Published 30 April 2026

Wärtsilä 20: Medium-Speed Four-Stroke Marine Engine

Contents

The Wärtsilä 20 is a medium-speed, four-stroke trunk-piston marine and stationary diesel engine produced by Wärtsilä Finland, with a 200 millimetre cylinder bore and a 280 millimetre piston stroke. The engine entered serial production in 1992 and remains one of Wärtsilä’s most widely sold smaller medium-speed products, with cylinder configurations from four to nine in-line and rated outputs from approximately 720 kilowatts to 1,980 kilowatts. The W20 is positioned principally as a marine auxiliary genset, a small-vessel main propulsion engine, and a stationary distributed-generation prime mover.

Cylinder data and outputs

The current rating of the Wärtsilä 20 produces 220 kilowatts per cylinder at 1,000 or 1,200 revolutions per minute, with available configurations of L4, L5, L6, L7, L8, and L9. Brake mean effective pressure is approximately 24 bar, with specific fuel consumption around 188 grams per kilowatt-hour at full load on diesel fuel. The engine is offered as a marine main propulsion unit, a marine genset (driving a 1,000 or 1,200 revolutions per minute alternator), and a stationary genset for land-based power generation.

Wärtsilä 20DF dual-fuel variant

The Wärtsilä 20DF is the dual-fuel variant capable of operating on liquefied natural gas in lean-burn Otto cycle with diesel pilot injection, or on diesel fuel in conventional diesel cycle. The 20DF entered production in the late 2000s and has been widely deployed on dual-fuel offshore supply vessels, gas tankers, and small LNG-fuelled cargo vessels as auxiliary genset units, complementing larger main-propulsion dual-fuel engines.

Fuel and emissions

The base diesel W20 runs on marine diesel oil, marine gas oil, heavy fuel oil, and biofuel blends. The engine meets IMO Tier II as standard. Tier III compliance is achieved either by selective catalytic reduction or by operation in gas mode on the 20DF. Wärtsilä supports the W20 with NOx Reducer SCR systems for Emission Control Area service.

Applications

The Wärtsilä 20 is deployed across a broad range of marine and stationary applications:

Marine auxiliary gensets on cargo vessels, tankers, ferries, and offshore vessels of medium to large size.
Small-vessel main propulsion for tugs, fishing vessels, and small ferries.
Naval auxiliary gensets on frigates, corvettes, and naval support ships.
Stationary distributed generation in 1 to 20 megawatt power plants, particularly for grid backup and island power.
Drilling rig and FPSO auxiliary power in offshore oil and gas applications.

The engine is typically paired with a Wärtsilä alternator and switchgear in pre-assembled genset packages, simplifying installation on shipyard new-build programmes and retrofits.

Production and service

The W20 is built principally at Wärtsilä’s Vaasa facility in Finland, with selected components sourced from Wärtsilä manufacturing partners in Asia. The engine is supported globally through the Wärtsilä Lifecycle services organisation, with parts inventory and service centres on every major shipping route. The W20 has undergone progressive output and efficiency upgrades through its more than three decades of production, with the current rating significantly above the original 1992 output.

Engineering significance

The Wärtsilä 20 is significant within the medium-speed four-stroke segment as a benchmark small-bore marine genset engine, with global installed base in the tens of thousands of cylinders. The product line predates the Wärtsilä-Sulzer merger and has continued through Wärtsilä’s evolution into the current corporate structure. The 20 sits below the Wärtsilä 26 and Wärtsilä 32 in the marine auxiliary range and is the smallest current Wärtsilä medium-speed marine product.