Wartsila 31 Medium-Speed Marine Engine

Background

The Wartsila 31 was launched on 2 June 2015 at Nor-Shipping in Oslo, with first commercial delivery in 2016. On the same day Guinness World Records issued an official certificate confirming the W31 as the most efficient four-stroke diesel engine, with diesel SFOC as low as 165 g/kWh. It represents the state of the art in medium-speed marine diesel efficiency. The “31” designation refers to the engine’s 31 bar BMEP (brake mean effective pressure) achieved at maximum continuous rating, a substantial step above the prior generation’s typical 22-25 bar.

In March 2023 Wartsila announced a power boost for the W31, raising peak output to up to 9.6 MW for 8-16 cylinder configurations.

The W31 broke records:

• 165 g/kWh SFOC at shop test (diesel mode), the lowest published value for any medium-speed engine
• 31 bar BMEP at MCR
• Thermal efficiency approaching 50% (compared to 45-47% for previous generation)

These improvements come from a combination of:

• Advanced combustion design optimised for low-load and full-load operation
• Higher peak cylinder pressures
• Improved fuel injection (common rail with multiple injection events)
• Variable valve timing
• Reduced friction
• Electronic engine management with cylinder balancing

The W31 family includes:

• W31 (diesel, single fuel)
• W31DF (dual-fuel: liquid + LNG)
• W31SG (single-gas: pure LNG operation)
• W31TF (tri-fuel: liquid + LNG + methanol, in development)

This article covers the W31 design, the variant families, applications, and operational considerations.

Design specifications

Geometry

• Bore: 310 mm
• Stroke: 430 mm
• Stroke-to-bore ratio: 1.39
• Displacement per cylinder: 32.5 litres
• Cylinder configurations: 8 inline, V12, V14, V16

Performance

• Speed: 720, 750, or 1000 rpm (depending on cylinder count)
• Power per cylinder: 600 kW (at 750 rpm) to 700 kW (V configurations)
• Total engine power: 4-11 MW depending on cylinder count
• BMEP at MCR: 31 bar

Mean piston speed

At 750 rpm, mean piston speed is 2 × 0.43 × 750/60 = 10.75 m/s. This is among the highest mean piston speeds for medium-speed engines.

SFOC

Shop test SFOC achievements:

• Single fuel diesel: 165 g/kWh at 85% MCR
• W31DF in gas mode: 7,300 kJ/kWh thermal energy specific consumption (equivalent to ~140 g/kWh on diesel basis)
• W31SG: similar to DF gas mode

These are the lowest values published for medium-speed marine engines.

Architectural features

Cylinder cover and fuel injection

The cylinder cover supports:

• Two intake valves and two exhaust valves
• Common rail fuel injector(s)
• Cylinder pressure transducer for combustion monitoring
• Indicator cock for diagnostic access

Fuel injection is common rail with rail pressure up to 1,800 bar, multiple injection events per cycle, and variable timing.

Combustion chamber design

The combustion chamber design optimises:

• Charge mixing through controlled swirl
• Spray pattern coverage of chamber volume
• Heat distribution to minimise hot spots
• Quenching reduction at chamber walls

The result is more complete combustion with lower heat losses, contributing to the high thermal efficiency.

Variable valve timing

The W31 includes variable valve timing for:

• Optimal valve overlap at each load
• Adjustment for ambient conditions
• Compatibility with different fuel modes (DF/SG variants)

Cylinder pressure transducers

Each cylinder has a pressure transducer (similar to slow-speed PMI systems) for:

• Continuous monitoring of cylinder behaviour
• Cylinder balancing through automatic adjustment
• Fault detection (misfire, knock)
• Combustion optimisation

Common rail

Common rail injection at 1,800 bar provides:

• Precise fuel quantity control
• Multiple injection events per cycle (pre, main, post)
• Variable injection timing
• Smooth combustion across loads

Engine control system

Wartsila UNIC engine management system:

• Integrates all engine functions
• Cylinder balancing automatic
• Diagnostic capability
• Network integration with ship management systems

Variants

W31 (diesel, single fuel)

The standard W31 burns marine diesel fuels (HFO with scrubber, LSFO, MGO):

• Single-fuel operation
• Tier II compliant out of box
• Tier III with SCR or EGR
• Best fuel efficiency for diesel-only operation

W31DF (dual-fuel)

W31DF is the dual-fuel variant:

• Liquid mode: same as W31
• Gas mode: LNG with diesel pilot ignition
• Tier III in gas mode without aftertreatment
• Methane slip approximately 1.5-2 g/kWh
• Dual-fuel switching during operation

The DF variant adds:

• Gas admission valves
• Pilot fuel injection (small)
• Gas valve unit (GVU) for fuel system management
• Methane slip optimisation

W31SG (single gas)

W31SG operates only on LNG:

• No liquid fuel capability
• Suitable for ships in continuous LNG operation
• Lower complexity than DF variant
• Slightly higher efficiency than DF on gas

W31TF (tri-fuel)

In development; expected to support:

• Liquid fuel
• LNG
• Methanol or other liquid alcohols

Applications

Cruise ships

The W31 has rapidly become the engine of choice for cruise ship power plants:

• Multiple W31 engines per ship
• Diesel-electric configuration
• Engines start/stop based on demand
• High fuel efficiency reduces operating cost

Cruise newbuilds have included W31 installations across multiple operators; specific ship-by-ship attribution is not summarised here, since cruise propulsion line-ups are documented per ship in operator and class-society publications. The W46DF and W32 remain the dominant Wärtsilä cruise platforms; the W31 has been selected for selected newbuilds where peak efficiency and compact footprint are decisive.

Offshore Support Vessels

OSV applications:

• 4-6 W31 engines per ship
• Diesel-electric for dynamic positioning
• High redundancy
• Best fuel efficiency in OSV class

Ferries

Modern fast and conventional ferries:

• W31DF (LNG) reducing emissions
• W31 single-fuel for retrofits
• 4-6 engines for redundancy

Gensets

W31 as auxiliary genset on ships with slow-speed two-stroke main engines:

• Highest efficiency genset
• Reduces total fuel consumption
• Common on new-build cargo ships

Performance comparison

The W31 leads its bore class significantly in efficiency.

Operational considerations

Maintenance

W31 maintenance is similar in pattern to other medium-speed engines but with:

• Higher precision components
• More electronic systems
• More sensors

Major intervals:

• Top overhaul: 16,000 hours (typical)
• Major overhaul: 32,000 hours
• Engine life: 200,000+ hours

Fuel quality

The high BMEP and tight tolerances make W31 sensitive to fuel quality:

• HFO usable but with care
• LSFO and MGO preferred
• Fuel additive use as needed
• Regular fuel testing essential

Cylinder oil

W31 system oil:

• Higher BN (40-60) for HFO operation
• Lower BN for LSFO/MGO operation
• Frequent change (1,500-3,000 hours)
• Premium quality oil

Spare parts

Spare parts inventory is comparable to other medium-speed engines but with:

• Some W31-specific items
• Electronic components
• High-precision injectors

Industry significance

The W31 has had several broader industry impacts:

Efficiency benchmark

The 165 g/kWh SFOC has set the benchmark for medium-speed marine engines. Competitors are working toward similar levels in their own designs.

Methanol and alternative fuels

The W31TF concept demonstrates that medium-speed engines can adapt to multiple alternative fuels. This is opening doors for shipping decarbonisation.

Emissions compliance

W31DF achieves Tier III in gas mode without separate aftertreatment, simplifying ECA compliance for cruise and OSV operators.

Operator economics

The W31’s record SFOC delivers a measurable fuel-cost reduction relative to earlier-generation medium-speed engines of similar output. The size of the saving depends on annual operating hours, fuel price, fuel mix (HFO / VLSFO / LNG), and the baseline engine being compared against; published case studies should be consulted for ship-specific figures rather than rule-of-thumb dollar bands. The same SFOC reduction also lowers CO₂ emissions per kWh across the operating envelope.

Future development

Wartsila continues to develop the W31 family:

• W31 Improvement: incremental efficiency gains
• W31 Methanol: methanol-capable variants
• W31 Ammonia: in development for net-zero operation
• Larger variants: possible expansion to higher bores

The W31 is expected to remain the medium-speed marine efficiency leader through the 2020s.

Related Calculators

• Engine Power Per Cylinder Calculator
• Brake Mean Effective Pressure Calculator
• Specific Fuel Oil Consumption Calculator
• Mean Piston Speed Calculator
• Wartsila 31 Performance Calculator

See also

• Medium-Speed Four-Stroke Marine Engines
• Four-Stroke Marine Diesel Engine Fundamentals
• Trunk Piston Engine Architecture for Marine Engines
• Tier III Compliant Two-Stroke Engines

References

• Wartsila. (2023). Wartsila 31 Product Guide. Wartsila Corporation.
• Wartsila. (2015). Wartsila 31 Launch Press Release. Wartsila Corporation.
• Wartsila. (2023). Wartsila 31DF Operation Manual.
• DNV. (2022). Marine Engine Efficiency Survey. DNV.
• Lloyd’s Register. (2022). Medium-Speed Engine Selection Guide.