marine engines

8/2/2019 Marine Engines

1/4

Marine Fuels and EnginesBy Chris Townsend, E.I.

Naval Architect, Guido Perla & Associates, Inc.

Marine propulsion starts at the primemover and ends at the propeller. In this,the second article in our series of marinepropulsion systems, we will discuss themost common types of marine primemovers and the different fuels that theyuse. Many factors go into deciding whichprime mover best fits a given vessel.These factors include the intended serv-ice of the vessel, as well as the weight,size, regulatory compliance, and cost ofthe prime mover. Cost considerationstypically include the initial cost of theengine, lifetime maintenance expenses,

and fuel cost.Optimization of fuel costs can beachieved one of two ways: choosingan engine that burns the cheapest fuelpossible, or choosing an engine that isparticularly fuel-efficient.

A combination of these factors canalso be cost-effective over the life ofthe engine.One of the flaws in choosingan engine that burns the cheapest fuelavailable is the unpredictable nature ofboth global and local marine fuel mar-kets. Prices could go up, prices could godown, or regulations could limit or pro-hibit the use of certain types of fuel.

Fuels

The selection of a primary fuel can be

influenced by other factors, such asstorage arrangements and auxiliary sys-tems associated with the chosen fueltype. Consider briefly the major typesof marine fuel in use today:

Heavy Fuel Oil (HFO)-Historically the least expensive of inuse marine fuels, heavy fuel oil is usedextensively in the low-speed recipro-cating engines that power many of theworlds largest commercial vessels.Though the cost of the fuel is low, theuse of heavy fuel oil for marine propul-sion requires a cleaning system to re-move excess impurities, as well as aheating system to lower the viscosityfor efficient combustion. The energy re-quired to deliver the fuel to the engineready for combustion is quite high. The

space and weight required for the heat-ing and treatment systems associatedwith HFO make using this type of fuelimpractical for smaller vessels.Ever growing emissions standards arealso making the primary use of HFO

SeaNG Rendering One of three Compressed Natural Gas (CNG) carrier versions designed by

Guido Perla & Associates, Inc. This 118-meter vessel carries 50 mmsCF of CNG in 16 coselles

and features a dual-fuel mechanical drive propulsion system.

Royal Belgian Institute of Marine Engineers
http://www.gallois.be/


2/4

more expensive and less practical.New regulations allow for the creationof Sulfur Emission Control Areas (SECA),in which levels of sulfur emissions mustbe below those achievable when burn-ing currently available HFO.This requires that a vessel be reroutedaround the SECA, install a scrubber sys-tem to remove sulfur from emissions, orswitch to a more expensive low-sulfurdistillate fuel while transiting the SECA.Reports from refiners indicate that thereis little interest in developing the tech-nologies necessary to dramatically re-duce thesulfur content of HFO.Instead, the refiners argue money shouldbe spent on increasing the overall refin-ing process, yielding more distillatesfrom residual wastes.

Marine Diesel Fuel/Marine Gas Oil

These fuels are blends of distilled fuelcombined with small amounts of HFO.They have lower levels of sulfur contentthan HFO and are cleaner burning.Though these fuels are more expensive,elimination of heating and extensivetreatment systems help justify the highercosts.

Current and future regulation will furtherrestrict the allowable levels of sulphur indistillate fuels, reducing pollution but al-most certainly increasing costs as well.

Natural GasFirst commonly used in marine propul-sion on LNG vessels, natural gas is nowbecoming a more popular marine fuel,primarily due to its clean burning char-ac-teristics, producing signi-ficantly lesscarbon dioxide than other fossil fuelsfor the same amount of heat generated.LNG additionally produces less NOx,SOx, and particulate matter than tradi-tional liquid fuels.Natural gas must be stored as a liquidunder pressure, and then warmed to agas before combustion, creating somechallenges in storage and piping ar-rangements.

NuclearNuclear fuel for marine propulsion, per-haps the most controversial of all pro-pulsion fuels, is typically limited to largecapital military vessels. These vesselstake advantage of the essentially unlim-ited cruising range nuclear propulsion

provides, without being unduly bur-dened by the size and weight associatedwith the nuclear reactor. The high powerdensity and high endurance capability ofa nuclear propulsion system also makesit an attractive choice for icebreakingoperations. However, there are certaindrawbacks to nuclear propulsion thatgenerally limit its use to military ves-sels.These constraints include the necessityfor specialized (and expensive) manning,high initial cost, the complexities of han-dling and disposing of nuclear materi-als, and extremely stringent worldwideregulation.

BiodieselBiodiesel is a clean burning alternativefuel that is gaining traction in the marineindustry. As its name suggests, biodiesel

is a fuel similar to diesel, but contains nopetroleum products. Biodiesel is createdthrough a chemical process called transesterification, which separates glycerinfrom fat (such as soybean oil). Whendone in the presence of alcohol, thisprocess leaves behind methyl esters,more commonly called biodiesel.This type of fuel can be used in a typicalcom-pression type reciprocating enginewith little or no modification.Biodiesel burns cleaner than conven-

tional diesel distillates, in addition tobeing made from renewable sources -these factors make biodiesel very envi-ronmentally friendly.

Engines

The generation of power, whether tosend down a shaft or an electrical cable,has been at the core of marine propulsionsince the application of steam power toships in the late 19th century. Years of

refinement and fine-tuning have resultedin a large array of possible prime moverssuited to many different ships in a widevariety of services.

Internal Combustion ReciprocatingThe most common type of marine pro-pulsion prime mover is a reciprocatinginternal combustion engine (such asa diesel engine). This type of engine ischaracterized by the intermittent rapidburning of air and fuel inside a com-

bustion chamber, releasing energy thatmoves a piston, yielding linear motionthat is converted into rotational motion.Reciprocating internal combustion en-

MAN Diesel Engine Cross Section

A cross sectional view of the vee configuration version of the MAN Diesel 51/60DF dual-

fuel gas engine. This versatile prime mover is being installed in LNG carriers, as well as

car and passenger ferries. (Image courtesy of MAN Diesel)


3/4

gines typically fall into one of three cat-egories describing the speed at whichthe engine rotates. Low-speed enginestypically rotate between 100 and 500RPM.Medium-speed engines rotate at speedsof 700-1200 RPM. High-speed enginesgenerally rotate at 1800-4000 RPM. Theparticular advantages and disadvantagesof each of these types of engines must beunderstood to properly match the engineto the application. Low-speed enginesare large, heavy, and expensive, but paydividends on lower fuel, operating, andmaintenance costs when compared to ahigh-speed engine of the same output.Installation of a low-speed engine canalso eliminate the need for a gearbox,saving further cost and weight.Perhaps one of the primary advantagesof a recipro-cating internal combustion

engine is the widespread acceptanceand use of this type of engine in the ma-rine industry. The high production num-bers drive down cost and promote reli-ability, while allowing for easy access toparts and trained mechanics.Reciprocating internal com-bustion en-gines are not without fault. At the largerend of the spectrum, they do not havegood power-to-weight ratios, thoughthis is generally offset by greater vesseldisplacement.

Reciprocating internal com-bustion en-gines are capable of burning a variety ofdifferent fuels. Use of heavy fuel oil istraditionally limited to use in low-speedengines, but marine diesel fuel and natu-ral gas are suited for a wider range of

engine speeds. Natural gas can be usedin a slightly modified diesel engine in adual-fuel arrangement. Diesel is injectedas pilot fuel to ignite the natural gas,which will not detonate under extremecompression alone. Alternatively, natu-ral gas can be used as a sole fuel sourcein a spark-ignited reciprocating internalcombustion engine. As mentioned previ-ously, diesel engines can also be madeto run on biodiesel with little or no modi-fication necessary.Diesel engines are well suited for a widevariety of marine applications - in fact die-sel engines can be found in nearly everytype of commercial marine vessel, frompassenger vessels to tug boats. Dieselengines have recently gained traction inLNG tanker propulsion, marking a shiftaway from burning boil-off gas. Thoughthe installation of a reliquefaction plant

adds complexity and cost to the ves-sel, rising market prices of LNG cargoand the greater achievable efficiency ofdual-fuel and diesel-electric propulsionsystems has made this option attractiveand financially viable.

Internal Combustion RotationalRotational internal com-bustion engines(such as a gas turbine) are also utilizedfor marine propulsion. This type of engineis charac-terized by continuous combus-

tion of air and fuel and conversion of thisenergy directly into rotational motion.The principle advantage of installing agas turbine engine is the large power-to-weight ratio this type of prime movercan achieve. The compactness of the

engine eases maintenance and down-time - the small size makes it possibleto quickly swap out turbines instead ofspending time at the dock doing in-placemaintenance or repair. The relative sim-plicity of the engine is another second-ary advantage - there are fewer movingparts, none that reciprocate.Some of the disadvantages of gas tur-bines are the strict inlet and exhaustpressure requirements, the need forlarge, heavy gearboxes - particularly ifused in a direct drive configuration, highexhaust gas temperatures, higher initialcost, and lower relative fuel economy- particularly at idle. A gas turbine canburn nearly any grade of refined petro-leum such as marine diesel fuel andkerosene, as well as natural gas. Thoughthese fuels are expensive, the gas tur-bines fuel flexibility makes it attractive

for volatile fuel markets.A gas turbine works well in high-speedapplications, where weight is always aconcern. Additionally, there is a growingmarket for gas turbines in vessels thatcannot afford excessive downtime, suchas shuttle tankers

External Combustion RotationalThough internal com-bustion engines(parti-cularly diesel internal combus-tion engines) are the dominant source

of power in the marine industry, a ro-tational external combustion propulsionplant (such as a nuclear-powered steamplant) still plays an important role in cer-tain services. An external combustionengine heats the working fluid (typically

MaK Diesel

The MaK 9 M 43C propulsion engine shown here is a four-

stroke medium-speed diesel engine, well suited to a variety of

common applications on intermediate-sized vessels.

(Image Copyright Caterpillar Marine Power Systems 2008)


4/4

steam) outside the mechanism (turbine), upon which theworking fluid expands, producing rotational motion.Like their gas turbine counterparts, a steam turbines advan-tages include the simplicity of fewer moving parts that rotate,a good power-to-weight ratio, and compact size. The steamboilers, however, which are neither small nor light, are a nec-essary compromise in this type of system.A steam system also requires constant upkeep and mainte-nance. Further disadvantages include a slower response timecompared to internal combustion engines, both from a coldstart and while underway.In most modern steam applications, steam is generated oneof three ways - burning marine diesel, burning natural gas, orusing nuclear fuel.Large vessels are the most practical platform for a traditionalsteam power plant, regardless of the heat source. The natureof their size requires a substantial installation of power, butalso allows for the installation of a large, heavy propulsionsystem without an excessive penalty to the vessels perform-ance.

ConclusionThe real change in marine propulsion in recent times hasntbeen the types of engines installed, but the fuels that powerthem. Regulatory and environ-mental changes are shiftingthe industry away from the exclusive use of HFO, towardscleaner burning distillates. LNG is undergoing a radical shiftas it becomes less popular as a fuel for LNG tankers butmore common in environmentally friendly ferry, tug, and PSVoperations.Even nuclear power for commercial ships is an old idea beingresurrected. In this day and age, prime mover selection mustbe a balance of modern environmental consciousness and old

world economic savvy.

References:

< http://www.naturalgas.org/environment/ naturalgas.asp>

Challenege. Surveyor. Fall 2008. 2-4.

2008. 21-23.

SOURCE: MARITIME REPORTER & ENGINEERING NEWS NOVEMBER 2008

WWW.MARINELINK.COM

Teamwork alone is not

enough you also have to

be in the right boat

marine engines

Documents