify the quality of the natural gas. Natural gas consist of lighter hydrocarbons compare to the other fuels there the octane number for the natural gas can exceed the maximum value of 120.
The methane number is derived experimentally correlating the engine performance and fuel gas composition. A 100% methane composition is given 100 methane number and as the higher hydrocarbons percentage increases the methane number decreases.
Every natural gas engine has a minimum methane number to prevent engine knocking. For most of the engine it is about 80 but can vary b/w 65 to 85 depending on the type of engine and manufacturer.