Ammonia marine engines are a class of dual-fuel marine diesel engines designed to operate on ammonia (NH3) as the primary fuel, with a small diesel pilot injection for ignition. Ammonia is a leading candidate alternative marine fuel for the post-2030 fleet, particularly for bulk carriers, tankers, and large container vessels where its higher volumetric energy density compared to hydrogen and its established global production infrastructure offer commercial advantages over alternative low-carbon fuels. The first commercial ammonia-fuelled marine engines entered prototype operation in 2023 and 2024, with first ship deliveries planned for 2026 and 2027.
Combustion characteristics
Ammonia is a low-cetane gas (or pressurised liquid) that does not auto-ignite reliably under typical marine diesel compression ratios and burns with a relatively low flame speed. Ammonia marine engines therefore use:
- Pilot-ignition dual-fuel cycle: a small quantity of conventional diesel or marine gas oil is injected to initiate combustion. This is the dominant approach used by MAN B&W ME-LGIA (slow-speed two-stroke) and Wärtsilä ammonia-ready medium-speed variants.
- High-pressure liquid ammonia injection: ammonia is stored and injected as pressurised liquid at approximately 50 to 100 bar, vapourising during combustion.
The combustion physics of ammonia presents specific challenges including:
- Slow flame propagation: requiring careful injection timing and combustion chamber geometry to ensure complete burn within the cylinder time.
- Nitrogen-rich exhaust: producing significantly higher nitrogen oxide (NOx) emissions than diesel combustion, requiring selective catalytic reduction (SCR) aftertreatment.
- Unburned ammonia slip: a small fraction of ammonia can pass through the engine without complete combustion, presenting an emission issue. Aftertreatment (typically SCR-based) is designed to oxidise this slip.
- Nitrous oxide (N2O) emissions: a potent greenhouse gas formed in low concentrations from ammonia combustion, requiring engine optimisation to minimise.
Major engine product lines
The principal ammonia marine engine variants in 2026 are:
MAN B&W ME-LGIA (slow-speed two-stroke): introduced in prototype in 2023, the LGIA (Liquid Gas Injection Ammonia) variant of the MAN B&W ME-C electronic two-stroke platform supports operation on ammonia with a pilot diesel injection. The first commercial sea trial was completed in 2024, and the first newbuild deliveries with ME-LGIA propulsion are scheduled for 2026 and 2027. Bore range covers the 50 to 95 centimetre band as standard ME-C engines.
Wärtsilä ammonia-ready 25 (medium-speed four-stroke): introduced as the Wärtsilä 25 platform with ammonia-ready provisions in 2024. The four-stroke variant supports auxiliary power generation and small main propulsion on ammonia.
MAN four-stroke ammonia variants: ammonia-ready and ammonia-fuelled variants of the MAN 32/40, 35/44, and 48/60 medium-speed engines under development.
WinGD X-DF ammonia variant: WinGD announced an ammonia variant of the X-DF dual-fuel platform in 2025 for slow-speed ammonia propulsion, with first commercial delivery scheduled for 2027.
HHI-EMD licensed production: Hyundai Engine Machinery licenses MAN B&W ammonia engines and is among the principal builders for the early ammonia orderbook.
Strategic positioning by vessel type
The ammonia commercialisation pattern differs from methanol:
- Container shipping (Maersk and others): predominantly methanol, due to existing methanol production scale and lower handling complexity.
- Bulk carrier and tanker shipping (NYK, Mitsui OSK, K Line, COSCO Bulk, Eastern Pacific): predominantly ammonia-ready and ammonia-fuelled, due to the alignment between bulk carrier business and the ammonia/fertilizer trade flow that already exists.
- Offshore vessels and selected ferries: mixed methanol and ammonia development.
The ammonia-bulk-carrier alignment reflects the natural cargo-and-fuel synergy: ammonia tankers can fuel themselves from cargo boil-off, and bulk carriers calling at ammonia/fertilizer ports can refuel where existing handling infrastructure is in place.
Bunkering infrastructure
Ammonia bunkering at scale is a critical infrastructure development for the late-2020s commercial fleet rollout. Existing ammonia transport infrastructure (tankers, terminals, pipelines) has been built up over decades to serve the global fertilizer industry, providing a foundation that methanol does not yet enjoy at comparable scale.
Major existing ammonia ports including Rotterdam, Singapore, Yokohama, Antwerp, Tampa, and several Middle Eastern terminals are commissioning marine bunkering capabilities. Bunkering is performed via truck-to-ship, barge-to-ship, or terminal-based pipeline-to-ship.
Like methanol, ammonia is sold today predominantly as fossil-derived (“grey ammonia”) with a progressive transition to bio-ammonia (limited supply path) and renewable e-ammonia (from green hydrogen and atmospheric nitrogen). The renewable ammonia production pathway is broadly parallel to e-methanol but does not require captured CO2, which is a structural advantage in the long run.
Safety and crew training
Ammonia fuel handling presents the most significant safety challenges of any major alternative marine fuel:
- Acute toxicity: ammonia is acutely toxic at low concentrations (50 ppm causes irritation; concentrations of 300 to 500 ppm cause severe respiratory damage; concentrations above 5,000 ppm are rapidly fatal). Crew exposure must be controlled through enclosed fuel handling, gas detection, and personal protective equipment.
- Asphyxiation risk: ammonia displaces oxygen in confined spaces.
- Corrosion: ammonia is corrosive to copper, brass, and certain other materials. Fuel system materials selection is constrained.
- Refrigerated and pressurised storage: ammonia is typically stored aboard ship as either refrigerated liquid at minus 33 degrees Celsius and atmospheric pressure, or as pressurised liquid at approximately 8.5 bar at ambient temperature. Both options require specialised tankage and fuel handling systems.
Class society guidance from DNV, Lloyd’s Register, ABS, ClassNK, and Bureau Veritas covers ammonia vessel design, fuel handling, crew training, leak detection, ventilation, and emergency response. The IGF Code (covered in a separate article) is being progressively extended to include ammonia provisions through IMO interim guidelines.
Outlook
Ammonia marine propulsion is at the very-early-commercialisation phase as of 2026. The first commercial ammonia ships will deliver in 2026 and 2027, with the bulk of the orderbook delivering through 2028 to 2030. Key uncertainties include:
- The rate of renewable ammonia production scale-up.
- The relative competitive positioning versus methanol for different vessel classes.
- The pace of bunkering infrastructure deployment.
- The crew training and safety regulatory framework, which is still being finalised by IMO and national administrations.
The orderbook position in 2026 suggests ammonia will be the dominant alternative fuel for bulk carriers and tankers through the 2030 to 2040 period, complementing methanol’s container ship dominance.