By Jim Shaw
Marine propulsion, the means of making a ship move, has become an increasingly complex segment of the maritime industry over the years as more power is squeezed from less fuel and new environmental regulations impact acceptable engine exhaust emissions. The size and power output of the largest marine diesels has also continued to increase, with Germany’s MAN Energy Solutions having developed the MAN B&W 14K98ME-C7 engine which can generate 115,000 BHP (84,280 kW) at 104 rpm, making it the most powerful marine engine to date. Like most modern marine diesels, the MAN B&W engine is considered more “environment friendly” than its predecessors in that electronic control of fuel injection and exhaust valves are used to provide lower fuel consumption as well as lower cylinder-oil consumption while improving emission characteristics. Even cleaner is the 12X92DF engine developed by WinGD, a dual-fuel powerplant rated at 63,840 kW that can burn LNG as well as traditional marine fuels. However, the dry weight of these massive engines is between 2,100 tons and 2,200 tons while their fuel requirements are tremendous with a 108,920 BHP 14-cylinder RTA96-C engine burning about 13.75 tons of fuel per hour or 330 tons per 24-hour period at a cost of approximately $375 per ton depending upon bunkering location.
Marine Propulsion Through the Years
The history of mechanical marine propulsion following centuries of reliance on sail is relatively short. In the late 1700s both John Fitch and James Rumsey in the United States came up with designs for rudimentary steam propulsion devices based on James Watt’s steam engine of 1769. Rumsey tested a small craft on the Potomac River in 1786 while Fitch followed with a larger 45-foot boat on the Delaware River in 1787. Neither was a commercial success and it was left to Robert Fulton to introduce his North River Steamboat on the Hudson River in 1807, the first commercially successful steamer. This progress was matched in Europe by William Symington’s towboat Charlotte Dundas on the Forth and Clyde Canal in 1802 followed by Henry Bell’s passenger-carrying Comet of 1812 on the River Clyde.
By 1819 the steam and sail powered City of Savannah was able to make a steam-assisted crossing of the Atlantic, followed by steam-only Cape Breton in 1833 and steamers City of Kingston, Sirius and Great Western in semi-regular service by 1838. Within another year John Ericsson had invented the first propulsion system based on the screw propeller and in 1884 the modern steam turbine was invented by Charles Parsons, a gentleman who dramatically unveiled his new idea at the 1897 Spithead naval review in England with his “speedboat” Turbinia. At the same time Rudolph Diesel was inventing his “diesel” engine, the first practical operating model of which was displayed in the same year that Turbinia made its famous run.
Although coal-powered steamers ended the age of sail in the late 19th and early 20th century, the steam turbine and diesel engine would have the world’s final coaling stations for these early steamers closed by the mid-1950s.
The first diesel engines to be placed in a ship’s hull were three 3-cylinder engines mounted aboard the 244-foot long Vandal in 1903. This electro-mechanical arrangement made Vandal, a shallow draft Russian petroleum tanker, the world’s first motor vessel as well as the world’s first diesel-electric powered ship. Within a year, Sautter-Harle of Paris, which had been granted a license by Diesel, built the first opposed piston, reversing diesel, a four-stroke model developing 25.5 HP. This engine was installed in the 124.5-foot-long canal boat Petit Pierre, a trendsetting vessel that also boasted a variable pitch propeller.
Larger versions of the Sautter-Harle diesel were then installed in several French submarines, a military application that may have played a part in Rudolph Diesel’s mysterious death when he disappeared from the cross-channel ferry Dresden in 1913, his body found floating a few days later by the Coast Guard. Ironically, only the year before, Denmark’s Burmeister & Wain had delivered the Selandia to the Danish East Asiatic Company as one of the world’s first ocean-going motor ships.
Two-Stroke and Four-Stroke
Selandia, a marvel of her time, was to spark a revolution in shipping and ship design, despite the lack of a proper funnel. She was powered by two four-stroke, reversible diesel engines developing 1,250 HP each at 140 rpm. Within months of Selandia’s introduction another diesel-powered ship was completed, Hamburg Sud’s 3,693-gt Monte Penedo, the first vessel to be powered by two-stroke diesels, her twin Sulzer-built engines developing a combined 1,675HP at 160 rpm.
The development of these large, commercially viable engines had been preceded by another Sulzer-built diesel which had been installed in the small cargo vessel Venoge in 1904, an engine that had to be stopped and then restarted for operating the ship in reverse. This problem was overcome a year later when the first two-stroke, direct reversible engine was built by Sulzer, a four-cylinder unit producing 88.5 HP.
The use of four-stroke and two-stroke engines, the power stroke taking place on every two revolutions of the crankshaft on the four-stroke model and every revolution of the crankshaft on the two-stroke model, continues down to this day, with the largest marine diesels normally being two-stroke while smaller and faster engines are normally four-stroke.
Although modern high-speed four-stroke diesel engines, capable of operating at 1,200 rpm and above, are available on the market they seldom see marine applications outside of the workboat, fishing and yachting sectors. Much more common on large ships are the medium-speed diesels, operating at approximately 300 to 1,200 rpm, which are the main drivers of electrical generating sets as well as prime movers for large tugs, river boats and coastal vessels.
The very largest engines in this category, capable of providing more than 30,000 HP (22,400 kW), are used to power ships, such as ferries and ro/ros, in which the vessel’s internal configuration would make use of tall two-stroke, low-speed crosshead engines difficult. The latter powerplant has come to be the engine of choice for most large commercial ships because of its power and fuel economy.
These engines, which can stand more than 50 feet tall and weigh more than 2,000 tons, have power outputs of more than 107,000 HP (80MW) and can operate in a range of approximately 60 rpm to 120 rpm. This low speed eliminates the need for gearing to the propeller, which higher speed engines require, and the big diesels can also burn the cheapest fuel available, Heavy Fuel Oil or Bunker C (No.6), although this requires a heated delivery system because of the oil’s high viscosity. Its use is also now restricted for environmental reasons unless an exhaust gas cleaning system is employed to remove contaminates.
In today’s marine market most of these large engines are built under license through MAN Energy Solutions, a subsidiary of the German MAN AG group, which acquired Burmeister & Wain (B&W) in 1980, and Winterthur Gas & Diesel Ltd, once known as Sulzer and broken away from Finland’s Wärtsilä group several years ago.
World’s Most Powerful Marine Diesel
In competition with the new MAN B&W 115,000 bhp engine is the 14-cylinder Wärtsilä RT-flex96C turbocharged two-stroke engine, which is capable of delivering 108,920 HP. This powerplant, used in some of the world’s largest container ships, including Maersk Line’s 157,000-dwt Emma Maersk class, features a cylinder bore of 96 cm (about 38 inches) and a stroke of 250 cm (a little over 98 inches), with each of the 14 cylinders displacing 1,820 liters (111,143 cubic inches) and producing 7,780 horsepower.
Full displacement of the powerplant, which is nearly 90 feet long, weighs over 2,100 tons and stands five stories high, is 1,556,002 cubic inches. Fuel consumption at maximum power setting is 0.278 lbs per hp per hour (Brake Specific Fuel Consumption or BSFC) while consumption at maximum economy is 0.260 lbs/hp/hour.
At maximum economy the engine exceeds 50 percent thermal efficiency, meaning that more than 50 percent of the energy in the fuel is converted to motion. This compares to most automotive engines that have BSFC figures in the 0.40-0.60 lbs/hp/hr range and a thermal efficiency of about 25 to 30 percent. Even at its most efficient power setting, however, the big Wärtsilä engine consumes considerable fuel, using between 270 and 330 tons per 24-hour period.
Short and Long Stroke
Like most modern marine diesels the Wärtsilä RT-flex96 engine has been developed with such parts commonality that it can be delivered in a number of cylinder variations, all using the same cylinder bore and basic engine bedplate. This holds true for Wärtsilä’s smaller RT82 engines, which feature an 82-cm (32.3 inch) cylinder bore, with the C version of this powerplant featuring a stroke of 2646 mm (104 inches) while the T version has a longer stoke of 3375 mm (133 inches). The short-stroke model, available in six to twelve cylinder versions, can cover a power range of 21,720 kW to 54,240 kW at 87 rpm to 102 rrpm, making it a good choice for Panamax-size container ships which require a service speed of around 24 knots.
The longer stroke engine, which can be built in six to nine cylinder models covering a power output range of 21,720 kW to 40,680 kW at 68 rpm to 80 rpm, is designed for shaft speeds required by large tankers in the 200,000-dwt to 350,000-dwt class. A step below this engine is the RT50 series, also available in multiple cylinder versions, which features power ranges from 5,800 kW to 13,280 kW and is well suited to the needs of bulk carriers in the Handymax to Panamax size range as well as product tankers and smaller container ships.
The Wärtsilä RT-flex series engines represent a move toward providing smokeless engine operation at all ship speeds, a feature that is now much in demand because of tightening environmental regulations, exhaust stack emissions being the most notable feature of ship pollution as seen by the general public.
The development of the Sulzer RT-flex system, which incorporates common-rail fuel injection, along with integral electronic control, began in 1993 and was first applied to a full-scale research engine in 1998. The first series-built engine, a Sulzer 6RT-flex58T-B of 11,275 kW (15,120 HP), was installed in the self-discharging bulk carrier Gypsum Centennial in 2001.
The “common rail,” long used in smaller diesels, is actually a tube that can carry fuel under very high pressure (up to 2,000 bar) to computer-controlled injector valves. It does away with the older distributor-type injection pump and several other mechanical components. Superior combustion performance is achieved by maintaining the fuel injection pressure at the optimum level across the engine speed range. In addition, a selective shut-off of single injectors and optimized exhaust valve timing is available to help keep smoke emissions below the visible limit, even at very low engine speeds.
Valve actuation is also electronically-controlled, doing away with the need for a mechanical camshaft and gear. MAN B&W and other diesel manufacturers have similar systems but the trade-off in these more environmentally-friendly engines is in engine construction cost and the more sophisticated maintenance requirements.
More Fuel Economy – Less Air Pollution
In the continuing development of the marine diesel two goals now reign supreme: better fuel economy and less air pollution. Space utilization is also a factor, with shipowners wanting minimal interference with revenue-earning spaces as well as higher horsepower engines that can be used to replace older engines but without any enlargement of the original machinery space. This is seeing more compact diesels developed as well as a move by more owners towards electro-mechanical propulsion in which diesel/generating sets can be placed almost anywhere in the ship and need only be connected to the motorized propulsion units, such as Azipods, by electrical cables.
Such a system, using four Wartsila 16V46 common rail diesels, each with a maximum continuous output of 22,840 bhp (16,800 kW) at 514 rev/min, is used to power the Queen Mary 2 in conjunction with two GE LM2500 gas turbine-generator sets and Mermaid propulsion pods. This combined diesel and gas turbine, or CODAG system, provides a total of 157,000hp and a top speed of better than 30 knots.
The IMO 2020 Mandate
Looking toward a cleaner future, the International Maritime Organization (IMO) instituted new regulations limiting the sulfur content of bunker fuel to 0.5 percent at the start of this year, a reduction of more than 80 percent from previous levels. This is part of the organization’s commitment to reduce greenhouse gas emissions, particularly CO2, from the world’s merchant fleet by at least 50 percent over the next three decades when compared to 2008 baseline figures.
Most commercial ships have been burning Heavy Fuel Oil (HFO) since the 1960s because it is the cheapest fuel on the market, with around 180 million tons consumed annually at a price of around $350 per ton. However, its use in certain areas has been limited over the past decade following the establishment of Emission Control Areas (ECAs) in North America, Northern Europe and Antarctica and, to a certain extent, in the Arctic.
To meet the ECA regulations, and more recent IMO mandates, ship operators have been following three basic paths: switching to low-sulphur fuel but at a higher cost (roughly $200 more than HFO per ton); continuing to use high-sulphur fuel but installing ‘scrubbers’ to clean the exhaust, or using a cleaner alternative fuel, such as liquefied natural gas (LNG) which requires engine modifications and the installation of large cryogenic fuel tanks.
Alternative Energy Sources
A number of auxiliary propulsion and fuel-saving devices such as Flettner rotors, fuel cells, solar cells, batteries, kites and sails are also being employed to cut down on fuel use and emissions. In addition, several companies have been testing the potential use of such “clean” fuels as alcohol, biomethane, hydrogen and ammonia. The IMO’s Energy Efficiency Design Index (EEDI), which indicates the energy efficiency of a ship in terms of CO2 per ton-mile at a specific draft and speed, is requiring ships completed since 2015 to achieve certain efficiency targets. A similar indexing method, EEXI, is now being examined for older vessels.
Beyond hardware, ship operators are also looking at operational refinements, including slow-speed steaming and the possibility of limiting the maximum main engine power of each type of vessel. Lloyd’s Register has even suggested that nuclear power be reconsidered as it is a “zero-emission” solution that does not emit any SOx, NOx, CO2 or particulates.
The London-based classification society notes that nuclear energy is millions of times more power-dense than fossil fuels, as well as all other alternative fuel options being considered, and is the only “proven solution” currently available that is capable of replacing fossil fuels in almost all marine applications. According to Lloyds there have been around 700 nuclear reactors operational at sea since the mid-1950s and about 100 are in operation today, largely in naval vessels.