Wärtsilä 25 ammonia engine gets a power rating increase

Wärtsilä 25 ammonia engine. [Image: Wärtsilä]

Wärtsilä has increased the power rating of its four‑stroke Wärtsilä 25 ammonia engine. As part of the company’s ongoing development of ammonia‑fueled engine solutions, the upgrade to the Wärtsilä 25 ammonia engine raises output to 315 kW per cylinder at 900 rpm and 345 kW per cylinder at 1,000 rpm, matching the power level of the LNG‑fueled Wärtsilä 25DF engine.

A key benefit of the upgrade is that fewer cylinders are required for a given power demand, which reduces installation complexity and simplifies maintenance, supporting more flexible and efficient vessel designs for ammonia‑fueled applications.

Wärtsilä says that the power increase “also supports the conversion of the engine between LNG and ammonia fuel operation. With this new power range of 1.9-3.1 MW, the Wärtsilä 25 ammonia engine is an ideal choice for a wide range of applications, both as a main engine or as an auxiliary engine. What’s more, by aligning with the power level of the LNG-fueled Wärtsilä 25DF, retrofitability is improved resulting in greater flexibility in fuel choice for shipowners.”

The increased power rating was demonstrated during type approval tests carried out in autumn 2025 under the supervision of classification society representatives.

“With decarbonization front and center of our strategy, we continue to enhance the capabilities of our ammonia engine solution,” says Stefan Nysjö, vice president of power supply, Wärtsilä Marine. “This power increase builds on our extensive testing and development work and helps make ammonia‑fueled ship operations more practical and affordable, while maintaining high standards of safety and reliability.”