Low-pressure dual-fuel engine technology comes to two-stroke market

The RT-flex50DF engine The RT-flex50DF engine
Industry Database

Wärtsilä’s long-anticipated dual-fuel two-stroke engine has now been introduced to the market; we saw the first example in operation at the company’s Trieste plant in Italy.

Recently we were privileged to see the first example of Wärtsilä’s gas-fuelled two-stroke engine in operation in Trieste. Although not the first two-stroke dual fuel marine engine to be made available, and not the first to pick up orders, the company is rightly proud of its achievement, describing it as a “game-changer for merchant shipping”.

The significant feature of this engine compared with its competitors is its use of low pressure gas injection technology. Wärtsilä has long experience of this with its dual-fuel four-stroke engines, but there was a widely-held belief in the industry that it could not successfully be applied to two-strokes. The first example, an RT-flex 50DF now running on the test bed, proves that viewpoint to be incorrect.

Engineers in Trieste have been working hard over the last few years to perfect the technology. At first sight the choice of the Italian location seems an odd one –two-stroke R&D is centred on Winterthur in Switzerland, and the four-stroke dual fuel engines were developed at Vaasa in Finland. Trieste, however, has a long two-stroke heritage, starting as a factory producing Fiat and IRI (Ansaldo and CRDA) large diesel engines under the Grandi Motori Trieste (GMT) name. With the exit of Fiat from the merchant marine market ownership of GMT was transferred to IRI, and in 1982 it took on a licence for Sulzer two-stroke and four-stroke engines. Two years later it was taken over by Fincantieri, which became a major shareholder in the New Sulzer Diesel business. Once Wärtsilä acquired all the NSD assets in 1999, GMT became a division of Wärtsilä rather than a licensee. Later it was renamed Wärtsilä Italia and production of the larger four-stroke engines – including deal-fuel – was concentrated on Trieste.

Although the original GMT plant has been considerably updated and redesigned under Wärtsilä it retained its large engine test beds suitable for two-stroke development, so provided a logical home for the dual-fuel two-stroke programme.

Rolf Stiefel, director of two-stroke sales for Wartsila Ship Power told us that the introduction of LNG as fuel was likely to represent the next step change in large ship propulsion. It has been supported by its environmental and economic benefits, but held back by safety considerations as well as concerns about fuel availability and the overall fuel pricing picture. He says that the wait for the low pressure solution will have been worth while, because there are no doubts about the environmental benefits: the engines will be optimised to run on gas with liquid fuel as a back up, and the ultra-clean combustion process means that the engines will meet IMO Tier III running in gas mode with no additional after-treatment such as exhaust gas recirculation (EGR) or SCR, as will be required by high-pressure systems. With bunkering developing rapidly – built on the experience in the Nordic countries, the expanding network of LNG terminals worldwide, and the number of LNG bunkering ships on order, supply prospects look bright.

One of the major uncertainties remains fuel costs. It is still far from clear whether this will be linked to the price of fuel oil, or determined in some other way. LNG bunker prices could be linked to the US Henry Hub system, with a margin to cover liquefaction and distribution, or could be fixed as a percentage of MGO or HFO prices. “The suppliers are still rolling the dice,” says Mr Stiefel, but he believes that economically LNG will definitely win through.

Another important question raised by potential customers is storage and handling of LNG on board. Here, Mr Stiefel points out that Wärtsilä, through its experience with four-stroke dual fuel engines, already has a track record, and is able to supply a complete package including tanks, evaporators, valves and controls. Moreover, the low pressure solution, with all on-board gas kept at 10bar or below, is well proven and far less complicated that the high pressure systems requiring 300bar pressures. As for location and type of gas tanks, these will vary according to ship type. For vessels like tankers, there are opportunities to site tanks on deck; LNG tanks are less easy to accommodate on container ships or bulkers but solutions are available.

R&D director Ingemar Nylund explained the technology behind the low-pressure dual-fuel two-stroke. As with the four-stroke equivalent, it uses lean-burn principles, with the gaseous fuel pre-mixed outside the combustion chamber, and runs according to the Otto cycle. The low pressure gas is admitted to the cylinder through injection ports in the cylinder liner, entering at mid-stroke. Ignition, using pilot fuel, takes place in the pre-chamber.

The merits of the system are that there is comparatively little additional capital expenditure involved, and the system occupies less space and consumes less energy itself, because no additional high-pressure equipment is needed, and neither is additional equipment required in order to meet future emissions limits. Capital costs some 15%-20% lower than high-pressure gas engines are forecast. With just 1% pilot fuel required when the engine is running normally, operational costs and emissions are well controlled, while the common rail injection system used by the pilot fuel is well-proven on the company’s electronic two-stroke engines, giving the possibility to run on HFO as well if required and emission limits permit. The pre-chamber principle is shared with the four-stroke dual-fuel engines so again uses well-proven technology.

The only real additional equipment is the gas admission valve, of which two are provided per cylinder. These are hydraulically activated from the exhaust valve servo oil system, and precisely controlled to achieve the optimum fuel/air mixture throughout the load range. The other key technology is the electronic control system, based on Wärtsilä’s UNIC system, which offers a full range of safety and redundancy controls as well as precise metering of both gas and pilot fuels.

The positive effect on NOx, SOx and PM emissions through using LNG as a fuel in an Otto cycle engine is well known, but greenhouse gas emissions, a result of the so-called ‘methane slip’ are less well controlled, methane being a particularly powerful greenhouse gas, some 20-25 times stronger than CO2. In the case of the RT-flex 50DF test engine, greenhouse gas emissions have been limited to 90% of the HFO equivalent, with further reductions likely as development progresses.

Methane slip can be reduced by further attention to the gas admission valve timing; the use of pre-chamber technology to achieve complete combustion; and optimising the combustion space to avoid ‘dead volumes’. The gas quality, measured by the ‘methane number’ is another potential limiting factor, with engines typically optimised for methane numbers of 70-90. Lower methane numbers, which cause knocking, can be accommodated by de-rating the engine. This can be carried out automatically by the control system, which will limit output to a safe level.

In normal operation, the engine will be started on liquid fuel (though starting on gas is said to be possible), and transferred to gas at a suitable load point – normally between 5% and 85%. In the event of a gas failure, the engine will automatically switch to diesel, which can be achieved virtually instantly, within one revolution.

The type approval process is expected to get underway in 2014, with the first RT-flex50DF engines operational at sea in 2015. Then, the technology will be extended to the entire range of larger-bore ‘Generation X’ engine types, and ultimately to the full two-stroke range.

“Describing this as a game-changing development for merchant shipping is certainly no exaggeration, since the many advantages of being able to use gas and LNG as primary fuel are now, for the first time ever, available to virtually all vessel types. Our well proven technologies for both the engines and the onboard gas and LNG handling systems, can now be applied to this wider market,” said Martin Wernli, vice president, two-stroke, Wärtsilä Ship Power.


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