Dual-fuel and gas engines
Updated 22 Oct 2019
LNG-burning engines have been used for onshore power generation for many years but their use for marine purposes is a more recent phenomenon. Initially, they were marketed almost solely as an alternative to the steam turbines in LNG carriers then later as a solution to meeting increasingly-stringent exhaust emission requirements.
Of the three options for meeting the 2020 sulphur rules, LNG would seem to be ideal as it contains no sulphur and thus engines running on it cannot produce SOx. Proponents of LNG have been forecasting its role as the fuel of the future for most of the 21st century but the lack of international standards and rules has been an impediment to a greater take-up, although that is now changing.
In spite of its attractions on environmental grounds, LNG has had a slower take-up than its supporters expected. There are many reasons for this including lack of bunkering infrastructure, higher capital outlays, LNG’s lower energy density compared to oil fuels and a lack of international regulation as to the use of gas as a fuel.
Those disadvantages are gradually being addressed and while the second two will remain an issue for shipowners to decide on merits, the first is underway and the fourth has been resolved by the IMO which in 2015 adopted the International Code of Safety for Ships using Gases or other Low-flashpoint Fuels (IGF Code), along with amendments to make the code mandatory under SOLAS with effect from 1 January 2017.
The impending introduction of the 2020 global sulphur cap has seen a growing acceptance of LNG as a marine fuel and the number and type of ships employing dual-fuel engines has advanced. Today there are dual-fuelled engines in the largest container carriers, bulkers, tankers and cruise ships as well as the smaller vessel types that were early adopters.
Because gas and other low-flashpoint fuels pose their own set of safety challenges and prior to the IGF Code their regulation was only possible by individual flag states, there had been no universal standard. The IGF Code addresses this and has led to more gas and dual-fuel ships being built.
The amendments to SOLAS chapter II-1 as a result of the IGF Code include changes to Part F ‘Alternative design and arrangements.’ These provide a methodology for alternative design and arrangements for machinery, electrical installations and low-flashpoint fuel storage and distribution systems while a new Part G ‘Ships using low-flashpoint fuels’, adds new regulations to require ships constructed after 1 January 2017 to comply with the requirements of the code, together with related amendments to chapter II-2 and Appendix (Certificates).
The code contains mandatory provisions for the arrangement, installation, control and monitoring of machinery, equipment and systems using low-flashpoint fuels, focusing initially on LNG with the intention to expand the provisions as new alternative fuels gain acceptance. It addresses all areas that need special consideration for the usage of low-flashpoint fuels, taking a goal-based approach, with goals and functional requirements specified for each section forming the basis for the design, construction and operation of ships using this type of fuel.
The MSC has also adopted related amendments to the STCW Code, to include new mandatory minimum requirements for the training and qualifications of masters, officers, ratings and other personnel on ships subject to the IGF Code. These amendments also entered into force on 1 January 2017, in line with the SOLAS amendments related to the IGF Code.
Wärtsilä had been developing dual-fuel engines for shore-based use since the late 1980s and was 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 and 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 spark-ignited gas versions of its Bergen Diesel engines. Regardless of maker, all gas-fuelled engines were medium-speed variants. That has changed and now there are dual-fuel low-speed two-stroke engines produced by MAN Energy Solutions and by Wärtsilä’s successor in the two-stroke sector, Winterthur Gas & Diesel, better known as WinGD.
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 as is one variant of the new Wärtsilä 31 series.
The four-stroke engines are being installed in many vessel types. Many of the engines are being installed in vessels that are ‘dual-fuel ready’ meaning they have the engines but not necessarily an LNG fuel system, which will be added later if the operating profile permits.
Dual-fuel engine types and development
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 engines that run on oil fuels (LFO and HFO) and can switch from gas to oil and vice versa smoothly during engine operation. The Wärtsilä dual-fuel engines are available in power range from 0.9-18.3MW having speed range from 500-1,200rpm. The Type 31 engine is also available in a spark-ignited pure gas variant that will not run on oil fuels.
MAN Energy Solutions was a later entrant to the dual-fuel market. Rather than concentrate on four-strokes, it 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 and for gensets in vessels with two-stroke dual-fuel propulsion engines.
The two-stroke engines in MAN Energy Solutions’ 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.
Currently, all MAN Energy Solutions 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.
In 2019, MAN Energy Solutions announced that it had initiated the development of a low-pressure gas engine as a supplement to its existing, successful dual-fuel ME-GI engine. The company cites market demand as the rationale behind the move and it aims to complete the development of the new engine during the first half of 2022. It sees the introduction of the low-pressure engine in terms of filling a gap within its portfolio, much akin to how its ME-LGIP engine – introduced in 2018 – added LPG to its roster, further broadening the list of liquid fuels its ME-LGI (Liquid Gas Injection) engine can exploit.
MaK is also a strong player in the dual-fuel sector and has contracted for at least several 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.
VOCs as a fuel complement
In any typical fuel system for oil-fuelled engines, the fuel is stored in bunker tanks on board the ship. The same is true for LNG fuel supplies except on LNG carriers where the fuel comes from the boil-off from cargo tanks. Ethane carriers with a dual-fuel engine adapted to run on ethane are similarly equipped.
In 2018, a project involving WinGD, Wärtsilä Gas Systems and shuttle tanker operator AET developed a system that makes use of a new source of fuel available to tankers. Most oil cargoes emit volatile organic compounds (VOCs) during a voyage and for safety purposes these must either be vented to the atmosphere or recirculated into the cargo.
In the project – which involves a WinGD X-DF engine – instead of returning or venting the VOCs from crude oil they were diverted to a holding tank and then injected into the natural gas supply to the engine. The engine was able to run normally with up to 20% VOCs in the fuel mix, which reduces LNG fuel consumption by a comparable amount.
The engine used in the tests was also running on fuel oil and the transitions between running on gas, gas/VOC and oil were all achieved easily and without problems. No changes were made to the engine’s normal operating parameters and there was no significant increase in NOx emissions. As a consequence of the tests, AET has ordered two vessels to make use of the new concept. Another tanker operator, Teekay, has also ordered vessels capable of using VOCs as fuel.
LNG and other gas fuel developments
In July 2016, a new impetus was given to promoting LNG with the formation of a coalition of partners known as SEA\LNG. The aim of the group is to accelerate the widespread adoption of LNG as a marine fuel and to break down the barriers hindering the global development of LNG in marine applications. The main areas of focus for the coalition include supporting the development of LNG bunkering in major ports, educating stakeholders as to the risks and opportunities in the use of LNG fuel and developing globally consistent regulations for cleaner shipping 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 Energy Solutions has secured an order for engines for eight ethane carriers belonging to German shipowner Hartmann Reederei. Their 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.
Engine conversion for LNG operation
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 was specifically developed for the retrofit of 32/44CRT2 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 remains 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.
Having shown the possibility of conversion, MAN Energy Solutions has followed through and contracted with German shipowner Wessels Reederei to convert the 8L48/60B main engine of the 1,000teu feeder container ship Wes Amelie to dual-fuel operation as an 8L51/60DF. This first conversion was completed in 2017 and others are now in progress.
Among the few 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 and 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.
A C-type gas tank was located in the forward part of the vessel under deck allowing containers still to be loaded on deck above. The reduction in overall power when converted and running in LNG mode is an almost 14% loss of power but this has no effect on the operation of the vessel which rarely needed all of the initial installed power.
After the successful conversion project was initiated, Germany’s Federal Ministry for Transport and Digital Infrastructure (BMVI) began promoting the upgrading and conversion of seagoing vessels to LNG. The support is made available from the mobility and fuel strategy (MKS) fund.
Other conversions that have been announced are a containership belonging to Hapag Lloyd and two ro-pax ferries owned by Spanish operator Baleària Eurolineas Maritimas. In the latter project, the sister ships Nápoles and Sicilia – each built with two MAN 9L48/60A main engines – will have the engines converted to 9L51/60DF units.
A twist on the theme of converting existing engines was made by the new Wärtsilä 31 engine which was unveiled in June 2015. It comes in three 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.
Both oil burning and dual-fuel versions have been contracted for use in newbuildings.