Wartsila 31: World-Record Medium-Speed Marine Engine

The Wartsila 31 is a four-stroke trunk-piston medium-speed marine and power-generation engine with a 310 mm cylinder bore and a 430 mm piston stroke. Wartsila launched it on 2 June 2015 at Nor-Shipping in Oslo, simultaneously obtaining a Guinness World Records certificate that declared it the most fuel-efficient four-stroke diesel engine ever tested, based on a measured brake specific fuel consumption of 165 g/kWh at 85% of maximum continuous rating in diesel mode. No medium-speed competitor had published a lower figure at that point. The engine family has since expanded to include dual-fuel LNG, spark-ignited pure-gas, and methanol variants, and in 2023 Wartsila uprated the peak output of the V16 configuration to approximately 9.6 MW.

The Wartsila 31 achieved a shop-test SFOC of 165 g/kWh at 85% MCR in diesel mode at its 2015 launch, a value Guinness World Records certified as the best for any four-stroke diesel engine. The figure corresponds to a brake thermal efficiency of roughly 50.1%, calculated from the lower heating value of marine diesel oil. The engine family includes diesel, dual-fuel LNG, spark-ignited gas, and methanol variants across 8-cylinder inline and V12, V14, and V16 configurations, covering a power range from approximately 4.2 to 9.8 MW.

The specific fuel oil consumption article explains how SFOC is defined, measured, and used as a performance benchmark across engine types. The W31 sits one bore-size below the Wartsila 32 (320 mm bore, 580 kW per cylinder) and well above the smaller Wartsila 20 (200 mm bore, 185 kW per cylinder). For the broader portfolio and competitor context, see the medium-speed four-stroke marine engines overview and the Wartsila corporate history.

The W31 MCR per Cylinder calculator computes the per-cylinder maximum continuous rating across the W31 configuration range. For the dual-fuel variant, the W31DF calculator covers both diesel and gas modes, and the W31SG calculator covers the pure-gas variant.

Launch and the efficiency record

Wartsila chose the 2015 Nor-Shipping trade fair in Oslo to introduce the W31. The event was the first time any engine maker had sought a Guinness World Record certification for fuel efficiency at a commercial product launch, a deliberate signal about how far the W31 stood from the existing generation of medium-speed engines. The certificate named the W31 “the world’s most efficient four-stroke diesel engine,” with the 165 g/kWh figure verified from witnessed shop tests conducted on the test bed at Wartsila’s Vaasa factory in Finland.

For scale: a typical medium-speed diesel of the early 2000s achieved 175 to 185 g/kWh at its optimal load point. The previous generation of Wartsila engines, including the W32 and W46, were operating at 175 to 178 g/kWh at best efficiency. The W31’s 165 g/kWh figure represents roughly a 6 to 7% improvement in specific fuel consumption over the best competitors at launch, which at 5 MW of continuous output across 6,000 operating hours per year translates to approximately 150 to 200 metric tonnes of fuel oil per engine per year.

The “31” in the engine designation refers to the 31 bar brake mean effective pressure at MCR. Medium-speed engines of the prior generation operated at 22 to 25 bar BMEP, making the W31’s 31 bar target a meaningful step up in cylinder loading and design complexity. Achieving it without compromising service life required new combustion chamber geometry, higher-rated pistons and cylinder covers, and a step change in the injection and engine management systems.

Bore, stroke, and dimensional specifications

The W31’s 310 mm bore and 430 mm stroke give a stroke-to-bore ratio of 1.39, which is on the long-stroke end for medium-speed engines in this bore class. The comparable Wartsila 32 has a 1.25 stroke-to-bore ratio (320 mm bore, 400 mm stroke). The longer stroke relative to bore on the W31 raises mean piston speed at a given RPM and increases the time available for combustion per revolution, both of which contribute to better combustion efficiency when matched with the right injection and air-handling parameters.

At 750 rpm, mean piston speed is:

That figure sits in the upper range for medium-speed trunk-piston engines, which typically operate between 7 and 12 m/s. The mean piston speed calculator computes this figure for any bore, stroke, and speed combination. Swept volume per cylinder is:

Configuration and power range

Note: power figures reflect Wartsila’s published range for the current production engine and the 2023 uprated outputs. The W31DF in gas mode operates at a lower effective BMEP due to the lean-burn Otto cycle’s knock constraints, giving approximately 85 to 87% of the diesel-mode output per cylinder at the same speed. For certified ratings at specific ambient conditions and fuel specifications, operators should consult Wartsila’s current project guide for the relevant variant.

The BMEP calculator computes cylinder loading from rated power, displacement, speed, and cylinder count. For the W31 at 750 rpm and 8 cylinders:

The 31 bar figure cited at MCR reflects Wartsila’s peak-rated point. The brake thermal efficiency from SFOC calculator converts the 165 g/kWh SFOC to efficiency, yielding approximately 50.1% on the lower heating value of MGO (42.7 MJ/kg).

Engine architecture

The W31 is a four-stroke trunk-piston engine: the piston crown faces the combustion chamber directly, the connecting rod attaches to the piston pin, and the piston serves as the gas-side seal without a crosshead. This is the standard architecture for medium-speed engines in the 250 to 600 mm bore range. Trunk-piston construction keeps engine height lower than a crosshead design and simplifies the lubrication circuit, which matters on vessels with restricted engine-room headroom.

Crankcase and cylinder block

The W31 crankcase is a ribbed nodular-iron casting. The inline 8-cylinder block is a single piece; vee configurations carry both banks on a common bedplate with bolted junctions. The main bearing arrangement places a saddle between every pair of crank throws and one at each end, and the stiffened lower bearing caps resist the approximately 220 bar peak cylinder pressure that accompanies the 31 bar BMEP design point. Wartsila engineered the block from scratch for this pressure level rather than uprating an existing casting; the W31 does not share its cylinder-block dimensioning with the W32.

The wet cylinder liner is centrifugally cast iron, sealed at its lower flange by O-ring grooves. A flame ring at the liner top limits wear in the top ring reversal zone, where temperatures and pressures peak during combustion. The liner bore is plateau-honed to the surface texture Wartsila specifies for the W31 piston-ring break-in sequence.

Crankshaft and connecting rod

The crankshaft is drop-forged from alloyed steel, finish-ground at journal and crankpin surfaces, and dynamically balanced. Vee configurations use a master-and-fork connecting rod arrangement per crankpin pair, matching the approach on the W32 and W46F families. Main bearings are tri-metal: steel back, lead-bronze intermediate, lead-tin running surface. The crankpin and main journal dimensions are sized for the higher peak pressure loading compared with the prior generation.

Cylinder head and four-valve configuration

The W31 cylinder head is a single casting carrying two inlet valves, two exhaust valves, the fuel injector (or dual-fuel injector/gas admission assembly on the DF variant), the starting-air valve, and the cylinder pressure transducer. Cooling water flows through a directed jacket toward the exhaust valve seat inserts, which bear the highest thermal load in the head. The four-valve layout maintains high valve-open area per unit bore area at low lift, important for the Miller-cycle early inlet valve closing strategy.

Exhaust valve inserts are Stellite-faced or equivalent hard-facing material, seated in nickel-alloy seat rings pressed into the head. On HFO operation the risk of high-temperature corrosion at the exhaust valve seat is managed through the valve-seat geometry and cooling design rather than requiring exotic alloy upgrades for standard service.

Common-rail fuel injection

The W31 uses a common-rail injection system operating at up to 1,800 bar rail pressure, supplied by a high-pressure pump driven from the camshaft. The high rail pressure decouples injection timing and rate shaping from pump-cam geometry: the engine management system can command any injection timing, duration, and split pattern (pre-injection, main, post-injection) by controlling the injector solenoid valve independently of pump delivery. This freedom is what allows the W31 to achieve its optimal combustion phasing across a wide load range without mechanical cam re-profiling.

Multiple injection events per combustion cycle reduce the rate of pressure rise (dP/dt), lowering combustion noise and mechanical loading on the crank gear. Pre-injection introduces a small fuel charge that begins burning before the main injection, moderating the sudden pressure rise at the start of main combustion. The marine engine common rail technology article covers the rail-pressure, injector, and control-system architecture in depth.

Variable valve timing

The W31 incorporates variable valve timing on the inlet valves, allowing the engine management system to adjust inlet valve closing relative to bottom dead centre. The Miller cycle in its aggressive form closes the inlet valve early, before BDC, so the charge trapped in the cylinder is at a lower pressure than the boost air. This reduces the effective compression ratio (from approximately 12:1 to 10:1 depending on the advance) without changing the geometric compression ratio. The thermodynamic gain is a lower peak combustion temperature, reducing NOx formation without requiring exhaust gas recirculation, while the two-stage turbocharger maintains sufficient trapped air mass per cycle. Variable valve timing lets the engine management system modulate the Miller advance as a function of load, ambient temperature, and NOx mode selection, optimising fuel consumption at each operating point.

Turbocharging: two-stage on higher configurations

The W31 uses exhaust-driven turbocharging throughout the range, with a charge-air cooler between the compressor outlet and the inlet manifold. On the 8L31, single-stage turbocharging is sufficient. On the V12 through V16 configurations, two-stage turbocharging provides the boost pressure needed to maintain adequate air-fuel ratio at the 31 bar BMEP design point. Two-stage arrangements require two turbocharger units in series on each bank: a low-pressure stage followed by an intermediate cooling stage and then a high-pressure stage. The combined pressure ratio achievable with two stages (typically 5:1 to 7:1) substantially exceeds what a single-stage unit can deliver at the same turbine-inlet temperature.

Wartsila designed the W31 turbocharger specification together with its preferred turbocharger supplier; the turbocharger is matched to the engine’s airflow map and the Miller-cycle operating envelope. Mismatching a replacement turbocharger during overhaul is a known cause of SFOC degradation and elevated exhaust temperatures, which is why Wartsila specifies OEM-sourced replacements rather than generic units of similar nominal capacity. The marine engine turbocharging article covers turbocharger selection and matching in detail.

UNIC engine management system

The Wartsila UNIC (Unified Controls) system is the engine management computer across the W31 family. UNIC integrates:

• Per-cylinder injection timing, duration, and rate-shaping commands to the common-rail solenoid valves
• Variable valve timing actuator control
• Cylinder pressure monitoring and automatic balancing: each cylinder has a piezoelectric pressure transducer whose output UNIC reads continuously, comparing the peak firing pressure and combustion phasing against target values and trimming the injection parameters to bring outlying cylinders back into balance within a few combustion cycles
• Exhaust gas temperature monitoring across all cylinders
• Speed governor with load-sharing capability for multi-engine genset installations
• Engine protection logic (overspeed, low oil pressure, high coolant temperature, high cylinder pressure)
• Data logging and remote diagnostic interface (Wartsila Expert Insight platform for fleet operators)

UNIC also manages the fuel-mode transitions on the W31DF and W31SG variants: the switch from gas to diesel mode or vice versa occurs under UNIC supervision and takes approximately 15 to 30 seconds without interrupting engine load, managed through coordinated gas-admission valve closure and diesel injection system activation.

The modular maintenance concept

One of Wartsila’s explicit design objectives for the W31 was a step change in maintenance time compared with the W32 and W46 generations. The approach centres on pre-calibrated replaceable modules: cylinder components (piston, rings, cylinder liner, and cylinder cover in some configurations) are replaced as factory-calibrated assemblies rather than individual parts that are disassembled and measured in situ.

The pre-calibrated module approach works as follows. Before an engine enters a planned maintenance period, Wartsila delivers a set of pre-assembled, bench-tested cylinder modules to the vessel or shore facility. The crew or service technician lifts out the worn assembly (piston with rings, liner, or cylinder cover depending on the maintenance type) as a single unit, installs the new pre-calibrated assembly, torques the fasteners per the specification, and the cylinder is ready to run without in-situ measurement of ring gap, piston clearance, or injection calibration. The calibration has already been done at the factory to the production tolerances.

Wartsila’s published maintenance-time claim is that a W31 cylinder top overhaul can be completed in approximately 6 hours per cylinder, compared with 10 to 12 hours for a comparable W32 top overhaul done in the traditional component-by-component rebuild approach. On a V16-31 with 16 cylinders, the cumulative saving across a full top-overhaul cycle is in the range of 64 to 96 man-hours, or roughly 8 to 12 technician-days. For vessels where off-hire time carries a commercial penalty (cruise ships during peak season, ferries on daily-schedule operations), this is a tangible value.

The modular concept also changes the parts-logistics picture. Instead of ordering individual piston rings, liners, and injectors separately and managing their individual tolerances, the operator orders complete cylinder kits from Wartsila. The tradeoff is that the kit cost is higher per unit than the sum of individual parts at list price, and that operators cannot substitute third-party rings or liners into the module system without losing the pre-calibrated status. For vessels on Wartsila Lifecycle Solutions service agreements, this tradeoff is absorbed into the agreement; for operators managing their own spare-parts inventory, it requires a capital commitment to a module stock.

Top-overhaul intervals for the W31 are typically 16,000 to 20,000 running hours for the diesel variant on LSFO or MGO, extending to approximately 24,000 hours on premium-distillate operation with UNIC condition monitoring active. The W31DF in gas mode runs cooler combustion temperatures (lean Otto-cycle burn versus diesel diffusion flame), which tends to extend cylinder-head and exhaust-valve service life versus diesel mode. Major overhaul intervals (crankshaft bearings, main bearing inspection) run in the 40,000 to 60,000-hour range. The W32, for comparison, has top-overhaul intervals of 12,000 to 16,000 hours on comparable fuels.

SFOC, thermal efficiency, and the world-record context

The 165 g/kWh SFOC recorded at the W31’s 2015 launch deserves careful context. This figure was measured on a shop test at 85% of MCR in diesel mode (marine diesel oil, standard ISO conditions: inlet air 25 C, cooling water 25 C). Shop-test conditions are favourable relative to in-service conditions: the test does not account for the parasitic power draw of the vessel’s sea-water pump system (which adds 1 to 2% to effective SFOC), the slight SFOC penalty for off-peak operating loads during manoeuvring, or the degradation that accumulates with liner wear and injector aging between overhauls.

In-service SFOC on a well-maintained W31 at 85% MCR on distillate fuel is typically in the range of 168 to 175 g/kWh during the first half of the overhaul cycle. By the end of the overhaul period (approaching 16,000 hours) injector wear, piston-ring wear, and minor liner deterioration will have degraded SFOC by a further 3 to 6 g/kWh from the early-service figure before the module replacement restores it. This in-service SFOC range is still materially lower than the 175 to 183 g/kWh typical of the in-service W32, W46, and competitor engines from the same era.

Converting SFOC to brake thermal efficiency:

(using LHV of 42.7 MJ/kg for marine diesel oil; result is slightly sensitive to the exact fuel specification used in the shop test). No four-stroke medium-speed engine had reached 50% BTE at the time of the W31 launch, and the 51% shop-test figure placed the W31 close to the efficiency of slow-speed two-stroke engines (which typically achieve 48 to 53% BTE at MCR) in a smaller, higher-speed package. For the comparison context, the medium-speed four-stroke marine engines overview and marine engine makers cover the segment’s performance history.

W31DF gas mode fuel consumption

In gas mode the W31DF is rated at approximately 7,300 kJ/kWh total thermal energy specific consumption (brake specific energy consumption, BSEC). This is the standard metric for dual-fuel engines in gas mode because the LHV of natural gas varies with composition. At a typical methane LHV of 50.0 MJ/kg and BSEC of 7,300 kJ/kWh, the equivalent SFOC in mass terms is:

That figure is not directly comparable to the diesel SFOC because the energy density and carbon content of natural gas differ from diesel, but on a CO2-per-kWh basis gas mode produces approximately 25 to 27% less CO2 than diesel mode at equal thermal efficiency, reflecting the lower carbon-to-hydrogen ratio of methane versus diesel fuel.

W31 family variants

W31 diesel

The base W31 diesel variant operates on heavy fuel oil (HFO), ultra-low-sulphur fuel oil (ULSFO), very-low-sulphur fuel oil (VLSFO), marine diesel oil (MDO), and marine gas oil (MGO). The UNIC system adjusts injection timing to optimise combustion across the viscosity range. On HFO, the fuel system includes a viscosity-controlled preheater maintaining the fuel at 12 to 15 cSt at the injector, and the cylinder lubricating oil system uses higher-BN oil (typically BN 40 to 70) to neutralise the sulphuric acid condensation from high-sulphur combustion gas.

The W31 diesel meets IMO Tier II NOx limits (14.4 g/kWh at 720 rpm, 13.8 g/kWh at 750 rpm under the NTC 2008 certification protocol) without aftertreatment, through the combined effect of Miller-cycle inlet valve timing, optimised injection timing, and the high air-fuel ratio made possible by two-stage turbocharging. For Tier III operation in designated ECAs, the W31 diesel requires an SCR system. Wartsila supplies its own NOx Reducer (NOR) SCR, which uses aqueous urea solution (32.5% by mass) ahead of a catalyst bed operating at 280 to 400 C. NOx reduction efficiency reaches 80 to 90% within the catalyst temperature window.

Biofuel blends are supported: FAME blends to B20 and hydrotreated vegetable oil (HVO) blends up to 100% (HVO100) in principle, subject to fuel-system compatibility verification. Wartsila has published compatibility guidance for its diesel engines with HVO and FAME blends.

W31DF: dual-fuel LNG variant

The W31DF operates in two modes: a lean-burn gas mode using liquefied natural gas with a small diesel pilot injection for ignition, and a diesel mode on any bunker distillate or residual fuel. In gas mode the engine meets IMO Tier III NOx limits natively without selective catalytic reduction, because lean premixed combustion keeps peak temperatures below the significant NOx formation threshold.

In gas mode, the UNIC system opens gas admission valves on the inlet manifold during the intake stroke, metering natural gas at low supply pressure (typically 4 to 6 bar gauge). The gas-air charge is compressed in a lean premix with an excess air ratio (lambda) typically between 1.8 and 2.2, above the stoichiometric ratio of 1.0. At top dead centre, a small diesel pilot injection (less than 1% of the energy equivalent at rated load) autoignites and triggers combustion of the surrounding lean mixture. The lean-burn cycle keeps peak combustion temperatures below approximately 1,700 K, well below the 2,200 to 2,500 K associated with significant thermal NOx formation in diffusion-flame diesel combustion.

The resulting NOx emission in gas mode is typically 0.5 to 1.5 g/kWh, compared with the IMO Tier III limit of 2.0 g/kWh for engines at 720 to 750 rpm per MARPOL Annex VI Reg.13. No urea or SCR catalyst is needed. Fuel-mode switching from gas to diesel (or the reverse) takes approximately 15 to 30 seconds under UNIC control and can be performed at any load without interrupting engine output.

The W31DF adds to the base diesel engine: low-pressure gas admission valves on the inlet manifold, a gas valve unit (GVU) for fuel-circuit management and emergency gas shut-off, a dedicated gas detection system in the engine room, and modified piston rings designed to limit gas leakage past the rings during compression.

Methane slip in the W31DF

Methane slip (unburned CH4 exiting in the exhaust) is the principal GHG trade-off of the Otto-cycle lean-burn approach. The W31DF methane slip at rated load in gas mode is in the range of 1.5 to 2.5 g/kWh, which is lower than the older W32DF range of 3 to 6 g/kWh as a result of the W31’s revised gas-admission timing, improved piston-ring pack geometry, and UNIC optimisation of the gas-air mixture. Methane’s 100-year global warming potential is 28 times that of CO2 (using IPCC AR5 figures as cited in MEPC.245(66)). At 2 g/kWh methane slip, the GHG benefit of gas mode over diesel narrows from the 25 to 27% CO2 reduction to roughly 10 to 15% on a GWP-adjusted basis, depending on the reference fuel and the ship’s operating profile.

At part load (below 60% MCR), methane slip on lean-burn dual-fuel engines typically increases as the air-fuel ratio moves further lean, reducing combustion temperature and leaving a greater fraction of the gas charge unburned. This is documented in the CIMAC WG17 position paper on methane slip and confirmed in Wartsila’s own published data. Operators accounting for the W31DF under the IMO CII or EU ETS GHG frameworks should use the emission factors specified in those regulations, which may not match measured in-service values, and should apply the MARPOL Annex VI Reg.27A provisions for methane GWP factor.

W31SG: spark-ignited gas variant

The W31SG replaces the diesel pilot injection with a spark plug in each cylinder, operating the engine solely on natural gas or bio-gas without any liquid fuel capability. The spark-ignited lean-burn cycle on the W31SG eliminates the pilot-diesel infrastructure (high-pressure injection pump, diesel fuel system, diesel pilot injectors) and the associated complexity, in return for full dependency on a continuous gas supply and no fallback to diesel mode.

The SG variant targets applications where a continuous gas supply is guaranteed and the installation economics of a single-fuel system are attractive: shore-based distributed power generation using pipeline gas, power plants co-located with gas processing or biogas facilities, and vessels on fixed gas-fuelled routes with assured bunkering at each port. Without the diesel backup, it is not appropriate for deepwater voyages or for vessels on routes where bunkering infrastructure is uncertain.

Thermal efficiency in SG mode is slightly higher than in DF gas mode because the SG combustion system can be optimised for gas-only operation without the design constraints imposed by dual-fuel compatibility. Wartsila publishes BSEC figures for the W31SG in the 7,000 to 7,200 kJ/kWh range at optimal load, compared with approximately 7,300 kJ/kWh for the W31DF in gas mode.

W31 methanol variant

Wartsila introduced a methanol-capable derivative of the W31 platform in the early 2020s, aligning with growing demand from ferry and cargo vessel operators pursuing methanol as a lower-carbon bunker alternative. Methanol (CH3OH) is liquid at ambient temperature and pressure, simplifying storage and bunkering relative to cryogenic LNG, and it can be synthesised from renewable hydrogen and captured CO2 as e-methanol, offering a path to near-zero tank-to-wake CO2 emissions.

Methanol has roughly half the energy density of marine diesel oil: approximately 19.9 MJ/kg lower heating value versus 42.7 MJ/kg for MGO. This means fuel tank volume and pumping rates must approximately double for an equivalent energy supply, which affects the vessel’s fuel-system design significantly. Methanol is also corrosive to certain elastomers and metals (particularly zinc and aluminium), and its toxicity and low flash point (11 C) require additional fuel-system sealing, ventilation, and leak-detection measures relative to diesel.

The W31 methanol variant uses a small diesel pilot injection, similar to the W31DF architecture, because methanol’s autoignition temperature (around 470 C) is higher than natural gas-air mixture autoignition under Otto-cycle lean-burn conditions. The pilot diesel initiates combustion, and the methanol-air charge then burns through the propagating flame front. The engine’s common-rail injection system is adapted to deliver methanol at the required pressure and timing; methanol’s lower lubricity relative to diesel requires additional injector-tip lubrication measures to prevent injector tip corrosion and erosion.

The W31 methanol variant is a recent product introduction. In-service experience is accumulating with initial fleet deliveries but is limited relative to the decade-long W31 diesel and DF track record. Prospective operators should request Wartsila reference vessel data and verify class-society type approval status for their specific installation.

IMO Tier III compliance paths

The W31 family offers three Tier III compliance routes, depending on the variant and fuel selection.

Diesel variant with SCR: The diesel W31 meets Tier II without aftertreatment. In designated ECAs (Baltic Sea, North Sea, North American ECA, US Caribbean Sea ECA) where the Tier III limit of 2.0 g/kWh (at 720 to 750 rpm) applies, the diesel W31 requires an SCR system. Wartsila’s NOR SCR injects 32.5% aqueous urea solution upstream of a catalyst bed at 280 to 400 C. NOx reduction efficiency is 80 to 90%, bringing the engine below Tier III. A 4,200 kW 8L31 genset requires approximately 1.5 to 2.5 m3 of catalyst volume. Urea consumption runs at roughly 3 to 5% of diesel fuel consumption by mass.

W31DF in gas mode: The dual-fuel variant meets Tier III natively in gas mode through lean-burn combustion, without SCR. Gas-mode NOx is typically 0.5 to 1.5 g/kWh, well within the 2.0 g/kWh limit. For Tier III in diesel mode the W31DF still requires SCR if operating within an ECA on liquid fuel.

W31SG: Spark-ignited gas operation meets Tier III natively in the same way as the DF gas mode. The SG variant produces no diesel fallback and therefore the Tier III question only arises if the engine is somehow expected to run on liquid fuel, which it cannot.

The MARPOL NOx Tier III calculator computes the applicable NOx limit for any engine speed under the NTC 2008 regime, covering the Tier I, II, and III bands as a function of rated speed.

Marine applications

Cruise vessels and expedition ships

The W31 has been selected for cruise vessel power plants where fuel efficiency and compact footprint are decisive. Diesel-electric cruise ship configurations typically deploy 4 to 8 genset engines, with the W31’s output range of 4.2 to 9.8 MW per engine unit well suited to providing the 20 to 80 MW of total installed capacity that modern cruise vessels require for propulsion, hotel load, and dynamic positioning.

Expedition cruise vessels (ice-class or polar-notation vessels for wildlife and polar tourism) have adopted the W31DF for its Tier III gas-mode compliance, which is required in polar ECAs and environmentally sensitive areas, and for its ability to switch to diesel when LNG bunkering is unavailable at remote ports of call. The W31DF’s mode-switching capability without interrupting load suits diesel-electric systems where the engines run at varying load percentages depending on the speed profile and weather conditions encountered on polar routes.

Ferries and ro-pax vessels

Ferry operators in the Baltic, Norwegian coastal routes, and North Sea crossings have deployed both the W31 diesel and W31DF variants. On routes where LNG bunkering infrastructure is available at both ends of the run (a growing number of Baltic and Norwegian routes since 2015), the W31DF in gas mode provides Tier III compliance without SCR, and the native lean-burn combustion’s lower combustion temperatures also reduce cylinder wear rates compared with diesel diffusion-flame combustion.

Ferry propulsion with the W31 is primarily diesel-electric: multiple W31 or W31DF gensets feed an AC busbar, with electric drive motors (direct AC or variable-frequency drive) on the shafts. Diesel-electric architecture suits ferries with significant hotel and car-deck loads because individual gensets can be loaded near optimal efficiency while unused gensets are shut down, matching total generation to the actual vessel load at each segment of the voyage.

Offshore support vessels

Offshore supply vessels (PSVs, AHTSVs) and multipurpose offshore vessels use the W31 in diesel-electric configurations for dynamic positioning. DP2 and DP3 vessels require at least two or three independent power plants for fault-tolerant DP capability, and the W31’s compact footprint at 4.2 to 9.8 MW per unit makes it practical to install three or four engines within the engine-room envelope typical of medium-size OSVs (65 to 95 m length).

The variable load profile of DP operation, ranging from near-zero in calm conditions to full thruster demand in North Sea winter sea states, favours diesel-electric architecture: gensets not needed are shut down and those running stay at 70 to 85% MCR where SFOC is near its optimum. The W31’s exceptional part-load efficiency curve (SFOC remains within approximately 5% of the optimal-load value down to around 50% MCR, owing to the UNIC variable valve timing and injection flexibility) is a measurable advantage in OSV service relative to earlier-generation medium-speed engines whose SFOC degraded more steeply at part load.

Auxiliary gensets on cargo vessels

Large container ships and bulk carriers in the Panamax-to-ULCV range typically use slow-speed two-stroke main engines (MAN B&W or WinGD classes) for propulsion, with medium-speed auxiliary gensets supplying the vessel’s electrical load. The W31 in 8L31 configuration (approximately 4.2 MW) is used as an auxiliary genset on vessels where total auxiliary demand during port stays (reefer container power, deck machinery, accommodation) reaches 3 to 5 MW per genset. Two to three W31 units in parallel provide adequate redundancy and allow single-genset operation during sea passages when shaft-generator or turbogenerator output reduces the genset duty.

Power plant and land-based generation

The W31 and W31DF are marketed by Wartsila as power-plant engines for island grids, remote industrial facilities, and fast-response peaking installations. The same engine block and cylinder components as the marine version are used; ancillary systems (cooling, exhaust, mounting) are adapted for stationary installation. The W31’s 15-second full-load acceptance capability from warm standby suits frequency-regulation and spinning-reserve roles on isolated grids. For land-based applications using pipeline natural gas, the W31SG (spark-ignited gas) eliminates the diesel pilot and simplifies the fuel system.

Comparison with principal competitors

The W31 competes in the 290 to 330 mm bore medium-speed segment. The most direct diesel competitor is the MAN L21/31 (210 to 310 mm bore range, overlapping at the 31 bore) and the MAN L32/44CR from MAN Energy Solutions. Bergen’s medium-speed product line (see Bergen B33/45) competes in the adjacent bore class.

The W31’s higher BMEP (31 bar versus 25 to 27 bar for competitors) delivers a higher power-to-weight and power-to-footprint ratio for a given cylinder count, which is relevant on vessels where engine-room volume is constrained. The SFOC advantage (165 g/kWh versus 174 to 178 g/kWh for competitors) is most valuable on vessels with high annual operating hours in the 70 to 90% MCR range: cruise ships, ferries, and deepwater OSVs. For vessels with highly variable duty cycles or frequent low-load operation, the in-service SFOC gap is smaller than the shop-test comparison suggests. The Wartsila 32 article covers the next-larger Wartsila bore in detail, including the 42-year development history of that platform.

Portfolio context

The W31 occupies the upper-mid range of the Wartsila medium-speed portfolio:

Below the W31 is the Wartsila 20 (200 mm bore, 185 kW per cylinder), which targets auxiliary gensets on smaller vessels, ferries, and offshore workboats. The W31 at 310 mm bore delivers three times the per-cylinder output at the same speed, serving a different size class of installation.

Above the W31 is the Wartsila 32 (320 mm bore, 580 kW per cylinder), with a larger installed base accumulated over four decades of production and a V18 configuration reaching 10,440 kW. The W32 offers a wider spread of cylinder counts (6 through 18 versus 8 through 16 for the W31) and a longer track record in FPSO topsides and naval applications. The W31 is the more efficient engine by SFOC, while the W32 covers a broader output range.

Further up the Wartsila scale, the Wartsila 46F (460 mm bore, 1,200 kW per cylinder) serves cruise vessel main power plants and large LNG carriers where total installed capacity requirements reach 50 to 80 MW, beyond practical reach of the W31’s output ceiling. The Wartsila 50DF (500 mm bore) is Wartsila’s dominant LNG carrier propulsion choice.

The W31’s place in the portfolio is the efficiency leader at medium bore: it does not cover the widest range of cylinder configurations or the highest total plant output, but it delivers the lowest SFOC per kilowatt-hour of any Wartsila medium-speed engine and of any four-stroke diesel engine worldwide by the Guinness World Records measurement.

The marine engine makers article provides the cross-manufacturer context including MAN Energy Solutions, Bergen, HiMSEN, Caterpillar, Cummins, and the two-stroke manufacturers. The marine engine model decoder explains how the “W31” designation fits into Wartsila’s naming convention and compares it with the coding systems from MAN, Bergen, and others.

Limitations

The rated power and SFOC figures in this article reflect Wartsila’s published project guide summaries for the current production engine and the 2023 uprating, at ISO standard reference conditions (inlet air 25 C, cooling water 25 C, standard atmospheric pressure at sea level). Certified performance for any specific application depends on ambient conditions at the installation site, the exact fuel specification, and the rated speed. Wartsila publishes rating curves for each variant against these parameters in the official project guide; the figures in this article are indicative rather than certification documents.

The W31 SFOC of 165 g/kWh was a shop-test result at 85% MCR in diesel mode at launch in 2015. In-service SFOC on a well-maintained engine over its overhaul cycle is typically 168 to 177 g/kWh, depending on fuel quality, load profile, engine condition, and whether Wartsila’s UNIC optimisation routines are active. The 165 g/kWh figure is not a guaranteed in-service value; it is a test-bench benchmark.

Methane-slip figures for the W31DF (1.5 to 2.5 g/kWh at rated load) are from Wartsila published data and independent test programme summaries available in the public domain. Actual in-service methane slip varies with engine load, engine age, gas composition, and the condition of the piston-ring pack. For GHG accounting under IMO CII, EU ETS, or FuelEU Maritime, operators should use the emission factors defined in those regulatory frameworks rather than in-service measured values.

The W31 methanol variant and the current methanol-capable configuration details are recent product introductions. Class-society type approval status, fuel-system requirements, and in-service performance data should be verified with Wartsila at the time of project specification, as the product continues to evolve.

Competitor SFOC figures (Wartsila W32, MAN 32/40, Bergen B33:45) in the comparison table are from published specification sheets for current or recent-production engines. Engine programmes update; verify against each OEM’s current project guide when making a newbuild or retrofit specification decision.

See also

• Wartsila 32: medium-speed four-stroke marine engine: the next-larger Wartsila bore with 42 years of production history
• Wartsila 20: medium-speed four-stroke marine engine: smaller-bore Wartsila genset engine for auxiliary duties
• Wartsila 46F: medium-speed four-stroke marine engine: larger-bore sibling for cruise, FPSO, and LNG-carrier duties
• Wartsila 50DF: dual-fuel medium-speed marine engine: 500 mm bore dual-fuel engine, dominant LNG carrier propulsion choice
• Wartsila corporate history: full lineage of the Wartsila group
• Specific fuel oil consumption: SFOC definitions, measurement methods, and benchmarks
• Medium-speed four-stroke marine engines: segment-level context across all major makers
• Marine engine common rail technology: common-rail injection architecture used in the W31
• Marine engine turbocharging: turbocharger selection and matching for medium-speed engines
• Marine engine makers: global OEM landscape including Wartsila, MAN, Bergen, HiMSEN
• Marine engine model decoder: naming conventions for Wartsila and 33 other makers
• Methanol as a marine fuel: fuel properties, bunkering, and regulatory status
• LNG as a marine fuel: LNG fuel properties, bunkering, and regulatory status
• MARPOL Annex VI sulphur cap: sulphur compliance context for the W31’s fuel range

Related calculators:

• Wartsila W31: MCR per Cylinder
• Wartsila W31DF: MCR per Cylinder
• Wartsila W31SG: MCR per Cylinder
• Engine: Brake Thermal Efficiency from SFOC
• Engine: BMEP from Output Data
• Engine: Mean Piston Speed
• MARPOL NOx Tier III limit calculator
• System: Auxiliary Engine, medium-speed 4-stroke