The fuel must be mixed with air to produce combustion. The amount of air that is required for efficient combustion will vary for different types of fuels because of the different compositions in the fuel. For optimum engine operation, the air/fuel ratio must be adjusted properly.
The methane number of the fuel indicates the tendency of the fuel to detonate. Fuel with a low methane number burns more quickly than fuel with a high methane number. Additionally, the heat that is produced by compression can ignite fuel with a low methane number sooner than fuel with a high methane number. If an engine is using low methane fuel and the timing is too early, detonation will occur. To avoid detonation, the engine timing must be retarded for low methane. The engine may also need a lower compression ratio.
An engine with a low compression ratio is able to utilize fuels with low methane. An engine with a high compression ratio can use a more limited range of fuels. However, a higher power output and greater fuel economy can be obtained. Operation without combustion problems and production of the required power from the available fuel depends on the correct engine configuration.
For a detailed explanation of methane numbers, see Application and Installation Guide, LEKQ7256, "Fuels/Fuel Systems". For information on acceptable fuels for the engine, refer to the engine's Operation and Maintenance Manual.