What methane slip is
In a gas or dual-fuel engine, a small part of the methane in the fuel leaves the engine unburnt. That is methane slip. The amount is small compared with the fuel used, but methane is a far stronger greenhouse gas than CO2, so it weighs heavily when emissions are reported as CO2 equivalents.
Methane slip applies to methane-containing fuels such as LNG. A methanol engine has no methane slip, but unburnt methanol and formaldehyde can be relevant to measure.
What the rules say
FuelEU Maritime uses default methane slip factors for each engine type. These defaults are conservative. FuelEU also allows a certified, measured value to be used instead of the default.
The European Commission has published guidelines for reporting and verifying actual methane slip. They are not legally binding, but they describe the route: measure the engine at several loads, monitor how the engine is actually loaded in service, and document it so that flag or the recognised organisation can approve it.
On the IMO side, MEPC.414(84) gives the guidelines for measuring methane (CH4) and nitrous oxide (N2O) from marine engines, onboard or at your test bed. It replaces MEPC.402(83). The reference method for methane is a heated FID with a non-methane cutter.
In the EU ETS, methane and N2O are covered from 2026. Whether measured values can be used in MRV and ETS reporting should be agreed with your verifier.
Why one number is not enough
Methane slip changes with load. At low load it can be higher, at higher load it can be lower. A single test point, or a test bed cycle, does not tell you what the ship emits on a real voyage.
That is why the measurement covers several load points, including the lowest load where the engine runs in gas mode. Combined with the vessel’s logged load profile, the measured curve gives a figure that reflects how the ship is actually operated.
Measure first, then decide
A measurement shows the real emission. With that in hand you can decide whether the measured value or the default factor serves you best, and whether the ship’s operating pattern supports the measured approach.
Either way, you know your figure, and it is one you can document and defend.
Reduce, not only report
The measured data is also the starting point for lowering the slip. Together with the engine designer, the results show where tuning can reduce it, within the emission limits.
If a methane catalyst is fitted, its effect can only be proven by measuring before and after.
What we recommend
Check whether the default factor fits your engine and the way your ship operates. Measure at the loads the ship actually runs, log the load profile, and agree the approach with your verifier before you rely on it.
Regulation (EU) 2023/1805 (FuelEU Maritime); European Commission, guidelines for reporting and verification of actual methane slip (October 2025); IMO resolution MEPC.414(84).
