Two-Stroke Engine Future Developments

The slow-speed two-stroke marine diesel engine is being systematically rebuilt for the post-fossil-fuel era. MAN Energy Solutions and WinGD have each developed production-ready dual-fuel engines for methanol, LNG, LPG, and ammonia, with ammonia units now entering commercial service. The 2023 IMO GHG Strategy targets net-zero shipping emissions by or around 2050, and the mid-term measures adopted at MEPC 83 in April 2025 introduce a carbon levy and a fuel intensity standard that translate that target into direct operating costs. These regulatory levers are running in parallel with the EU Emissions Trading System for shipping and FuelEU Maritime, both of which add a price penalty to every tonne of CO2-equivalent emitted in European waters. The combination makes continued operation on heavy fuel oil progressively more expensive through the 2020s and into the 2030s, and it is the immediate economic force reshaping new-build engine specifications.

This article covers the principal development tracks: the alternative-fuel dual-fuel engine families (methanol, LNG, LPG, ammonia), onboard carbon capture, the efficiency and hybridisation improvements, digital and AI engine control, and the practical challenges across all of them. The underlying engine architecture is described in the two-stroke marine diesel engine fundamentals article.

Decarbonisation drivers and the regulatory framework

The slow-speed two-stroke operates in a regulatory environment that has moved from a single NOx/SOx focus to a full carbon-intensity framework on a tight timeline.

IMO 2023 GHG Strategy

MEPC 80 in July 2023 replaced the 2018 GHG strategy with a revised target: at least 20% reduction in total annual GHG emissions from shipping by 2030 relative to 2008, at least 70% by 2040, and net zero by or around 2050. The 2023 strategy for the first time defines a “Lifecycle GHG Intensity” metric, which means the well-to-wake emission of a fuel matters, not just the tank-to-wake combustion emission. This has direct consequences for how methanol and ammonia qualify: a tonne of fossil methanol or fossil ammonia provides little well-to-wake benefit, while green variants (synthesised from renewable hydrogen) can approach zero. Engine technology alone is not enough; the fuel supply chain determines compliance.

IMO mid-term measures: GFS and carbon levy

MEPC 83 (April 2025) adopted the two mid-term measures under the IMO Net-Zero Framework. The Global Fuel Standard (GFS) sets a declining greenhouse gas intensity limit expressed in grams CO2-equivalent per megajoule of energy supplied, tightening from 2028. Ships exceeding the limit pay into a central fund; ships meeting a more stringent zero-emission threshold earn credits they can sell. The IMO Carbon Levy adds a direct price per tonne of CO2-equivalent emitted, with rates structured to make green alternatives progressively cost-competitive with fossil HFO. Both instruments are embedded in MARPOL Annex VI, making them mandatory for all MARPOL signatories.

FuelEU Maritime and EU ETS

FuelEU Maritime applies from January 2025 to ships above 5,000 GT calling at EU/EEA ports. It sets a declining limit on the annual average well-to-wake GHG intensity of the energy used, starting at 2% below the 2020 baseline in 2025 and reaching 80% below by 2050. The EU Emissions Trading System for shipping has applied to large ships on EU voyages since January 2024, with full inclusion (100% of verified CO2 from all voyages between EU ports, plus 50% of voyages to/from non-EU ports) from January 2026. Together these two instruments mean that a ship burning fossil fuel on EU routes already faces a carbon cost per voyage, with that cost rising each year.

CII and EEXI as interim signals

The Carbon Intensity Indicator and the Energy Efficiency Existing Ship Index (both mandatory from January 2023 under MARPOL Annex VI) already rank every ship’s efficiency and require corrective action for poor ratings. These don’t yet carry a direct carbon price, but a C or D CII rating reduces the ship’s charter appeal and is increasingly referenced in BIMCO clause packages. They push owners toward speed and efficiency measures now, ahead of the heavier mid-term cost instruments.

Alternative-fuel two-stroke engine families

The two major slow-speed two-stroke engine makers, MAN Energy Solutions and WinGD, have each developed dual-fuel variants for the four alternative fuels with commercial prospects before 2030: LNG/gas, methanol, LPG, and ammonia. The table below summarises the principal engine designations and combustion types.

LNG dual-fuel: ME-GI versus X-DF

LNG remains the most deployed alternative fuel in the slow-speed two-stroke fleet. The design philosophies from MAN and WinGD diverge at a fundamental combustion level, and that divergence produces different trade-offs for operators.

The MAN B&W ME-GI (Gas Injection) compresses natural gas to approximately 300 bar in a high-pressure supply system before injecting it directly into the cylinder near the end of the compression stroke, where it ignites through the heat of the compressed charge alone, with no separate pilot fuel. The combustion mechanism is identical to the conventional diesel cycle. Methane slip is essentially zero: the gas burns inside a hot high-pressure environment from which it cannot escape unburned in any meaningful quantity. The CO2 reduction versus HFO is approximately 20 to 25% on a tank-to-wake basis. The capital cost of the high-pressure gas supply system (compressors, high-pressure gas pump, high-pressure piping) adds roughly $3 to 5 million to a new-build compared with an ME-GI-ready ship using HFO, and the high-pressure equipment requires more stringent maintenance disciplines.

The WinGD X-DF (Dual-Fuel) premixes natural gas with the scavenge air before the combustion chamber, operating on the Otto cycle where ignition is triggered by a small quantity of pilot fuel injected by a conventional diesel injector. The low-pressure gas supply system is simpler and less costly than the ME-GI high-pressure arrangement. However, in Otto-cycle operation at partial loads, unburned methane passes through the cylinder during scavenging, producing methane slip of roughly 2 to 4 g/kWh depending on load. WinGD’s X-DF 2.0 upgrade reduces this via a reformed injection strategy and gas admission valve improvements, targeting below 1.5 g/kWh. The well-to-wake value of the fuel supply matters here: LNG’s CO2-equivalence advantage can be reduced by roughly 30 to 50% if methane slip is not controlled, because methane carries a 20-year global warming potential of approximately 82 times CO2.

Both engine families are in large-scale service. As of 2025, over 700 two-stroke LNG dual-fuel engines are on order or in service from MAN and WinGD combined, covering container ships, LNG carriers, tankers, and bulk carriers. See the LNG as marine fuel article for the fuel-side context.

Methanol: ME-LGIM and X-DF-M

Methanol was the first liquid alternative fuel to see commercial slow-speed two-stroke deployments at scale. The MAN B&W ME-LGIM (Liquid Gas Injection Methanol) uses a dual-fuel injection system: a small quantity of conventional diesel pilot fuel ignites, and the combustion event propagates into the methanol charge injected through a dedicated high-pressure injector. The system runs entirely on the diesel cycle. Methanol injection occurs at roughly 600 bar through a stainless-steel low-flashpoint fuel system that complies with the IGF Code Chapter 7 requirements for methanol as a fuel. The engine can switch between methanol mode and conventional diesel (HFO/VLSFO) mode without interrupting the combustion event, typically via a change-over sequence taking 10 to 15 seconds.

Laura Maersk, delivered in September 2023, was the first methanol-fuelled container ship to enter service, powered by an ME-LGIM. By end of 2024 MAN had orders for ME-LGIM installations on over 130 vessels, covering container ships, tankers, and car carriers. WinGD’s X-DF-M follows the same pilot-ignition diesel-cycle architecture, with commercial orders from 2023 and first deliveries expected in 2026.

Fossil methanol from natural gas provides a tank-to-wake CO2 reduction of approximately 7 to 10% versus HFO (better than nothing, but marginal). Green methanol, synthesised from electrolytic hydrogen and captured CO2, can reduce well-to-wake emissions by over 90%. The economics of green methanol are currently unfavourable: green methanol production costs sit at roughly $800 to$1,400 per tonne versus $350 to$600 for fossil methanol. However, the renewable production pathway scales directly with electrolyser capacity and renewable electricity cost, and the IMO carbon levy creates an increasing incentive to shift to green feedstock over time. See the methanol marine engines overview and methanol as marine fuel for detailed treatment.

Methanol is a safety challenge for different reasons than LNG. It’s colourless, so leaks are invisible. It’s miscible with water, which affects fire-fighting. Its flashpoint (approximately 11 degC) classifies it as a low-flashpoint fuel requiring IGF Code compliance, including gas-safe machinery spaces, detection systems, and double-walled piping for the low-flashpoint supply system. The ME-LGIM fuel system addresses all of these through an enclosed vented second-barrier piping arrangement.

LPG: ME-LGIP and X-DF-LPG

Liquefied petroleum gas (LPG, primarily propane or butane) has an established global bunkering infrastructure tied to the petrochemical and domestic-gas trade. MAN’s ME-LGIP (Liquid Gas Injection Propane) shares the ME-LGIM architecture: diesel-cycle combustion with a pilot ignition and a high-pressure low-flashpoint fuel injection system. LPG is stored at modest pressure (about 8 bar at ambient temperature for propane) with no cryogenic requirement, which simplifies the fuel tank arrangement relative to LNG.

The ME-LGIP is in commercial service primarily on very large gas carriers (VLGCs), where the cargo boil-off gas can also feed the main engine. This makes LPG dual-fuel on a VLGC operationally analogous to steam LNG carriers, but on the diesel cycle with far better fuel efficiency. WinGD’s X-DF-LPG offers the same fuel flexibility for LPG. The tank-to-wake CO2 improvement from fossil LPG versus HFO is roughly 15 to 20% because propane has a lower carbon-to-hydrogen ratio. A bio-LPG pathway exists (from waste fats and oils, via hydrotreating), which could push well-to-wake intensity much lower. See the LPG well-to-wake article for the lifecycle numbers.

Ammonia: ME-LGIA and X-DF-A

Ammonia is the fuel most technically capable of delivering zero-carbon shipping at scale, because green ammonia (produced by the Haber-Bosch process using electrolytic hydrogen from renewable electricity) contains no carbon. Its combustion produces nitrogen and water in theory, though in practice a hot high-pressure flame also oxidises some nitrogen to form nitric oxide (NOx) and small quantities of nitrous oxide (N2O), which is a potent greenhouse gas with a 100-year global warming potential approximately 273 times that of CO2. Ammonia also burns slowly and has a narrow flammability range, which makes ignition and stable combustion in a high-speed injection event difficult.

MAN Energy Solutions addressed these challenges in the ME-LGIA (Liquid Gas Injection Ammonia) through a dual-injection arrangement: a small quantity of diesel pilot fuel ignites the cycle, and liquid ammonia is injected at high pressure into the burning combustion chamber through a separate injector that uses a corrosion-resistant material specification (ammonia attacks copper and its alloys). The system maintains the diesel combustion cycle. After extensive shop trials at MAN’s Copenhagen facility, the ME-LGIA completed full-scale running on ammonia in 2024 and the first commercial installation entered sea service in early 2025. The pilot fuel fraction is approximately 5 to 7% of energy content; on green ammonia this fraction must also come from a low-carbon source to fully realise the lifecycle benefit.

WinGD’s X-DF-A (Dual-Fuel Ammonia) uses a similar pilot-ignition approach and entered prototype testing in 2024. Commercial deliveries from WinGD are expected from 2026. Both manufacturers publish ammonia conversion kits for existing X-DF (gas) platforms, which shares much of the gas valve and cylinder-head infrastructure.

The ammonia safety challenge is substantial. Ammonia is acutely toxic: the IDLH (Immediately Dangerous to Life or Health) concentration is 300 ppm. A bunkering spill or a fuel-line leak on a ship presents a shipboard emergency in a way that a methane leak (which disperses rapidly) does not. IGF Code Chapter 12 provides the regulatory framework for ammonia as a fuel, but the class-society implementation guidelines (Lloyd’s Register, DNV, Bureau Veritas) extend those rules considerably in practice. Crew training for ammonia emergency response is a non-trivial programme requiring simulator time and shore-based drills. N2O production in the combustion process is actively being quantified: MAN’s shop-test data from the ME-LGIA suggest N2O formation is low (below 10 mg/Nm3 in the exhaust) at the optimised injection timing, but this remains an area of active measurement and research.

See the ammonia marine engines overview and ammonia as marine fuel for the combustion chemistry and safety systems in detail.

Ethane and other developing fuels

MAN’s ME-LGIFE covers ethane, a component of LPG streams and a significant cargo on ethane carriers. Ethane’s CO2 intensity is between LNG and LPG on a tank-to-wake basis. The engine architecture is the same ME-LGI diesel-cycle pilot-ignition approach. Hydrogen as a pure combustion fuel in a two-stroke is under research but faces severe volumetric storage constraints: liquid hydrogen requires cryogenic storage at -253 degC, and gaseous storage at practical ship pressures provides very limited range. No commercial slow-speed two-stroke hydrogen engine programme has a confirmed delivery date as of mid-2025.

Methane slip reduction and NOx management

The methane slip problem

For the WinGD X-DF and similar low-pressure Otto-cycle gas engines, methane slip is the dominant lifecycle performance issue. At 25% engine load (which occurs frequently during port approaches and manoeuvring), X-DF engines at their original design specification produced slip in the range of 5 to 8 g/kWh. The X-DF 2.0 upgrade package, available from WinGD from 2022 onward as a retrofit or factory-fit, introduces a new gas admission valve with faster closing action and a reformed gas injection strategy that reduces slip to below 2 g/kWh across the load range, including at low load. This matters for the CII rating calculation under MARPOL Annex VI Regulation 26, which will incorporate methane factors when the IMO updates its CII calculation guidance.

NOx in ammonia and LPG engines

Ammonia combustion at diesel-engine temperatures and pressures produces elevated NOx from thermal nitrogen oxidation, because N-H bonds in the ammonia molecule also produce reactive nitrogen intermediates. ME-LGIA test data indicates that NOx emissions are manageable with injection timing tuning, but exhaust gas recirculation (EGR) or selective catalytic reduction (SCR) may be required to meet MARPOL Annex VI Tier III NOx limits in nitrogen emission control areas (NECAs). The pilot-injection in dual-fuel engines article describes the general pilot-fuel combustion dynamics that apply across these fuel types.

Onboard carbon capture

A parallel technology path targets the exhaust stream from engines that continue to burn fossil or blended fuels: onboard carbon capture (OCC) systems capture CO2 from the engine exhaust and compress or liquefy it for offloading in port. The concept has been piloted since 2021, with the most advanced commercial project being the Cargill-chartered bulk carrier Pyxis Ocean, which ran with a Compact Carbon Capture unit installed for evaluation in 2024.

OCC adds weight, volume, and power consumption. A system capable of capturing 70 to 80% of exhaust CO2 from a large two-stroke installation requires a post-combustion absorption column (typically monoethanolamine or potassium carbonate solvent), a regeneration heater that consumes part of the ship’s waste heat, and CO2 compression or liquefaction equipment. The total power penalty is estimated at 3 to 8% of main-engine output, and the captured CO2 must be stored in liquid form at approximately -20 degC or at high pressure, requiring dedicated cargo-hold volume or deck tanks. The mass of CO2 produced by a large two-stroke is substantial: at 175 g/kWh SFOC burning HFO with a carbon content of approximately 86%, a 60,000 kW engine produces roughly 550 tonnes of CO2 per day at full load.

Class society rules for OCC are being developed. DNV’s class guideline DNVGL-CG-0659 (2024) covers the structural, fire safety, and chemical hazard requirements. Lloyd’s Register and Bureau Veritas have issued equivalent guidance notes. Whether OCC counts fully toward IMO GHG compliance depends on the offloading and permanent storage chain, which the IMO is still formalising under the CCS pathway in the mid-term measures framework.

Efficiency developments: ultra-long stroke, derating, and turbocompounding

Ultra-long stroke designs

The thermodynamic efficiency of a diesel engine improves as the expansion ratio increases, and in a two-stroke the expansion ratio is directly linked to the stroke-to-bore ratio. MAN’s current extreme long-stroke designs (the G-type and S-type series with stroke-to-bore ratios of 3.5:1 to 4.5:1) already extract about as much work per unit fuel as is physically achievable in a conventional diesel cycle at the operating conditions available. Cycle efficiency at maximum efficiency point is approximately 55% on the best current engines (equivalent to SFOC of roughly 155 to 162 g/kWh on diesel fuel). Further gains of 2 to 4% are being targeted through:

• Increased peak firing pressure (current designs up to 200 bar; next-generation targeting 220 to 240 bar), which requires stronger cylinder heads and piston crowns using advanced alloy steel or nickel superalloy inserts.
• Improved turbocharger efficiency: the MAN TCA and ABB Power2 turbocharger series used on ME-GI and ME-LGIM installations achieve turbine efficiency in the 91 to 93% range; any further gain extends the working range of the engine’s scavenging efficiency.
• Variable compression ratio concepts, achieved by modifying the effective clearance volume through a hydraulic piston-crown mechanism; these are at laboratory/prototype stage for two-stroke scale as of 2025.

Engine derating and slow steaming

Intentional derating, operating an engine at a significantly lower load than its rated maximum continuous rating (MCR), has been the main short-term efficiency measure for many fleets since slow steaming became widespread after 2008. The efficiency benefit comes because a two-stroke at 50 to 70% load typically sits close to its specific fuel oil consumption minimum; at 100% load the SFOC rises slightly due to heat losses and combustion timing constraints. The engine derating for slow steaming article covers the formal derating process under engine makers’ instructions, including the need for revised layout diagrams and the impact on turbocharger matching.

The CII framework adds an administrative incentive to derating: a ship with a good CII rating can accept slightly longer voyage times without losing charterer confidence, while a ship approaching a D or E rating needs speed reduction (and therefore load reduction) to bring its intensity back into the acceptable band.

Waste heat recovery

A waste heat recovery system (WHRS) captures energy from the engine exhaust gas that would otherwise be lost to the atmosphere. On a modern large two-stroke, the exhaust energy available after the turbocharger turbine represents approximately 10 to 15% of the fuel energy input. WHRS architectures used in commercial service include:

• Power turbine (PT): a second turbine connected to the exhaust manifold downstream of the main turbocharger turbine drives a generator or a hydraulic coupling to the shaft. Typical output: 2 to 4% of main engine power. MAN and WinGD both offer integrated PT packages.
• Steam cycle WHRS: exhaust heat raises steam in an exhaust gas economiser (EGE), and the steam drives a turbogenerator or supplements the main engine shaft through a turbine wheel. Typical power recovery: 4 to 7% of main engine output at full load. The steam cycle also captures jacket-water and scavenge-air cooler heat in larger installations.
• Organic Rankine cycle (ORC): uses a low-boiling organic working fluid (typically pentane or refrigerant) in place of steam, enabling heat recovery at lower exhaust temperatures and from cooling-water sources. ORC units are more compact than steam systems and are being fitted on medium-sized ships where the steam cycle’s capital cost is harder to justify.

An integrated WHRS on a large container ship can recover 3 to 8 MW of additional shaft power, reducing net fuel consumption by 4 to 8% on a voyage. At current bunker prices this represents $1.5 to$3 million per year on a heavily laden ULCS running near full load.

Hybridisation and shaft generators

PTO/PTI shaft arrangements

The most common hybridisation path for slow-speed two-stroke ships is a shaft generator / shaft motor mounted on the propulsion shafting between the main engine and the propeller, operating in power take-off (PTO) or power take-in (PTI) mode. In PTO mode the engine turns the propulsion shaft and also drives the generator, replacing the auxiliary diesel generators during sea passage and eliminating their fuel consumption (which might be 2 to 4 tonnes per day on a large ship). In PTI mode the same machine operates as a motor, boosting shaft power from battery or shore connection during harbour manoeuvring or in slow-steaming conditions where the main engine would otherwise run at a very low load with poor efficiency.

The battery-hybrid propulsion article describes the BESS (battery energy storage system) integration that can work alongside a PTO/PTI arrangement. Battery capacity on current large ships is typically limited to 1 to 5 MWh, which is enough for harbour zero-emission periods of 2 to 4 hours or for peak-shaving during acceleration demands, but is not sufficient to replace the main engine for blue-water propulsion.

Shaft-power limitation and EPL

The Energy Power Limitation (EPL) and ShaPoLi (Shaft Power Limitation) programmes, introduced alongside the EEXI under MARPOL Annex VI, limit the maximum continuous power available to the propeller shaft in order to reduce a ship’s carbon intensity index. On some vessels this is done through an engine derating, but on others it is done through a shaft power limiter that caps the electrical or mechanical power output available to the shaft drive, even if the main engine is physically capable of more. See the EEXI EPL and ShaPoLi article for the technical and regulatory detail.

Digital control and AI optimisation

Electronic fuel injection and adaptive control

The MAN ME-C electronic control system replaced the camshaft-driven mechanical injection and exhaust valve timing on two-stroke engines from the early 2000s. The electronically controlled system allows injection timing, injection pressure, exhaust valve opening/closing, and cylinder lubrication dosing to be varied independently and continuously. On the ME-GI and ME-LGIM this extends to the gas admission valve timing and the pilot-fuel injection sequence. The control granularity this provides is the foundation for all subsequent AI and optimisation developments.

WinGD’s Integrated Intelligent Operating Concept (iCON) platform, introduced from 2022, embeds a digital model of the engine’s thermodynamic behaviour directly into the engine control unit. iCON uses real-time sensor data (cylinder pressure, exhaust temperature, turbocharger speed, fuel flow) to continuously adjust operating parameters, targeting a defined combination of SFOC, NOx, and component-life metrics. In WinGD’s internal tests, iCON reduced SFOC by 1 to 3 g/kWh across typical load profiles.

Digital twins

A digital twin is a real-time simulation of the engine’s thermodynamic and mechanical state, running on either the ship’s onboard computing system or at a shore-based operations centre, continuously updated from the engine’s live sensor data stream. The twin runs the same physics-based or data-calibrated model as the design-stage simulation tool, but now reconciled to the actual engine’s condition: worn cylinder liner geometry, actual piston ring condition, actual turbocharger efficiency at its current age.

MAN’s CoCoS-EDS (Computer Controlled Optimisation System, Engine Diagnostics System) and WinGD’s digital reporting interfaces both feed shore-based fleet management platforms. The twin compares measured exhaust temperatures and cylinder pressures against the model prediction; a deviation of more than 2 to 3% in peak pressure or a consistent exhaust temperature difference across cylinders triggers an automated alert and a root-cause diagnostic tree. Class society condition monitoring rules, such as ClassNK’s PM-CAIMS programme, increasingly accept continuous condition monitoring data from digital twins as a basis for extending survey intervals.

Predictive maintenance

Predictive maintenance on two-stroke engines uses the combination of digital-twin deviation monitoring, high-frequency vibration sensors on bearing housings and the crosshead, and periodic cylinder-pressure indicator analysis to predict the remaining useful life of components subject to wear and fatigue. The principal targets are:

• Cylinder liner wear: liner bore measurements at each piston inspection interval, combined with cylinder oil consumption data, allow a wear rate model to project the remaining liner life and optimise the cylinder oil feed rate. MAN’s Alpha Lubricator system varies the lubricant dosing per cylinder based on the sulphur content of the current fuel and the instantaneous load, cutting cylinder oil consumption by 20 to 40% compared with a fixed-rate system without increasing ring/liner wear.
• Crosshead bearing: vibration signatures from accelerometers on the crosshead guide shoes correlate with bearing clearance and oil-film breakdown, enabling bearing replacement to be scheduled at planned port calls rather than as an emergency breakdown.
• Exhaust valve seat wear: continuous exhaust-valve lift monitoring combined with temperature and cylinder-pressure data allows the valve seat wear to be tracked; re-grinding intervals are optimised accordingly.

The commercial outcome is measurable. A fleet operator with a large VLCC fleet reported in 2024 that predictive maintenance integration across 40 ships running MAN ME-GI engines reduced unplanned engine stoppages from 1.8 per vessel per year (the fleet average before the programme) to 0.4 per vessel per year after 24 months, citing MAN’s CEON fleet analytics platform.

AI-assisted combustion tuning

Machine-learning models trained on fleet-wide operational data can identify combustion tuning optima that are not apparent from single-ship or test-bed data. Because cylinder condition varies between ships of the same class (different liner wear profiles, slightly different injection-nozzle wear), the optimum injection timing and pressure for minimum SFOC without exceeding the thermal load limit is ship-specific and shifts over time. AI-assisted tuning continually re-identifies the optimum as the engine ages. WinGD’s Merlin analytics platform (the commercial name for its fleet-learning system) uses this approach across the X-DF fleet; MAN’s equivalent is the CEON platform.

Fuel availability and bunkering challenges

The technical readiness of the alternative-fuel engines is not the limiting factor for adoption in 2025. Fuel availability is.

LNG bunkering is available at approximately 135 ports worldwide as of early 2025, covering most major container routes. Methanol bunkering is available at fewer than 30 ports with any regularity, though this is expanding: Maersk has publicly committed to securing green methanol supply contracts for its new-build ME-LGIM fleet, and port authorities in Rotterdam, Singapore, and Ulsan have commissioned methanol bunkering infrastructure.

Ammonia bunkering for ships does not yet exist as a commercial service anywhere in the world. The IGF Code framework for ammonia is in place at the regulatory level, but no major port has commissioned the quayside handling equipment, vent-mast infrastructure, safety detection systems, and emergency response procedures required for ammonia ship-to-ship or terminal-to-ship bunkering transfer. Industry working groups led by the Ammonia Energy Association and the Global Maritime Forum have published port readiness frameworks, but first commercial ammonia bunkering is unlikely before 2027 or 2028 at the earliest established port.

The economics of green alternative fuels remain the dominant barrier. As of mid-2025:

• VLSFO (very low-sulphur fuel oil) trades at approximately $550 to$650 per tonne.
• Fossil LNG on an energy-equivalent basis: approximately $500 to$700 per tonne equivalent, depending on LNG spot price.
• Fossil methanol: approximately $350 to$550 per tonne.
• Green methanol (e-methanol): approximately $800 to$1,400 per tonne depending on electrolyser vintage and renewable electricity cost.
• Grey ammonia: approximately $250 to$350 per tonne.
• Green ammonia (electrolytic): approximately $650 to$1,100 per tonne.

The IMO carbon levy, starting from 2028, will add a cost to fossil fuel that narrows these gaps. At the levy rate structure adopted at MEPC 83, the effective cost addition to VLSFO at the proposed levy entry level is estimated at $60 to$90 per tonne of fuel, rising over time. This alone doesn’t make green fuels economically superior in 2028, but combined with FuelEU Maritime penalties and the EU ETS allowance cost, the total carbon cost burden on a fossil-fuel ship operating in European trades moves into a range where green methanol becomes competitive at the upper end of its current production cost.

Limitations of this article

This article describes the principal development tracks for slow-speed two-stroke marine engines as of mid-2025. Several caveats apply:

• Delivery and service data are rapidly changing. The ammonia-engine programme in particular is moving from shop tests to initial commercial service on a timeline of months, not years. The figures cited (pilot-fuel fraction, N2O emission concentration, bunkering availability dates) reflect published test data and industry announcements available at the time of writing and should be verified against current OEM technical documentation for any specific project.
• Well-to-wake emission factors are fuel-chain-specific. The lifecycle CO2-equivalent savings attributed to any alternative fuel depend on the specific production pathway (fossil, bio, electrolytic) and geographic grid carbon intensity. The figures in this article are indicative ranges, not regulatory compliance values. Regulatory compliance must use the default factors published in MEPC.1/Circ.795 or the well-to-wake emission factors approved under the FuelEU Maritime Delegated Regulation.
• Engine designations and availability change by year. The ME-LGIM, ME-LGIA, X-DF-M, and X-DF-A product lines are under continuous development. Engine bore variants, stroke-to-bore ratios, and available cylinder counts for each fuel type change with each new product revision. Consult the current MAN Engine Programme and WinGD Engine Programme documents for specific project specifications.
• Bunkering and safety standards are still being finalised. IGF Code Chapter 12 (ammonia) was adopted at MSC 107 in 2023, but the supporting MSC circulars on risk assessment methodology and port bunkering procedures are still being developed by the IMO. Similarly, the class-society onboard carbon capture guidelines cited are first-generation documents that will be revised as project experience accumulates.
• Economics depend heavily on the carbon-levy design. The IMO mid-term measures adopted at MEPC 83 are subject to a review clause in 2028. Changes to the levy rate structure or the GFS limit trajectory would materially shift the break-even economics of each alternative fuel.

See also

• Two-Stroke Marine Diesel Engine Fundamentals
• Ammonia Marine Engines Overview
• Ammonia as Marine Fuel
• Methanol Marine Engines Overview
• Methanol as Marine Fuel
• LNG as Marine Fuel
• WinGD X-DF Dual-Fuel Engine Architecture
• MAN B&W ME-C Electronic Control Overview
• Onboard Carbon Capture
• IMO Net-Zero Framework
• FuelEU Maritime Explained
• EU ETS for Shipping
• Waste Heat Recovery System
• Battery-Hybrid and Full-Electric Propulsion
• Pilot Injection in Dual-Fuel Engines
• What is CII?
• What is EEXI?
• Slow Steaming and CII