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    Wärtsilä 25 – Fuel flexibility and low emissions for your vessel

    Choose adaptable power for mechanical, diesel-electric, or hybrid propulsion. The Wärtsilä 25 is engineered for evolving fuels, efficient operation, and easy integration - helping you prepare your vessel for the future.

    Configure your Wärtsilä 25 engine

    Wärtsilä 25 – Engineered for fuel flexibility, efficiency, and future needs

    The Wärtsilä 25 is a next-generation medium-speed marine engine designed for adaptable power, efficient operation, and easy integration. Available in diesel, LNG (standard or NextDF for ultra-low methane slip), and ammonia variants, it supports lower emissions and future fuel transitions. With a modular design and 1.7–3.4 MW output, Wärtsilä 25 delivers proven reliability for a wide range of vessels and propulsion arrangements.

    Wärtsilä 25 engine highlights

    Power range: 1.7–3.4 MW (6L–9L configurations)

    Fuel flexibility

    • Diesel
    • LNG: choose standard DF or NextDF for ultra-low methane slip
    • Ammonia variant for up to 90% GHG reduction
    • Modular design enables easy conversion to future fuels

    Compliance and emissions

    IMO Tier III compliant natively in LNG mode, and with Wärtsilä NOR when operating on diesel or ammonia. Low emissions performance with minimized NOx and methane slip as low as ~0.9% with NextDF - best-in-class for LNG engines.

    Key benefits

    • Low methane slip and upgradeable platform for evolving fuel regulations
    • Future-ready performance for mechanical, diesel-electric, and hybrid propulsion
    • Proven reliability for small merchant ships, LNG bunkering vessels, and other demanding applications
    • Lifecycle efficiency with TBO up to ~32,000 h

    Why choose Wärtsilä 25?

    Fuel flexibility today and adaptability for tomorrow — all in one versatile engine platform. Open the Wärtsilä 25 brochure

    Specs at a glance

    • Power range 1.7 –  3.4 MW
    • Configurations 6L – 9L
    • Nominal speed 900 or 1000 rpm
    • Fuel options Diesel, LNG (standard DF or NextDF for ultra-low methane slip), Ammonia
    • Compliance IMO Tier III compliant natively in LNG mode, and with Wärtsilä NOR when operating on diesel or ammonia
    • Emissions highlights Methane slip as low as 0.9% with NextDF ; up to 90% GHG reduction with ammonia
    • Lifecycle Time between overhaul (TBO) up to 32,000 h

    For detailed Wärtsilä 25 specifications and integration data, use the Engine configurator 

     

    Access 3D models and drawings
    Download Product guide
    nextdf

    Wärtsilä 25 engine: Fuel flexibility and low-emission performance

    The Wärtsilä 25 combines fuel flexibility with advanced emission control, helping shipowners reduce greenhouse gases and meet IMO Tier III requirements without compromising efficiency.

    LNG options: standard DF or NextDF

    • Both options meet IMO Tier III and current FuelEU Maritime requirements
    • NextDF: methane slip as low as 0.9%, efficiency gain up to 0.5 percentage points
    • NextDF offers the most future-compatible LNG choice for tightening emission regulations
    Ammonia
    • Up to 90% GHG reduction at 95% ammonia energy share
    • IMO Tier III compliance with Wärtsilä NOR" as below in diesel and biofuels
    • Integrated safety systems and Wärtsilä AmmoniaPac ensure safe operation

    Learn more about Wärtsilä 25 Ammonia:

    Step inside a virtual environment to explore the real-like engine

    Diesel and biofuels

    • IMO Tier III compliance with Wärtsilä NOR
    • Compatible with carbon-neutral biofuels for future sustainability

    Future-ready design

    • Modular platform enables easy conversion to new fuels
    • Supports mechanical, diesel-electric, and hybrid propulsion arrangements with straightforward installation and conversion procedures.

    Download product guides for detailed fuel specifications

    Download Product guide

    What makes Wärtsilä 25 future-ready?

    Discover Wärtsilä 25: Fuel flexibility, low emissions, and future-ready performance for mechanical, diesel-electric, and hybrid propulsion.

    Hybrid propulsion with Wärtsilä 25 boosts efficiency

    The Wärtsilä 25 is designed for seamless integration into diverse propulsion concepts. Integrate the engines with batteries and this flexible, hybrid propulsion with Wärtsilä 25 will give you:

    • Better fuel efficiency. The hybrid system will enable the engines to run at optimal load, which saves fuel – and money. You can also save on maintenance because there will be less wear and tear on the engines.
    • Lower emissions. Less fuel burned means that the hybrid system generates less emissions, too.
    • Better operational flexibility. The smart hybrid system makes it possible to switch smoothly between power sources. This will deliver consistent, reliable performance even under varying operational conditions.

    Learn more about the benefits from a smart hybrid solution.

    Versatile integration gives long-term value

    The Wärtsilä 25 is optimised for lifecycle efficiency, offering a modular design that simplifies maintenance and makes future fuel conversions straightforward. Advanced automation and control systems ensure seamless integration with vessel power management and you can gain further fuel savings with smart propulsion control systems such as EcoControl.

    An expert reveals: What makes Wärtsilä 25 future-ready and fuel-flexible? Watch this video now.

    The engine is fully compatible with the Wärtsilä NOx reducer (NOR) for IMO Tier III compliance. It is also compatible with the LNGPacTM fuel gas supply system and the AmmoniaPac fuel supply system which support safe supply and handling of these fuels.

    With a Time Between Overhaul (TBO) of up to 32,000 hours, you benefit from reduced downtime and lower lifecycle costs. Predictive maintenance and digital solutions further optimise performance and reliability, while the platform is built to adapt to evolving fuel regulations and sustainability targets. Retrofit paths for new fuel technologies and emission control systems are easy to implement, ensuring your vessel remains future-ready.

    Optimal performance for the vessel’s lifetime

    Keep your Wärtsilä 25 engine in top condition and maximise performance while minimising operating costs. With a Wärtsilä Lifecycle Agreement, our experts support you in planning, preparing, and carrying out maintenance - so you can focus on your core operations.

    We can coordinate spare parts and make sure that a skilled maintenance workforce will be available. The AI-powered solutions and predictive analyses combined with expert knowledge will ensure the optimal performance of your vessel. This proactive approach helps extend service intervals, reduce unplanned downtime, and adapt to new fuels or regulations as they emerge.

    A Lifecycle Agreement with Wärtsilä means true peace of mind and optimal performance for your vessel’s entire lifetime.

    Learn more about Wärtsilä Lifecycle Agreements

    Proven performance in demanding applications

    The Wärtsilä 25 is already powering vessels that require efficiency, compliance, and fuel flexibility in real-world operations. See below for few great examples for fuel efficient Wärtsilä 25 configurations:

    LNG bunkering vessels

    Equipped with Wärtsilä 25DF featuring NextDF technology

    • Achieves ultra-low methane slip (as low as 0.9%) and IMO Tier III compliance
    • Demonstrates readiness for FuelEU Maritime requirements

    Success story: Ibaizabal case study 

    Small merchant ships

    Wärtsilä 25 engines selected for reliable propulsion and lifecycle efficiency

    • Supports mechanical and hybrid configurations for versatile operations

    Success stories: Vertom, James Fisher and sons plc, and Erik Thun Ab

    Other demanding applications

    Modular design makes it easy to adapt the engine for different vessel types, special requirements, or future fuel upgrades.

    Case examples: Fishing vessels for Vardin, First PSV conversion to ammonia – Viking Energy 

     

    Customer cases

    • 3D illustration of Ibaizabal’s new LNG bunkering vessel
      Ibaizabal

      Discover how Ibaizabal’s new LNG bunkering vessel sets new standards in methane slip reduction using the Wärtsilä 25 marine engine with NextDF technology

      Learn more
    • Geared Tweendecker Vertom Render © Vertom Group
      Vertom Group

      Vertom Group chose integrated hybrid propulsion for four new tweendecker vessels as commitment to sustainable shipping

      Find out more
    • 3D illustration of new James Fisher and Sons plc tankers
      James Fisher and Sons plc tankers

      James Fisher is future proofing its new tankers with Wärtsilä dual-fuel engines – the perfect way to cut emissions and boost efficiency.

      Find out more about the solution
    • Erik Thun AB

      Erik Thun Group’s four coastal tankers benefit from the Wärtsilä 25 engine and can achieve high environmental performance in the long term.

      Learn more

    Resources

    • Fuel specifications and detailed data:
    Download Product guide
    • Engine drawings:
    3D models & drawings

    Frequently asked questions

    What is the power range and configuration of the Wärtsilä 25 engine?

    The Wärtsilä 25 offers 1.7–3.4 MW output across 6L–9L configurations, operating at 900 or 1000 rpm.  View full specifications in the Engine configurator.

    Which fuels does the Wärtsilä 25 support today, and can it be converted to other fuels in the future?

    It supports diesel, LNG (standard DF or NextDF), and ammonia. Thanks to its modular design, the engine can be converted to new fuels as the market evolves.
    Learn more about fuel flexibility and upgradeability 

    How does the Wärtsilä 25 meet IMO Tier III and other emission regulations?

    • LNG mode: IMO Tier III compliant natively
    • Diesel and ammonia modes: require Wärtsilä NOR

    Both LNG options (standard DF and NextDF) meet current FuelEU Maritime requirements, and NextDF offers methane slip as low as 0.9% for superior and longer-term compliance.

    What is NextDF technology, and why should I consider it for LNG operations?

    NextDF is Wärtsilä’s advanced LNG combustion technology that reduces methane slip to 0.9% and improves efficiency by up to 0.5 percentage points. It’s the most future-compatible LNG choice for meeting tightening emission regulations.
    Read about NextDF and its benefits 

    Is the Wärtsilä 25 ammonia variant commercially available, and what emission benefits does it deliver?

    Yes, the ammonia variant is available today and can achieve up to 90% GHG reduction at 95% ammonia energy share. The Wärtsilä 25 Ammonia solution includes Wärtsilä NOR for full IMO Tier III compliance and integrates with Wärtsilä AmmoniaPac for safe fuel supply and operation. Learn more in the Wärtsilä Ammonia lounge.

    Ready to future-proof your newbuild?

    The Wärtsilä 25 gives you fuel flexibility and upgrade options to keep pace with changing regulations and sustainability goals. Talk to our experts about the best solution for your next project.

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