State-of-the-art large two stroke engines

There are only three manufacturers that spring to mind when talking about large two-stroke engines: MAN B&W, Mitsubishi Heavy Industries and Wärtsilä. The most successful company is MAN B&W with a record result of engines delivered and on order. At the beginning of this year the company recorded about 8,000 engines of the MC series on order or delivered with an historical output figure of 100 million kW.

About 20 years ago MAN B&W had modified the engine programme, introducing in 1982 the MC engine family with the first engine built in Japan by licensee Makita as sub-licensee of Mitsui. Since that time engines of this family have consistently been developed to meet market needs, including environmental regulations. Today the output range spans from about 1,000 (series S26MC) to 80,000 kW (series K98MC) with a speed range between 94 and 250rpm. This includes engines with piston diameters from 260 to 980 mm and strokes from 980 to 2400 mm. With about 20 years of further development, the power output could be increased in some cases by more than 40 %.

In 1996 the MC-C family was introduced. Recent market demands led to the decision to increase the output of all engines in series by approximately 5%. This applies also to the corresponding ME-C engine range which has the same basic design but is equipped with electronic control and has no camshaft. The power increase corresponds to a mean effective pressure of 20 bar. With an increase of the maximum pressure to 160 bar, the fuel oil consumption remains unchanged.

Because of the power increase, some design changes had to be introduced. All engines will have a so-called triangular frame box with a lower stress level and a more rigid structure of the cross-head guides. This design will be standard for all up-rated engines in the family.

MAN B&W has taken into account the possibility of most demanding load condition with an engine running continuously at full-speed. This requires strong bedplates and thin-shell bearings in a rigid housing. The company recorded so far no cracks in bedplates or frame box.

The increased heat in the combustion chamber led to the introduction of a modified chamber. All larger engines will be equipped with the so-called OROS combustion chamber offering the benefits of more space for fuel injection and less heat load for the components.

Instead of having three different piston versions, standard equipment will be, in future, the well proven bore cooled piston. For the liners, small engines with the original rating will be equipped with slim liners and larger engines with new rating bore with cooled liners.

Wärtsilä, with its Sulzer 14RT-flex96C, can offer an output in the order of 80 MW and is thus prepared, like MAN B&W, to power the jumbo container vessels. The company says: “The safety factors of all the components of this engine are better or the same as for the 12-cylinder engine.” Since the introduction of the RT-flex 96C engine series at the beginning of 2003, 64 engines have been ordered.

The first 12-cylinder Sulzer RT-flex 96C marine engine has successfully completed its official shop tests in June this year. With a maximum continuous power output of 68,640 kW, it is so far the most powerful engine equipped with common rail technology.

Another new engine in the Wärtsilä portfolio is the ?small? RTA 50 engine, jointly developed with Mitsubishi Heavy Industries. This engine will also be fitted with common rail technology and, shop testing of the RT-Flex 50 will start at the end of this year.

Beyond that there has been a development programme for up-rating the RTA 58T-B and the RTA 84T-D series by 2.6 to 2.8 % to meet market demands. Together with the power increase, some improvements have been carried out to eliminate design shortcomings and to improve manufacturing processes.

The TriboPack, a package of design measures to improve piston running behaviour, introduced 1999 for all RTA engines, is now standard. The task to extend the TBO to three years was reached with stable wear rates of less than 0.04 mm/1000 hours using feed rates for cylinder lubrication between 1.0 and 1.4 g/kWh.

With standard components and lubrication specification the feed rate for cylinder lubrication is down to 1.1 g/kWh. This rate can be reduced even further and, under test conditions, engines were running without any problem with a feed rate of only 0.8 g/kWh.

Some problems with main bearings led to an ?improvement package?. It includes an optimised horizontal bearing clearance, precise centering of the bearing cover with the shell and back relief of the bearing shell to reduce edge pressure and measures to ensure white metal quality and bonding according to specification. Results from shipboard service are very promising and these improvements are now a design standard.

The Mitsubishi UEC low-speed two-stroke engine family was introduced in 1954. Beginning in the early 1980s the development of the UEC-LS II was started and in 1997 the series was completed. It comprises of engines with a bore between 330 and 850 mm and a stroke between 1,050 and 3,150 mm and output spans from 1,230 to 34,740 kW. More than 300 units are in service. In 1998 the new UEC-LSE series was released and, to date, is available in 52, 60 and 68 cm bore with others to follow. Compared to the LS II engines, LSE engines with the same output run at a higher speed and feature a higher MEP. They are thus more attractive for the propulsion of large fast vessels, offering a power range between 3,700 and 23,520 kW at speeds between 81 and 127 rpm.

Mitsubishi claims that the complete UEC engine family has the lowest fuel oil consumption among low-speed marine diesel engines. To decrease the lube oil consumption, the company adapted the Danish SIP system for the UEC engines. This system features a high-pressure injection to spray the lube oil into the cylinder and ensures a thin and even distribution. Under service conditions the feed rate of cylinder lube oil can be reduced to 0.8 g/kWh. In spite of the very low feed rate, the wear rate of piston rings and cylinder liner is low as before.

Although the existing range of Mitsubishi engines meet the current IMO regulations on emissions, the company has decided to develop an electronically controlled engine, the UEC Eco, to cater for more stringent requirements in the future.