Wärtsilä has announced a higher power rating for its four-stroke ammonia-fuelled engine, bringing its output in line with that of its LNG-fuelled equivalent.

The upgraded Wärtsilä 25 Ammonia engine now delivers 315 kW per cylinder at 900 rpm and 345 kW per cylinder at 1,000 rpm, matching the output of the LNG-fuelled Wärtsilä 25DF engine.

According to the company, one of the main advantages of the increase is that fewer cylinders are needed to meet a given power requirement, thereby reducing installation complexity and simplifying maintenance. Wärtsilä said this should support more flexible and efficient vessel designs for ammonia-fuelled applications.

The revised power range of 1.9-3.1 MW means the engine can be used in a broad range of roles, both as a main engine and as an auxiliary engine. Wärtsilä also said the higher rating supports conversion between LNG and ammonia operation and improves retrofittability by aligning the ammonia engine with the LNG-fuelled Wärtsilä 25DF.

The increased power output was demonstrated during type-approval tests conducted in autumn 2025 under the supervision of classification society representatives.

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.”

Wärtsilä said ammonia is increasingly being recognised as an important alternative fuel as shipping works towards international emissions reduction targets, with pressure growing to move away from conventional fossil fuels.

The Wärtsilä 25 platform is a medium-speed marine engine designed for power generation, low emissions, efficient operation and easier integration. The ammonia version has a modular design intended for a wide variety of vessel types and propulsion arrangements.

The upgraded Wärtsilä 25 Ammonia engine has now been released to the commercial market, with deliveries scheduled for 2028.