The Wärtsilä 50DF is a medium-speed, four-stroke trunk-piston dual-fuel marine and stationary engine produced by Wärtsilä Finland, with a 500 millimetre cylinder bore and a 580 millimetre piston stroke. The engine entered serial production in the early 2000s as the principal large-bore dual-fuel product in the Wärtsilä range and has been one of the most widely deployed dual-fuel engines on large LNG carriers, floating storage and regasification units (FSRUs), and selected merchant vessels. The 50DF is configured as L6, L8, L9, V12, V16, and V18, with rated outputs from approximately 5,700 kilowatts to 17,550 kilowatts.
Cylinder data and outputs
The Wärtsilä 50DF is rated at approximately 950 to 975 kilowatts per cylinder depending on configuration (the higher figure applies to the larger V configurations at 514 revolutions per minute; the lower applies to the in-line variants and to the 500 rpm rating). The engine is rated at the same output in gas mode and in diesel mode, allowing seamless switchover between fuels without load reduction. The V18 configuration delivers up to 17,550 kilowatts (18 cylinders at 975 kilowatts each), placing the 50DF in the upper output range of the medium-speed segment, overlapping with the lower end of slow-speed two-stroke output and competing with Wärtsilä’s own larger four-stroke products.
Dual-fuel operation
The 50DF operates on three distinct modes:
- Gas mode: lean-burn Otto cycle on natural gas (or LNG boil-off gas) with a small pilot diesel injection (approximately 1 per cent of the energy input) for ignition. NOx emissions are very low, meeting IMO Tier III without aftertreatment. SOx and particulate emissions are negligible due to the absence of sulphur in pipeline-grade natural gas.
- Diesel mode: conventional diesel cycle on heavy fuel oil, marine diesel oil, or low-sulphur fuel oil. Standard IMO Tier II compliance.
- MDF mode: backup operation on marine diesel oil with conventional diesel cycle, used when gas supply is unavailable.
Switchover between modes is automatic and occurs without load interruption, a key operational advantage on LNG carriers using boil-off gas as the principal fuel.
LNG carrier propulsion
The Wärtsilä 50DF is one of the dominant medium-speed prime movers on large LNG carriers built from approximately 2005 onward. The dual-fuel diesel-electric (DFDE) propulsion architecture, in which four 50DF engines drive electrical generators feeding propulsion motors, became the standard for new large LNG carriers in the late 2000s and 2010s, displacing the steam turbine that had been the dominant LNG carrier propulsion system for the previous forty years. The 50DF DFDE arrangement offers significantly better fuel efficiency than steam turbines and the ability to use boil-off gas as fuel without reliquefaction.
Notable LNG carrier classes using the 50DF include vessels built at Hyundai Heavy Industries, Daewoo Shipbuilding and Marine Engineering, Samsung Heavy Industries, and selected Japanese and Chinese yards. Approximately 300 to 400 LNG carriers are estimated to use 50DF main generation as of the mid-2020s.
FSRU and FPSO applications
The 50DF is also widely deployed on:
- Floating storage and regasification units (FSRU) for power generation and process load.
- Floating production, storage, and offloading vessels (FPSO) in offshore oil and gas applications.
- Selected dual-fuel cargo vessel newbuilds at the upper output range.
- Land-based combined cycle power plants competing with gas turbines in the 50 to 200 megawatt range.
Successor and related products
The 50DF has been progressively succeeded in new orders by:
- The Wärtsilä 31DF (smaller bore, higher efficiency) for medium-output applications.
- The Wärtsilä 46DF (closer competitor in output) for selected applications.
- The Wärtsilä 34DF for smaller dual-fuel installations.
The 50DF remains in production in 2026 for selected applications, and the global installed base supports continued aftermarket service and parts supply.
Production and service
The 50DF is built principally at Wärtsilä’s Vaasa and Trieste (Italy) facilities. Service is supported globally through Wärtsilä Lifecycle services with specialised LNG carrier service expertise concentrated at strategic LNG hubs. Major class-wide service campaigns and condition-based maintenance programmes are managed through the Wärtsilä Expert Insight digital platform.