Wärtsilä has upgraded its four-stroke ammonia-fuelled engine solution, increasing its power rating as part of ongoing development of alternative fuel propulsion systems.
The updated Wärtsilä 25 Ammonia engine now delivers 315 kilowatts (kW) per cylinder at 900 rpm and 345 kW per cylinder at 1,000 rpm, aligning its output with the LNG-fuelled Wärtsilä 25DF.
The enhancement reduces the number of cylinders required for a given power demand, lowering installation complexity and simplifying maintenance for shipowners.
The adjustment also improves flexibility in vessel design, particularly for operators seeking dual-fuel capability.
The engine can be converted between LNG and ammonia operation, with a new overall power range of 1.9–3.1 MW, enabling deployment across both main propulsion and auxiliary applications.
By matching the LNG-fuelled variant’s power profile, retrofit potential is strengthened, offering shipowners greater optionality in fuel strategy as regulatory pressure on emissions intensifies.
READ: Wärtsilä maps 2026 trends in decarbonisation and digitalisation
The upgraded performance was validated during type approval tests conducted in autumn 2025 under the supervision of classification society representatives.
Stefan Nysjö, Vice President of Power Supply at 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.”
Ammonia is gaining traction as a potential zero-carbon marine fuel, with industry stakeholders increasingly exploring it as part of longer-term compliance pathways under tightening International Maritime Organization (IMO) emissions targets.
READ: Wärtsilä to equip new LNG bunkering vessel for SIPG
The Wärtsilä 25 platform is designed as a modular medium-speed engine focused on efficiency, low emissions and flexible integration across vessel types.
The ammonia variant is intended to support a broad range of propulsion configurations while maintaining operational reliability.
The upgraded ammonia engine has now been released for commercial orders, with deliveries scheduled from 2028.
For more information:
Wärtsilä – https://www.wartsila.com/
