Wärtsilä has announced a power rating increase for its Wärtsilä 25 Ammonia engine, matching the output of LNG-fuelled models to support more efficient vessel designs. The upgrade, offering up to 345 kW per cylinder, simplifies installation and maintenance while facilitating easier fuel retrofits for global shipowners. This development marks a critical step for the maritime industry’s transition to carbon-free fuels, with commercial deliveries set for 2028.
Helsinki | April 16, 2026 – In a major technical milestone for the maritime energy transition, technology group Wärtsilä has announced a significant power increase for its 4-stroke ammonia-fuelled engine solution. The upgrade to the Wärtsilä 25 Ammonia engine now brings its output in line with the LNG-fuelled Wärtsilä 25DF, offering shipowners a more efficient and flexible pathway to zero-carbon operations.
The power upgrade raises the engine’s output to 315 kW per cylinder at 900 rpm and 345 kW per cylinder at 1000 rpm. By matching the power density of LNG alternatives, the Wärtsilä 25 Ammonia now requires fewer cylinders to meet specific power demands, a shift that drastically reduces installation complexity and maintenance overhead for next-generation vessel designs.
Closing the Gap: Ammonia vs. LNG
Historically, alternative fuels have often faced “power penalties” compared to traditional or LNG-fuelled counterparts. This latest development eliminates that gap for the 1.9–3.1 MW range.
The increased rating was successfully demonstrated during rigorous type approval tests in late 2025, conducted under the watchful eye of international classification societies. This alignment of power levels is particularly critical for retrofitting, as it allows shipowners to design vessels with the flexibility to switch between LNG and ammonia operation without sacrificing performance or necessitating massive structural redesigns.
Driving Decarbonization Through Practicality
The Wärtsilä 25 platform is designed as a modular, medium-speed solution. With this upgrade, the ammonia-specific variant is positioned as a primary choice for both main propulsion and auxiliary power across a wide variety of vessel segments.
Stefan Nysjö, Vice President of Power Supply, Wärtsilä Marine, said “With decarbonisation front and centre of our strategy, we continue to enhance the capabilities of our ammonia engine solution. This power increase builds on our extensive testing and development work and helps make ammonia-fuelled ship operations more practical and affordable, while maintaining high standards of safety and reliability.”
The Ammonia Advantage in Global Shipping
As the International Maritime Organization (IMO) tightens the screws on greenhouse gas (GHG) emissions, ammonia has emerged as a frontrunner in the “green fuel” race due to its carbon-free combustion profile. However, the industry has long called for solutions that don’t compromise on the space-to-power ratio, a challenge Wärtsilä is directly addressing with this release.
Key Technical Benefits of the Wärtsilä 25 Ammonia Upgrade:
- Reduced Installation Footprint: Higher power per cylinder means fewer cylinders are needed, saving valuable engine room space.
- Simplified Maintenance: Fewer moving parts directly translate to lower long-term OpEx.
- Interchangeability: Power parity with the 25DF (LNG) model streamlines the transition from transitional fuels to net-zero fuels.
- Market Availability: The high-power version is now commercially available, with deliveries scheduled to begin in 2028.
About Wärtsilä Marine
Wärtsilä Marine is a global pioneer in power, propulsion, and lifecycle solutions for the marine market. The company develops industry-leading technologies to advance maritime’s transition to new fuels and supports an end-to-end digital ecosystem connecting vessels and ports. Their portfolio includes engines, propulsion systems, hybrid technology, exhaust treatment, and digital technologies.
About Wärtsilä
Wärtsilä is a global leader in innovative technologies and lifecycle solutions for the marine and energy industries. With a team of 17,900 professionals operating in 199 locations across 78 countries, Wärtsilä is shaping the decarbonization transformation. In 2025, Wärtsilä’s net sales totaled EUR 6.9 billion. The company is listed on Nasdaq Helsinki.
Source: Wärtsilä