Two-Stroke diesel engines on ships
These 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 high-speed engine operating as a genset.
At the engine’s optimum point the 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 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.
Three names dominate this sector for main engines, MAN Diesel & Turbo, Winterthur Gas & Diesel (WinGD) which has acquired the two-stroke business formerly owned by Wärtsilä and Japan Engine Corporation (J-ENG) another change of ownership after Mitsubishi Heavy Industries spun off its engine business which integrated with the former licensee Kobe Diesel.
MAN has an almost 90% market share, WinGD around 9% and J-ENG the small remaining balance.
Crosshead and Trunk differences
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 will use a common lubricating oil for all aspects of the engine and the oil will splash up to lubricate the liner. Trunk two-strokes are rarely used these days as prime movers.
Crosshead engines have much longer strokes and. 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.
Replacing the camshaft
Just under two decades ago in October 1998, the first electronic 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 but in 2016 less than one in ten of all two-strokes were 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 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.
Of the other means, Selective Catalytic Reduction (SCR) is a form of exhaust gas cleaning and is carried out after the engine. 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 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.
Improving two-stroke efficiency
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 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.
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 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 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.
In 2016, MAN B&W introduced the Mk 10 engine which included several new developments such as 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.
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.