LNG-Fuelled Ships and the Methane Slip Problem
Dual-fuel engine types, what methane slip does to the carbon calculation, the FuelEU limit timetable, fuel tanks, the rules and the limits of LNG as a transition fuel.
An LNG-fuelled ship burns liquefied natural gas in its main or auxiliary engines, and most such ships are dual fuel, able to return to MGO or fuel oil when required. Gas mode cuts sulphur oxides and particulates sharply and reduces combustion CO₂ by about 27% per unit of energy against heavy fuel oil. Whether those gains survive a full climate calculation depends on how much methane leaves the funnel unburned. For that reason the engine matters as much as the fuel.
DNV's Alternative Fuels Insight counted 914 LNG-capable ships in service and 654 on order in mid-2026.[1] LNG also accounted for 63% of the 242 alternative-fuel newbuildings ordered in the first eight months of that year, with container ships taking 59% of those orders and car carriers 30%.[2]
Tailpipe CO₂ Arithmetic
Combustion CO₂ follows directly from a fuel's carbon content. FuelEU Maritime's default values give 2.750 tonnes of CO₂ and 49.1 GJ of energy per tonne of LNG, against 3.114 tonnes and 40.5 GJ per tonne of heavy fuel oil (HFO).[3] Dividing one by the other gives the comparable figures.
LNG: 2.750 / 49.1 = 56.0 g CO₂/MJ. HFO: 3.114 / 40.5 = 76.9 g CO₂/MJ. The gap is about 27%.
Two further items complete the picture: upstream emissions from production and transport, known as well-to-tank (WtT), and methane that escapes combustion.
Methane Slip
Methane slip is the share of gas entering an engine that reaches the exhaust unburned. Because methane is a potent greenhouse gas, small percentages move the result. FuelEU assigns it a 100-year warming potential of 25 times CO₂.[3] EU MRV and ETS rules draw on a different reference, and the Commission's guidance warns that the factors may not match.[3]
The regulation sets default slip by engine type, and the table below converts each into methane and CO₂ equivalent per tonne of LNG.
| Engine type | Default slip | Methane per tonne of LNG | CO₂ equivalent (tonnes) |
|---|---|---|---|
| Otto, medium speed, dual fuel | 3.1% | 31 kg | 0.775 |
| Lean-burn spark ignition (LBSI) | 2.6% | 26 kg | 0.650 |
| Otto, slow speed, dual fuel | 1.7% | 17 kg | 0.425 |
| Diesel cycle, slow speed, dual fuel | 0.2% | 2 kg | 0.050 |
Source: FuelEU Annex II defaults, calculated with a GWP of 25.[3]
Otto and Diesel Cycle Engines
An Otto-cycle engine admits gas before compression, forming a lean mixture that a small pilot injection then ignites. The burn is relatively cool. WinGD's X-DF engines take gas at under 13 bar, which keeps the supply system simple, and the maker states that they meet IMO Tier III NOx limits with no EGR or SCR.[4] The drawback is that some of the mixture survives combustion in the cylinder.
A Diesel-cycle engine injects gas at high pressure late in compression, so it burns as it arrives. MAN's ME-GI is the main example, with a supply pressure of about 300 bar and a stated methane slip of 0.2 g/kWh.[5][6] The cost lies in a high-pressure pump or compressor and, because combustion runs hotter, an additional system for NOx in most installations.
| Characteristic | Otto cycle | Diesel cycle |
|---|---|---|
| Gas admission | Before compression, low pressure | End of compression, high pressure |
| Methane slip | Relatively high | Very low |
| NOx Tier III | Usually no extra system | Usually an extra system |
| Fuel supply system | Simpler | More complex |
| Fuel quality sensitivity | Knock depends on methane number | Less sensitive |
Well-to-Wake Comparison
Since 2025, FuelEU Maritime has capped the greenhouse gas intensity of energy used by ships above 5,000 GT calling at EU ports, measured against a reference of 91.16 g CO₂e/MJ. The defaults yield the following totals.
| Fuel and engine | Well-to-tank | Tank-to-wake | Total (g CO₂e/MJ) |
|---|---|---|---|
| HFO | 13.5 | 78.2 | 91.7 |
| MGO | 14.4 | 76.4 | 90.8 |
| LNG, Otto, medium speed | 18.5 | 70.7 | 89.2 |
| LNG, Otto, slow speed | 18.5 | 64.4 | 82.9 |
| LNG, Diesel cycle, slow speed | 18.5 | 57.6 | 76.1 |
Source: calculated from FuelEU Annex I and II values. The Diesel-cycle result matches the worked example of 76.08 in the Commission's guidance.[3]
On this basis a medium-speed Otto engine leaves LNG only about 3% ahead of HFO, while a slow-speed Diesel-cycle engine leaves it about 17% ahead. Set beside the regulatory timetable, that difference determines how long a ship can comply on fossil LNG alone.
| Year | Reduction from reference | Limit (g CO₂e/MJ) |
|---|---|---|
| 2025 | 2% | 89.34 |
| 2030 | 6% | 85.69 |
| 2035 | 14.5% | 77.94 |
| 2040 | 31% | 62.90 |
| 2045 | 62% | 34.64 |
| 2050 | 80% | 18.23 |
Source: Commission guidance; the 2045 value calculated as 91.16 × 0.38.[3]
Using defaults, a medium-speed Otto ship sits just under the 2025 limit and over the 2030 one. A slow-speed Otto ship clears 2030 and misses 2035. A slow-speed Diesel-cycle ship clears 2035 and misses 2040. The calculation is annual and per ship, and pooling and other flexibility mechanisms can alter the outcome.[3]
Measured Slip Values
Commission guidance lets an owner replace the default with a certified measured value. The procedure rests on IMO resolution MEPC.402(83), combining spot measurements with continuous load monitoring and applying 30-minute load averages to emission curves. DNV reports that improved slow-speed Otto engines can reach 0.7% against the 1.7% default.[7] WinGD has said that 0.5% is feasible on its X-DF 2.0 engines.[4]
Four-stroke engines have been measured too. Wärtsilä 46DF installations on MSC World Europa and MSC Euribia were verified at 1.67% and 1.48%, both well below the 3.1% default.[8] These figures come from makers and owners and vary with ship and operating profile. Time spent at low load tends to raise slip, so the load distribution behind any published number deserves attention.
Methane and nitrous oxide entered the EU Emissions Trading System in 2026,[7] so engine choice now feeds directly into compliance costs on EU trades.
Fuel Containment
At 49.1 MJ/kg and about 0.45 kg/L a litre of LNG holds about 22 MJ, compared with about 40 MJ for heavy fuel oil, so equal energy takes about 1.8 times the volume. The IGF Code also limits filling to 98% at the reference temperature. Insulation and tank geometry add more on top, and the space usually comes out of cargo.[9]
The Code defines four types of tank.
| Tank | Construction | Design vapour pressure | Secondary barrier |
|---|---|---|---|
| Membrane | Non-self-supporting, thin barrier supported through insulation by the hull | Normally up to 0.025 MPa, below 0.070 MPa with reinforcement | Complete |
| Independent Type A | Classical structural analysis, mainly plane surfaces | Below 0.07 MPa | Complete |
| Independent Type B | Model tests plus stress, fatigue and crack propagation analysis | Below 0.07 MPa if prismatic | Partial |
| Independent Type C | Pressure vessel rules | By formula, based on a minimum of 0.2 MPa | Not required |
Source: IMO, IGF Code, 6.4.3 and 6.4.15.[9][10]
Membrane tanks make good use of the hull, and the large container ship CMA CGM Jacques Saadé carries about 18,600 m³ of LNG.[11] Type C tanks need no secondary barrier and can contain boil-off for some time by letting pressure rise, at the cost of unusable space around the cylinder.
Rules and Safety
The IGF Code, adopted through resolution MSC.391(95) on 11 June 2015, took effect with the related SOLAS amendments on 1 January 2017.[9] It covers ships other than gas carriers, which remain under the IGC Code when burning their own cargo.[10]
One of the Code's core requirements is that systems must prevent venting in normal operation unless safety demands it.[9] In practice boil-off has to be burned, reliquefied or held in the tank within its pressure limits. Other shipboard hazards are cold embrittlement of carbon steel, gas accumulation in enclosed spaces and rollover, a sudden release of vapour when stratified layers mix. The properties of the fuel and its accident record are covered under liquefied natural gas.
How Far LNG Goes as a Transition Fuel
Owners who choose LNG often point out that the same tanks and engines can take bio-LNG or synthetic LNG, for which the regulation provides separate values. The limit of the fossil product is visible in the tables above. The 2040 cap of 62.90 g CO₂e/MJ is out of reach for fossil LNG on default values in every engine type.[3] For a ship with a long working life, the decision therefore rests on the future cost and supply of those alternatives.
Global rules remain open. Adoption of the IMO Net-Zero Framework was postponed for a year in October 2025 by 57 votes to 49.[12] MEPC 84 in April 2026 left the framework as the main basis for talks without a decision. MEPC 85 is scheduled for 30 November to 3 December 2026, with the adjourned extraordinary session resuming on 4 December. That was the position on 3 November 2026, and the outcome is uncertain.[13]
The LPG option and the wider range of marine fuels are treated separately.
Frequently asked questions
What is an LNG-fuelled ship?
An LNG-fuelled ship burns liquefied natural gas in its main or auxiliary engines, and most such ships are dual fuel, able to return to MGO or fuel oil when required.
Does an LNG ship need low sulphur fuel in an ECA?
Gas mode meets the sulphur limit by itself. Ships still carry suitable liquid fuel for pilot injection and for changeover.
How can methane slip be reduced?
The main levers are engine selection, improved in-cylinder combustion, exhaust gas recirculation and operating profiles that avoid long periods at low load.
Is the FuelEU default slip mandatory?
No. An owner can use a measured value certified through the approved procedure.
Can any ship be converted to LNG?
Tank space, engine compatibility and time out of service decide it for each ship.
Sources
- ENGINE. LNG Leads Shrinking Alternative Fuel Vessel Orderbook in June: DNV, 3 July 2026.
- LNG Industry. DNV Alternative Fuel Vessel Order Figures for August 2026, 4 September 2026.
- European Commission. Guidance on the FuelEU Maritime Regulation for Shipping Companies, 8 October 2025, Section 4.
- WinGD (D. Kadau). WinGD's X-DF Low-Pressure Dual-Fuel Marine Propulsion Engines Technology, IMO Technical Seminar on Methane-Based Fuels, May 2026.
- Everllence (MAN Energy Solutions). ME-GI Product Page, 2026.
- Sea-Man. ME-GI Engine (secondary source for gas supply pressure).
- DNV. EU Guideline for Reporting and Verifying Actual Methane Slip for FuelEU and EU MRV/ETS, 17 November 2025.
- Wärtsilä. MSC Cruises Verified Methane Slip Data, Case Study, 2026.
- International Maritime Organization (IMO). Resolution MSC.391(95), IGF Code, 11 June 2015, paragraphs 3.2.9, 6.4.3, 6.4.15, 6.8.1.
- International Maritime Organization (IMO). IGF Code Overview Page; Resolution MSC.551(108), 2024.
- Wikipedia. CMA CGM Jacques Saadé (secondary source).
- Ship & Bunker. IMO's MEPC Meeting Votes to Delay Adoption of Net-Zero Framework, October 2025.
- Ammonia Energy Association. MEPC84: Support for Net Zero Framework Stabilizes ahead of Key Decision Point in Late 2026, 2026.
