Wärtsilä engines for LNG carrier run on natural gas, diesel

Later this year, Wärtsilä will deliver its first four Wärtsilä 50DF engines for a 74,000 cu m LNG carrier.

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Later this year, Wärtsilä will deliver its first four Wärtsilä 50DF engines for a 74,000 cu m LNG carrier. Wärtsilä designed the dual fuel engines to run on natural gas with a liquid fuel backup. In January, Wärtsilä completed its three-month factory acceptance test on the first 6LWärtsilä 50DF engine for Gaz de France's LNG carrier.

The vessel, under construction at the French shipyard Chantiers de l'Atlantique and slated for delivery in 2004, will be the first LNG carrier to be powered by electric propulsion. The engines will each develop 5,700 kw at 514 rpm, and the LNG carrier will be able to make its planned weekly roundtrip route between Algeria and Marseilles at a service speed of 16 knots, even with three of the four generating sets working.

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Wärtsilä tests its first Wärtsilä 50DF dual-fuel engine for a 74,000 cu m LNG carrier.
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Wärtsilä tested the functioning of the engine during factory acceptance testing.

The company tested the engine mostly on natural gas, but it also ran on low heating value gas and on gas with a low methane number. The tests included operation on gas fuel and light fuel oil (LFO) with tripping to diesel operation, transfer to gas fuel operation, load acceptance, and load rejection. Following factory acceptance testing, the engines were to remain in Wärtsilä's Vaasa, Finland, facility for endurance testing before delivery in August to the French shipyard for installation on the LNG carrier.

The four-stroke engine can run either on natural gas or LFO and can operate on heavy fuel oil (HFO) with some modifications. Wärtsilä said the engine can smoothly switch between gas and LFO while operating. The engine's thermal efficiency is 47%, higher than for any other gas engine, the company said. The Wärtsilä 50DF was designed to yield the same output on natural gas or on LFO or HFO. When running on gas, NOx and CO2 emissions are substantially lower than for an HFO engine. Wärtsilä said its Wärtsilä 50DF engine will have a NOx emissions rate about 10% of equivalent diesel engines.

How it works

The fuel system in the engine is divided into two: one for gas and the other for diesel oil. Filtered natural gas is supplied to the engine through a valve station. A pressure control valve, mounted on the engine, controls the gas pressure. The system includes the necessary shut-off and venting valves to ensure safe and trouble-free gas supply. On the engine, the gas is supplied through large common-rail pipes running along the engine. Each cylinder has an individual feed pipe to the gas admission valve on the cylinder head.

The fuel oil supply is split into pilot fuel and backup fuel systems. An engine-driven pump elevates the pilot fuel to the required pressure before distributing it through a common-rail pipe to the injection valves at each cylinder. Pilot fuel is injected at 900-bar pressure, and the timing and duration are electronically controlled. The backup fuel is fed to a normal camshaft-driven injection pump. From there, the high-pressure fuel goes to a spring-loaded injection valve of standard design for a diesel engine.

In gas mode, the Wärtsilä 50DF engine runs according to the lean-burn Otto process. Gas admitted to the air inlet channels of the individual cylinders during the intake stroke gives a lean, premixed air-gas mixture in the engine combustion chambers. Injecting a small quantity of diesel oil directly into the combustion chambers as pilot fuel reliably ignites the compression ignition, as in a conventional diesel engine. The engine features micro-pilot injection with less than 1% of the fuel energy being required as liquid fuel at nominal load.

The engine can switch automatically from fuel oil to gas operation at maximum 80% load. Transfer automatically occurs after the operator's command without load changes. Switchover lasts about one minute.

"The transfer from gas fuel to liquid fuel can be done at any load," said Mikael Wideskog, Wärtsilä 50DF engines product manager, adding there will be no obvious change in speed.

Destination: LNG carriers

Wärtsilä said the engine is particularly beneficial for LNG carriers because they make maximum use of the gas fuel, or boil-off gas, from the LNG cargo, for useful power. The efficient machines call for lower fuel consumption overall, lowering operating costs in contrast to the conventional steam turbine plant, the company said. This onboard available fuel should be enticing to LNG carriers, Wärtsilä believes.

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Mika Laurilehto, product and application development director, said there has been a push for use of LNG as a bunker fuel. A drawback, he said, is the limited distribution network for global use of LNG in marine applications.

Currently, he said, international conventions do not cover the use of gas as fuel in diesel engines, so gas installations need additional acceptance by flag authorities. Individual countries are likely to implement their own requirements until an international standard is set.

Creating the Wärtsilä 50DF

While "it's clearly a new product," Wideskog said, Wärtsilä drew on its proven Wärtsilä 32DF and Wärtsilä 46 engines to design the Wärtsilä 50DF. In all, about 40% of the Wärtsilä 50DF components are identical to the Wärtsilä 46, 40% of the Wärtsilä 50DF components are based on the Wärtsilä 46, 10% of the Wärtsilä 50DF components are identical to the Wärtsilä 32DF, and 10% are up-scaled from the Wärtsilä 32DF.

"We used a lot of the design philosophy and tools from the other engines," Wideskog said.

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