Dual fuel engines – An Overview

Although Diesel engines are designed to run on a variety of fuels, natural gas was until very recently not one of those that engine developers had considered as being suited to ships. The main reason for this was the need for highly sophisticated fuel storage and delivery systems as well as the obvious lack of bunkering infrastructure.

Wärtsilä had been developing dual-fuel engines for shore-based use starting in the late 1980s and were the first maker to transition the idea to marine applications. In 2001, Wärtsilä was contracted to supply the FPSO Petrojarl I with a pair of its 18V32DF dual-fuel engines.

The choice of engines for the Petrojarl I was because it was planned to operate under Norwegian jurisdiction, where a CO2 tax levied on power production in the offshore sector was due to come into effect. As the operator’s requirements also include low NOx emissions, the Wärtsilä 32DF dual-fuel engine became a consideration for the installation which being on a FPSO would be a stationary situation and with good supplies of LNG from the Norwegian offshore gas fields.

Until the announcement concerning the construction of the LNG carrier Gaz de France energY was made in 2002, almost every commercial vessel afloat was powered by one or more two-stroke or a four-stroke diesel engine running on some type of oil or was an LNG carrier with a steam turbine set-up. Gaz de France’s choice of a dual-fuel engine on its new LNG carriers was driven more by profit motive than environmental reasons.

As an expensive commodity, LNG was too valuable to wasted on powering the vessel when cheaper fuel oil was available. A dual-fuel engine that could run on oil and which was more efficient than a steam turbine when using gas provided one solution, the other was a conventional diesel engine coupled with re-liquefaction system for the boil off gas from the cargo. Those economic assumptions would later be reversed when fuel oil prices skyrocketed and boil off gas from the cargo once again became cheaper.

The first dual-fuel engines ordered for ships were Wärtsilä 6L50DF types for the Gaz de France energY and the same engine type was chosen for the larger 154,000 m3 LNG carrier Gaselys also owned by Gaz de France. That vessel was fitted with three 12-cylinder and one 6-cylinder 50DF engines with an aggregate power of 39.9 MW.
Although the first dual-fuel engines ordered were 50DF types, it was the smaller Wärtsilä 32DF engines installed in the world’s first gas-driven PSVs Viking Energy and Stril Pioner that became the first engines in operational use. From LNG carriers and PSV’s the attractions of the engines have also been noted and employed by owners of passenger vessels including cruise ships and ferries.

More recently the range of ship types with dual-fuel engines has expanded to include tankers, tugs, car carriers and other offshore types. As well as adding different ship types, the range of fuels has been extended. In addition to LNG, dual-fuel engines are now in operation burning methanol, ethane and propane.

For many years, Wärtsilä was the main proponent of dual-fuel engines although Rolls-Royce was also promoting a spark-ignited gas version of its Bergen diesel engines. Regardless of maker all gas-fuelled engines were medium speed variants.

That has been changed and now there are dual-fuel low-speed two stroke engines produced by MAN Diesel & Turbo and by Wärtsilä‘s successor in the two-stroke sector, Winterthur Gas & Diesel.
The third two-stroke manufacturer, Mitsubishi did announce a dual-fuel version of its own design in 2013, but the company has now shed its engine manufacturing arm and although development is planned to continue by the new owner, no announcement has yet been made as to when the first will be produced.

In the four-stroke sector, the number of makers producing dual-fuel engines is higher. Wärtsilä, MAN, MaK, EMD, ABC, Himsen, and Niigata all have dual-fuel engines in their ranges and more makers are soon to join the list. The four-stroke engines are being installed in many vessel types but LNG carriers and cruise ships account for the majority of engines.
For most manufacturers, their range of dual-fuel engines is smaller than for their diesel offerings although if the choice of LNG as a marine fuel accelerates, this is likely to change.
It is not certain that all dual-fuel engines will be operated on both fuel types. Many of the engines are being installed in vessels that are ‘dual-fuel ready’ meaning they have the engines but not necessarily a LNG fuel system which will be added later if the operating profile permits.

Wärtsilä’s range of dual fuel engines currently comprises five basic models, the longer established 20DF, 34DF, 46DF and 50DF and the most recent the 31DF launched in 2015. All are four-stroke engine that run on oil fuels (LFO and HFO) and can switch over from gas to oil and vice versa smoothly during engine operation.

The Wärtsilä dual-fuel engines are available in power range from 0.9–18.3 MW having speed range from 500–1200 rpm. The 34DF was first introduced in 2008 as the successor to the 32DF engine. The upgraded engine, produced 11% more power than the original version.
As an early mover in the dual-fuel sector it should be no surprise that the company has sold the majority of dual-fuel propulsion engines.

The exact number changes regularly as new orders are received but as at June 2017, it would appear Wärtsilä has sold or contracted for more than 860 dual-fuel engines for merchant ship propulsion, the vast majority of which – 684 engines – are variants of the 50DF model mostly installed in LNG carriers. As of yet no contracts for the dual-fuel versions of the Wärtsilä 31 have been announced although some negotiations are apparently in progress. When Wärtsilä first began promoting dual-fuel vessels for LNG carriers, MAN responded by offering conventional diesels suggesting that the boil-off gas from LNG cargoes was too valuable to be used for fuel.

Table 6. Wärtsilä dual-fuel propulsion engines contracted.

As the economics and value of different fuels changed so MAN’s strategy changed. Rather than concentrate on four-strokes, MAN has played to its strength and is the undisputed leader in dual-fuel two-strokes although it does have four-stroke dual-fuel offerings and has sold several for propulsion engines in LNG carriers as well as for gensets in vessels with two-stroke dual-fuel propulsion engines. The two-stroke engines in MAN’s portfolio are identified by four different suffixes to the engine designation. GI engines are intended for gas fuels particularly methane, GIE engines for ethane, LGI engines are designed for liquid gas fuels with LGIM indicating methanol and LGIP indicating LPG such as propane or butane. Before Wärtsilä transferred its two-stroke business to WinGD it was developing two-stroke dual-fuel engines. As its successor in the market, WinGD has continued the development and has contracted for 18 engines. Before Wärtsilä transferred its two-stroke business to WinGD it was developing two-stroke dual-fuel engines. As its successor in the market, WinGD has continued the development and has contracted for 18 engines in total.

Table 7. MAN dual-fuel propulsion engines contracted

The MAN and WinGD two-strokes employ different technologies and understandable each maker claims certain benefits for its system. MAN dual-fuel two-strokes operate according to the high-pressure Diesel principle while the WinGD engines employ the low-pressure Otto cycle. The higher temperatures of the Diesel cycle means more NOx formation but the lower pressure of the Otto cycle can lead to methane slip where unburned fuel passes out in the exhaust. Methane has a higher greenhouse gas potential than CO2 and so is considered undesirable. The Diesel cycle is considered more energy efficient but the higher pressures employed mean more costly and complex fuel systems.
WinGD’s portfolio comprises several engine sizes although half of these have yet to achieve initial orders. The smallest is the RT-flex50DF ranging in output from 4.8MW to 11.5MW depending on cylinder count. The other engines in the range are the successor to the RT-flex50DF the W-X52DF and the larger W-X62DF, W-X72DF, W-X82DF and W-X92DF types.