What are the engine choices for LNG propulsion?

Malcolm Latarche
Malcolm Latarche

14 February 2018

LNG has been used as a marine fuel for more than 50 years but for most of that time it was as boil off gas from the cargo used to fuel the boilers for steam turbines. Its use in internal combustion engines at sea is more recent and dates to 2001.

It was then that Wärtsilä equipped the FPSO Petrojarl I with a pair of its 18V32DF dual-fuel engines. This was followed by contracts for a series of LNG carriers built in France and two offshore ships.

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.

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. Rolls-Royce is following a different path with its Bergen engines offering them only as oil burning or pure gas engines. Dual-fuel engines ordinarily make use of a pilot ignition system using diesel fuel but the Rolls Royce engines are spark ignited.

A growing range of engines

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.

In June this year, a count of contracted Wärtsilä dual-fuel engines suggested that almost 900 had been installed or contracted for. That figure is soon likely to reach the 1,000 milestone although if all other makes of dual-fuel engines is taken into account that mark has already been exceeded.

MAN diesel & Turbo was a later entrant to the dual-fuel market. 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 variants of its oil burning ME engines and 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.
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. 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.

MaK is also a strong player in the dual-fuel sector and has contracted for at least 15 engines of different variants of its M46 range. MaK has traditionally enjoyed good support from owners in the cruise market. The cruise market has been targeted by MAN Diesel & Turbo for its new 45/60CR.

A twist on the theme of variants engines was made by the new Wärtsilä 31 engine which was unveiled in June 2015. It comes in three alternative versions; Diesel, Dual-Fuel and Spark-Ignited Gas. The multi-fuel capabilities extend the possibilities for operators to burn different qualities of fuels, from very light to very heavy diesel, and a range of different qualities of gas. Its fuel consumption efficiency in its diesel version is as low as 165g/kWh. The first engines have been ordered but these are as yet only the diesel burning version. Four of them will be used in fishing vessels, two in a ropax ferry being built for Mols Linien and three for Russian icebreakers.

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.

The decision to build dual-fuel ready ships is mostly as a precaution for the impending 2020 global cap on sulphur levels permitted in fuels under MARPOL Annex VI. The uncertainty as to what fuels will be available and at what price means that in most instances decision will not be made until nearer the time or soon after.

Other gases as fuels

Recently two other fuels have been added to the list of alternatives to oil with successful use of both ethane and methanol. Both fuels have been on the horizon for some time and although their use may be limited to certain vessel types, ensuring the engines run correctly is a vital precursor to their wider adoption.

In May 2015, Wärtsilä announced that its four-stroke 50DF engine has been certified to run on liquid ethane gas fuel after a successful testing programme in collaboration with petrochemical and gas shipping company Evergas. The engines can switch between LNG, ethane, liquid fuel oil and heavy fuel oil with uninterrupted operation.

Just as with LNG carriers, the ability for ethane carriers to burn ethane boil-off gas as engine fuel significantly reduces the need for gas re-liquefaction during the voyage, meaning that less power is needed for the cargo handling.
MAN diesel has secured an order for engines for eight ethane carriers belonging to German shipowner Hartmann Reederei. The G50ME-C9 engines will run on boil-off gas when running in gas mode and can also operate on the full range of fuel oils from HFO to MGO.

Methanol is a fuel that avoids some of the problems associated with LNG and ethane because
It is liquid at ambient temperature and so does not need such specialised fuel storage systems. The issues with methanol are not related to its environmental impact as it is considered as a clean fuel on a par with LNG and unlike fuel oil requires no exhaust treatment to meet MARPOL requirements.

Converting to gas

The advent of dual-fuel engines has raised the possibility of converting some existing diesel engines to the new configuration. The modular aspect of engines aids in this regard allowing newer versions the potential although converting older versions may present more difficulties. At SMM in 2012 MAN Diesel exhibited an engine showing how a conversion could be achieved.

The L35/44 engine on view at SMM was specifically developed for the retrofit of 32/44CR-T2 engines where it can avail of a high level of component synergies and the same crankcase, which could be re-machined on board.

Subsequent engine operation would mainly be intended for gas mode with a separate pilot ignition system that is independent of the primary, common rail injection system. However, the common rail system is retained and fully functional as a back-up system in the event of any problem while operating in gas mode. Similarly, Caterpillar’s MaK M46DF is a development of the M43 C engine which has become a popular choice for cruise ships and which was involved in the first conversion of an engine to dual-fuel on the tanker Fure West in 2015.

Having shown the possibility of conversion, MAN Diesel & Turbo has followed through and contracted with German shipowner Wessels Reederei to convert the 8L48/60B main engine of the 1,000-teu feeder container ship Wes Amelie to dual-fuel operation as an 8L51/60DF.

Almost the only major components of the original engine that were re-used were the main casing and the crankshaft. The increased bore obviously signifies that cylinder jackets, liners pistons and piston rings must all be different and gas injection and fuel lines needed to be added. The combustion chambers and cylinder heads were replaced because of the additional fuel feed. The pilot oil system necessary for gas operation was completely rebuilt. To allow for the changed ignition timing with a 51/60DF engine, new valve cams and a new turbocharger rotor assembly were required. Controlling the multi-fuel engine is more complex than the original running on HFO making conversion of the engine sensors and new instrumentation necessary. This allows switching between fuels automatically if the supply of fuel is interrupted without any interruption in the engine loading.

This first MAN conversion was completed in 2017 and is to be followed by several more on ships belonging to the same owner. Around the same time as the new conversions of the German vessels was announced, MAN Diesel & Turbo also reported that more conversion had also been contracted.

TOTE Maritime Alaska, a daughter company of TOTE Inc. is to convert two of its 2003-built Orca Class ro-ros to dual-fuel operation. The ships were built with four MAN 58/64 engines and will be retrofitted to MAN 58/64 retrofit units. Since the engines were not designed with conversion to LNG in mind as some later engine models have been, the conversion contract covers the design, development and testing of a first-of-its-kind dual-fuel kit.

TOTE Maritime Puerto Rico, another TOTE company, is notable for ordering the world's first LNG-powered container ships, the Isla Bella and Perla del Caribe, launched in 2015 and both featuring single MAN B&W 8L70ME-GI engines.