Two-Stroke Engines on ships

Two-stroke engines on ships could be considered as the main drivers of world trade as they are fitted as standard to all the largest cargo vessel types including LNG carriers (although not exclusively in that sector where steam turbines are still employed).

The dominance of the MAN B&W brand is quite clear from the table below and while the vast majority of the engines were conventional diesel engines, the figures do include the variants such as the GI, GIE or GIM models which are dual-fuel types capable of running on methane, ethane or methanol respectively.

Two-strokes are favoured over any other propulsion systems for large vessels for three main reasons; efficiency, reliability and ability to use cheaper HFO. In addition they have a much better power to weight ratio increasing cargo capacity, generally lower maintenance is needed and because they are used in a direct drive situation there is no gearbox to add to cost and weight.

Just under two decades ago in October 1998, the first electronic controlled intelligent engine – a MAN B&W 6L60ME – was installed on the chemical tanker Bow Cecil heralding the gradual demise of the camshaft controlled two-stroke. That process is still continuing but in 2016 less than one in ten of all two-strokes were camshaft models.

The following table shows the breakdown by engine designer rather than licensee. Table 4: Two-Stroke engines on ships delivered in 2016

The development of electronic engine control

As can be expected, development of electronic control has not ceased and improvements to valves and openings are regularly made. MAN 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.

The development of electronic engine control has allowed the two-stroke to meet the challenges posed by the NOx Code but the requirements of phase III which came into effect in 2016 for new vessels operating 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.

The two technologies are Selective Catalytic Reduction (SCR) or Exhaust Gas Recirculation (EGR). The choice of SCR or EGR can vary depending upon engine size and other factors. Both methods require additional equipment to treat the exhaust gases. SCR is not entirely suited to use with two-strokes because the catalytic reaction requires a high temperature to take place. To meet this requirement the SCR reactor should ideally be placed between the engine and the turbocharger thus reducing the turbocharging effect. This configuration is referred to as high pressure SCR.

Integrating the SCR reactor before the turbine allows the reactor to be designed in the most compact way due to the higher density of the exhaust gas. Importantly, even if placed before the turbocharger, the temperature may still be too low if the engine is operating at low-load conditions as it may well be when manoeuvring in an ECA. There are means of overcoming this problem but they reduce efficiency and add to the cost of treatment.

Alternatively, The SCR reactor is put on the low-pressure side, after the turbine. A low-pressure SCR system is typically larger in volume but has the advantage of being less complicated to integrate into the exhaust stream.

EGR is much more suited to NOx reduction on all engine types especially when using low sulphur fuels. It works by way of 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.

There is a problem with EGR in that the levels of sulphur in fuels means the exhaust gas also contains sulphurous compounds which 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. This is achieved by an additional system such as Alfa Laval’s PureNOx developed in association with MAN.

If high sulphur fuel is to be used EGR can also be combined with an exhaust gas scrubber after the main engine exhaust receiver (and most likely installed higher up in the ship or in the funnel) to achieve full MARPOL Annex VI SOx compliance in an ECA.

Meeting EEDI regulations on two-stroke engines

For meeting EEDI purposes, the developments to the two-stroke 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 on ships 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 sub-contractors.

With a larger catalogue of designs, MAN B&W engines delivered in 2016 included models with long, super-long and ultra-long strokes, WinGD has lengthened the stroke of its main designs but does not offer the same choice of its rival.

MAN B&W offerings also demonstrate a wider choice of bores with models ranging from 35cm to 95cm and nine intermediate sizes. WinGD is not far behind offering nine bore sizes in all ranging from 35cm to 92mm. These sizes fall short of previous generations of produced and designed engines which have fallen out of favour as container vessel speed has decreased and the EEDI makes such engines unviable for a variety of reasons.

In practice, 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 on the larger bulkers and tankers. Most engines are five or six-cylinder versions. 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 as on the need to meet EEDI regulations. It should not be imagined that efficiency was never a consideration, engine designers have long boasted the efficiency of their products when comparing with competitors. 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.

The designations given to some manufacturers products do not make following the evolution a simple process but MAN B&W make use of a Mk number increasing for each generation. In 2016, engines of the Mk7, 8 and 9 types were delivered with the majority of shipowners opting for the latest version. In 2016, MAN B&W introduced the Mk 10 engine.

Two-stroke engines on ships – Improvements

Improvements include a complete redesign of the valve systems including the new FBIV (Fuel Booster Injection Valve) and TCEV (Top Controlled Exhaust Valve). This means that this engine does not have a hydraulic cylinder unit (HCU), a base plate and long high-pressure pipes between the actuators and the fuel and exhaust valves. In addition, the engine features a new bedplate design, optimised cylinder frame and a new connecting rod design. The overall benefits mean that the new version of a 90cm bore engine can deliver a 2.3% power increase for a 1g/kW SFOC decrease and a 10% weight reduction.

WinGD’s predecessor began development of a new range of engines designated X types in 2015 shortly before the transfer of business. Different variants have made their debuts since with the latest the WinGD X52 passing its factory and type approval tests in July this year.

Including the test engine, a total of 14 engines are already on order. The major benefits ship owners wish to leverage are the low specific fuel consumption and reduced service costs of the Generation X diesel engines. The reduced fuel consumption results primarily from the longer stroke configuration of WinGD’s Generation X engines, but they also have a relatively light structure and are designed to have low maintenance costs.

The X-prefix engines can also be offered with dual ratings which can be accessed via a minimum of modifications to engine and turbocharger components, enabling ship operators to readily employ a fuel-saving slow steaming mode, according to market and contract conditions.
Of the thirteen X52 engines still on order, all will be six-cylinder 6X52s to be built in Korea. They include both IMO Tier II and Tier III emission compliance, with the Tier III engines featuring both low and high-pressure SCR. Eight of the engines include six 6X52 engine rated 7180 kW at 86.9 rpm and employing high-pressure SCR to achieve IMO Tier III compliance.

The Tier III engines will power a series of six 49,000 DWT petroleum products tankers. The other two engines with the same rated output are Tier II compliant and will be installed in two 50,000 DWT product tankers. Finally, five 6X52s rated 8200 kW at 80 rpm will power a series of five 60,000 DWT open hatch general cargo vessels. These X52 engines will be Tier III compliant thanks to the use of a low-pressure SCR system.

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