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The various types of engines used on ships

Engine overview

Internal combustion engines were first used for ships in 1912 but took more than 50 years to fully replace steam engines. Early motors were not necessarily more efficient than steam but did allow owners to employ far fewer crew, as stokers to carry coal from the bunkers and shovel it into boilers of steam ships were replaced by simple pumps to move oil from tanks to the diesel engine. Thus, there was an immediate saving in crew numbers that will not be repeated if oil is replaced by alternative fuels.

Diesel engines can be either two-stroke or four-stroke with the former being low-speed engines and the latter either medium- or high-speed. Two-strokes are used for direct mechanical propulsion only, but four-strokes can be used either in mechanical propulsion systems – using a gearbox to reduce the engine speed to one more suited to driving a propeller – or in a diesel-electric configuration where the engine drives a generator to produce electricity which is then directed through cables to electric motors to drive the propeller or another type of propulsor. Either type of engine may also have a shaft generator to produce electric power when the engine is running.

Until the mid-1930s, marine diesels were invariably four-stroke and ran on distillate rather than residual fuels. Today, the giant two-strokes with their better power to weight ratio are the engine of choice for most large cargo vessels. High-speed diesels are rarely encountered on commercial ships except as generators or for life- and rescue boats but are regularly used for propulsion in small tugs, work boats and ferries.

Diesel engines can run on many types of oil fuel from heavy residuals through to light distillates but, although it is possible (and frequently necessary) to switch between fuel types, ideally an engine performs best when all parameters are matched to a single fuel type. Because of the polluting effect of engine exhaust when burning oil fuels, there has been a move to persuade owners to switch to burning LNG. This can be done either using a pure gas engine or a dual-fuel engine that is capable of running on either oil or gas fuels.

The imposition of a 0.5% m/m sulphur limit on marine fuels used outside of ECAs in 2020 is changing the driving factors in the choice of fuel type, although most analysts expect that residual fuels will still be the preferred choice of most operators of the largest ship types.

This implies that either sufficient compliant residual fuel will be available or sales of  exhaust gas cleaning systems will accelerate, which was certainly the case in the two-year period leading up to the 2020 deadline.

The development of dual-fuel engines is relatively recent but, although most major engine manufacturers now have models in their portfolios, they are still not fully accepted by the majority of shipowners and operators despite being heavily promoted. There has been a gradual acceleration in their take-up over time and there is now no major shipping sector where they are not being used.

The Diesel and Otto cycles

Back in the 19th century when internal combustion engines were in the very early stages of development, two men – Nikolaus Otto and Rudolph Diesel – devised different means of initiating combustion of the fuel. Otto’s method was to compress the fuel to a particular volume and to then apply a source of ignition in the form of a spark. Diesel’s method was to continue to compress the fuel until it ignited spontaneously due to the heat produced by the higher compression used.

At similar pressures, the Otto engines are considered more efficient but, because they make use of much higher pressures, in practice Diesel engines are more efficient and consume less fuel.

Modern oil burning engines mainly rely on the Diesel cycle, but dual-fuel engines need an alternative ignition source when operating in gas mode. Wärtsilä dual-fuel engines and those produced by their successor in the two-stroke sector WinGD make use of the lean-burn Otto process in which gas is admitted into the air inlet channels of the individual cylinders during the intake stroke to give a lean, premixed air-gas mixture in the engine combustion chambers and ignition is obtained by injecting a small quantity of diesel oil directly into the combustion chambers as pilot fuel which ignites by compression ignition as in a conventional diesel engine.

By way of contrast, in MAN Energy Solutions’ high-pressure ME-GI DF engines the gas is injected only after the combustion air is compressed, after which it is ignited by the pilot oil injection. In 2019 MAN Energy Solutions announced that it too would be developing low-pressure engines making use of the Otto cycle.

As the 2020 deadline for the introduction of a 0.5% global cap on sulphur in fuels drew ever closer, it had been thought that the number of ships being ordered that are powered by dual-fuel or pure gas engines would dramatically take off. It is probably fair to say that this has not happened, although there has been a steady increase in the number of such ships being ordered.

Diesel engines come in various basic types. Direct mechanical propulsion systems are the most common, while diesel-electric, combined diesel and gas and other hybrid variants are alternative options that maybe encountered. 

Two-stroke diesel engines

Large marine engines are impressive examples of the engineer’s art and have improved in thermal efficiency over time to today’s level which is above 50%. Most wasted energy is in the form of heat, some of which is recovered through the use of heat exchangers to provide hot water and domestic, cargo and fuel heating as necessary. It is possible for more to be recovered through waste heat recovery systems that can power small turbines to produce electricity removing the need to run an auxiliary engine.

The lost energy may seem excessive, but it is on a par with other methods of producing energy from combustion of fuels and a diesel engine is as efficient as current fuel cell technology in practice, even if considerably below the theoretical efficiency of such systems.

Two-strokes are the engines of choice for almost all large ships and, with the exception of a relatively small number of recent newbuildings, run on heavy fuel oil. Those that do not use oil are either of the dual-fuel type or are running on ethane or methanol. Two-strokes are used as prime movers only and ships equipped with them will also have at least one medium- or highspeed engine operating as a genset.

After 2020, when the MARPOL Annex VI global cap on sulphur in fuels was reduced to 0.5%, only ships equipped with SOx scrubbers are permitted to run on any fuel that does not meet the requirements. It is in fact be an offence for ships that do not have a scrubber to have non-compliant fuel on board other than as cargo except when compliant fuel is unavailable.

After 2020 most ships will have to run on distillate fuel or one of the new fuel types developed that retain most of the properties of HFO but without the typical sulphur levels associated with it.

Two-strokes are the operators’ engine of choice because, at the engine’s optimum point, their speed is most commonly around 90-110rpm which by coincidence is the preferred speed for propellers, so direct connection without a gearbox is the usual configuration.

The choice of engine model will depend upon the chosen power output selected by the owner and with so many overlapping options available, several factors will come into play including experience with different engine types and the benefit of similar engines across a fleet. The latter point ensures crew experience and the advantage of a reduced spare part stock.

The power available for the largest engines of the type can be astonishing: more than 6,000kW per cylinder. However, the race for power of the early 2000s has given way to a more conservative approach and many of these most powerful engines look destined to be supplied in de-rated versions.

Three names dominate this sector for main engines: MAN Energy Solutions, Winterthur Gas & Diesel (WinGD) – which has acquired the two-stroke business formerly owned by Wärtsilä – and Japan Engine Corporation (J-ENG), which marks another change of ownership after Mitsubishi Heavy Industries spun off its engine business which integrated with the former licensee Kobe Diesel. MAN Energy Solutions has an almost 90% market share, WinGD around 9% and J-ENG the small remaining balance.

There are two variants of a two-stroke diesel engine: the trunk type or the crosshead type. Trunk engines have a shorter stroke than a crosshead and have the piston connected to the crankshaft by a simple connecting rod. They use a common lubricating oil for all aspects of the engine and the oil splashes up to lubricate the liner. Trunk two-strokes are rarely used these days as prime movers.

Crosshead engines have much longer strokes. In these engines, a diaphragm plate separates the crankcase from the cylinder liner space and the piston has a long rod passing through the plate using a stuffing box that separates the upper cylinder lubricant from the system oil. The piston rod is connected at the crosshead to the connecting rod attached to the crankshaft. These are the engines that power the vast majority of bulk carriers, container ships, PCTCs, container ships and general cargo vessels. There are also a significant number of LNG carriers that have two-stroke diesel engines.

Development of two-stroke engines accelerated around 20 years ago when it became clear that the IMO would limit NOx output from engines. It was also a period of rising fuel prices when shipowners began demanding more efficient engines.

Camshaft-less engine development

For most of the history of the internal combustion engine, mechanical control of inlet and exhaust valves has been by way of a camshaft. In October 1998, the first electronically-controlled intelligent engine – a MAN B&W 6L60ME – was installed heralding the gradual demise of the camshaft-controlled two-stroke. That process is still continuing and now less than one in ten of all two-strokes are camshaft models.

The electronic control that replaced the camshaft allows for more flexibility in valve timing, permitting improved flexibility in power output and reduced environmental impact. As can be expected, development of electronic control has not ceased and improvements to valves and openings are regularly made.

MAN Energy Solutions has two main types of electronic control in regular use. On the newer ME-C engines, the electronic control includes flexible control of fuel injection timing and actuation of exhaust valves, starting valves and cylinder lubrication whereas on earlier type ME-B engines, which are still favoured by some owners and remain in production, the injection timing is electronically-controlled but actuation of the exhaust valves is camshaft-operated, but with electronically-controlled variable closing timing.

Simultaneous developments were also taking place in the fuel injection systems with individual cylinder injector pumps giving way to common rail systems where a single high pressure pump and common feed were employed. On some large multi-cylinder engines, a segmented common rail system is employed rather than a single unit to eliminate stresses in materials and pressure fluctuations.

NOx control measures in two-strokes

The development of electronic engine control has allowed the two-stroke to meet the challenges posed by the NOx Code. However, the requirements of Phase III which came into effect in 2016 for new vessels operating in ECAs mean other measures are needed as well. Unlike with four-stroke engines, Miller timing is not possible on two strokes so although the electronic control can allow the Tier II requirements to be met quite easily by way of variable exhaust valve closing, two other means are employed on new vessels either alone or in combination depending upon the engine and the operating parameters need to meet the trading strategy of the ship.

Of the other means, Selective Catalytic Reduction (SCR) is a form of exhaust gas cleaning and is carried out after the engine. It can be used on both two-stroke and four-stroke engines. In this system, the exhaust gas is directed through a catalytic reactor unit usually with a vanadium catalyst where it meets an injected stream of 40% urea solution. The injection of the reductant can be done in two ways; either airless or air-assisted. High-speed engines usually have airless systems while low- to medium-speed engines use air-assisted injection to dose the exhaust stream. The resultant reaction reduces the NOx in the exhaust gas to nitrogen and water.

Applying SCR to two-stroke engines has presented several engineering problems because under normal conditions, the exhaust gas temperature after the turbocharger would be in the 230-260°C range, dependent on load and ambient conditions. These low temperatures are problematic for the SCR when HFO is employed. Thus, in order to achieve the highest possible fuel flexibility, it has been necessary to ensure that the engine produces an exhaust gas with the right temperature for the SCR system. The SCR inlet gas temperature should ideally be around 330-350°C when the engine is operated on HFO.

The alternative of Exhaust Gas Recirculation (EGR) is much more suited to NOx reduction on all engine types, especially when using low-sulphur fuels. It works by recirculating around 40% of the exhaust gas into the engine thus reducing both the temperature and the amount of oxygen in the combustion chamber. Since NOx forms when fuel is burned at high temperatures in air, the system reduces NOx formation. With an EGR system in place, there is no need for catalytic reduction.

EGR has been used very successfully in motor vehicles for some time but for two-stroke marine engines it is a relatively new technology and one that is still being developed. One of the challenges for this technology is the positive scavenging differential pressure. For this reason, an EGR blower is necessary to realise exhaust gas recirculation.

The purpose of the blower is to raise the pressure of the cooled and cleaned exhaust gases so that recirculation through the engine inlet is possible. In this way, a reduction of combustion-temperature peaks – and a subsequent reduction in NOx formation – can be achieved. The required EGR flow varies, depending on load and ambient conditions.

MAN Energy Solutions has developed an electrical turbo blower (ETB) that plays an important role in the operation of the EGR system by providing active speed-control. It is derived from the company’s turbocharger portfolio. The desired EGR operating condition is achieved by using an electrical, high-speed motor directly coupled to the compressor wheel and driven via a frequency converter. A casing unit holds the stator of the motor and provides a supply for cooling water and lube oil for the journal bearings.

The interface between the ETB, frequency drive, instruments and control panel in the engine control room is hardwired. Since May 2015, two ETB18 prototypes have run successfully on an 82,000dwt bulk carrier equipped with an MAN B&W 6S60ME-C 8.2 Tier III engine. The first fully-commercial version of an ETB40 passed its factory acceptance test in October 2018.

Although EGR can allow two-stroke engines to meet Tier III NOx requirements, there is a problem with it in that the levels of sulphur in fuels means the exhaust gas also contains sulphurous compounds that are corrosive to the engine. To overcome this, the exhaust gas being recirculated passes through a scrubber system to remove some of the corrosive compounds. The wash water from the EGR scrubber needs to be treated before it can be disposed of.

At MEPC 73 in October 2018, IMO adopted new guidelines for EGR bleed-off water previously agreed at PPR 5. These guidelines are contained in MEPC.307(73) and apply to a marine diesel engine fitted with an EGR device having a bleed-off water discharge arrangement, for which the EIAPP Certificate was first issued on or after 1 June 2019. The whole question of discharges of exhaust treatment systems is a topic for debate because of the use of scrubbers for cleaning SOx from exhausts and could be subject to future changes.

Efficiency improvements for two-strokes instigated due to EEDI rules

For meeting EEDI purposes, the developments to the two-stroke crosshead engine in recent years have centred around increasing the stroke, reducing rpm and matching the engine to a larger propeller designed to match the operating profile of the vessel. For many years, two-stroke engines were generally available in two variants – short or long stroke – the difference being self-explanatory. To meet the greater efficiency requirements demanded by EEDI regulations, most engine designers agree that a longer stroke, which increases compression, is advantageous. The designers and manufacturers have responded by designing super- and ultra- long stroke variants of their engines.

As these have been adopted by customers, the short and long stroke variants are gradually disappearing from engine catalogues although they are still available if required. The new longer-stroke engines do raise some issues: engines are necessarily taller and manufacturing suitable crankshafts requires re-tooling by makers and subcontractors.

With a larger catalogue of designs, MAN Energy Solutions engines delivered since 2016 have included models with long, super-long and ultra-long strokes while WinGD has lengthened the stroke of its main designs but does not offer the same choice of its rival.

Two-stroke crosshead engines are produced in a range of bore sizes from 35cm to 95cm. The smaller 35-45cm bore sizes are found on Handy and Handymax bulkers and similar size product tankers, the 50-60cm bores on Panamax size bulkers and tankers and the 70-80cm versions on larger bulkers and tankers. Most engines have five or six cylinders while the largest 90-95cm engines are found in 10- and 11-cylinder versions on the largest container ship types where more speed is still considered a desirable characteristic.

Today the focus of development is less on absolute power and more on the need to meet EEDI rules. Different generations of engines appear from time to time with the improvements almost always aimed at improving efficiency as well as ease of maintenance and reducing complexity and weight.

Four-stroke diesel engines

In recent years it would seem that most technology advances have taken place on two-stroke engines. In part this is due to the fact that marine two-strokes are very specialised engines with few if any counterparts in other industries and rapid change has been needed to ensure that the most popular engine type for medium to large vessels can remain viable in an increasingly regulated environment.

With at least 25 different manufacturers plus a number of licensees, the four-stroke market is much more competitive than the two-stroke market. That could make survival for some makers difficult, but four-stroke engines are also used much more extensively for non-marine applications. The same engine that is used in a marinised version on a ship or smaller vessel might be found in a power station, for powering a train or industrial plant, on a truck or bus and many more applications beside.

That diversity helps in engine development and it is often in the four-stroke sector that innovations such as electronic valve timing, common rail, Miller timing and variable and two-stage turbocharging take place. Usually those developments migrate into the marine sector from land-based uses.

In terms of vessel numbers, four-stroke engines are much more prominent than two-strokes, but the ships concerned are, with a few notable exceptions such as multi-engined cruise ships, much smaller and include craft such as tugs, workboats and similar.

Four-strokes are higher speed engines encompassing both the medium- and highspeed types and as such are not suited to being directly connected to the propeller so require a gearbox, thus complicating the power transmission. Alternatively, the engines can be linked to generators providing their power through electric rather than mechanical means.

They are frequently installed in multiples on a ship in both configurations although in many ships, a single four-stroke will be the main engine. Because of the more situations that four-strokes are employed in, the number of the various models produced is generally higher than for two-strokes. This can mean faster development and series production of the engines. Four-strokes also account for the majority of dual-fuel engine types having been the first types to offer this additional flexibility, although this numerical domination is slowly disappearing as dual-fuel two-strokes become more popular.

Medium-speed four-strokes are able to run on all oil fuel types from HFO to MGO. In smaller ships, the use of HFO is less common, mostly because of the need for separate tanks for different fuel types and for the extra fuel treatment needed for HFO. Adding LNG as a fuel type obviously requires even more additional equipment for fuel storage and handling.

For meeting the most stringent Tier III NOx requirements, four-strokes can employ Miller timing, SCR or EGR, which are described in greater detail in the section on two-stroke engines.

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 systems that store excess power in batteries for use when power demand increases. It is the latter type that are now most commonly being referred to when the term ‘hybrid’ is used. Power take-in and power take-out systems are most often built around a medium-speed diesel.

Generally speaking, it is accepted that four-strokes as a type are marginally less efficient than two-strokes. They also tend to be squarer with bore and stroke much closer in dimension than the two-strokes where long and ultra-long strokes are the norm.

Four-strokes for propulsion purposes come in many sizes with the larger bore sizes from 400mm and up generally revving slower at up to 500rpm than the smaller sizes, which have speeds between 1,600rpm and 3,000rpm. Between the two are the intermediate bore sizes such as Caterpillar’s very popular 3500 series models with bores of 170mm and through to the 320mm-plus bore engines produced by the likes of MaK, Wärtsilä, MAN Energy Solutions and Rolls-Royce among others. Although under the same ownership, there is some overlap in the Caterpillar and MaK lines with the C280’s 280mm bore being larger than MaK’s smallest M20 engine.

The high revolutions of four-stroke engines mean they are too fast for direct mechanical drive and so will require a gearbox for mechanical drive or must be connected to a generator in a diesel-electric set-up. Although the large-bore four-strokes may come in six-cylinder in-line variants that mirror typical two-stroke configurations, most have many more cylinders and vee models to give more power output but with only a small additional length. For example, the MAN 48/60CR engine range has four in-line models with the smallest being a six-cylinder version producing 7,200kW and the largest a nine-cylinder version producing 10,800kW. The same range has 12-, 14-, 16- and 18-cylinder vee variants with power outputs of 14,400kW through to 21,600kW. The length difference between the in-line and their respective vee versions is in the region of 1.6 and 2.0m.

Multiple engine systems

With the notable exception of a small number of twin-propeller tankers and container vessels, most two-stroke engines are installed as the sole prime mover on the vessel. By contrast, on ships with highly variable power demands, such as a cruise ship or offshore vessel, it would be quite common to find four, six or occasionally more four-stroke 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.

A multiple engine arrangement also means that failure of a single engine will rarely have disastrous consequences. 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.

Diesel electric systems

Unlike the mechanical power that is delivered directly to a propeller or through a gearbox, electric power produced by four-stroke gensets needs to be managed to allow for safe and efficient distribution to all the consuming systems. Opting for a diesel-electric propulsion system does mean that electric motors must be used to power any propulsors.

This has advantages and disadvantages. Unlike the mechanical drive systems, the cables from the electric distribution system to the electric motors can be routed in any direction without any problems and with far less space needed than for mechanical drives. This can allow engines and motors to be isolated from each other and permit power to be generated from any genset on the ship. The most obvious disadvantages are that electric motors are not cheap and when installed they are an additional point of potential failure.

Recent developments in electric distribution have focussed on the benefits of direct current (DC) rather than alternating current (AC) systems.

Another major advantage of diesel-electric is the potential for 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 energy from solar panels, 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.

Latest new models

In such a competitive field, regular releases of new models are essential to keep pace although it can be two or three years following the launch of a new engine before orders begin to be placed. Most of the new engine types announced over the last five years have been models that are available in gas burning versions as well as oil burning. That is not seen as marking the end of oil-powered ships as the sales figures show that the majority of orders are for single fuel oil variants.

Most makers have opted for a modular design for new engines as a means of both reducing production costs and facilitating maintenance. On the 31 series engine, Wärtsilä has taken this to a new level by shifting from single parts to exchange units. For example, the powerpack unit now consists of a connecting rod, piston, cylinder liner and cylinder head with related pipes combined in one single exchange unit.

Standardisation also makes conversion of the engine from one version to another a simple matter of swapping components without the need for machining.

Dual fuel and gas engines

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 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.

Dual-fuel market

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.

 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.

A relative newcomer to the scene is the JV Belgian company BeHydro in which ABC is a partner. This company has developed a new range of hydrogen-fuelled engines based upon ABC’s DZ engines. These new engines announced in 2020 will come in both dual-fuel (with MDO) or pure gas engines.

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.

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