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Wärtsilä sets new benchmark in four-stroke design

Marine Power & Propulsion: October 2015

Representing the culmination of five years’ development work, the Wärtsilä 31 engine sets a new industry standard in four-stroke efficiency and is also exceptional in providing a concurrent design platform for three distinct products, in diesel, gas-only and dual-fuel variants.

As with the Finnish company’s ubiquitous 320mm-bore engine family, the new generation will serve an extremely wide range of sectors within one of the most intensely competitive segments of the market as regards engine size and power.

The diesel version of the W31 offers the highest power output in its class, at 610kW per cylinder, and is claimed to return an average 8-10g per kWh lower fuel consumption rate than that of its closest contender. At its optimum point, the fuel burn can go as low as 165g/kWh.

Moreover, Wärtsilä’s technicians have engineered a design promising a reduction of some 20% in maintenance costs, with significantly longer overhaul intervals. While compliant across-the-board with existing IMO Tier II criteria, Tier III compliance is assured by the pure gas version and when running the DF variant in gas mode.

Targeted at both propulsion and auxiliary applications, the W31 has been developed exclusively as a V-form engine, a factor which underpins the modular build strategy, favouring parts commonality and production economics. Available in 8V, 10V, 12V, 14V and 16V configurations, it is suited to diesel-mechanical, diesel-electric and hybrid drive installations.

The engine series is pitched at various types of vessel in the offshore, cruise, ferry, short-sea and special purpose categories requiring unit powers from 4,200kW to 9,800kW. Multi-fuel flexibility is among the design’s strongpoints, as it spans heavy fuel oil (HFO), marine diesel oil (MDO), low-viscosity or low-sulphur fuels, LNG, ethane gas (LEG) and petroleum gas (LPG), by way of three alternative versions of the engine, namely diesel, dual-fuel (DF) or spark-ignited gas (SG). The DF variant enables automatic switchover to gas when entering Tier III waters.

Moreover, in the context of ‘future-proofing’, such as for a change of vessel operating area, the modular design and common technologies across the different versions allow conversion from one version to another with only minor mechanical alterations.

To illustrate the engine’s relative frugality in fuel usage, Wärtsilä stated at the W31’s unveiling in June that it would potentially yield a saving of about €10,000 (US$11,065) per day in operating expenses for an anchor-handling tug/supply vessel.

Ulf Aastrand, Wärtsilä’s director for product development programmes, observed that a fuel efficiency improvement of the magnitude secured with the W31 had never before been accomplished in a single step change.

"Today, fuel efficiency is the ultimate mark of technological achievement," added Giulio Tirelli, the company’s director, engines portfolio and applications. "And delivering a 10g/kWh upgrade in a single product launch is a dramatic improvement. This engine has reached a level of efficiency that, just a few years ago, was not considered physically possible."

Ensuring efficiency across a ship’s operating profile is key to vessel economics, especially so in sectors characterised by frequent speed and load changes, and where protracted periods are spent under low load, as in offshore work. The W31 readily adapts to different running profiles through the sophisticated UNIC control system, the advanced injection system, and variable valve timing.

"Thanks to an extremely high level of automation, we have been able to optimise several points that we would not have been able to adapt in the past," explained Giulio Tirelli the company’s director engines portfolio and applications.

Maintenance has been addressed not only in the cause of reducing through-life expenditure but also out of acknowledgement of the crucial need among operators to minimise unscheduled downtime. Whereas ‘standard’ marine engines of similar output require a first maintenance interval, i.e. with regard to the injection valves, after about 2,000 running hours, the first service on the W31 is only necessitated after 8,000 hours.

Similarly, injection pump TBO (time between overhaul) is 32,000 hours rather than the 16,000 required for the previous generation diesel engine, while the TBO for cylinder head and piston overhaul is also extended to 32,000 hours, from 20,000 hours hitherto.

Furthermore, downtime stands to be reduced because the modular design, with its lower component count, facilitates a shift from dismounting and overhauling individual parts to exchanging whole assemblies, such as power units, injectors and high-pressure fuel pumps.

The modular design will also allow for technological upgrades over time, particularly with regard to evolving emission controls and decisions to opt for different fuel types.

The W31 will be produced in Vaasa, Finland, where the W32 is also currently assembled and tested. For the time being, Wärtsilä does not licence four-stroke engine building to external parties, confining manufacture to fully-owned factories or joint ventures.

It is anticipated that the first deliveries of the new type will be made in 2016. At the time of writing, The Naval Architect was advised by Wärtsilä that many projects were under negotiation, and that the initial orders could be expected fairly soon. Factors attracting market interest were said to be the engine’s extremely high efficiency and new approach to maintenance, aswell as its low emissions and scope for conversion.

In terms of power band coverage, the new design sits between the 320mm-bore W32 and 380mm-bore W38 types, with a top power rating slightly in excess of that of the enduringly popular W32. The latter dates from 1997 as the follow-on to the Vasa 32, which was first introduced in 1978 and had collective sales numbering several thousands.

The W31, in all its versions, is seen by Wärtsilä as an alternative, rather than as a successor to, the W32 and W34DF the dual-fuel version. It will be for the market to ultimately decide if the W31 is the long-term successor to the august 320mm-bore family.

Affording added credibility to the company’s claims for its new offering, Guinness World Records has rated the W31 as the "most efficient four-stroke diesel engine". The prestigious achievement was verified on 26 May this year.

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