Methane Slip from LNG Dual-Fuel Engines

Methane slip is the fraction of fuel methane that exits a dual-fuel marine engine as unburned CH4 through the exhaust rather than oxidising during combustion. Expressed as a percentage of total methane supplied to the cylinder per cycle, it ranges from roughly 0.2 percent for the best high-pressure diesel-cycle engines at full load to over 5 percent for first-generation low-pressure spark-ignition four-stroke engines at part load. That gap matters because methane carries a 100-year global warming potential (GWP-100) of 28 relative to CO2 under IPCC AR5, and a GWP-20 of 84. At a 3 percent slip rate, the additional warming from unburned methane adds roughly 42 g CO2eq per MJ of LNG burned, cutting and potentially reversing the well-to-wake (WtW) climate advantage that LNG holds over heavy fuel oil.

The WinGD X-DF dual-fuel architecture article covers the engine-cycle mechanics in detail; this article addresses the slip rates by engine type, the GWP arithmetic that converts slip percentages into CO2eq intensities, the regulatory frameworks that count or ignore methane slip, the abatement options, and the measurement approaches available in service. The companion methane slip GWP calculator computes the CO2eq contribution of any measured slip rate at GWP-20 or GWP-100. For the broader LNG fuel picture see LNG as marine fuel.

Combustion mechanism and slip pathways

LNG as marine fuel is essentially methane (typically 85 to 99 mole percent CH4 depending on source field) delivered to the engine after vaporisation. Under ideal conditions the combustion is clean:

In practice, several mechanisms prevent complete oxidation of every methane molecule in each cylinder cycle.

Crevice slip is the largest contributor in low-pressure engines. The small volumes between the piston crown and the cylinder liner, and around the piston ring pack, are narrow enough that the flame front cannot propagate into them. Methane trapped in these crevices at the time of combustion is expelled during the exhaust stroke as unburned fuel. Crevice volume as a proportion of displacement is fixed by engine geometry; reducing it requires tighter machining tolerances.

Wall quenching arises because the cylinder liner is cooled to protect its structure. Near the cool metal surface, the flame is extinguished before the reaction completes. The quench distance for methane-air mixtures is roughly 2 mm at atmospheric pressure, shrinking under compression. Even so, the thin unburned layer at the wall contributes measurably to total slip, particularly at reduced load when liner temperatures are lower.

Valve-overlap slip is specific to two-stroke uniflow-scavenged designs such as the WinGD X-DF and the MAN ME-GI. In these engines there is a brief period when the scavenge ports are still open and the exhaust valve has not yet fully closed, allowing fresh gas-air charge to short-circuit directly to the exhaust manifold. In low-pressure engines where gas is already present in the scavenge charge at this moment, valve-overlap slip is a first-order contributor to total methane emissions. This mechanism does not apply to four-stroke engines (no scavenge ports) or to the MAN ME-GI (gas injected at top dead centre, well after the overlap period).

Lean-limit misfire is significant at part load. Low-pressure engines run lean (excess air ratio lambda typically 1.8 to 2.2 at full load) for thermal NOx control. As load falls, the charge becomes leaner still, approaching the lean flammability limit for premixed methane-air mixtures (lambda around 2.5 for natural gas at ambient conditions). At or near this limit, combustion in individual cylinders becomes intermittent or partial, releasing unburned methane in discrete pulses. This produces the steep upswing in measured slip at loads below 50 percent MCR observed across all low-pressure engine types.

High-pressure diesel-cycle engines (MAN ME-GI) avoid crevice slip and valve-overlap slip at near-full load because gas is injected directly into the compressed cylinder charge at roughly 300 bar, ignites immediately on contact with the hot air, and burns as a diffusion flame near the injector tip. Gas exposure to crevice volumes is brief. At part load, as injection timing and quantity change, small amounts of gas do reach colder regions; this explains why even ME-GI slip rises at low load, though it remains below 1 percent in most practical operating conditions.

Temperature and composition effects

Exhaust methane concentration is not constant even at a fixed load. At steady-state cruise, an LBSI engine’s slip fluctuates on a cylinder-by-cylinder, cycle-by-cycle basis because combustion quality in individual cylinders varies with the methane number of the incoming fuel, turbocharger lag, and minor asymmetries in gas admission timing. FTIR measurements on in-service LBSI vessels show cycle-to-cycle variability of plus or minus 30 percent around the mean slip value. This variability matters for in-service measurement protocols: a short spot measurement campaign may over- or under-represent steady-state slip by a significant margin if it happens to coincide with a transient event.

LNG methane number (MN) is the fuel-quality parameter most directly linked to knock risk and to slip from lean-limit misfiring. A high-MN fuel (MN 80 to 90, typical of pipeline-quality gas or Norwegian LNG) allows the engine to run at a leaner air-fuel ratio for a given load before approaching the knock or misfire boundary. A low-MN fuel (MN 60 to 70, possible for LNG with elevated ethane or propane content) requires richer combustion or retarded injection timing; both increase slip. Bunker LNG composition is reported in the ship-to-ship bunkering documentation; operators tracking in-service slip should log the methane number alongside the CEM data.

Ambient conditions affect exhaust temperature and therefore MOC performance. At cold ambient temperatures (Arctic or North Atlantic winter operations), exhaust temperature at part load can fall below the MOC light-off threshold even at moderate ship speeds, causing the catalyst to contribute nothing to slip reduction during significant fractions of a voyage. Operators with MOC retrofits should map the exhaust temperature profile against their typical trade routes and seasons to estimate effective annual methane conversion rates rather than relying on design-point conversion figures.

Slip rates by engine type

The four principal dual-fuel marine engine architectures produce markedly different slip rates. The table below draws on manufacturer bench-test data and the in-service measurement campaigns published by ICCT (2020) and SEA-LNG (2023).

Sources: MAN Energy Solutions ME-GI product specification; WinGD X-DF engine family documentation; ICCT 2020 in-service campaign; SEA-LNG 2023 multi-vessel campaign. In-service rates for LBSI, LBDF, and LPSI engines are typically 20 to 80 percent above bench-test values because of variable LNG composition, engine wear, and operating profiles skewed toward low load (slow-steaming, dynamic positioning).

The iCER effect on WinGD X-DF

WinGD introduced the X-DF2.0 generation in 2019 with iCER (Intelligent Control by Exhaust Recycling), a low-pressure exhaust gas recirculation circuit that extracts a fraction of exhaust gas after the turbocharger, cools and scrubs it, and re-introduces it into the scavenge air receiver. The diluted charge has two effects on slip: it reduces valve-overlap short-circuit by increasing charge density during scavenging, and it lowers the lean-limit instability that drives part-load misfiring. WinGD’s published data states that iCER reduces methane slip by roughly 50 percent compared to the first-generation X-DF1.0, bringing the X-DF2.0 well-to-wake performance closer to the ME-GI. The WinGD X-DF dual-fuel architecture article covers the iCER circuit design in detail.

Load dependence

Across every engine type, slip increases sharply as load falls below 50 percent MCR. For HPDF engines, slip at idle (10 to 15 percent MCR) can reach 0.8 to 1.5 percent, three to five times the full-load value. For LBSI engines without iCER, slip at idle can exceed 5 percent. This load dependence matters operationally: vessels spending significant time at part load (slow steaming, dynamic positioning, port maneuvering) have materially higher average slip rates than their bench-test figures suggest.

GWP weighting and the CO2-equivalent calculation

The climate impact of methane slip depends directly on the global warming potential (GWP) value applied. Two IPCC GWP values are in active regulatory use:

• GWP-100 = 28 (IPCC AR5 Table 8.7, fossil methane, without climate-carbon feedback): the value adopted by FuelEU Maritime (Regulation (EU) 2023/1805 Annex II), the IMO LCA Guidelines (MEPC.391(81)), and the IMO GFI metric under the 2023 IMO Strategy (MEPC.376(80)).
• GWP-20 = 84 (IPCC AR5 Table 8.7): used in academic analyses focused on near-term warming and by some voluntary frameworks evaluating short-lived climate pollutants.
• GWP-100 = 27.9 / GWP-20 = 82.5 (IPCC AR6 Table 7.SM.7, fossil methane): marginally lower than AR5; regulatory frameworks have not yet migrated from AR5 to AR6 as of the date of this article.

The WtW CO2-equivalent intensity from engine methane slip is computed by:

where $s$ is the slip fraction (dimensionless, e.g. 0.03 for 3%), $e_{\text{CH}_4}$ is the methane content of the fuel per unit of energy (approximately $50\,\text{g CH}_4 / \text{MJ}$ for typical LNG), and $\text{GWP}_{100} = 28$. At $s = 0.03$:

Add the combustion CO2 from oxidised methane (approximately 56 g CO2/MJ for LNG) and a representative upstream well-to-tank (WtT) intensity (18 g CO2eq/MJ from FuelEU Maritime Annex II for pipeline LNG, or 12 g CO2eq/MJ for LNG from Norway) and the total WtW intensity for a vessel with 3 percent slip is roughly 116 g CO2eq/MJ. That compares with a HFO WtW baseline of approximately 93 g CO2eq/MJ: LNG at 3 percent slip is about 25 percent worse than HFO, not better. Use the methane slip GWP calculator to compute this for any measured slip rate.

GWP-20 and the near-term warming argument

Under GWP-20 = 84, the same 3 percent slip adds $0.03 \times 50 \times 84 = 126\,\text{g CO}_2\text{eq/MJ}$ from slip alone, pushing the WtW total to roughly 200 g CO2eq/MJ, more than double the HFO baseline. The GWP-20 framing is common in climate-advocacy analyses that emphasize the near-term (20-year) warming impact of short-lived gases and the urgency of cutting CH4 to slow near-term temperature rise. For regulatory compliance under FuelEU Maritime and IMO, GWP-100 = 28 is the mandated value; GWP-20 does not change a vessel’s compliance status today.

Net climate benefit by engine type

Combining representative in-service slip rates (75 percent MCR, mid-range of in-service data), GWP-100 = 28, combustion CO2 of 56 g/MJ, and FuelEU Annex II WtT of 18 g CO2eq/MJ:

HPDF delivers a genuine climate benefit over HFO. LBSI with iCER is modestly better. LBSI without iCER sits at approximately break-even. LBDF and LPSI Gen 1 are net climate negatives under GWP-100 = 28 and the FuelEU Annex II WtT value.

Upstream methane leakage

The table above accounts only for engine exhaust slip. Methane is also released during LNG production, liquefaction, shipping, and bunkering. Estimates of upstream leakage rate vary:

• FuelEU Annex II / IMO LCA default: roughly 0.5 percent of production volume (about 7 g CO2eq/MJ at GWP-100).
• US Environmental Protection Agency national inventory: approximately 1.4 percent for US natural gas systems.
• EDF ground-based US measurements (2018): approximately 2.3 percent for natural gas at the wellhead and midstream, significantly above the EPA inventory.
• Field measurements, Permian Basin and Vaca Muerta (2021 to 2023): 4 to 9 percent in specific production regions, as documented by independent monitoring campaigns.
• Norwegian North Sea (Equinor published data): 0.1 to 0.3 percent, the world’s lowest, due to near-complete capture infrastructure.

At an upstream rate of 2.3 percent (EDF figure, US-sourced LNG), the WtT contribution rises from 18 to roughly 30 g CO2eq/MJ. An LBSI vessel running on US Gulf Coast LNG with 1.2 percent in-service slip then reaches a WtW intensity of approximately 118 g CO2eq/MJ, 27 percent worse than HFO. FuelEU Maritime allows producers to certify batch-specific WtT values below the Annex II default; Norwegian LNG operators can use this route to claim a materially better WtW score.

Regulatory accounting of methane slip

FuelEU Maritime (Regulation (EU) 2023/1805)

FuelEU Maritime is the first binding regulation to account for methane slip in shipping compliance. Article 4 sets a maximum annual average WtW GHG intensity for energy used on board, expressed in g CO2eq/MJ and tightening in five-year steps: 89.34 g CO2eq/MJ from 2025; 85.69 from 2030; 77.94 from 2035; 62.90 from 2040; 48.33 from 2045; 27.18 from 2050. These reference a 2020 baseline of 91.16 g CO2eq/MJ.

The Annex II default WtW values for LNG embed engine-type-specific methane slip assumptions:

An LPSI-engined vessel at 98.5 g CO2eq/MJ is already non-compliant under the 2025 limit of 89.34 without any additional non-LNG fuels or compliance measures. An LBDF vessel at 88.5 is marginally compliant through 2030 but faces exposure from 2030 onward. HPDF and LBSI + iCER have compliance headroom through at least 2030 under the default values.

Owners can certify a vessel-specific WtW intensity lower than the Annex II default if they install class-approved continuous emission monitoring (CEM) for methane slip and submit verified data. This creates a direct financial incentive to fit MOCs or switch to HPDF: every gram saved per MJ avoided translates directly into FuelEU compliance value and avoided pooling penalty.

IMO LCA Guidelines (MEPC.391(81))

IMO Resolution MEPC.391(81), adopted at MEPC 81 in April 2024, provides the LCA Guidelines for marine fuels that feed into the IMO’s GFI (Greenhouse Gas Fuel Intensity) metric under the 2023 IMO Strategy (MEPC.376(80)). The LCA Guidelines apply GWP-100 = 28 for methane, consistent with FuelEU Maritime. They provide WtW default values for LNG by engine type broadly aligned with the FuelEU Annex II values, with the same engine-type differentiation between HPDF and LPSI. The default values are subject to revision as in-service measurement data matures.

These Guidelines supersede the earlier MEPC.376(80) interim guidance; operators should reference the MEPC.391(81) document for current default values when constructing GFI compliance submissions.

IMO CII: the known gap

The Carbon Intensity Indicator (CII) is defined in MEPC.336(76) as the ratio of annual CO2 emissions from fuel combustion to transport work. The CO2 figure is derived from fuel consumption multiplied by the carbon content factor Cf from MEPC.364(79): for LNG (methane), Cf = 2.750 g CO2/g fuel. This calculation covers only the CO2 from the fraction of methane that does combust; unburned methane exhausted as slip is not counted.

The practical consequence: a vessel burning LNG on LPSI engines with 4 percent slip attains a CII rating based on the 96 percent of methane that did combust, with no penalty for the 4 percent that escaped unburned at a GWP 28 times higher than CO2. A comparable HFO vessel burning fuel with Cf = 3.114 might have a worse CII rating despite a better actual WtW GHG intensity. The IMO has acknowledged this gap in the context of the IMO Net-Zero Framework deliberations; a Cf for methane that incorporates a slip factor has been discussed at MEPC but was not included in MEPC.336(76) or its subsequent amendments as of MEPC 81.

This gap means CII cannot be used as a reliable proxy for the climate impact of LNG dual-fuel vessels. Operators and charterers relying on CII ratings alone may underestimate actual GHG exposure.

EU ETS and EU MRV

The EU Monitoring, Reporting and Verification Regulation (EU MRV, Regulation (EU) 2015/757 as amended) covers CO2, N2O, and methane from ships above 5,000 GT calling at EU ports. From 2024, methane is included in the MRV scope, meaning operators must monitor and report methane emissions including exhaust slip. For most operators, slip is calculated from fuel consumption and an engine-type default emission factor rather than direct CEM measurement.

The EU ETS for shipping entered full scope for shipping CO2 from 2024. Under the EU ETS Directive amendment of 2023, methane and N2O come into ETS scope from 2026, adding a direct carbon price on exhaust slip. At EUR 60 per tonne CO2eq (a rough mid-range estimate for 2026 EU ETS pricing), a vessel emitting 500 t CH4 per year from slip faces an additional compliance cost of roughly EUR 840,000 per year from 2026 for that slip alone (500 t CH4 × 28 GWP × EUR 60/t).

Abatement technologies

Methane oxidation catalyst (MOC)

A methane oxidation catalyst is a palladium-based (or platinum-palladium-rhodium) catalytic converter fitted in the exhaust path, downstream of any SOx scrubber and SCR catalyst, that oxidises unburned methane to CO2 and water:

Methane conversion efficiency in service: 50 to 90 percent at operating temperature of 350 to 550 degrees C. The light-off temperature is approximately 350 degrees C; below this, conversion collapses. Marine engines at part load produce lower exhaust temperatures, which limits MOC effectiveness precisely at the operating points with the highest slip rates.

Sulphur in the pilot diesel poisons the palladium active sites progressively. Current marine MOCs require pilot fuel sulphur below approximately 50 ppm for stable performance; periodic thermal regeneration (a high-temperature burn-off at roughly 600 degrees C) partially restores activity. Commercial marine MOC systems are available from Yara Marine Technologies, Caterpillar, MAN Energy Solutions, and Wartsila. Capital cost runs USD 100,000 to USD 500,000 per installation; the fuel-consumption penalty from backpressure is roughly 0.5 percent.

As of 2024 only a small number of in-service LNG vessels carry MOCs, mostly on commercial pilot programmes. Uptake is expected to accelerate as FuelEU Maritime 2030 tightening approaches and as MOC supply chains scale.

Engine combustion redesign

WinGD’s iCER system (described above) represents the most commercially significant combustion-side slip reduction deployed so far. The X-DF2.0 programme targeted a 50 percent slip reduction vs the first-generation X-DF1.0, achieved through EGR-modified scavenging dynamics.

MAN Energy Solutions introduced the ME-GA (announced 2022) as a low-pressure variant with revised combustion chamber and injection timing targeting 1.0 to 1.5 percent slip at typical operating points, sitting between HPDF and LBSI. WinGD’s development roadmap includes X-DF2.2 variants targeting below 0.5 percent slip across the full operating envelope for delivery from 2026 to 2028. Wartsila’s 50DF Mk3 achieves around 1.5 percent in steady-state operation compared to 2.5 to 5.0 percent for original 50DF Gen 1.

These development programmes pursue slip reduction at source, eliminating the cost and complexity of aftertreatment. However, they apply to new engines and to major overhauls; they do not reduce slip on the existing LPSI and first-generation LBSI fleet.

HPDF retrofit

Converting an LBSI engine to HPDF is technically possible but requires replacement of the cylinder cover, injection system, and potentially gearbox and shafting modifications. Estimated cost: USD 5 to USD 15 million per engine. Payback against avoided FuelEU penalties at representative penalty rates is 8 to 15 years, marginal. For LPSI four-stroke engines, the architectural gap is too large for retrofit; only engine replacement is practical.

Operational measures

Without capital investment, operators can reduce average slip by keeping engines at higher load fractions. Running two auxiliary engines at 70 percent load rather than three at 45 percent is a specific application: for typical LBSI auxiliary engines the 70 percent load saves approximately 1.5 percentage points of slip per engine, sacrificing one redundancy unit. Voyage planning that minimizes the time spent in dynamic positioning, and maintenance scheduling that avoids allowing engine condition to deteriorate before overhaul, each typically deliver 10 to 30 percent total slip reduction across the operating profile.

Bunker fuel quality matters too. LNG with elevated nitrogen content (inerts) or unusual heavy hydrocarbon fractions burns less completely in premixed engines and raises slip above bench-test values. Quality monitoring at the bunkering point and selection of sources with stable methane numbers reduces this contribution.

In-service measurement

Continuous emission monitoring

Direct measurement of exhaust methane concentration combined with exhaust flow measurement is the most accurate method. The exhaust flow is measured by hot-wire anemometer or by ultrasonic transit-time meter at the funnel. Methane concentration is measured by Fourier-transform infrared spectroscopy (FTIR) or by tunable diode laser absorption spectroscopy (TDLAS). CO2 concentration (NDIR) is measured simultaneously to allow calculation of the carbon balance.

The sample point must be downstream of any MOC (to capture net slip after aftertreatment) and upstream of the funnel cap to avoid dilution. Marine CEM systems require class-approved enclosures, vibration-tolerant mounting, and ATEX/IECEx certification for the fuel-gas area. Commercial marine FTIR systems from vendors such as Servomex, MKS Instruments, and Sintrol achieve detection limits below 1 ppm CH4, well within the range needed to quantify 0.1 percent slip.

Periodic spot measurement and modelled estimation

For owners not investing in continuous CEM, annual or biennial spot measurement campaigns (USD 20,000 to USD 50,000 per campaign by a third-party contractor) provide less granular but still class-reportable data. The lowest-cost approach is modelled estimation from engine performance simulators (MAN, WinGD, and Wartsila each publish slip estimation modules) combined with voyage operating-profile data. Model accuracy is typically within 30 to 50 percent of in-service measurements for well-maintained engines at moderate loads; accuracy degrades at low load and for worn engines.

SEA-LNG and ICCT measurement campaigns

The SEA-LNG industry coalition published results from a 30-vessel, multi-architecture CEM campaign in 2023. Headline results: HPDF in-service slip of 0.3 to 0.5 percent at operating loads; LBSI in-service slip of 1.0 to 2.0 percent; LBDF in-service slip of 1.5 to 3.0 percent. The campaign confirmed a 2 to 3-fold increase in slip from typical cruising load (75 percent MCR) to idle across all engine types. SEA-LNG used this data to argue that HPDF and LBSI (with iCER) deliver genuine WtW improvements over HFO even under GWP-100 = 28, a claim the data supports at moderate operating loads.

The ICCT 2020 study reached similar conclusions for HPDF and LBSI but reported higher in-service LPSI slip rates (4 to 6 percent at typical operating points) than earlier manufacturer claims had suggested. The ICCT findings were influential in the EU and IMO regulatory decisions to use GWP-100 = 28 and to differentiate Annex II default WtW values by engine type, rather than applying a single LNG default that would have masked the large spread.

Class-approved measurement protocols

Classification societies including Lloyd’s Register, DNV, Bureau Veritas, and ClassNK have published or are developing type-approval requirements for marine methane CEM systems used for FuelEU Maritime compliance monitoring. The key requirements are: demonstrated calibration traceability to certified reference gas mixtures; maximum measurement uncertainty of plus or minus 5 percent of reading at the ship’s expected operating concentration range; data availability of at least 95 percent on an annual basis; and integration of the methane mass flow calculation with the fuel mass flow metering system. Systems that do not meet these requirements cannot be used for the certified actual-value route under FuelEU Maritime Annex II; operators fall back to the (typically worse) default WtW values.

The practical measurement chain is: exhaust volumetric flow (m3/s) from the ultrasonic or anemometry meter, multiplied by the FTIR-measured CH4 mole fraction, multiplied by the molar mass of methane (16.04 g/mol) and divided by the molar volume at exhaust conditions. The resulting mass flow of CH4 (g/s) is integrated over the voyage and divided by total LNG energy consumed (derived from the flow-meter-certified LNG mass and a certified LHV for the bunkered composition) to give the slip fraction. The methane slip GWP calculator accepts any measured slip percentage as input and applies the GWP-100 or GWP-20 weighting to produce the CO2eq contribution for reporting purposes.

GWP-20 vs GWP-100: why the metric choice is not neutral

The choice between GWP-100 and GWP-20 is not just an academic question; it determines whether LNG is characterised as a transition fuel or a climate liability in policy and finance.

Methane is a short-lived climate pollutant: its atmospheric lifetime is roughly 12 years, compared to centuries for CO2. Over a 100-year integration window, each tonne of methane causes 28 times the warming of a tonne of CO2 (AR5). Over a 20-year window, before decay reduces the concentration, the same tonne causes 84 times the warming. The ratio of GWP-20 to GWP-100 for methane is therefore roughly 3:1.

The GWP-100 framework was adopted by the UNFCCC for national GHG inventories primarily because it reflects long-run equilibrium climate sensitivity. For shipping specifically, the IMO 2023 Strategy and FuelEU Maritime both use GWP-100 = 28 because the long-run trajectory matters for 2050 net-zero targets. Under GWP-100, HPDF LNG can be defended as a genuine decarbonisation step.

The GWP-20 framework emphasises near-term warming. The 2021 IPCC AR6 report notes that limiting near-term methane emissions has a proportionally larger effect on temperature in the 2030-2040 window than cutting CO2 alone, because CO2 already in the atmosphere will cause warming for centuries regardless. Several environmental NGOs, including the Environmental Defense Fund, argue that for a regulatory regime targeting 1.5 degrees C by 2050, GWP-20 is the more policy-relevant metric for assessing methane-slip impacts.

Under GWP-20, no commercially available LPSI or LBDF engine passes a WtW comparison with HFO. Even LBSI without iCER at 1.2 percent slip produces a GWP-20-weighted WtW intensity (roughly $0.012 \times 50 \times 84 + 56 + 18 = 124\,\text{g CO}_2\text{eq/MJ}$), 33 percent worse than HFO. Only HPDF at 0.3 percent slip ($0.003 \times 50 \times 84 + 56 + 18 = 86\,\text{g CO}_2\text{eq/MJ}$) remains marginally better than HFO under GWP-20. This analysis informs the position of some Poseidon Principles member banks that apply internal portfolio-level GWP-20 assessments alongside the GWP-100 regulatory default.

Implications for fleet operations

Newbuild engine selection

The methane slip data now available shapes engine selection for LNG dual-fuel newbuilds decisively. For vessel types where the engine spends significant time below 50 percent MCR (container feeders, platform supply vessels, car carriers, LNG bunkering vessels) the choice between HPDF and LBSI is directly a FuelEU compliance decision. An LBSI vessel without iCER on a 2025-2030 trade cannot rely on reaching the 2030 FuelEU target on default values alone; it needs either a MOC retrofit, HPDF upgrade, or blended biofuel use to maintain compliance.

For LNG carriers and VLCCs, HPDF (ME-GI) has been standard since the mid-2010s. Boil-off gas re-liquefaction economics and voyage regularity make low-load operation infrequent; the slip argument reinforced rather than changed that engine selection.

The n2o-emissions-marine-engines article covers the parallel GHG issue for dual-fuel engines running on ammonia, where N2O slip plays the role that CH4 slip plays in LNG dual-fuel. The two problems are structurally similar: a short-cycle combustion residual with a GWP far higher than CO2 that current CII accounting ignores.

Existing fleet: compliance exposure

Owners of first-generation LBSI and LPSI engines face two distinct risk windows. The first is the 2030 FuelEU tightening (85.69 g CO2eq/MJ); the second is the introduction of EU ETS CH4 pricing from 2026. For LPSI owners specifically, the Annex II default of 98.5 g CO2eq/MJ is already above the 2025 FuelEU limit of 89.34 g CO2eq/MJ, meaning every EU-reporting voyage consuming LNG on those engines generates a compliance shortfall from 1 January 2025.

The FuelEU penalty structure (Article 23 of Regulation (EU) 2023/1805) sets a pooling-adjusted shortfall penalty equivalent to EUR 2,400 per tonne CO2eq deficit for the 2025 to 2030 period. A 5,000 GT vessel with LPSI engines running on LNG year-round might consume 8,000 tonnes of LNG annually. At the Annex II default of 98.5 versus the 2025 limit of 89.34, the annual shortfall is roughly 9 g CO2eq/MJ across approximately 320,000 GJ of energy, equivalent to a CO2eq shortfall of about 2,880 tonnes. Penalty: roughly EUR 6.9 million per year. MOC retrofit at EUR 300,000 capital and 0.5 percent added fuel cost is clearly preferable.

Charter party considerations

Long-term charterers increasingly require certified in-service slip data when fixing dual-fuel vessels. The mismatch between CII rating (CO2 only) and actual WtW GHG exposure (CO2 plus methane) creates a potential dispute between owners and charterers under time charters where the charter party allocates FuelEU compliance costs. BIMCO has been developing a FuelEU Maritime clause since 2024 that addresses slip-rate certification and cost allocation; operators should verify whether their current charter forms address methane slip explicitly rather than relying on generic “compliance costs” allocations.

Banks signed up to the Poseidon Principles report annual portfolio WtW intensity that includes methane slip. LNG dual-fuel vessels with high in-service slip rates reduce the portfolio climate-alignment score, which increasingly affects refinancing terms and loan pricing.

E-LNG and the long-term trajectory

The longer-term mitigation for any LNG-fuelled engine is to displace fossil LNG with bio-LNG or e-LNG (synthetic methane via the Sabatier process from green hydrogen and captured CO2). E-LNG is chemically identical to fossil LNG; the engine’s slip mechanism and slip rate are unchanged. But the upstream WtT carbon intensity falls to roughly 5 to 15 g CO2eq/MJ (versus 18 g for fossil LNG), and the combustion CO2 is biogenic or e-fuel origin and conventionally accounted as carbon-neutral upstream. A 3 percent slip on e-LNG still adds 42 g CO2eq/MJ from slip at GWP-100 = 28; the total WtW intensity falls to roughly 61 g CO2eq/MJ, better than HFO (93 g) but still affected by slip. Methane oxidation catalysts and combustion-cycle improvements remain relevant investments even after a switch to e-LNG.

FuelEU Maritime includes bio-LNG and e-LNG in its fuel scope with certified WtW values. Vessels burning certified e-LNG can claim GHG intensity below 20 g CO2eq/MJ for the e-LNG portion (subject to RFNBO certification under Article 9 of Regulation (EU) 2023/1805 and the associated delegated acts on renewable fuel definitions).

Limitations

Bench-test vs in-service gap. The slip percentages cited in this article rely on manufacturer bench-test data and the published measurement campaigns. In-service slip on individual vessels varies with fuel composition, engine age, maintenance state, and operating profile in ways not fully captured by any single dataset. The ICCT and SEA-LNG campaigns cover a limited number of vessels; fleet-wide in-service data at the granularity needed for individual compliance decisions remains sparse.

GWP metric choice. GWP-100 = 28 is the regulatory default but is not a universal scientific consensus. AR5 and AR6 report slightly different values; the AR5 and AR6 values with climate-carbon feedback are higher (36 and 34.7 respectively). GWP-20 analysis would frame several currently compliant LBSI vessels as significantly worse than HFO. The choice of GWP is a policy decision embedded in the regulatory framework, not a physical constant.

Upstream leakage uncertainty. The WtT default values in FuelEU Annex II and the IMO LCA Guidelines rely on aggregate estimates that conceal wide regional variation. Vessels bunkering US Permian-sourced LNG face a materially different WtW reality from vessels bunkering Norwegian LNG, but the regulatory framework does not automatically distinguish between them unless the operator certifies a batch-specific WtT value.

MOC field data. Published MOC performance data is largely from bench tests and early commercial pilots. Long-term catalyst durability, regeneration interval requirements under varying pilot-fuel sulphur levels, and performance across the full marine operating envelope (cold start, rapid load cycling, dynamic positioning) are not yet well characterized from fleet-scale in-service data.

Regulatory evolution. The MEPC methane work stream and the FuelEU review clause in Article 33 of Regulation (EU) 2023/1805 could change default values, GWP assignments, or measurement requirements before 2030. Compliance planning should track these deliberations and not rely solely on current default values.

See also

Marine fuels

• LNG as marine fuel
• LNG fuel system
• Methanol as marine fuel
• Ammonia as marine fuel
• Heavy fuel oil
• Well-to-wake intensity
• N2O emissions from marine engines

Engines and exhaust treatment

• WinGD X-DF dual-fuel architecture
• Marine diesel engine
• Exhaust gas cleaning system
• NOx Tier I, II, III

Regulatory frameworks

• MARPOL Annex VI
• IMO Net-Zero Framework
• IMO GHG Strategy
• EEXI, EPL and ShaPoLi
• SEEMP I, II, III
• CII corrective action plan
• EU MRV Regulation
• EU ETS for shipping
• FuelEU Maritime
• FuelEU penalties, pooling and multipliers
• Emission control areas
• IMO 2020 sulphur cap

Voluntary frameworks

• Poseidon Principles

Calculators

• Methane slip GWP calculator
• Methane slip CO2-equivalent calculator
• CH4 methane slip calculator
• LNG well-to-wake calculator
• Bio-LNG well-to-wake calculator
• CO2 from fuel calculator (MEPC.364(79) Cf)
• WtW fuel blend intensity calculator
• CII attained calculator
• GFI compliance calculator