Methane number
The knock resistance rating of a gaseous fuel, and the acceptance limit an LNG bunker delivery must meet before a lean-burn dual-fuel engine can run on it.
The methane number rates the knock resistance of a gaseous fuel on a scale where pure methane is 100 and pure hydrogen is 0. It is the gas-fuel analogue of the octane number, and it is the property that decides whether a delivered LNG parcel can be burned in a given lean-burn dual-fuel engine without derating.
The number falls as the heavier hydrocarbon content rises, so an LNG rich in ethane, propane and butane can sit below an engine maker declared minimum even though it is entirely within the specification of the supply contract. That is why the certificate of quality is checked against the engine acceptance window before transfer rather than after it.
The full article will cover the calculation methods and why they disagree, the relationship to the Wobbe index and to calorific value, engine maker minimum values, the derating and load limitation that follow a low-methane-number delivery, and the contractual handling of an off-specification parcel. See LNG fuel system and LNG as a marine fuel .