Other than cruise ships and the majority of LNG carriers, almost every vessel of Handymax size and above will be powered by a low-speed two-stroke engine normally linked directly to a fixed-pitch propeller. That means that more than 80% of commercial ships built are powered by this type of engine. The rpm of the engine at its optimum point is most commonly around 90-110rpm.
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 at 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.
MAN Diesel & Turbo
The acknowledged market leader in this sector with around 80% of the market MAN Diesel & Turbo has the biggest range of engines of this type with 12 different bore sizes between 30mm and 98mm. Most engines have a choice of stroke length with short (K) and long (L) being available across most of the range and super long stroke (S) and ultra-long stroke (G) being available on some – notably the larger sizes.
The longest stroke, coupled with large diameter propellers and engine speeds below those that have been considered normal until recently, are seen as being one of means of ensuring more efficient operation called for by the advent of EEDI.
At the lower end and middle of the bore sizes, engines are available with between five and eight or nine cylinders. Above 80mm bore the maximum number of cylinders can be 12 or 14 depending on engine type.
Low-speed two-strokes are commonly run using heavy fuel oils which are the cheapest marine fuels available. At the height of the market in 2013/14, prices for IFO 380 reached as much as $600 per tonne. This has since tumbled along with crude oil prices and today the average cost is less than half what it was. The drop in fuel prices may reduce the urgency of finding efficiencies temporarily but the demands of regulation will not allow a long pause in development.
Most of MAN’s engines can also be supplied in either mechanical or electronic variants and are based on the ubiquitous MC type. The electronic control of ME/ME-C/-GI engines includes the combustion process, i.e. fuel injection timing, actuation of exhaust valves and starting valves, and cylinder lubrication. On ME-B/-GI engines, the combustion process is electronically controlled while the actuation of exhaust valves and starting valves is hydraulically or mechanically controlled.
The GI suffix indicating gas injection is relatively new having been introduced in 2011. This series of engines are the first to bring a dual-fuel option to two-strokes and have found their first reference in the award-winning TOTE container vessels constructed in the US.
In July, 2013, MAN Diesel & Turbo announced the development of a new ME-LGI dual fuel engine. The engine expands the company’s dual-fuel portfolio, enabling the use of more sustainable fuels such as methanol and Liquefied Petroleum Gas (LPG). The ME-LGI engine was developed in response to interest from the shipping world in operating on alternatives to heavy fuel oil. The engine is targeted at the growing fleet of LPG carriers since, as with LNG carriers, exploiting a fraction of the cargo to power a vessel makes sense.
At SMM in 2016 MAN announced a significant step forward in two-stroke engine design with its new type 10 platform. A new engine designated G90ME-C10 has been added to the company’s large-bore engine programme. It is weight-optimised, compared to its predecessor. A new S70ME-C10, which is currently under consideration, will be based on this platform. The G90 will be based on a much more mass-optimised design platform that results in a lighter engine with a specific mass of 26.5 kg/kW for the G90 compared to 29 kg/kW for the S90ME-C10. The overall length, width and height have also been reduced compared to the S90ME-C9/10.
As the first engine in the engine programme, the G90 has been designed from the outset with a new valve system featuring 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. The purpose of the TCEV is to integrate the exhaust actuator, the hydraulic push rod and the HCU block into the exhaust valve. By doing so, the dynamic behaviour is improved (no long hydraulic push rod).
The FBIV and the TCEV technologies are well-suited for integration on the cylinder cover of the engine. Similar to the G95, the control of the valves for fuel injection is separated from the control of the exhaust valve, however, now by using the well-known electronic fuel injection valve (ELFI) and the new proportional exhaust valve actuator (PEVA).
Integration of FBIV and TCEV will lead to a considerable weight reduction, because the baseplate, HCU, pressure booster, high-pressure fuel oil pipes and exhaust actuator can be eliminated. For a G95, the weight reduction going from the traditional HCU setup to the TCEV/FBIV concept is about 2 tonnes per cylinder equating to around 28 tonnes on a large container ship. More details of the new arrangement can be found in the ShipInsight Knowledge Base.
Win GD as the second place designer and builder in this sector has an equally extensive range of bore sizes and both mechanical RTA engines and electronic RT Flex versions of most sizes. The electronically-controlled common-rail systems for fuel injection and valve actuation at the heart of the camshaftless RT-flex series, which made its seagoing debut in September 2001, have been shown to deliver an array of benefits deriving from operating flexibility, whereby engine settings can be far more closely matched than before to actual operating requirements.
In 2011 Wärtsilä launched its new low-speed X-generation engine series employing the common-rail RT-flex technology, and incorporating additional features designed to meet the emerging needs imposed by EEDI. The X series engines feature an extra-long stroke that gives improved fuel economy and operational flexibility. The first two engines in the series were originally introduced as the Wärtsilä RT-flex35 and RT-flex40 but it was decided to change the names. Since then the range has been expanded to include 620mm, 720mm, 820mm and 920mm bore versions. The current range reflects the Wärtsilä heritage but the company has plans to introduce innovations and designs of its own in the future.
Win GD also includes dual-fuel two-strokes in its portfolio and as well as providing fully operational DF versions All Generation X engines can be converted to use LNG as fuel. To meet the potential demand for conversions WinGD has introduced the DF-ready version as an option. The DF-ready engines can be easily converted to dual-fuel, as no major structural components need to be modified. All parts, which are to be replaced at a later conversion, are either typical wear parts or specific X-DF components and systems.
The only other two-stroke engines manufactured in any numbers for the marine sector are the UE engines of Mitsubishi Heavy Industries in Japan. The company has a fairly extensive range with bores ranging from 330mm to 800mm. In 2012, Mitsubishi announced the beginning of a programme aimed at developing a dual-fuel version of the UE engines and in 2015 suggested that an initial order was under discussion for an engine designated the UEC-LSGi but nothing more has been confirmed. As with the two market leaders, Mitsubishi has evolved its range to include super long-stroke engines identifiable by the UE-LSH designation. The first of these was produced in 2015.