07 Fuel Oil Systems

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• This fuel is not suitable for burning directly in the

diesel engine because it has some solids and

water as impurity, which may cause damage to

the engine parts and also has a very highviscosity, which makes it difficult for atomization

of fuel in the combustion process. To make this

fuel suitable for burning, it has to go through a

conditioning process consisting of settling,centrifuging, boosting of pressure, filtering and

heating.



Fuel Oil SystemsHeavy residual fuel consists ofresidues left after lighter and costlierdistillates fuels and gases are removedfrom petroleum crude oil in an oil

refinery. Marine diesel engines aredesigned to burn heavy residual fuelblended with distillate gasoil to meetthe specification of fuel oil ordered,

especially viscosity and density. This ispopularly known as “heavy fuel oil”.



Fuel oil supply system of a ship.



Specification Data for Fuel Oil

• (a) Density: It is the relationship between massand volume at 15C and is measured byhydrometer. This value changes withtemperature, depending upon the coefficient ofexpansion of the substance. For marine fuels thevalues are 800-1010 kg/m3.Knowledge ofdensity is needed for quantity calculations and toselect the optimum size of gravity disc for

purifiers. 991 kg/m3 is the accepted limit fornormal centrifugal purification and 1010 kg/m3 in

ALCAP purifier.



• As already mentioned density is the ratio of themass of a substance to its volume, but not itsweight to volume ratio and therefore, density bydefinition is in vacuo. The term „density in air”,although often used, is incorrect and should bereferred to as “weight factor”. This is due to thefact that a substance weighed in air issupported, to a small extent, by the buoyancy of

the air acting on it. In effect therefore, the weightof a liquid in air is slightly less than its weight invacuo.



• Viscosity : Viscosity can be termed as resistance toflow. Viscosity is measured in a viscosimeter . Thekinematic viscosity is obtained by dividing dynamicviscosity by density of the fluid and its unit ofmeasurement is stoke or centistoke and is quoted with areference temperature. For distillate fuels the referencetemperature is 40C and for residual fuels the referencetemperature is usually 50C or 100C. Each fuel has itsown temperature viscosity relationship and although oil

suppliers publish temperature/viscosity charts, it shouldbe understood that these charts are based on averagedata of large number of representative fuels. Preciserelationship would depend upon crude oil source andrefining process. In general, for lower viscosity fuels thedifference is small, but it becomes wider as viscosity ofthe fuel increases. A knowledge of viscosity is necessaryfor the determination of the heating required for a fuel fortransfer purpose and the temperature range required forsatisfactory injection and combustion at the fuelatomiser.



• In order to ensure efficient atomization of the charge,when burning residual fuels it is essential to inject thefuel at the most suitable viscosity. Despite widedifferences in engine and fuel system designs there isconsiderable agreement that the most suitable viscosity

of the fuel leaving the injector nozzle lies between 12.5 -18.0 cSt. In well designed systems, the viscosity iscontrolled automatically within fairly close limits bymeans of viscosity controllers.

• Pressure/viscosity characteristics : The viscosity of

hydrocarbon oils increases under pressure. The veryhigh fuel injection pressures now employed will increasethe fuel viscosity markedly. This should be allowed forwhen preheating the fuel.



• Cloud and pour points : The cloud point

of a distillate fuel is the temperature atwhich wax starts to crystallise out, and this

is seen when the clear fuel becomes

opaque. For marine fuels this

characteristic is only applicable to somelight grades.



• The pour point of an oil is the lowesttemperature at which the oil remains fluid.

It is determined by cooling the oil in a testtube having a diameter of approx 30 mm.The pour point is 3C higher than thetemperature at which the glass can be

held in the horizontal position for 5 secondwithout any visible signs of movement ofthe oil surface.(Solidifying temperature is3C below pour point). The pour pointresult will give guidance regarding thelowest temperature at which a fuel may bestored. If fuels are held at temperaturesbelow pour point, wax will begin toseparate out.



• This wax may cause blocking of filters and can

deposit on heat exchangers. In severe cases the

wax will build up in storage tank bottom and onheating coils, which can restrict the coils from

heating the fuel. When dealing with heavy

marine fuels, both the pour point and the

viscosity of the fuel need to be considered, if thefuel is to be maintained at a temperature to

prevent wax formation and allow pumping. For

efficient pumping the viscosity of the fuel should

not be above approximately 600 cSt. If the

suction line from the pump to the tank is very

long the viscosity should be lower.



• Flash Point: The flash point of a fuel is the lowesttemperature at which sufficient vapour is given off to

produce a flash on application of flame under specifiedtest condition. The flash point may be measured as aclosed or open cup figure and for marine fuels the closedcup figure is used. The test method uses the Pensky-Marten apparatus. The minimum flash point for fuel in

the machinery space of a merchant ship is 60C. Forfuels used for emergency purposes, external to themachinery space, for example the lifeboats, the flashpoint must be greater than 43C. The purpose of defininga minimum flash point is to minimise fire risk during

normal storage and handling. The general rule is thatfuels should not be heated above 10C below the flashpoint, unless specific requirements are met. (SolasChapter II-2, Regulation 15)



• Fire Point : It is the lowest temperature at which

vapour is generated at a rate sufficient to sustain

combustion for 5 second. The same equipment

which is used for determining flash point is usedfor this test also.

• (f) Auto-ignition temperature or Self ignition

temperature : It is the lowest temperature at

which the generated vapour will ignitespontaneously without any source of ignition.



• Calorific Value or Heat of Combustion or

Specific Energy : Heat of combustion of a fuel

is the amount of heat released duringcombustion of a unit mass under following

circumstances:

• (a) The temperature of fuel before combustionand that of the combustion products after

combustion is 20C.

• (b) The combustion products from carbon and

sulphur are solely gaseous carbon dioxide andsulphur dioxide and no oxidation of nitrogen has

occurred.



• In gross heat of combustion, the water existing beforecombustion as well as the water generated by thecombustion process is to be found in the combustion

products in liquid state. In net heat of combustion theabove mentioned water is to be found in the form ofvapour at 20C. The gross heat of combustion can bedetermined by Berthlot-Mahler calorimeter. The net heatof combustion(hi) is calculated if the gross heat ofcombustion(hs) is known. hi = hs - 25 (f+w) kJ/kgwhere water content of fuel is f% by mass, and that w%by mass of water is generated by combustion ofhydrogen in fuel.

• Heat of combustion can be calculated with a degree ofaccuracy sufficient for normal purposes from the density

of the fuel and the application of corrections for anysulphur, water and ash that are present. On a world-widebasis the heat of combustion does vary slightly,depending mainly on density and sulphur content of thefuel.



• Water - Normally the water content in the

fuel oil is very low and 0.1-0.2% by volume

is typical. Ingress of water can come from

tank condensation, tank leakage andheating coil leakage. Water is normally

removed by gravitational separation in fuel

oil tanks and centrifugal purificationsystem.



• Ash : Nickel, Aluminium, Silicon, Sodium andVanadium

• The ash content is defined as the residue left after all the

combustible components of the oil has been burnt. Indistillate fuel this quantity is negligible. The ashconstituents are concentrated in residual fuels. The ashconsists generally of oxides and/or sulphates of nickel,aluminium, silicon, sodium and vanadium. The sources

of these are (a) inorganic material naturally present inthe crude oil, (b) Catalytic fines picked during refiningprocess ( Catalytic fines are particles arising from thecatalytic cracking process in the refinery and are in theform of complex alumino-silicates) (c) Contamination by

sand, dirt, rust scale and sea water subsequent torefining process.



• Sodium and Vanadium - Fuels leaving refinery havesodium level below 50 mg/kg. If contaminated withsea water subsequently, sodium level will increase.

A 1% sea water contamination represents a potential100 mg/kg increase. Normally sea water can beremoved by gravitation separation in settling tankand centrifugal separation. Vanadium is present inall crude oils in an oil soluble form and the levels

found in residual fuels depends mainly on the crudeoil source, with those from Venezuela and Mexicohaving the highest levels. The actual level is alsorelated to the concentrating effect of the refiningprocesses used in the production of the residualfuel. There is no economic process for removingvanadium from either the crude oil or residue.



• During combustion of the fuel, vanadium andsodium constituents form a mixture of sodiumsulphate and vanadium pentoxide. This mixture has

a low melting point (approx 500-600 C)corresponding to the temperature of the exhaust

valve seating. The semi-fluid particles of ash adherefirmly to the surfaces they touch, gradually forming avery hard, thin layer of slag which, after havingreached a certain thickness, allows the hotcombustion gases to leak out, the result being thatthe slag melts forming a narrow channel. If the layerof slag is of sufficient thickness, the channel growsand the combustion gases heat up the seatingmaterial, causing what is known as high-temperature

oxidation, which in turn results in the seatingmaterial melting in the vicinity of the channel. Themost critical sodium to vanadium ratio is about 1 to3.



• Silicon and aluminium :Silicon may be present in the fuelin form of sand and aluminium may also be present in

very small quantities, having been picked up by thecrude oil in sub-surface rocks. However presence ofaluminium and silicon is mainly due to catalytic finesdiscussed earlier. Catalyst is an expensive material forthe oil refiner and stringent methods are taken for its

retention but some still find their way in residual fuel.Excessive catalytic fines can lead to high wear of pistonrings and liners, fuel pump barrels and plungers, and fuelinjector nozzle needle and guide. The level of catalyticfines in delivered fuels can be significantly reduced by

efficient centrifugal purification prior to combustion in theengine.



• Carbon Residue: The carbon residue of a fuel

is the tendency to form carbon deposits under

high temperature conditions in an inert

atmosphere, and may be expressed as eitherRamsbottom carbon residue, Conradson carbon

residue (CCR) or micro carbon residue (MCR).

This parameter is considered by some to give an

approximate indication of thecombustibility/deposit forming tendency of the

fuel.



• Sulphur: Sulphur is naturally occurring elementin crude oil which is concentrated in the residualcomponent. The amount of sulphur in fuel oil

depends mainly on the source of crude oil and toa lesser extent on the refining process. Sulphurcontent is typically 1.5-4% wt in residual fuelworld wide. In the combustion process in adiesel engine the presence of sulphur in the fuelcan give rise to corrosive wear. This can beminimised by suitable operating conditions, andsuitable lubrication of the cylinder liner withalkaline lubricant. MARPOL Annex VI limits the

sulphur content of marine oil to reduceatmospheric pollution, in the form of sulphurdioxide, from international shipping.



Amendment to Marpol Annex VI

• Regulation 14 of MARPOL Annex VI hasbeen significantly revised. For the GlobalCap, the sulphur content limits are as

follows:• 4.5% prior to 1 January 2012

• 3.50% on and after 1 January 2012




Special Emission Control Areas

• For the Special Emission Control Areas,

the sulphur content will be as follows:

• 1.50% prior to 1 March 2010

• 1.00% on and after 1 March 2010


• The existing Emission Control Areas(ECAs) are the North Sea and the Baltic

Sea.



Review provision

• The amended Regulation 14 has a “reviewprovision” which requires the IMO tocomplete by 2018 a review of theavailability of the 0.50% sulphur contentfuel. Based on the results of such areview, the Parties to MARPOL Annex VIwill decide whether the global cap of0.50% can be enforced from 1 January

2020. If not, the 0.50% sulphur global capwill be enforced on 1 January 2025 withoutany additional review.



• Ignition Quality : Cetane number - The cetanenumber for any fuel is a measure of the oil‟s

readiness to ignite, under conditions prevailingin the diesel engine. This number is determinedby comparing the oil with a mixture of cetaneand heptamethylnanone. Cetane, which has avery high spontaneous combustion ability is

rated at 100 and the corresponding cetanenumber for heptamethylnonane is 15. The oil forwhich cetane number is to be determined isused as fuel in a so-called CFR (Co-operationFuel Research) engine, which is a singlecylinder diesel engine with a variable andcontrollable compression ratio.



• Fuel injection and combustion timing are controlled byelectronic equipment. When these have beendetermined then engine is run with different mixtures of

of cetane and heptamethylnanone until a mixture givessame results. Cetane number = a -0.15 x b, where a isthe volume % of cetane and b the volume % ofheptamethylnanone. For high speed diesel engines, acetane number of over 50 is desirable. Medium speeddiesel engines require a fuel with a cetane number ofaround 40-50. Large, slow speed diesel engines operatesatisfactorily with fuels having a cetane number ofapprox 30. However slow speed engines are said to benot so sensitive to with regard to the cetane number andit is not normally specified for these engine types.

• (b) Calculated Ignition Index (CII) and Calculated Carbon Aromatic Index(CCAI) :These are calculated byempirical equations ,where use is made of the densityand viscosity of the residual fuel.



Standards of Fuels - Need for

quality control in bunker fuel

• The cost of bunker fuel is one of the most

significant components of a ships operating cost.

Ship owners in their effort to limit this cost have

preferentially turned to the use of heavier andthus less expensive bunker fuels. Technology

developments in petroleum refining, such as in

vacuum distillation, catalytic cracking etc, often

result in a deterioration of the characteristics ofheavy fuels as lesser volumes of residues are

left after petroleum refining.



• These residuals may contain elevated levels of

undesirable constituents such as Aluminium and

Silicon, compounds that could result to

significant engine wear and damage. In additionto the above the supply of marine bunker fuels is

nowadays often the result of a complex

sequence of buying, selling and mixing of fuels

of different origins. The use of poor quality fuel isknown to result to the serious damage of boilers,

fuel pumps springs, pistons and cylinders.



ISO 8217 • During 2005 and 2006 the set of regulations included in

MARPOL Annex VI that relate to the use of the marine

bunker fuels came into effect. The sampling of the

bunkered fuels became mandatory, following a detailed

list of requirements listed in the above document and in

the MEPC.96(47) IMO document.• To obviate dispute between ship owners and bunker

suppliers and also to meet MARPOL Annex VI

requirements, International Organisation for Standards

published the first edition of International fuelspecification ISO 8217 known as “Petroleum products -

Fuels (class F) - Specifications of marine fuels” in 1987.

It was revised in 1996 and again in 2005. ISO 8217-2005

defines four distillate grades (DMX, DMA, DMB, DMC)

and ten residual grades.



• Distillate grades remain same and the main changes arein marine residual fuels:

• Reduction of residual fuel grades from 15 to 10 - With

the viscosity classification of residual fuel grades beingmeasured at 50 °C (instead of 100°C as under ISO8217-1996), the names of the 10 residual fuel gradeshave been changed as follows – RMA30, RMB30,RMD80, RME180, RMF180, RMG380RMH380,RMK380, RMH700 AND RMK700.

• Maximum sulphur limit reduced to 4.5% - for all theresidual fuel grades with viscosity higher than that ofRMD 80. For RMA, RMB and RMD grades the previouslower sulphur limits have been retained.

• Fuel to be free of ULO (used lubricating oil)

• Reduced water content - from 1.0%v to 0.5%v.

• Reduced ash content



CHARACTERISTIC LIMI

T

CATEGORY ISO - F

DM

X

DM

A

DMB DMC(a)

Density at 15°C (Kg/m3) max. --- 890,0 900,0 920,0

Viscosity at 40°C (mm2/s

b)

min.

max.

1,40

5,50

1,50

6,00

---

11,0

---

14,0

Flash Point (°C) min. 43 60 60 60

Sulfur (% m/m) max. 1,00 1,50 2,00

(e)

2,00 (e)

Cetane index min. 45 40 35 ---

Carbon residue (%m/m) max. --- --- 0,30 2,50

Carbon res. on 10% (V/V) distillation

bottoms

(% m/m) max. 0,30 0,30 --- ---

Ash (% m/m) max. 0,01 0,01 0,01 0,05

Appearance (f) Clear and

bright

(f) ---

Total Sediment Existent (% m/m) max. --- --- 0,10

(f)

0,10

Water (% V/V) max. --- --- 0,3 (f) 0,3

Vanadium (mg/kg) max. --- --- --- 100

Aluminum plus silicon (mg/kg) max. --- --- --- 25

Aluminum plus silicon (mg/kg) max. --- --- --- 25

Used lubricating oil (ULO)

1.Zinc

2.Phosphorus

3.Calcium

mg/kg max. ____

_

____

_ _____

The fuel shall be free of ULO

(g) 15

15

30







• Fuel Testing: Analysis of particularcharacteristics of the fuel delivered may becarried out by some independent shore basedlaboratory or by tests carried out on board.

Testing of fuel on board may range from one ortwo tests to fully automated online monitorswhere direct read out of viscosity, density andelemental analysis (e.g. sulphur, silicon,

vanadium) as well as derived parameters suchas „ignition index‟ expressed as CII or CCAI areavailable.



• Storage and Transfer : The pump for fuel transfer is ofthe positive displacement type and are usually of screwor gear design. The temperature of fuel in the storageshould be maintained 5C above its pour point otherwise

there is a possibility of wax formation and in case of highwax content, if left to cool, it may be difficult to reheat thefuel to a temperature above the pour point. Also thetemperature has to be raised for higher viscosity fuel to45C to bring it below 500 cSt for pumping it. Fuel oil is

heated in storage tanks by low pressure steam, but insome ships thermal fluid heating is used.



STORAGE• Bunkering: Marine heavy fuel oils are blends of

viscous residues from various refinery operations,cut back with distillate cutter stock. The growingtrend is towards cracked residues of a highlyaromatic/asphaltic nature to be cut back to anacceptable viscosity with cracked aromatic

distillates. Both components have a highcarbon/hydrogen ratio, cracked distillates havinggood solvency properties for large-asphaltenehydrocarbons. In a stable fuel the asphaltenes arecarried in a colloidal dispersion in the lighter phase.If the equilibrium between the two phases is

disturbed the asphaltene will agglomerate to a sizewhich can no longer be maintained in suspension,and they will tend to separate out as ‘sludge’. Ifsludge deposition does occur this is made worse,not better, by the addition of more distillate,



• This is particularly true if a high-quality straight-runparaffinic distillate is added to a cracked, highasphaltenic content, residual fuel. It is possible thattwo residual fuels, each stable by themselves, whenmixed together can prove to be incompatible andthrow down objectionable sludge or sediment. Ifcompatibility tests have not been carried out

beforehand, when bunkering, every effort should bemade to segregate bunkers from different source indifferent tanks to avoid potential problems ofincompatibility. In such a case an unstable blendmay occur in the ship’s tanks, which could result in

precipitation of asphaltenic deposits as sludge in thetanks, pipes, filters and centrifuges.



TREATMENT OF FUEL OIL

• Before the fuel is burnt in diesel engine or aboiler, a shipboard treatment takes place.Distillate fuels are generally filtered through acoalescer type filter to remove water and solid

impurities. For boilers burning residual fuels, inaddition to settling tanks, cold and hot filters areinstalled in the system prior to boiler. In case ofdiesel engines burning residual fuel, in additionto settling tanks and filters, centrifuges are

installed to clean the fuel to take account of thefine clearances which exist in fuel system ofdiesel engine.



• Treatment of High Density Fuel: As the densityof fuel oil increases and exceeds 991kg/m3, thedensity difference between the fuel oil and freshwater is so small that any change in oiltemperature, viscosity or flow rate will cause theoil/water interface to fluctuate leading to apotential failure of water seal. For residual fuel

having density above 991 kg/m3, alternativearrangements to traditional purifier are used.One such arrangement called ALCAP system isused, where fuels of density upto 1010 kg/m3can be treated. The centrifuge operates as a

clarifier and clean oil is continuously dischargedfrom clean oil outlet, and any free water andseparated sludge accumulate at the periphery ofthe bowl.



• When the sludge space is filled up, theseparated water approaches the disc and traceswater start to escape with clean oil. Increased

water content in clean oil is sensed by the watertransducer in the clean oil outlet side. Theelectrical signal from water transducer arecontinuously transmitted to and interpreted bythe control unit. When the water content in clean

oil reaches a specific „trigger‟ point, the controlunit determines, based on the time elapsedsince the last sludge sequence, which of the twomethods it will use to empty the bowl. This caneither be through a water drain valve or with the

sludge through the sludge ports at the peripheryof the bowl.



• Fuel Heating : Residual fuels have to be heatedto reduce the viscosity to that required foratomisation. In case of boilers this is in the rangeof 15-65 cSt, whilst for diesel engines the

injection viscosity is usually 12.5 -18 cSt. Fuelheaters may be operated by low pressuresaturated steam, a thermal fluid or electricalelements. It is important to maintain correctviscosity range under all conditions. Local

overheating may cause cracking of fuel, whichmay lay down deposits on the heating surface,impairing efficient operation of the heater.



• Viscosity Controller : A viscosity controlleris often installed downstream of a fuel oilheater so that a constant injection

viscosity can be maintained. There arevarious types of these. One of thesemeasures the differential pressureresulting from laminar flow through a

capillary tube and compares this value to aset point, generating a signal to control thetemperature of fuel oil heater.



• Additives :There are two types of additives: (a) Whichreduce problem in pre-combustion phase

• (b)Which react during post combustion phase

• (a)With normal fuel handling procedures, with respect tocorrect heating, and avoidance of mixing of fuels fromdifferent bunkering, no problem should occur. In theevent of problems, an effective additive shouldcontribute as follows :

• (1) Dispersion of possible sludge in fuel oil tanks.

• (2)Promotion of separation of any dispersed water.

• (3) Prevention of sludge formation.

• (b)An additive which has the effect of an ash modifier(ability to raise the melting temperature of ash) may be

beneficial. Slagging and high temperature corrosionoccurs when molten ash adheres to the metal surface.By increasing the melting temperature the ash is not inmolten form and less likely to stick to metal surfaces.



Combustion In Diesel Engine• For combustion of fuel in a diesel engine, the air charge

is highly compressed to a temperature well above thespontaneous ignition temperature (SIT) of the fuel. Asthe piston approaches TDC fuel is injected at highpressure and suitable viscosity. This continues for 14-28degrees of crankshaft rotation, depending upon enginespeed and design. The fuel passes through the followingphases :

1. A delay period between the commencement of injectionof the very finely divided fuel droplets and thecommencement of ignition.

2. Rapid combustion of the fuel accumulated in the cylinderduring the initial delay period, accompanied by a rise inpressure.

3. Steady combustion of the remainder of the fuel chargeas it is mixed.

4. An after burning period during which remaining unburntfuel finds oxygen and combustion is completed.

f



Factors Influencing ignition

1. Exactly when ignition commences is dependent uponseveral factors, the most important being:

2. The size of the droplets injected into the cylinder;

3. The pressure of the fuel at the injector tip;

4. The velocity of the droplets entering the dense air mass;5. The air pressure and temperature in the cylinder;

6. The air turbulence in the cylinder;

7. The ignition delay properties of the fuel;

8. The surface tension of the fuel;9. The chemical composition of the fuel;

10.The engine design.



Droplet formation and size

• The size of the droplets in the injected fuel spray iscontrolled primarily by the size, shape and number ofholes in the injector tip, their position and the fuelinjection pressure and the viscosity of the fuel leavingthe injector. The higher the viscosity, the larger will be

the droplet size. As the fuel leaves the small injectororifices at pressures in modern pressure-chargedengines of upto 1500 bar the pressure falls sharply as itenters the cylinder, in which the charge-air pressure ismuch lower. The pressure energy is converted intokinetic energy, so that there is a sharp rise in velocity.Both the fall in pressure and the shearing action as thefuel passes through the dense air charge at high velocitybreak up the liquid stream, while its viscosity and surfacetension form the mechanically disrupted liquid into smalldroplets.



• The droplets sprayed into the cylinder are of varyingsizes; the higher the injection pressure, the higher thepercentage of small droplets. With current trend towardsmuch higher injection pressures fuel droplet sizes will bereduced correspondingly. The droplet size decreases asthe compression pressure increases. The increaseddensity of the air charge helps to break up the spray intosmaller droplets. This is beneficial, as the smaller thedroplets, the quicker they will vaporize as there is a

greater overall area of the oil charge exposed to the hotcompressed air. This reduces the ignition delay period,measured either in milliseconds or degrees of crankangle.



Importance of high fuel pressure

• If the droplets leaving the injector have a diameter ofabout 20-40m, there is minimum delay in combustion.Conversely, if the droplet diameter exceeds some 100-120m, the combustion period is so long that even aslow-speed, two-stroke engine runs the risk of someparticles remaining unburnt when the exhaust ports orvalves open. Below 20m droplet size there isinsufficient kinetic energy in the tiny droplet to penetratethe dense air mass in the cylinder, resulting in poor

fuel/air admixture. In order to ensure the required finedroplet size, an injection pressure exceeding 1200 bar isbeing used by some engine manufacturers.



The effect of air temperature • The temperature of the air compressed in the cylinder

has a major effect upon ignition delay. The higher thetemperature, the shorter the delay period, everythingelse being equal. Several factors determine the aircompression temperature, the main ones being theengine compression pressure, which, in turn, is

determined by the charge air pressure, the compressionratio and the volumetric clearance, the temperature ofthe induction air entering the cylinder, and thetemperature of the cylinder head, liner and piston crown.In turn, the combustion chamber and pistontemperatures are controlled by the temperature of the

cooling water, or oil, and by the design of the combustionchamber and piston components. Compressiontemperatures in normally aspirated engines are in theorder of 500-600C, but in modern highly pressure-charged medium and large output engines, they may be

as high as 700C.



Compression Pressure

• Increased compression pressure (or

densities), which are now as high as 90-

110 bar in modern crosshead and trunk

piston engines, not only promote theformation of more, smaller fuel droplets

but, equally important, reduce the

spontaneous ignition temperature of thefuel appreciably.



Air Turbulence

• Turbulence or swirl, in the compressed air

charge promotes efficient distribution of

the fuel spray droplets throughout the

combustion chamber, ensuring thoroughmixing of the fuel and clean air (increasing

the rate of heating and vaporization) thus

tending to reduce ignition delay andassisting in efficient burning of the fuel

charge.



Droplet combustion process • If an individual fuel droplet is considered it will be found

to be very small, the size depending upon factorsdiscussed previously but, as compared with the physicalsize of individual hydrocarbon molecules which form thedroplet, they are relatively large. Even the smallestdroplets in the fuel spray contain thousands ofhydrocarbon molecules having widely different chemical

structures. This is particularly true of heavy residual fuelswith high carbon-numbers. The molecules varyappreciably in their volatility, ignition temperature, rate ofburning, the completeness of burning and their tendencyto release carbon and associated organometallic

compounds. The heating of the spherical droplet occursfrom the outer surface inwards to the centre, so thatevaporation and subsequent ignition commences at thesurface. The more volatile constituents with the lowestignition temperatures burn first, leaving the lesscombustible hydrocarbon constituents to find clean airand burn slowly.

Advanced injection timing



Advanced injection timing• During a long ignition delay, injection of fuel into the

cylinder continues, so that the longer the delay, the

greater is the amount injected before ignitioncommences. When ignition finally occurs, theaccumulated fuel ignites violently with a very rapid, high-pressure rise. The resultant high pressure causes shockloading on the piston and running-gear bearings. With apoor equivalent Cetane Number residual fuel, within

fairly narrow limits one way of reducing this harmfuleffect is to advance the ignition timing. In case of low-speed crosshead engines, upto 2 degrees crank anglemay be adequate, with a somewhat greater advance formedium-speed engines - possibly 3 to 6 degrees,

depending upon engine design and, in particular, enginespeed. Advancing the injection timing enables ignition tooccur at maximum compression pressure andtemperature and smooth combustion to be completedearlier in the stroke. The manufacturer's maximum firingpressure, related to load conditions, should be

maintained.



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07 Fuel Oil Systems