If the low-speed two-stroke engine market includes the most powerful engines available and dominates the large ship sector, the medium speed four-stroke market is probably bigger in terms of engine numbers and is also highly competitive with more makers involved. The makers of low-speed engines are all involved in this sector also and are complemented by a number of European, US and Asian makers.
Medium speed engines may be found both as propulsion engines and also as gensets in diesel-electric ships. They are also increasingly being offered in dual-fuel versions or as pure gas burners. The power output of the genset versions is generally a little less than the mechanical versions because of losses in the attached alternator. As gensets they will also be used as auxiliary engines on vessels with low-speed two-stroke main engines.
Hybrid drive systems
Where the normal choice would once have been mechanical drive or diesel-electric, today many more hybrid drive systems are being used and experimented with. There are for example combined diesel and diesel-electric drives, combined diesel and gas turbine drives, permanent magnet drives and also systems that store excess power in batteries for use when power demand increases. Power take in and power take out systems are most often built around a medium speed diesel.
The medium-speed sector features a much wider choice in terms of basic engines configurations from each maker than the two-stroke sector. The use of licensees is less common for medium-speed engines especially among the smaller producers who tend to keep production within their own hands.
Modular design and production has reached a peak in this sector. Most engines come in a choice of in-line or vee versions, the latter being used where the power required is high but the length of available engine room space low. The size of the bore in these engines varies usually between 200mm and 600mm but the number of bore sizes offered by each manufacturer is frequently less than for two-strokes because there are far more cylinder number versions available – up to 24 cylinders in some cases.
Where the engine is to be used for mechanical propulsion, the operational speeds of around 500rpm to 1,000rpm for this type of engine are far too high for a propeller so the use of gearboxes becomes essential. This is not the case for the diesel-electric configuration where the final drive comes from an electric motor either inside the vessel or in a podded propulsor outside of the hull.
In small cargo ships it is not unusual to find a single medium-speed engine as the vessel’s main engine but the majority of ships employing this type of engine do have multiple engine set ups. On ships with highly variable power demands such as a cruise ship or offshore vessel for example, it would be quite common to find four, six or occasionally more engines installed. Because all engines have an optimum load at which they operate most efficiently, operating below this will increase fuel consumption. In such cases, the power required will be provided by an appropriate number of engines operating at near to optimum speed with perhaps another engine operating at low load as a spinning reserve.
The power arrangements on multiple-engined vessels will normally mean that engines of different outputs are available. This can be achieved by having engines of the same type but with different cylinder numbers or larger bore engines supplemented by smaller bore types.
The modular design of engines and common parts across a range mean that ships can benefit from carrying lower numbers of spares. Even when vessels are mechanically driven, often an owner will choose to have the same basic engine for propulsion and smaller version of the same type as a genset for electrical power. A multiple engine arrangement also means that failure of a single engine will rarely have disastrous consequences.
Unlike the mechanical power which is delivered directly to a propeller or through a gearbox, electric power produced by gensets needs to be managed to allow for safe and efficient distribution to all the consuming systems. Recent developments in electric distribution have focussed on the benefits of direct current (DC) rather than alternating current (AC) systems. Based on a common DC-distribution inside the system, power generation and consumption is decoupled. Another major advantage is the integration of energy storage sources such as batteries. The energy storage sources reduce the transient loads on the diesel engines and give much better system dynamic response times. Also, emission-free propulsion can be realised when running on batteries especially when they are topped up using power from solar, recovered from waste heat onboard or even when charged in port using a local grid. The footprint of such a propulsion plant is up to 30% less compared with a classic diesel-electric propulsion plant.
Battery systems are not small. In most instances a lithium ion battery pack will likely require as much space as a diesel genset of similar output. With a battery pack replacing one of a ship’s many gensets, the need for a spinning reserve generator can be done away with. Batteries also require less maintenance and will continue to supply power even if individual cells fail and need replacing.
Battery technology has been embraced by the offshore industry and by the ferry sector, particularly in Norway where the government is keen to see ferries drawing on power from the shore grid system which is powered by hydroelectricity rather than relying on hydrocarbon fuels.
Tugs are another vessel type that is well represented in the growing number of battery fitted vessels. In fact the first vessel sporting a hybrid battery system was the 2009-built harbour tug Carolyn Dorothy operated by Los-Angeles based Foss Maritime. Its batteries were of the lead acid type but all of the commercial vessels fitted with batteries since have been Lithium ion types.
So far competition to supply batteries has not been fierce. Most of the major projects have used batteries supplied by Corvus Energy founded in 2009 in Richmond, British Columbia. Corvus claims a 95% market share. European Batteries, based in Finland was declared bankrupt in July 2013 after less than four years in business. Gaining ground in the marine sector are ZEM based in Norway which has notched up a number of ferry references and the French company Saft which has supplied systems to some domestic ferries but is now targeting a wider market.
As things stand, battery technology does not warrant its use as the sole power source except where voyage times are very short and measured in minutes or hours rather than days. However, except in a very few cases this has not been a design aim and their use is more as a power storage solution and brief periods of use to meet sudden high demand situations.
Medium speed diesels can run on a wide range of fuels and although many operators will prefer to use heavy fuel oil on cost grounds, the smaller engines are usually set up for running on MDO. For smaller vessels operating into small ports where bunkering facilities are poor this strategy allows them to take fuel supplied from road tankers. Running on distillates also means that ships operating in ECAs do not have to concern themselves with installing exhaust gas cleaning systems to remove SOx.
Medium speed engines were the platform on which the use of dual-fuel systems was first pioneered. The first ships to be equipped with dual fuel engines were the LNG carriers Gaz de France Energy and Provalys which were equipped with Wärtsilä 50DF type engines in 2006.
The idea of equipping LNG carriers – traditionally powered by steam turbines burning boil off gas from the cargo – with dual-fuel or diesel engines was initially to preserve as much of the valuable cargo as possible. Within just a few years the main selling point of the dual-fuel versions was as a means of meeting NOx regulations and other environmental aspirations.
Most medium-speed engine makers feature at least one dual-fuel engine in their portfolio. Aside from Wärtsilä; MAN Diesel & Turbo, Caterpillar (MaK), Cummins, ABC, Yanmar and Himsen all have – or will soon have – a dual-fuel engine but 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.
Most LNG powered vessels other than the LNG carriers have been domestic ferries operating in Norwegian waters where the NOx tax and NOx fund favours this type of engine. More recently some offshore vessels and cargo ships – notably the first ro-ro/container variants of Rolls-Royce’s Environship offerings and the converted tanker Bit Viking have been added.
In the larger vessel sphere, there are the ferries Viking Grace, Stavangerfjord and its sister Bergensfjord. The first being fitted with dual-fuel engines and the latter two with pure gas burning Rolls-Royce Bergen engines. In addition one of the four engines on each of the cruise ships which Mitsubishi Heavy Industries is building for Carnival Group subsidiary AIDA Cruises is a dual-fuel MaK engine. In the two-stroke arena the most notable is perhaps the feeder container ships for US operator TOTE. The number and type of vessels installing engines able to run on LNG and even LPG as alternatives to HFO is definitely increasing and is likely to accelerate further as bunkering infrastructure projects come to fruition.
Currently only LNG carriers with dual-fuel engines can make long international voyages because they are carrying the fuel onboard as cargo. Other ship types can be fitted with dual-fuel engines but operators have explained their scepticism by saying that the cargo carrying capacity lost because of the need of two different fuel storage systems, the lower power density of LNG and other gas fuels and lack of experienced crew.
The scepticism is not universal as this year a 50,000dwt bulk carrier powered by a dual-fuel MAN B&W 6G50ME-C9.5-GI engine has been ordered by a Korean shipowner. In July 2016 a new impetus was given to promoting LNG with the formation of a coalition of partners known as SEA/LNG. Partners in the coalition include Wärtsilä, Carnival Corporation, DNV-GL, ENGIE, ENN Group, GE Marine, GTT, Lloyds Register, Mitsubishi, NYK Line, Port of Rotterdam, Qatargas, Shell Downstream and TOTE.
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 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. Being liquid within the normal temperature range of ship operation transport and bunkering is simpler than LNG but methanol is toxic, corrosive and because of its energy density takes up twice as much space as MDO. It also has a very low flash point which at 11°C is far below the 60°C associated with oil fuels.
The first vessel to make commercial use of methanol is the ferry Stena Germanica on which the first of its four Wärtsilä 8ZAL 40S MD engines was converted in 2015 in Poland to run on methanol. After successful operation of the converted engine the remaining engines will be converted. Stena Line has also announced that it has signed a contract for four new ro-pax ferries to be delivered in 2019 and 2020. The Deltamarin designed ships will be built by AVIC Shipyard in China and will be designated as “gas ready”, to be a fuelled by either methanol or LNG.
A factor holding back full-scale acceptance of dual-fuel and pure gas engines has been the issue of fuel storage on board. LNG tanks need high levels of insulation or refrigeration plant to maintain the extremely low storage temperatures and pose a number of safety-related questions. Until the adoption of the IMO’s IGF Code this year there was no universal regulation of the fuel storage and delivery systems for LNG-fuelled vessels and all installations have needed to be agreed on a case by case basis by flag states.
The conversion option
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.
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.
This will be the first such conversion and it will be taking place on an engine that has been specifically designed for the possibility of conversion to dual-fuel operation. Conversion of ships to LNG is frequently promoted by environmentalists and regulators alike but it is doubtful if many of them have a full understanding of what is involved. For a start, any ship that is converted may have to comply with new regulations. As an example, *Wes Amelie* was originally built in 2011 and therefore its engine would not have been subject to the Tier III requirements of the NOx Code.
With the retrofit due to take place next year, the engine will be subject to the new requirements if operating in a NOx emission control area. That is looking probable as the ship’s normal sphere of operation is the Baltic and North Seas where the emission control only currently applies to SOx but which are slated for becoming NOx control areas in the near future.
Almost the only major components of the original engine that will be re-used are 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 need to be added. The combustion chambers and cylinder heads need to be replaced because of the additional fuel feed. The pilot oil system necessary for gas operation will be completely rebuilt. To allow for the changed ignition timing with a 51/60DF engine, new valve cams and a new turbocharger rotor assembly are required. Controlling the multi-fuel engine is more complex than the original running on heavy fuel oil making conversion of the engine sensors and new instrumentation necessary. This permits switching between fuels automatically is the supply of fuel is interrupted without any interruption in the engine loading.
It is not planned to reduce the present volume of tank space for diesel fuel and a C-type gas tank is being located in the forward part of the vessel under deck allowing containers still to be loaded on deck above. The loss of container space was well within limits that the owner considered acceptable. Aside from fuel lines, the remainder of the LNG fuel system comprises a bunkering point located on the port side of the vessel a little aft of the fuel tank, and a control station also on the port side adjacent to the main engine.
The reduction in overall power when converted and running in LNG mode is an almost 14% loss of power. In order to ensure that this would not be a deal breaker for the ship and to ensure it could continue to operate the same routes with the same speed, the operating profile of the Wes Amelie was recorded over a period of time. From analysis of the data, it transpired that the ship rarely if ever operated at or near full power and could even be considered a little over-powered.
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 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. It is available for applications in mechanical drive installations, as a genset, for hybrid installations, as well as heavy duty installations or as an auxiliary engine and is designed to serve a variety of vessel types requiring main engine propulsion in the 4.2 to 9.8 MW power range.