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Extensive testing of Wärtsilä's 4-stroke ammonia engine demonstrates GHG emission reductions by up to 90 percent
Wärtsilä Unveils Major Leap in Ammonia Engine Efficiency, Promises Near-Zero GHG Emissions for Shipping
(FINLAND, 5/28/2025)
Extensive Testing Shows Up to 90% Greenhouse Gas Reduction for 4-Stroke Solution, Boosting Decarbonisation Hopes for Maritime Sector.
HELSINKI – Technology group Wärtsilä has announced a significant breakthrough in its 4-stroke engine-based solution for ammonia fuel, demonstrating unprecedented levels of efficiency and environmental performance. Following intensive testing since its commercial marine introduction in November 2023, the latest results show the solution can achieve up to a 90% reduction in greenhouse gas (GHG) emissions compared to equivalent diesel engines.
This impressive figure, assessed against the FuelEU Maritime reference on both a Well-to-Wake and Tank-to-Wake basis with a 95 percent ammonia energy share, represents a further 18 percent improvement on the solution's already strong emissions performance.
The Wärtsilä 25 ammonia engine, part of the broader Wärtsilä 25 engine platform, leverages the company’s extensive safety and operational experience gained from low-pressure dual-fuel (DF) engines running on LNG. Recent comprehensive tests across various engine sizes have confirmed ammonia as a suitable fuel for the low-pressure Otto cycle concept, which Wärtsilä chose for its optimal balance of safety, efficiency, and overall GHG reduction potential. The tests also highlighted ammonia's strong knock resistance and robust combustion capabilities.
The Wärtsilä 25 is a medium-speed 4-stroke marine engine with a future-proof, upgradeable modular structure that makes it easier than ever for you to target net-zero. Available in cylinder configurations from 6L to 9L and with a power output ranging from 1.7–3.4 MW, the Wärtsilä 25 provides reliable performance whether you are using it as a robust main propulsion engine or as a cost-efficient and flexible auxiliary engine. The engine is available in diesel and dual-fuel versions and now introducing ammonia.
 "With decarbonisation front and centre of our company strategy, we have continued to enhance the capabilities of our ammonia engine solution – adding even more safety features and further improving its performance," commented Stefan Nysjö, Vice President of Power Supply, Wärtsilä Marine. He added that this reflects Wärtsilä's commitment to continuous improvement, developing technologies that enable meaningful reductions in GHG emissions for the industry.
The enhancements to the Wärtsilä 25 ammonia engine benefit the entire integrated solution, which includes the AmmoniaPac fuel gas supply system, the Wärtsilä Ammonia Release Mitigation System (WARMS), and the Wärtsilä NOx Reducer (NOR) for optimal exhaust after-treatment. Safety, efficiency, and reliability are core to the design, supported by a sophisticated automation system and maintenance agreements. The improved performance will  be integrated into future engine orders and applied to existing contracts, with first deliveries slated from 2026 onwards.
Ammonia is increasingly seen as a critical alternative fuel as the shipping industry strives towards net-zero emissions by mid-century, a target set by the International Maritime Organization (IMO). A recent Wärtsilä report underlined that while existing decarbonisation solutions can cut shipping emissions by up to 27 percent, sustainable fuels like ammonia will be indispensable for eliminating the remaining 73 percent, cementing its vital role in the maritime sector's green transition.
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