Methane Slip and N2O: The Non-CO2 GHGs

A ship can burn a fuel that emits no carbon dioxide at the funnel and still warm the planet more than the heavy fuel oil it replaced. That is not a paradox; it is the arithmetic of methane and nitrous oxide. When an LNG dual-fuel engine lets a few percent of its gas slip past the pistons unburned, or an ammonia engine throws off a trace of N2O, the climate effect of those two gases, weighted against CO2, can swallow the saving the switch was supposed to deliver. This is the hub for the two non-CO2 greenhouse gases that decide whether LNG and ammonia actually decarbonize a ship or only appear to. It routes down to the two deep-dive leaves, methane slip from LNG dual-fuel engines and N2O emissions from marine engines, and across to the regimes that price the problem. The methane slip CO2-equivalent calculator converts a measured or assumed slip rate into the CO2-equivalent that lands in the regulatory intensity.

The logic of the cluster is one number repeated: the global warming potential. CO2 is the reference at 1. Methane is 28 times CO2 over a century; nitrous oxide is 265 times CO2 over the same century, both on the IPCC Fifth Assessment Report (AR5) basis the IMO and the EU adopted. So a gas that leaks in grams can outweigh a gas that vents in kilograms. The whole cluster is the study of when a small mass of a potent gas beats a large mass of a weak one, and what the engine, the fuel, and the regulation do about it. The companion N2O CO2-equivalent calculator does the same weighting for nitrous oxide.

Methane slip: the unburned gas problem

Methane slip is exactly what the name says: methane (CH4) that enters a gas-burning engine and leaves it through the exhaust without combusting. It is not a leak in the fuel system or the storage tank, though those exist too; it is unburned fuel that the combustion process itself failed to consume, passing through the cylinder and out the stack. The mechanism is specific to how the gas meets the air and the flame, which is why slip is a property of the combustion cycle, not of LNG as a molecule. An engine that admits gas before the cylinder fires and relies on a flame front to sweep through a lean premixed charge will always leave some gas in the cold crevices the flame cannot reach.

The reason slip matters at all is the GWP100 weighting. Burned methane becomes CO2 and water and counts as the modest CO2 the fuel was meant to deliver. Slipped methane stays methane, and at 28 times CO2 it does 28 times the climate work per tonne. A slip of even 3% of the fuel, multiplied by 28, adds a CO2-equivalent burden that can rival the CO2 the engine emits from the 97% it did burn. The whole climate case for LNG turns on keeping that slip percentage small, and the field data says it is not always small. The methane slip from LNG dual-fuel engines article works the combustion physics in full.

Three engine architectures, three slip outcomes

The marine gas engine comes in three combustion architectures, and they sit at opposite ends of the slip spectrum. The low-pressure dual-fuel Otto-cycle engine admits gas into the cylinder at low pressure (typically below 16 bar) and burns a lean premixed charge ignited by a small pilot of liquid fuel. This is the premixed lean-burn (Otto) route, and it is the one with the slip problem: gas trapped in the piston-ring crevices and gas short-circuited during the valve-overlap window escapes the flame and goes out unburned. The four-stroke medium-speed engines on many LNG-fueled cruise ships, ferries, and gas carriers work this way.

The high-pressure dual-fuel diesel-cycle engine takes the opposite approach. It injects gas at high pressure (around 300 bar) directly into the cylinder late in the compression stroke, after the intake valve has closed and against an already-hot charge, so the gas burns on a diffusion flame as it injects, the same way diesel burns. There is no premixed charge sitting in the crevices waiting to escape, so the slip is small. The MAN ME-GI two-stroke is the production example, and field measurement found this diesel-cycle two-stroke effectively eliminates methane slip. The trade-off is a high-pressure gas supply system and a modest fuel-economy penalty against the Otto engine, the engineering price of solving the slip at source.

Between them sit the slow-speed two-stroke Otto engines (the low-pressure premixed two-stroke, such as the WinGD X-DF type) and the lean-burn spark-ignited (LBSI) engines, which run the Otto cycle but at lower slip than the four-stroke medium-speed Otto because of their scavenging geometry and longer combustion window. The pattern to carry away is that the combustion cycle, not the fuel, sets the slip: diesel-cycle high-pressure injection is the low-slip route, premixed Otto-cycle low-pressure admission is the high-slip route, and everything else is a matter of degree.

What the field data actually shows

For years the regulatory slip assumptions were desk estimates, and the ICCT’s Fugitive and Unburned Methane Emissions from Ships (FUMES) campaign put instruments on real ships to test them. The result was uncomfortable for the LNG case. Across 22 measurements of 18 unique ships running four-stroke low-pressure Otto-cycle LNG dual-fuel engines, the average slip was 6.42% of the fuel, with a median of 6.05%. Restrict the sample to measurements at or above 50% combined engine load, the operating range a ship spends most of its time in, and the average was 6.07%. The IMO assumes 3.5% by default and the EU’s FuelEU regime assumes 3.1%, so the measured slip on this engine class ran roughly twice the regulatory default.

The load dependence is the sharpest finding, and it has direct operational consequences. Brake-specific methane slip on those four-stroke Otto engines ran 2.3 to 3.0 grams per kWh at 54 to 80% engine load, rose to 10 g/kWh at 25% load, and reached 21 g/kWh at 12% load. So slip is worst exactly when the engine is lightly loaded: maneuvering, slow-steaming, or running on auxiliary power in port. A ship that slow-steams to cut CO2 can drive its methane slip up, an interaction that a CO2-only fuel-saving calculation misses entirely. On the strength of the FUMES data the ICCT argued the default factor for this engine class should be raised to at least 6%, which would close much of the gap between the regulatory model and the funnel.

The brake-specific figures (2.3 to 3.0 g/kWh at high load, 10 g/kWh at 25%, 21 g/kWh at 12%) and the FUMES averages come from the ICCT field campaign cited below; the FuelEU defaults are the Annex II Cslip values of Regulation (EU) 2023/1805. Read the table as a spectrum, not a set of fixed constants: an individual engine’s slip depends on its tuning, its load profile, and its maintenance state, which is why both regimes let a certified measured value replace the default.

Mitigating the slip

The slip can be cut, and the engineering falls into three families. The first is combustion tuning: shaping the gas admission timing, the pilot injection, and the valve overlap to leave less gas in the crevices and short-circuit less during scavenging, work the engine makers have done across successive engine generations. The second is after-treatment: a methane oxidation catalyst in the exhaust burns slipped methane to CO2 and water after the cylinder, though methane is a hard molecule to oxidize at exhaust temperatures and the catalysts are sensitive to sulfur and to the low exhaust temperatures of a lightly loaded engine. The third is architecture: choose the high-pressure diesel-cycle engine, which sidesteps the slip rather than treating it, at the cost of the high-pressure gas system.

The measurement lever matters as much as the hardware. Because both FuelEU and the IMO let an operator substitute a certified measured slip value for the conservative default, and the EU published a 2025 guideline on measuring and verifying an actual methane-slip emission factor, an owner who fits a genuinely low-slip engine can prove it and be credited for it rather than carry the default. That turns slip from a fixed regulatory assumption into a number worth engineering down, which is the policy intent behind allowing measured values at all. The methane slip CO2-equivalent calculator shows how a change in the assumed slip percentage moves the CO2-equivalent that lands in the intensity.

The engine makers have put numbers on the gains. WinGD reports brake-specific slip of roughly 1.0 to 1.2 g/kWh on its X-DF2.0 two-stroke against 2.0 to 2.5 g/kWh on the first-generation X-DF, achieved through Intelligent Control by Exhaust Recycling (iCER), which recirculates a portion of exhaust to slow and complete combustion and cuts slip by up to 50% in gas mode while trimming gas-mode fuel consumption by about 3%. Layering variable compression ratio (VCR) on top took a six-cylinder 62-bore engine to roughly 30% fewer methane emissions than the same engine without VCR, a total slip near 0.83% of gas consumption, well under the FuelEU slow-speed Otto default. On the four-stroke side the medium-speed Otto engines remain the hard case the FUMES data exposed, which is why an exhaust methane-oxidation catalyst is the route under most study there. The catalyst burns slipped methane to CO2 & water downstream of the cylinder, but methane is the most stable of the hydrocarbons and resists oxidation at the 350 to 450 degree exhaust temperatures a lightly loaded engine produces, so the catalyst works least well at exactly the low loads where slip is worst.

The upstream leak: well-to-tank methane

Slip at the funnel is only half the methane story. The full well-to-wake intensity also counts the methane that leaks before the fuel reaches the ship: at the gas field, through processing, in the liquefaction plant, during transfer, and as boil-off in transport and bunkering. This upstream or well-to-tank methane is fugitive emission, not combustion slip, and it is counted in the well-to-tank term of the life-cycle intensity rather than the tank-to-wake slip term. The IMO LCA Guidelines (MEPC.391(81)) carry default well-to-tank emission factors for each fuel pathway, and for LNG that factor reflects an assumed upstream leakage rate that the same ICCT analysis argued is set low against field measurement of the gas supply chain. The practical effect is that the slip a ship can measure at its own funnel does not bound its real methane footprint: a low-slip engine fed by a leaky supply chain still carries the upstream methane in its well-to-wake number, and the operator controls only the tank-to-wake half. The well-to-wake fuel pathways hub separates the well-to-tank and tank-to-wake terms in full.

Nitrous oxide: the small mass, large potency gas

Nitrous oxide (N2O) is the other non-CO2 greenhouse gas that decides a fuel’s real footprint, and it is more dangerous per gram than methane. On GWP100 it is 265 times CO2, against methane’s 28, so a slip measured in grams of N2O carries the weight of a quarter-tonne of CO2 per gram-times-265. Marine N2O comes from two distinct places, and the second is about to matter far more than the first as the fleet looks at ammonia. The N2O emissions from marine engines article treats both sources and the abatement options.

The first source is combustion and after-treatment on conventional and gas engines. Any high-temperature combustion forms a trace of N2O, and the amount is small on a well-run diesel or gas engine. The complication is the NOx after-treatment: a selective catalytic reduction (SCR) system, fitted to meet the IMO Tier III NOx limit, reduces NOx by dosing urea over a catalyst, and an off-design catalyst, a wrong dosing rate, or operation outside the catalyst’s temperature window can produce N2O as an unwanted byproduct. So a device fitted to cut one regulated emission (NOx) can lift an unregulated greenhouse gas (N2O) if it is run badly, a trade-off that only surfaces when the GHG accounting counts N2O at 265 times CO2.

The SCR temperature window is the practical crux. An SCR catalyst reduces NOx best in a band roughly 300 to 400 degrees, and a marine engine spends real time below that band at low load and during maneuvering, the same operating points where methane slip peaks on a gas engine. Outside the window the urea can break down to N2O instead of cleanly reducing NOx to nitrogen and water, so the unregulated greenhouse gas rises exactly when the regulated one is hardest to control. On a conventional fuel-oil engine the N2O term is small enough that it is a footnote in the well-to-wake intensity; the reason it earns a section here is that the same after-treatment chemistry sits in front of the ammonia engine, where the fuel itself feeds the nitrogen and the stakes climb.

Ammonia and the N2O question

The second source is the one that turns N2O from a footnote into the central emissions question: ammonia fuel. Ammonia (NH3) contains no carbon, so an ammonia-fueled engine emits no CO2 at the funnel, which is the entire point of the fuel. But ammonia is a nitrogen molecule, and burning it produces N2O directly from the fuel nitrogen, alongside the NOx that any nitrogen-bearing combustion produces. Because N2O is 265 times CO2, even a small fraction of the fuel nitrogen converting to N2O can create a CO2-equivalent footprint that undercuts the zero-carbon claim, the same way methane slip undercuts LNG.

The numbers make the stakes plain. A fuel that emits zero CO2 can still post a meaningful CO2-equivalent intensity if it slips N2O, because the multiplier is so large that a tenth of a percent of the relevant mass matters. This is why ammonia-engine development centers on combustion control and after-treatment to suppress N2O formation, and why the unburned-ammonia slip (NH3 itself, which is toxic and a strong indirect concern) sits alongside N2O as a thing the engine must control. The climate case for ammonia, like the case for LNG, depends on a non-CO2 slip staying small, and the regulatory intensity counts it whether the engine controls it or not. The decarbonization and alternative marine fuels hub places ammonia among the candidate fuels and the trade-offs each one carries.

The abatement chain for an ammonia engine therefore runs across three streams at once, where a conventional engine worried about one. The engine must keep N2O low, because at 265 it dominates the CO2-equivalent; it must keep unburned ammonia slip low, because NH3 is acutely toxic & tightly limited around crew and in port; and it must keep NOx within the IMO Tier III limit, because nitrogen-rich combustion makes NOx readily. These three pull in different directions: a combustion setting that suppresses NOx can favor N2O formation, and after-treatment fitted for one can shift the balance of another. The leading ammonia-engine designs pair a small pilot of liquid fuel to stabilize combustion with a tailored selective catalytic reduction system and, in some designs, an ammonia slip catalyst, tuned together rather than separately. Until field measurement of production ammonia engines at sea exists in the way FUMES measured LNG engines, the N2O numbers carried in the regulatory defaults are modeled rather than measured, which is the same gap the LNG case had before 2024.

Why N2O is the harder gas to dismiss

Methane slip is a known quantity with a clear engineering fix at one end of the architecture spectrum: choose the high-pressure diesel-cycle engine and the slip largely goes away. N2O has no equally clean structural escape. It forms wherever nitrogen meets high-temperature combustion, so any engine burning a nitrogen-bearing fuel, or fitted with an SCR that can over-reduce, carries some N2O risk that must be managed by control rather than avoided by architecture. The multiplier compounds the problem: at 265 times CO2, the mass of N2O that matters to the intensity is so small that it sits near the detection limit of routine emissions monitoring, which makes it both hard to measure and easy to underestimate. For a zero-carbon fuel whose entire value proposition is the absence of CO2, an unmeasured N2O term is the single largest uncertainty in its real climate footprint.

The GWP100 weighting: how the gases are added

Three greenhouse gases, three different potencies, one common currency. To add CO2, CH4, and N2O into a single climate figure, each non-CO2 gas is converted to the mass of CO2 that would do the same warming over a chosen time horizon, its global warming potential. The maritime regimes use the 100-year horizon (GWP100), and they fixed the values from the IPCC Fifth Assessment Report (AR5): methane at 28 and nitrous oxide at 265 times CO2. The IMO LCA Guidelines (Resolution MEPC.391(81), adopted 22 March 2024, succeeding the first guidelines in MEPC.376(80) of 7 July 2023) write those AR5 GWP100 values into the well-to-wake intensity that the GFI runs on.

The choice of AR version and time horizon is a policy lever with real consequences, not a neutral technicality. The IPCC Sixth Assessment Report (AR6) updated the numbers and split methane by origin: fossil methane sits near 29.8 and biogenic methane near 27 on GWP100, and N2O rose to about 273. The IMO and the EU stayed on the AR5 values (28 and 265) for consistency and to avoid moving the goalposts mid-regulation, so the maritime figure is deliberately a step behind the latest science. The time horizon matters even more: on a 20-year basis (GWP20) methane is roughly 80 times CO2, nearly three times its GWP100 weight, because methane is short-lived and front-loads its warming. A regime that chose GWP20 would treat methane slip as far more damaging than GWP100 does, which is part of why the choice of 100 years is itself contested.

The AR5 GWP100 values of 28 and 265 are the figures fixed in the IMO LCA Guidelines and used in FuelEU; the AR6 values come from IPCC AR6 Working Group I, Chapter 7. The AR6 methane figure carries a wrinkle worth stating: the headline GWP100 for methane is 27.9 without including the carbon-cycle feedback of the CO2 that methane oxidizes into, and about 29.8 for fossil methane once that feedback is included, against roughly 27 for biogenic methane. So even within AR6 the methane number depends on the origin of the gas and on which feedbacks are counted, which is part of why the maritime regulators chose a single fixed AR5 value of 28 rather than tracking the more conditional AR6 numbers. The practical lesson is to state the AR version and the horizon whenever a CO2-equivalent figure is quoted, because the same physical slip yields a different CO2-equivalent under AR5 versus AR6 and a very different one under GWP20 versus GWP100. The well-to-wake intensity hub carries the full intensity calculation that these weightings feed.

A worked CO2-equivalent: what a few percent of slip costs

The arithmetic is worth doing once with numbers, because it shows why a slip of single-digit percent is not a rounding error. Take a tonne of LNG burned. About 75% of LNG by mass is carbon, and complete combustion of that carbon yields roughly 2.75 tonnes of CO2 per tonne of methane, so a tonne of LNG burned cleanly puts out on the order of 2.75 tonnes of CO2 at the funnel. Now slip 3.1% of that tonne, the FuelEU medium-speed Otto default: 31 kg of methane escapes unburned. Weighted at 28, those 31 kg carry the climate work of 868 kg of CO2-equivalent. So a 3.1% slip adds about 0.87 tonnes of CO2-equivalent on top of the roughly 2.66 tonnes of CO2 from the 96.9% that did burn, lifting the tank-to-wake figure by close to a third. Run the same arithmetic at the FUMES-measured 6.42% and the slip term becomes 64.2 kg of methane, or about 1.80 tonnes of CO2-equivalent, which now rivals the CO2 from combustion itself. That is the mechanism by which a measured slip roughly double the default can erase the funnel-CO2 advantage LNG holds over heavy fuel oil, which emits around 3.1 tonnes of CO2 per tonne of fuel with no methane term at all. The methane slip CO2-equivalent calculator runs this conversion for any slip percentage and fuel mass.

How slip and N2O enter the GFI and FuelEU intensity

Both the IMO’s GFI mechanism and the EU’s FuelEU Maritime regime measure a ship’s fuel in the same unit: grams of CO2-equivalent per megajoule of fuel energy (gCO2eq/MJ), on a well-to-wake basis that runs from extracting the feedstock to burning the fuel on board. The intensity is the sum of three weighted streams: the CO2, the methane, and the N2O, each in CO2-equivalent after the GWP100 multiplier. So methane slip and N2O are not side calculations bolted onto a CO2 figure; they are line items inside the headline intensity number that decides compliance.

The tank-to-wake part of the intensity is where slip lives. The model takes the fuel burned, accounts for the CO2 from combustion, then adds a slip term: a fraction of the fuel (the coefficient FuelEU calls Cslip) is treated as slipped methane rather than burned carbon, multiplied by 28. FuelEU’s Annex II sets the default Cslip by engine type: 3.1% of fuel mass for an LNG Otto medium-speed engine, 1.7% for an LNG Otto slow-speed engine, and 0.2% for an LNG diesel slow-speed engine, the diesel-cycle figure low because that architecture barely slips. The IMO LCA Guidelines carry their own default slip factor (3.5% on the comparable Otto class). N2O enters the same way, as an emission factor multiplied by 265, with the largest future weight on ammonia engines.

This is the precise point at which the engine choice becomes a compliance choice. A shipowner picking a low-pressure Otto engine inherits the high default slip and the CO2-equivalent it carries; an owner picking the high-pressure diesel-cycle engine inherits the 0.2% default and a much lower methane burden in the intensity. Because both regimes let a certified measured value replace the default, an owner who engineered the slip down can prove a lower figure and improve the compliance position, the lever that turns a fixed assumption into a number worth reducing. The IMO Net-Zero Framework and the GFI hub covers how the intensity becomes a compliance balance and a price, and the methane slip CO2-equivalent calculator and the N2O CO2-equivalent calculator compute the two non-CO2 contributions a ship adds.

Measuring slip at sea, not assuming it

The reason the default-versus-measured distinction has teeth is that measuring methane slip on a working ship is now a defined procedure rather than a research exercise. The FUMES campaign that produced the 6.42% mean did it by flying a drone through the exhaust plume of ships at sea and by placing instruments in the funnel, sampling methane against CO2 to derive a fuel-normalized slip across real load profiles rather than a single bench point. That field method is what exposed the load dependence: a bench test at rated load misses the 10 g/kWh at 25% load and 21 g/kWh at 12% load that the plume sampling caught. In 2025 the EU published a guideline setting out how an operator and a verifier establish an actual methane-slip emission factor for use under FuelEU and the EU MRV and ETS systems, which turns the measured-value option from a stated possibility into a documented pathway with a verification standard behind it.

The early adopters have begun to use it. In 2026 a cruise operator running four-stroke Otto LNG engines secured independent verification and flag-state recognition of a measured methane-slip value below the FuelEU default, the kind of certified figure the regulation was built to reward. The lesson for an owner weighing an LNG newbuild is that the default Cslip is a conservative starting point, not a fixed fate: an engine that genuinely slips less can be proven to, but only through a verified measurement campaign, & the cost and effort of that campaign is itself part of the LNG compliance calculus. An owner who cannot or will not measure carries the default, and on a four-stroke Otto engine that default may itself rise toward the 6% the ICCT proposed.

Regulatory basis and currency

The figures in this hub rest on instruments with dates and version numbers worth keeping straight. The IMO LCA Guidelines were first adopted as Resolution MEPC.376(80) on 7 July 2023 and revised as MEPC.391(81) on 22 March 2024; the AR5 GWP100 values of 28 for methane and 265 for N2O are written into them, along with the default well-to-tank and tank-to-wake factors by fuel pathway. FuelEU Maritime is Regulation (EU) 2023/1805, in force for ships from 1 January 2025, and its Annex II carries the default Cslip values by engine type. The IPCC AR5 values date from the 2013 Fifth Assessment Report; the AR6 update (27.9 to 29.8 for methane by feedback treatment, 273 for N2O) is the 2021 Sixth Assessment Report, Working Group I, Chapter 7. The ICCT FUMES report dates from 2023, with the 2024 publication of the slip findings, and the EU measurement guideline from 2025. Each of these can be revised: the IMO defaults sit under periodic review, the FuelEU Cslip table can be amended, and the ICCT has formally argued the four-stroke Otto default should be raised to at least 6%. Always cite the resolution or regulation number and its date, not the category, when a figure feeds a compliance filing.

Why a zero-CO2 fuel can still fail the test

The unifying idea across LNG and ammonia is that a well-to-wake CO2-equivalent intensity does not care which gas does the warming, only how much warming the fuel causes. LNG cuts the CO2 at the funnel by roughly a quarter against heavy fuel oil but adds a methane stream; ammonia cuts the funnel CO2 to zero but adds an N2O stream. In both cases the non-CO2 gas, weighted at 28 or 265, can claw back the CO2 saving, and on a high-slip Otto LNG engine running at the measured 6%-plus rate the well-to-wake intensity can approach or pass the oil fuel it replaced once the upstream methane leakage is counted too. The fuel’s marketing measures CO2 at the funnel; the regulation measures CO2-equivalent across the life cycle, and the gap between those two views is precisely the methane and N2O this hub covers. The alternative marine fuels hub sets the candidate fuels side by side on that well-to-wake basis.

What the operator actually controls

Across the two gases the levers split into ones an owner sets at the drawing board and ones an operator works day to day. Engine architecture is the drawing-board lever, and it is the strongest one for methane: choosing a high-pressure diesel-cycle two-stroke over a low-pressure Otto engine moves the default Cslip from 1.7% or 3.1% to 0.2%, a difference that survives the whole life of the ship. For ammonia the architecture choice is less decisive because no clean low-N2O structure exists, so the lever shifts to combustion control and after-treatment tuning, which are commissioning-and-maintenance levers rather than design ones.

The operational levers are subtler and sometimes cut against other goals. Load profile is the clearest: because four-stroke Otto slip rises from 2.3 to 3.0 g/kWh at high load to 21 g/kWh at 12% load, a ship that runs its engines lightly to save fuel can raise its methane slip enough to erase the CO2 saving the slow steaming bought. A CO2-only fuel-saving calculation cannot see that trade because it does not weight the methane, which is exactly the blind spot the well-to-wake intensity closes. Maintenance is the other operational lever: an engine drifts off its commissioning tune, a catalyst ages, an SCR dosing system goes off-design, and the slip or the N2O rises with it, none of which shows up unless the ship measures rather than assumes. The honest position for an owner is that the default factors are a conservative floor the ship pays unless it measures, and the measured number is only as good as the verification behind it and the maintenance that holds the engine at the tune it was measured in.

These choices feed the wider fuel decision rather than standing alone. An owner is not choosing methane slip or N2O in isolation; they are choosing a fuel, an engine, and a compliance position together, and the non-CO2 slip is one input among several including fuel price, bunkering availability, tank volume, and the well-to-tank emissions the supply chain carries. The two deep-dive leaves work each gas in full: methane slip from LNG dual-fuel engines on the combustion physics and the architecture split, and N2O emissions from marine engines on the combustion, SCR, and ammonia sources and their abatement. The IMO Net-Zero Framework and the GFI and the well-to-wake fuel pathways hubs show where the weighted slip and N2O land in the number that decides compliance and cost.

Limitations

This article is the cluster hub for methane slip and N2O; it is not a substitute for the IMO LCA Guidelines text (MEPC.391(81)), the FuelEU Maritime regulation and its Annex II, or the engine maker’s emissions data sheet for a specific engine. The slip percentages quoted are field-measurement averages and regulatory defaults, not the slip of any one engine: an individual engine’s slip depends on its model, its tuning, its load profile, its maintenance state, and the ambient conditions, and the only authoritative figure for a given installation is a certified measurement on that engine. The ICCT FUMES averages (6.42% mean, 6.05% median on four-stroke low-pressure Otto engines) describe the measured population in that campaign and should not be read as a guaranteed value for every engine of the type.

The GWP100 values used here (CH4 at 28, N2O at 265) are the AR5 figures the IMO and the EU adopted for their regulations, and they are deliberately a step behind the AR6 update (fossil methane near 29.8, N2O near 273); a footprint computed for a different framework, a corporate inventory, or a scientific study may use the AR6 values or a different time horizon, and the resulting CO2-equivalent will differ. The regulatory defaults (FuelEU Cslip of 3.1%, 1.7%, and 0.2% by engine class; the IMO 3.5% Otto default) are the values in force as cited and are subject to revision: the ICCT has argued the four-stroke Otto default should rise to at least 6%, and both the GWP basis and the default slip factors can change as the regulations are reviewed. None of the linked calculators replaces a certified emissions measurement, the regulation as written, or the engine maker’s data for a specific compliance filing.

See also

• Methane slip from LNG dual-fuel engines: the combustion physics of slip, the Otto-versus-diesel-cycle split, and the load dependence in detail.
• N2O emissions from marine engines: combustion and SCR N2O, the ammonia-engine N2O question, and abatement.
• Decarbonization and alternative marine fuels: the candidate fuels (LNG, ammonia, methanol, hydrogen) and their trade-offs.
• Well-to-wake fuel pathways: the full life-cycle intensity calculation these weightings feed.
• Alternative marine fuels: the candidate fuels set side by side on a well-to-wake CO2-equivalent basis.
• IMO Net-Zero Framework and the GFI: how the well-to-wake intensity becomes a compliance balance and a price.
• Methane slip CO2-equivalent calculator: converts a slip rate into the CO2-equivalent that lands in the intensity.
• N2O CO2-equivalent calculator: weights an N2O emission factor at GWP100 into CO2-equivalent.