Four-stroke marine engines drive cruise ships, ferries, offshore vessels, tugs and the generator sets of every large ship, with the Wartsila 31 holding a Guinness record at 169.4 g/kWh. Between 2024 and 2026 Wartsila, Everllence, Rolls-Royce mtu, Caterpillar MaK and HD Hyundai HiMSEN all added methanol or ammonia versions of their base platforms.
What it is and how it works
Four-stroke marine engines complete the intake, compression, power and exhaust cycle over two crankshaft revolutions and run at medium speed (roughly 500-1,000 rpm) or high speed (above 1,000 rpm). They drive propellers through reduction gearboxes or through generators feeding electric motors. They power cruise ships, ro-pax ferries, offshore vessels, tugs, naval auxiliaries and superyachts, provide the generator sets on every large cargo ship and drive diesel-electric LNG carriers.
Efficiency figures come from maker brochures. Wärtsilä lists the Wartsila 31 at 169.4 g/kWh in diesel form and calls it the world's most efficient four-stroke engine, with a Guinness World Record for the diesel version; the 31DF is listed at 176.5 g/kWh in diesel mode. Everllence rates its L+V49/60DF at 7,800-18,200 kW at 600 rpm with 171 g/kWh liquid-fuel consumption at 85% MCR.
Why it matters now
Four-stroke makers are not building clean-sheet ammonia engines. They are offering base platforms, the Wartsila 31, Everllence 49/60, mtu Series 4000 and MaK M32/M43, with methanol or ammonia variants and retrofit kits. Methanol dominates the near-term retrofit market because it needs the least engine-room change, and the research found no measured methanol-mode or ammonia-mode fuel consumption figures for any of these engines. HiMSEN and the Wartsila 25 are the exceptions with dedicated, class-approved ammonia products.
Key players and products
Wärtsilä offers the 31 Methanol at 4.7-9.8 MW in 8V to 16V form and lists methanol upgrades for the 31, 32, 46F, 46TS and ZA40S; it is also converting Stena ferries to methanol. Everllence (formerly MAN Energy Solutions) sells the 48/60, 49/60DF and 175D; its PrimeServ arm offers conversions of older 48/60 engines to 51/60DF with methanol capability from 2025 (MAN Energy Solutions), and the first 175DF-M methanol order came in July 2025 for a superyacht at a European yard (Turbomachinery Magazine).
HD Hyundai's HiMSEN H22CDF-LA ammonia engine won type approval from seven class societies in 2024 (Clean Shipping International), and the larger H32CDF-LA, rated up to 5.4 MW with high-pressure direct ammonia injection, obtained approval from KR, ABS, DNV, BV, ClassNK, LR and RINA in October 2025. Rolls-Royce Power Systems (mtu), Caterpillar (MaK), Yanmar and Daihatsu complete the field.
Figures that matter
Three performance claims stand out, all from makers or their test programmes. Wartsila's ammonia four-stroke ran at 95% ammonia energy share with up to 90% GHG reduction in a 2025 CIMAC test (Ammonia Energy Association). Caterpillar's MaK M32 and M43 HPDI conversions run up to 95% methanol or ethanol by volume for up to 80% GHG reduction depending on fuel carbon intensity.
| Metric | Value | Source type | Source |
|---|---|---|---|
| Wartsila 31 SFOC, diesel version | 169.4 g/kWh | vendor | Wartsila brochure |
| Wartsila 31DF SFOC, diesel mode | 176.5 g/kWh | vendor | Wartsila brochure |
| Everllence L+V49/60DF | 7,800-18,200 kW; 171 g/kWh at 85% MCR | vendor | Everllence |
| HiMSEN H32CDF-LA maximum output | 5.4 MW, seven class approvals | verified | — |
| Upgraded Wartsila 25 Ammonia range | 1.9-3.1 MW, deliveries 2028 | vendor | Wartsila |
| Wartsila ammonia engine, 2025 CIMAC test | 95% ammonia share, up to 90% GHG cut | vendor | Ammonia Energy Association |
| MaK M32/M43 HPDI alcohol share | up to 95% by volume | vendor | — |
| mtu Series 4000 methanol test hours | 5,000 h, DNV certification | verified | Rolls-Royce |
Developments 2023-2026
- August 2024: Wartsila and Eidesvik agree to retrofit the Equinor-chartered PSV Viking Energy with a Wartsila 25 Ammonia engine after a planned solid-oxide fuel cell slipped; the aim is to replace at least 70% of the vessel's fuel with ammonia (Wärtsilä). Bunkering of up to 73 t at Stord has been authorised (Inspenet) and the ship is due to be available for ammonia operation later in 2026.
- 2025: Rolls-Royce tests what it calls the first high-speed marine engine running exclusively on methanol.
- 16 April 2026: Wartsila releases the upgraded 25 Ammonia with a 1.9-3.1 MW range for commercial deliveries in 2028 (Wärtsilä).
- 31 August 2026: Rolls-Royce reports DNV Methanol Readiness Certification for current mtu Series 4000 engines and 5,000 hours of methanol test running (Rolls-Royce).
- September 2026: Caterpillar debuts its MaK dual-fuel alcohol conversion kits at SMM Hamburg; DNV had granted approval in principle for the 35% blending option (MarineLink).
Outlook
Ammonia four-strokes are entering service in niches such as offshore supply vessels and gas-carrier auxiliaries rather than mainstream propulsion, and the 2028 date for Wartsila's upgraded engine signals that volume ammonia four-stroke deployment is a post-2028 story. Methanol retrofits, from Stena ferries to MaK kits and mtu certification, are the near-term volume business.
