FUEL OIL SPECIFICATIONS

FUEL OIL SPECIFICATIONS

In order to fully define the grade of fuel loaded by the vessel, a fuel standard or specification should always be used. The old method of using the criteria of density and viscosity to specify the fuel required does not provide enough detail of the limits that must be adhered to in order to prevent premature damage to the main engine.

There is only one standard which is ISO 8217 (2005). The attached fuel standard indicates the full specification for residual fuels of varying grades.

To specify which fuel should be ordered the engine makers books should be consulted, and this will indicate their suggestion to the permissible grade of fuel.

For example:

MAN B&W MC RMH 55, with 1010 density if correct purifier units installed

B&W GF Reference in manual only to limits of 600cSt at 50^oC, and 0.990 density

SULZER RTA RMH 55, with additional limit on sodium at 100ppm

MITSUBISHI UET45/80 RMD 15, with limits of 0.2% water, 3.5% sulphur, 10% carbon residue, and max viscosity 650cSt at 50^oC

WÄRTSILÄ VASA 32 RMH 55, with additional limits on sodium of 100ppm, and asphaltenes at 14%. Allows 1010 density with correct purifiers.

MaK M35 & M551 RMH 55

Thus we can see that the engine builders have constructed recent engines to consume the average `worst-grade' fuels in order to remain competitive, although continuously operating the engine at the fuel maximum limits will increase engine fuel related problems and fuel preparation difficulties.

The oil refiners also attempt to produce oil that is close to these standards, depending ultimately on the crude oil base. However the pricing of fuels is still governed by the final viscosity, with about a $4 discount per tonne for 380cSt oil against 180cSt. Both of these fuels is produced by `cutting' a high viscosity fuel (over 600cSt) with a lighter fraction to produce the desired viscosity, and hence the higher viscosity fuel used will often dictate the impurities in both oils. Thus buying a lighter grade fuel does not imply a better grade fuel, i.e. one with fewer impurities; in fact it may even produce more problems than the original residual stock.

Publications such as Lloyds List indicate the average grade of fuel in different areas of the world, and Northern Europe and the US Gulf have been chosen to show that bunker grades differ slightly around the world.

Density Water MCR Ash Alum/Si Sediment

Europe 992.2 5.0 17.5 0.11 88 0.09

US Gulf 994.5 0.4 19.3 0.08 58 0.14

Hence although you may specify the correct grade of fuel, the supplier may deliver (sometimes unintentionally) fuel that can severely damage the engine. Fuel could be classified as unsuitable if the damage occurs over a small period of time, or if the fuel was obviously totally unusable, such as too low a flash point. Poor qualityfuel would indicate that the fuel could damage the engine, but this would occur over a much longer period.

The only way of monitoring the delivered fuel quality would be to test the fuel using a recognised fuel testing agency. The results obtained from this sampling could be used in

1. Disputes over quality with the supplier

2. Highlights areas where operational adjustments of the main engine or preparation equipment may reduce damage

3. Reduce frequency of poor bunkers, (supplier more likely to supply `other' vessel)

In order to obtain effective results, the sampling of the fuel must follow accepted guidelines to ensure that a representative sample is obtained. This sample should be obtained at the point of transfer, usually the ship’s rail or the closest point, (i.e. the ship’s manifold). Sampling using either the “Continuous Drip System”, or a “Flow Proportional Automatic Continuous Sampler” would be acceptable. This ensures that the small sample and the subsequent fuel oil analysis results give a true representation of the fuel oil quality that has been bunkered.

Once the sample has been taken, the sample bottle is sealed and signed by the engineer, before being sent to a recognised fuel testing agency. The test results will normally be available within three days of the test sample arriving at the testing agency, and will specify some of the following results:

à Viscosity at 100⁰ C. This will allow the engineer to find the required heating temperature required for transfer of the oil within the vessel, and the injection temperature. Temperature ~ viscosity charts held on-board will be used to find these temperatures.

à Viscosity at 50⁰ C. This is an alternate temperature for measuring the viscosity. For residual fuel oils the 100⁰C measurement is considered more accurate. The 50⁰ C temperature is used as its results are still widely used within the marine industry i.e . when 180cSt fuel is referred to, it will be the viscosity of the fuel at 50⁰C; hence the viscosity measurement should always be read in conjunction with the measuring temperature.

à Density at 15⁰ C. This measurement is required by the engineer to calculate the quantity of fuel in tonnes within the vessel’s tanks from volume measurement calculated from the tank gauging. There is a density limit for the fuel treatment plant i.e. purifiers, with the modern purifiers capable of cleaning oils up to a density of 1010 kg/m³. Older purifiers using a water seal can only accept oils up to a density of 991 kg/m³, and still maintain effective operation. Ignition and combustion characteristics of higher density fuels may be inferior and be an indication of poor ignition quality.

à Water. The water limit of 0.5% for residual and less for distillate is defined in all fuel specifications. Quantity shortfall, gassing up, poor lubrication and reduction of combustion energy being some of the problems. Salt water being the main problem resulting in deposits and corrosion. It is possible to homogenise after centrifuging to emulsify (NO_x reduction).

à Ash. This is a measure of all the incombustible material free of carbonaceous matter contained within the fuel. High ash content fuels will lead to abrasion of the fuel injection equipment, and fouling of the exhaust gas path. Ash levels can contain -

· natural organic compounds of the oil such as silicon, sodium, vanadium etc

· Contamination from the refining process, such as aluminium and silicon which are present in the catalytic fines used in some refineries

· Contamination from the storage of the fuel, such as sand, scale, rust, etc

à Micro Carbon Residue. This test replaces the Conradson Carbon Residue test (USA) and Ramsbottom (Europe), but gives the same readings. The carbon residue indicates the difficult to burn carbon and fouling nature of the fuel, and residual fuels always have higher carbon residue levels than marine diesel oils. As the combustion of the residual fuel always produces heavier exhaust fouling, then more frequent in service cleaning of the T/C and EGB and out of service maintenance will be required for these fuels.

à Sulphur. This measurement of the mass of sulphur in the fuel indicates the corrosive nature of the fuel’s reactants once the combustion process is complete. When the sulphur in the fuel has been converted to a sulphur di or trioxide, then condensation of these products will form a highly corrosive sulphuric acid. This produces corrosive wear on components at a temperature of 170⁰ C or lower, such as the cylinder liner wall, piston rings, exhaust valve stems, and the cooler regions of the waste heat boiler. As discussed in the Emission section, most residual fuel oils have a sulphur level between 1 and 4%.

à Total Sediment – Accelerated. This test is primarily for the residual fuels, and gauges the stability of the asphaltene phase of the fuel. When a fuel becomes unstable in storage, the various components of the fuel break-down, and the resultant sludge formed at the base of the tank is high in asphaltenes, and produces heavy filter blockage and fouling/damage to the components in the fuel injection system. The accelerated nature of the test attempts to replicate the long term instability of a fuel using a test, which only lasts one hour.

à Pour Point. This is a measurement of the temperature at which the fuel oil ceases to flow. The pour point indicates the minimum temperature that a fuel has to be held at in order to avoid filter plugging from cold oil. Once a fuel starts to solidify, due to the formation of waxes within the fuel structure, reheating will not totally convert the fuel back to its original state (re-dissolve). Hence it is imperative that sufficient heat is given to the fuel in storage to maintain the oil temperature at 5-7⁰ C above the quoted pour point (transfer pumps designed with a max 800-1000cst). Residual and diesel fuels can have pour points up to 30⁰ C and 15⁰ C respectively.

à Nett Calorific Value. This measures the energy of the fuel as determined by actual combustion of the test sample. Although it is desirable to have a high energy fuel, the chemical structure of the hydrocarbon dictates the energy available, with the residual fuels having a lower calorific value than the distillate fuels.

à Flash Point. The international accepted minimum temperature for the flash point of a marine fuel is 60⁰ C. This limit is stipulated for safe storage of the fuel, store 10⁰ C below, as most fuels are heated in the tanks. Generally fuels have flash points in excess of 100⁰ C, thus it is only when the fuels have been mixed with volatile substances such as kerosene, crude oil, etc that the flash point will be low when tested.

à Silicon. This element can naturally occur in crude oils but usually at low levels. When testing indicates a high level of silicon, then the presence of highly abrasive catalytic fines should be suspected within the fuel.

à Aluminium. This element can naturally occur in crude oils but usually at low levels. When testing indicates a high level of aluminium, then the presence of highly abrasive catalytic fines should be suspected within the fuel. Note the catalyst would normally increase both aluminium and silicon levels.

à Iron. This can include both the naturally occurring irons within the oil, but also the abrasive rust and scale particles present from the transportation of the oil.

à Vanadium. Another natural occurring element, but this will cause corrosive deposits to be formed on the high temperature components of the engine, such as the exhaust vale and turbocharger exhaust gas side.

à Sodium. High levels of this element usually indicate salt water contamination, which should be reduced by an effective purifier stage. It is important that the sodium level is reduced before the fuel is injected into the cylinder, as sodium will combine with any vanadium present to form highly corrosive deposits, as well as heavy fouling of the turbocharger unit.

à Phosphorus (P), Lead (Pb), Calcium (Ca), zinc (Zn), iron (Fe). All of these elements are measured to test for the presence of waste lubricating oils within the fuel oil. Some suppliers add waste automotive lube oil for easy disposal and cost gains. The presence of such waste oil is highly undesirable, as the additives present in the lubricating oil are soluble and will prevent the dirt and abrasive particles being removed by the purifier unit. Hence abrasive damage of the fuel injection equipment, rings/liner and formation of deposits on components such as turbochargers will take place. Environmental issues involve the increase of particulates in the exhaust gas.

Note that none of the specifications include any limits on mixing compatibility, or sodium content, and only the DO grades have a reference to ignition quality.

For compatibility, the aromaticity reserve in the fuel is the important factor. The ashphaltene phase is kept in suspension by the aromatic of the fuel, and extreme care should be taken when mixing residuals with fuels of low aromaticity levels such as distillates. If this were to occur then the ashphaltene phase would precipitate causing filter blockage, and purifier difficulties. The biggest culprits of poor compatibility fuels were residuals from vis-breaker refineries, but as these units are being used less frequently this problem may not increase. Straight run residuals are fuels, which are highly paraffinic oils with the correct viscosity without light fuel dilution, and these tend to be the most stable fuels.

Stability is the resistance to chemical breakdown on its own due to heating /moving which can form sludges.

ISO 8217 (2005) specification tables

Courtesy of Lloyds Register Fuel Testing service - FOBAS

COMPONENTS OF FUEL

The majority of the fuel oils, which we burn, derive their existence from a crude oil base stock. Although all crude oils differ in their make-up, the main constituent levels are similar; eg Carbon 83-87%, and Hydrogen 11-14%, with the remainder made up of sulphur, oxygen, nitrogen and other trace elements.

But although the chemical ingredients are similar, the atomic structures that can be produced vary greatly.

There are three main types of hydrocarbons in crude oil

1. Paraffins or alkane series

2. Naphthaltenes or cyclo-alkanes

3. Aromatics or arenes

Paraffins or alkane series (C_nH_2n+2)

These compounds are the simplest hydrocarbons. They start with the simplest member METHANE, and by adding H-C-H units to the middle of the compound, increase in size and complexity.

The molecules always form straight chains, and the longer molecule units with 5-16 carbon atoms have increased interlinking with one another and exhibit more `viscous' characteristics. This 5-16 carbon group forms the liquid state. Increasing the carbon number count above 16 forms the semi-solids such as waxes.

All members form straight chain, fully saturated (i.e. each carbon atom is attached to 2 hydrogen atoms) paraffins, which end in the suffix `ane' i.e. pentane, hexadecane (the last of the liquids at 16 C atoms). The Self Ignition Temperature for this hydrocarbon group is 220-250^oC.

Naphthenes or cyclo-alkanes (C_nH_2n)

In this structure the fully saturated carbon and hydrogen combination forms closed cyclic or ring structures.

The number of carbon atoms in the ring structure varies from three to seven, but six is the most common e.g. cyclohexane C₆H₁₂.

Although cyclic units can join together, it is more common for straight chain paraffins to attach themselves; e.g. C₆H₁₂ changes to dimethylcyclohexane C₈H₁₆

The Self Ignition Temperature for this hydrocarbon group is 380-420^oC.

Aromatics or arenas (C_nH_2n-6)

These are a series of polyunsaturated hydrocarbons, having a ring structure. The format of `non-localised' double bonds is used to explain how the unsymmetrical, therefore possible chemically unstable unit, has high stability.

These `delocalised' electrons from the double bonds act as grabbers, which allows other elements to form substitution products by attaching themselves to the main primary benzene ring.

The Self Ignition Temperature for this hydrocarbon group is 500-550^oC.

In order to investigate the composition of fuels formed by the residual of refining, we can study three of the main residue streams, namely Wax, asphaltenes and resins

Wax

These are the semi-solid residuals formed when a high proportion of paraffinic substance is present. Due to the inherent value of waxes to form more saleable products, modern refineries extract the majority of the wax from the base stock. Waxes are readily soluble in a petroleum oil base, and only start to crystallise out when the cloud point is reached, which may be as high as 35^oC.

Asphaltenes

These are highly complex structures, with high carbon/hydrogen ratios, and hence high molecular weights. Asphaltenes can be considered to be condensed aromatic rings linked by paraffinic chains. The high quantity of aromatic rings present means that small but significant amounts of complex combined oxygen, nitrogen, vanadium, sodium, water, and sulphur products are locked into the asphaltene structure.

Due to their complex nature, it is difficult for the oxygen and hydrocarbon elements to mix, and hence combustion is prolonged. This tends to produce black smoke (partly burnt hydrocarbons); fouling and high exhaust temperatures which is an indication of incomplete combustion.

Resins

These are low molecular weight asphaltenes with a higher proportion of naphthenic and aliphatic structures. Resins can increase in carbon numbers to form asphaltenes, and are also absorbed into many substances including metals and asphaltenes.

Resinous hydrocarbon compounds, particularly asphaltenes, formed by incomplete combustion, will readily adhere as sticky, semi-solids to metal surfaces and act as a flypaper to trap other deposits.

FUEL CONTAMINANTS

Within the complex fuel structure, various metal elements exist such as vanadium, sodium and nickel. These contaminants do not aid the combustion process, and high levels can damage engine components.

Vanadium

This not only acts as a catalyst in the formation of sulphuric acid, but it also oxidises at high temperature to form a series of compounds of which vanadium pentoxide V₂O₅ has the lowest melting point at 675^oC.

There are no economic methods of removing vanadium from hydrocarbons due to its solubility, although some additives are claimed to react with sodium and vanadium to form other oxides and salt with high melting points, which will exit the engine as very fine solids.

Vanadium compounds can react with relatively soft carbon particles to form clinker-like, abrasive agglomerates, which can increase ring/liner wear.

Sodium

This reacts with both oxygen and vanadium compounds to form highly corrosive deposits. The temperature at which a critical mixture of 3:1 vanadium to sodium (Pento sodium vanadate - 5Na₂O V₂O₄ 11V₂O₅) can adhere to a metal surface can be as low as 450^oC. These deposits build-up on hot surfaces such as exhaust valves or turbine blades, necessitating an increase in turbocharger cleaning. The deposit itself is corrosive and can attack many metals (produces characteristic cobblestone effect), and if the deposit were to break away from a small area of exhaust valve, then a gas passage is formed where hot, high pressure (and hence velocity) gases will escape causing wire-drawing erosion.

The amount of sodium occurring naturally in residual fuels is quite low (35ppm max), and the major increase will come from sodium chloride which is present in large quantities in salt water. 1% salt water ≈ 100 mg/Kg (or ppm) Na. Sodium can be removed by centrifuging from the fuel when it is present in large quantities such as when the fuel contains seawater, with results 1550 to 97ppm, & 88 to 42ppm.

Nickel

In itself this does not cause problems, although it acts as a catalyst in corrosive element production.

Nickel is partly soluble in oil and water, thus only small reductions can be achieved by centrifuging.

Aluminium

This will occur naturally in the oil in very small quantities. However aluminium can be used to indicate the level of oil refining catalyst which is present. This catalyst consists mainly of aluminium trioxide (Al₂O₃) and silica (SiO₂) and is extremely hard and abrasive. Fuel containing a large (over 30ppm) quantity of aluminium could cause rapid liner, piston ring & fuel pump wear.

30ppm Al equates to 250ppm catalyst, and as the substance is not linked with the fuel, it can be nearly totally removed (18 to 4ppm) by correct use of the purifier/clarifier set, especially at low throughputs (hydroscopic).

Sulphur

This is present naturally in the oil chain, with the quantity dependant upon the crude oil base stock, with the `sour' crudes having the higher sulphur levels. Sulphur is converted by combustion into SO₂, then SO₃, before mixing with water to form sulphuric acid H₂SO₄. This is highly corrosive in the liquid state, so if the dew point of the acid/gas mixture is reached, then acid attack on the metal surfaces will result. For 2% sulphur the dew point lies between 154-162^oC, so the low temperature components should be kept above this temperature and other methods used, such as neutralisation of the liner wall by alkaline cylinder lube oil.

Sulphur is combined with the fuel structure and can not be removed by normal methods.

Sulphur can also react with carbonaceous matter at high temperature to form very hard, compact, abrasive deposits especially in the ring zone.

Additives are marketed which claim to eliminate the formation of sulphur trioxide and hence reduce acid production. However on tests the product works on some engines/fuels, whilst not on others.

IGNITION QUALITY OF FUELS

The ignition quality of a fuel dictates the time delay that occurs between the start of fuel injection, and the commencement of ignition in a cylinder. Thus a fuel with a high ignition quality will have a shorter delay period, and visa versa for the low ignition quality fuel.

The ignition of the fuel is dependant on the ease of which the hydrocarbon molecule can be broken away or 'attacked'. The long thin chains of the paraffin are more easily broken down than the compact ring structures of the aromatics (benzene ring).

Thus the amount of aromatics within the fuel structure has a direct relationship on the ignition performance. However measuring fuel aromaticity is difficult, so the concept of CCAI (Calculated Carbon Aromaticity Index) has been used for indicating ignition quality.

CCAI is a ranking of ignition qualities of different types of residual fuels on the basis of known specification properties of density and viscosity. The rating of a high CCAI value 870-950 indicates a low ignition quality fuel, which will give long ignition delays.

Different delays in the ignition of the fuel will affect the rate of pressure rise during the initial period of combustion.

Low CCAI rating - This will have a shorter delay between injection and ignition. The rate of pressure rise within the cylinder will be lower than normal producing a lower peak pressure.

High CCAI rating- This will have a greater delay between injection and ignition. The rate of pressure rise once ignition does occur will be quicker and greater due to the larger amount of vapour in the cylinder. Peak pressure will be greater, although there is little change on the power developed or fuel consumption, if the fuel injection point remains the same.

The most noticeable effect of a long delay period is increased engine noise, diesel 'knock', and 'rougher' running particularly at the lighter loads.

To reduce the ignition delay the following changes can be made:

1. Increasing the compression ratio

2. Increasing the load on the engine

3. Retard the fuel injection point

4. Increasing the charge air temperature

5. Reduce the amount of fuel injected during the ignition delay-cam profile

The ignition delay period is short compared to the injection period for low speed engines, meaning that only little fuel is injected before ignition. Faster running engines may inject all the fuel before ignition causing detonation. Both NSD and MAN B&W have stated that their slow speed engines are not affected by the effects of poor ignition quality fuels, and all the published data relating to damage to engines comes from the medium speed engine manufacturers.

COMBUSTION PERIODS OF FUEL

There are 4 main phases to the combustion of fuel.

1. Injection delay. This is the small delay between closing of the spill ports/valve and the opening of the fuel injector. This delay is affected by the rate of pressure rise within the pumping element (influenced by wear in the pump), the pressure in the injector line (influenced by the delivery valve), very slight compression of the fuel and expansion of the HP pipe.

2. Ignition delay. This is the delay between the start of injection and the start of combustion. In order that a fuel can burn, it must be transformed into a vapour and mixed with the correct ratio of air. In order to do this the fuel must break up into small droplets (10-20mm), and heat up to its self ignition temperature once in the cylinder (cylinder air temp 550-700^oC), therefore dictated by physical and chemical processes.

3. Pre-mixed combustion. This phase of combustion commences immediately after ignition. The fuel which combusts in this phase is:- all the fuel which has been injected, and the fuel which is still being injected. Thus the severity of this uncontrolled combustion is dictated by the length of ignition delay, and the quantity of fuel injected during this delay. Large pre-mixed combustion produces increased max cylinder pressure and rate of pressure rise. This large pressure rise produces diesel `knock', which increases engine noise, and shock loading to bearings, piston rings, combustion cylinder, etc.

4. Diffusion combustion. This phase of combustion commences when the rate of fuel combusting is the same as that being injected. The length of this period is dictated by the type of fuel burnt (HFO takes longer to evaporate or prepare), and the rate at which fuel is injected (fast rate produces small fuel droplets and shorter time to burn). Depends on how rapidly the oxygen and fuel can be mixed, as the initial supply of oxygen near the fuel droplets will have been used during the pre-mixed combustion phase.

If this period of combustion is too long, exhaust soot particles and temperature will increase, and useful power output will fall.

FUEL COMBUSTION

Efficient combustion

Is achieved when the fuel has been thoroughly burnt in the time available, achieved by:-

1. Atomisation of the fuel. High pressure injection through small nozzle holes converts the liquid fuel into a vapour (which has a high surface area to volume ratio) due to high velocity and friction drag with the dense air. This reduces the ignition delay, and the time required to complete combustion. Atomisation is affected by the following factors:-

where p = pressure difference between fuel line and cylinder

m = mass flow rate of fuel

m = fuel viscosity

A = Nozzle hole area

2. Distribution/penetration of the fuel. To avoid areas with too low air/fuel ratios, the fuel injector must distribute the fuel sprays evenly without overlapping, and penetrate far enough into the air (60%). Note that the air will centrifuge towards the liner wall under high swirl conditions. Good, even distribution will reduce the time required to complete combustion. Penetration is affected by the following factors:-

where p = Fuel pressure

d = Nozzle hole diameter

t = Time of injection BTDC

r = density of air in the cylinder

We can see that the diameter of the nozzle hole has a large influence on penetration. Note that the density of air will increase as the piston approaches TDC, thus the fuel initially injected at 15^oBTDC will travel further, as there is less air resistance.

Increasing the length of the nozzle holes will also increase penetration, as the jet is more stabilised, but L/d is usually designed at around 3:1, thus included as a constant in penetration calculations.

Increases in fuel viscosity will increase droplet size, but only slightly increase penetration. The more compact fuel jet will increase penetration, but should not cause excessive over penetration.

3. Sufficiently high air temperatures. In order that the fuel will ignite, the air temperature at the end of compression must be higher than the auto-ignition temperature. Low air temperature will increase ignition delay, and can lead to diesel knock.

4. Air turbulence. To increase air/fuel mixing, especially after the initial ignition has occurred, some turbulence of the cylinder contents are required. This is provided by the burning fuel/air mixture, swirl from the intake air, and squish from the piston shape. This allows the areas of high fuel/air ratios to be diluted so that combustion can be completed. If low swirl conditions exist, soot or carbonaceous particles in the exhaust stream and the diffusion period of combustion will increase.

5. Ample excess air. Diesel engines operate under conditions of high excess air, as the time available for a completed combustion cycle is relatively short (11ms for a 600rpm 4/S engine). Hence in order that the majority of fuel particles can find an oxygen molecule to burn with, there must be an oversupply of air, as combustion can rely on cylinder turbulence alone.

Poor combustion

The desirable properties above can deteriorate when each/all of the following occur:-

A. Incorrect fuel distribution within the cylinder, due to:-

a) Incorrect fitting of injector nozzle during overhaul.

b) Part blockage of injector nozzle due to carbon trumpets.

c) Excess wear of injector holes, which will also increase spray penetration.

B. Incorrect temperature of the fuel. If too low then,

a) Fuel droplets become larger, thus much slower burning. The droplet can form a hard layer around it which reduces evaporation has hence burning.

b) Fuel sprays become more compact, which reduces mixing, and hence increases combustion time and increases exhaust smoke levels.

C. Incorrect fuel pressure due to fuel pump internal wear. This will reduce the maximum pressure delivered by the pump, as well as producing late injection, thus:-

a) Fuel droplet size will increase.

b) Penetration will reduce, as fuel supply now injected later, against higher gas pressures in the cylinder.

D. Incorrect out flow of exhaust due to fouling of the turbocharger or exhaust gas boiler. This will reduce cylinder content purity.

E. Incorrect charge air pressure. This will:-

a) Reduce the pressure and hence temperature at the end of compression, increasing ignition delay.

b) Reduce the quantity of excess air supplied which will increase smoke levels and combustion time.

c) Reduce air swirl, as velocity through the scavenge ports will be reduced. Smoke levels will also increase.

Causes of poor combustion

Can be traced on all cylinders, requiring measurements of the following:-

1. Scavenge air pressure and temperature. Turbo-charger, cooler etc performance will affect delivery. The correct air pressure for the engine load could be found from test bed or previous records. Scavenge air temperatures should be above 35^oC

2. Fuel rail pressure and temperature (viscosity). Checks to be made of the fuel pressure, and viscosity using viscosity/temperature charts, or a calibrated viscometer.

Can be traced on individual cylinders, requiring measurements of the following:-

1. Indicator cards should be taken to record injection timing, Pmax, Pcompr, and/or afterburning, and check the power balance between cylinders.

2. Exhaust, liner and cooling water temperatures. Deviation from normal will indicate problems.

If poor combustion is evident whilst the engine is running it should be shut down and the cause confirmed through visual inspection i.e. scavenge port inspection, removal of suspected components for signs of wear, erosion, corrosion etc- injector, pump, rings, exhaust valve etc.

Hence anything which causes the atomisation number to decrease will affect the efficiency of burning. The factors of p, m and A are all monitored and controlled by the engineer on board. Hence he must ensure that close control of the fuel pump wear (for p), the fuel temperature (for m), and the injector nozzle wear (for A) is carried out.

As with the control of the atomisation, the engineer on board can control the size of the nozzle holes (by discarding worn holes and ensuring clean, non-abrasive fuel), and the density of the air (by ensuring that the air supply system is clean, and that the engine is not operated for long periods on loads below 50%)

FUEL PUMPS

The fuel pumps are used to control when, and how much fuel is injected into the cylinder, from the joint inputs of governor (fuel rack) and camshaft (fuel cam). It is the fuel pump that generates the high fuel pressure that enables the fuel injector to operate correctly to atomise and distribute the fuel within the cylinder.

The governor input to each fuel pump will be identical when new, and this output quantity can be changed by adjusting the fuel rack to each pump. This operation would be carried out to power balance the engine, and compensate for internal leakage within the pump element.

NB When a fuel pump is replaced the governor linkage should be checked so that no fuel injection occurs at zero position, and that full movement is possible. Also check that both the governor and fuel cut-off can reduce the fuel rack to zero.

The point in the engine cycle at which the fuel is injected into the cylinder is critical. Early fuel injection (usually achieved only by timing mal-adjustment) will increase mechanical stresses on the cylinder components, whilst late fuel injection decreases fuel efficiency, and increases smoke and exhaust gas temperature levels.

Fuel pump timing is carried out whilst the engine is stopped. There are various methods by which this can be accomplished, and two manufacturer's methods are detailed below.

MaK medium speed engine types M452/453C

a) Bar the engine to the position of commencement of fuel delivery for that cylinder

b) Check the window on the fuel pump to ensure that the upper mark on the body coincides with the moving mark on the tappet bucket (FIG b)

c) If the two marks do not coincide bar the engine away from top dead centre and check that the lower mark on the body coincides with the moving mark on the tappet bucket (FIG c)

d)

FIG c

Remove the fuel pump and slacken the locking screw (4) and adjust the thrust screw (5) (FIG d). Replace the fuel pump and check the commencement of fuel injection using methods a) and b) above. Continue to adjust until the fuel setting is correct.

FIG b

FIG d

FIG d

FIG e

a) Shut off the fuel to that cylinder, and drain the fuel pump

b) Disconnect the puncture valve at the top of the fuel pump, and the two small plugs to allow the measuring tool to be inserted into the fuel pump

c) Turn the engine to TDC for that cylinder, ensuring that the engine is in the correct direction of rotation (i.e. Ahead or Astern)

d) Place the measuring tool into the top of the fuel pump plunger

e) Ensure that the measuring pin rests into the bottom of the hole in the top of the plunger

(Fig e)

f) Read the measurement from the top of the gauge. For the example shown the reading is –5mm. This means that the fuel pump plunger is 5mm below the spill ports when the piston is at TDC

g) Compare this reading with the measurement given by previous or even test bed readings. Also note the position of the timing rack with all fuel pump measurements

h) If the reading is incorrect, or should you wish to adjust the timing of the fuel pump, then the barrel position can be adjusted using the individual adjustment of the timing rack.

MAN B&W MC slow speed engine series

To increase the maximum pressure by advancing the fuel timing requires the timing rack to be moved in. Increasing the index by 1 will drop the pump barrel by 1mm and increase the peak pressure by 3 bar. This adjustment is carried out by disconnecting pin B and adjusting link E. (FIG i)

FIG i

Following the basic fuel pump setting as stated in the manufacturers manual, the engine should be operated and the various parameters that indicate cylinder combustion should be checked, such as

1. Fuel rack setting

2. Exhaust gas temperature

3. Maximum cylinder pressure

4. Point of fuel injection using the draw card

5. Cylinder power, from indicator card

VARIABLE INJECTION TIMING

The next stage of adjusting the fuel pump timing was to allow the fuel pump timing to be adjusted whilst the engine is running. This is the Variable Injection Pump that is used on all the slow speed engines, and some medium speed engines.

The traditional fuel pump helix had a flat top, which meant that fuel injection was commenced at the same time irrespective of the engine load. However improvements in engine efficiency would occur if the fuel were injected into the cylinder earlier, which would lead to higher cylinder temperatures and pressures. The limit imposed on the maximum pressure was dictated by engine design strength, but once the engine operated below maximum power this maximum pressure would also fall. Hence for an engine operating away from the MCR, there was the opportunity of injecting the fuel into the cylinder earlier to improve the specific fuel consumption of the engine.

The first fuel pumps to achieve this had the upper part of the fuel pump plunger modified to provide early fuel injection from 50% to 85% engine load. If the fuel is injected too early at low powers, then the engine could stall, whereas at high loads early fuel injection would cause excessive cylinder pressures.

This method can control the start point of injection from outside the fuel pump, and therefore is adjusted whilst the engine is running for different operating conditions. The fuel pump is predominately a fixed start of injection, but whilst operating in the `upper load range’ (50-85%), the start of injection is automatically advanced. This produces an increase in maximum pressure from 85-100%, and a constant 100% maximum pressure from 85% to 100% engine load.

MAN B&W Super VIT fuel pump

The diagram shows the main features of the fuel pump that will allow adjustments to be made in service.

Main design points:

· Fuel pump quantity automatically (from govr) adjusted by standard method of rotating the plunger (rack position). This allows the effective length of the fuel pump stroke to be increased.

· Individual/manual adjustments of the quantity can be made at the turnbuckle arrangement at the input/rack to each fuel pump.

· Fuel pump timing is changed by moving the fuel pump timing control lever in and out of the fuel pump. This timing is automatically adjusted by the air servo fitted at each fuel pump. The air pressure for this servo is controlled by the VIT timing servo fitted at the fuel rack. When the fuel rack is rotated to admit more fuel into the cylinder, the output pressure of the VIT timing servo will increase as its piston is depressed. This will advance the fuel timing. However the construction of this VIT timing servo will mean that the change in fuel pump timing is dictated by the cut off/break point of the pivot arrangement and can be adjusted.

· Individual timing adjustment of the fuel pump to correct for worn pumps or to balance Pmax will be via the turnbuckle adjustment at the timing rack of each fuel pump.

Advantages:-

1. Reduced fuel consumption especially in the important 65-85% power range

2. Adjustments to fuel timing can be easily carried out, and without stopping engine, which can:-

i) allow balancing of individual engine cylinder p_max levels,

ii) allow fuel quality effects to be countered.

Disadvantages:-

A. More complex pump unit, requiring greater amount of maintenance. Most VIT pump units do not operate,

B. If incorrectly adjusted, p_max will become excessive, which results in high mechanical stresses and shock.

Another type of VIT unit is used by the MAN B&W medium speed engines. In this unit the separate fuel camshaft is effectively rotated radially by the application of oil pressure.

FUEL PUMP FAULTS

Barrel/plunger wear

Although internal wear will always occur, the wear rate is strongly influenced by the cleanliness of the fuel. The purifier and filtration units should remove abrasive particles in the fuel. A worn pump will:-

· Reduce the max fuel pressure achieved by the pump which influences the atomisation achieved,

· Inject a reduced amount of fuel, resulting in lower power developed by that cylinder,

· Retard the fuel injection.

Pump wear can be compensated for by increasing the fuel rack setting and retiming the fuel pump using indicator cards as the datum.

Cavitation damage

This is caused by an excessive pressure drop at the spill ports, and produces erosion damage just above the helix control edge. This is usually a design problem (fuel flow reflected back against plunger), and will increase plunger wear, reducing sealing efficiency. The only option for the engineer with this fault is to ensure that the fuel pump surcharge pressure is correct, and change the plunger/barrel when excessive damage is present. (Double helix/baffle screw/erosion plug/ shock absorber are all designs to help reduce).

Excess leakage of fuel into camshaft system

Occurs when the barrel is excessively worn, or the sealing arrangement of the barrel is leaking with the drainage holes blocked. Obviously less of a problem with umbrella type and space beneath plunger is exposed (NSD and later MAN B&W), but can introduce low flash point oil with abrasive particles into camshaft/sump system. This will increase cam wear, and increase risk of crankcase explosion. Regular tests of the lube oil system should monitor for fuel ingress, and the seals and possible barrel change will repair this fault.

Wear at discharge valve

The discharge valve is provided to maintain the required pressure in the discharge pipe between fuel pump and injector. If this pressure is too high then the reflection of the pressure pulse in the fuel line against the closed discharge valve will reopen the injector causing secondary injections. These injections will increase the trumpet formation at the tip, and smoke levels from the engine. However if the pressure is too low then there will be an excessive injector delay, which may even prevent fuel injection at low loads. The control of this pressure is achieved by the discharge valve, which causes a controlled reduction in the discharge pipe contents by means of the piston fitted beneath the valve sealing faces. If the valve piston is worn this will reduce the quantity of fuel it removes from the pipeline when seating (& increase seating velocity), and hence secondary injections are possible. If valve leaks or spring is broken, then a greater quantity of fuel will `escape' and hence less fuel will be injected (compared to other cylinders) at next injection, producing cylinder imbalance. Both faults can be easily be repaired by exchanging the valve and spring.

Seizure

Fuel pump seizure may occur if the clearances between the plunger and barrel are reduced, water in fuel or no fuel (the lubricant!) is present. Obviously the fuel should never be closed to an operating fuel pump to stop fuel delivery, but the most frequent seizure cause is excessive temperature changes when changing between fuel grades. MAN B&W recommend that the rate of fuel temperature change should not exceed 2^o C per minute, and this can only be ensured if the temperature control is measuring temperature rather than viscosity. If there is only a viscosity control on your vessel, then a manual change of the fuel temperature should be carried out.

A fuel pump element that has either started or is fully seized should be replaced.

OTHER FUEL INJECTION SYSTEMS

Pilot injection system

A number of manufacturers have investigated a pilot injection system in which two pulses of fuel are injected by the one injector. The first small charge has a long delay period, but when it burns it heats up the cylinder so that the second charge burns with much reduced delay, which produces a smoother combustion sequence. This can be achieved either by an electronic control of the injector, or by a double injection profile on the single Bosch type fuel pump plunger.

Twin injection system

This system is used by Wärtsilä and utilises two fuel injectors per cylinder, a small valve for the pilot injection, and the larger main injector. The pilot injector injects the same amount of fuel each cycle (just below that require to idle the engine under no-load) and is used to reduce diesel knock and ignition delay on the main charge. This mechanism uses the pilot charge to heat up the main charge, similar to pilot injection, and allows the engine to operate for `unlimited' periods on minimum load, and burning of the highly aromatic fuels in medium speed engines. Note that medium speed engines are much more sensitive to changes in fuel quality (i.e. CCAI index)

Twin fuel pump barrels

This system is used by Wärtsilä on its largest medium speed engine 640mm bore, and utilises two fuel pumps per cylinder. The main reason is to ensure high pressures for the fuel injection, which they say can not be achieved using a single pump plunger. Thus two pump plungers operate in parallel, and deliver to the same fuel injector. However one plunger controls the start of the fuel injection (timing), and the other controls the end of fuel injection (quantity).

FUEL INJECTORS

The function of the fuel injector is to atomise and disperse the fuel evenly within the combustion space, which is achieved by the size, position, and orientation of the injector nozzle. It also acts as a non-return valve to prevent gas blow-back into the fuel system.

In order to achieve satisfactory atomisation at the commencement/end of injection, the valve should not open until a pre-set pressure is reached. This pressure should ensure that the fuel droplet size at the start of injection is not too large, which will increase “slow burning” of the fuel.

Another requirement of the valve is a prompt opening action, to prevent throttling (i.e. pressure loss) during the needle valve opening. This is achieved by using a differential needle valve, which opens with a snap action when the high pressure fuel acts on the full cross section area of the needle. One drawback of using this differential valve, is the lower closing pressure of the valve, but the larger droplets injected at this point will be injected into a hotter cylinder and should ignite and burn relatively fast.

The injector must be cooled in service to prevent softening of the valve and seat, and to minimise expansion of the trapped fuel in the fuel sac. Cooling can be achieved either by separate oil or water system, or more recently using the bore cooling system within the cylinder head to cool the injector.

To minimise the need for engines to run on distillate fuels, modern engines are designed to operate on residual fuel even whilst manoeuvring.

For the MAN B&W slow speeds, the fuel injector is provided with a method of spilling/recirculating the hot system oil when the injector is not injecting fuel. This is carried out using a pressure sensitive re-circulation system that only allows the fuel injector to recirc fuel when the pressure is between 2 and 8 bar. Hence once the fuel pump starts to inject fuel, this re-circulation line is closed.

NSD use a similar approach on their fuel injectors, in that the re-circulation unit (4) allows heated oil to flow through the injector when the high injection pressure is not present. Also note on this sketch the close proximity of the cylinder head cooling passages, which effectively cool the injector.

FUEL INJECTORS FAULTS

Over heating/under cooling

If cooling of the injector is reduced, either by fuel valve cooling system or poor heat transfer to the cylinder head, then the working temperature of the injector will rise. This can cause:-

· Softening of the needle and seat which increases the possibility of nozzle leakage and/or,

· Fuel to expand/boil out of the fuel sac, leading to carbon trumpet formation, and increased levels of HC and smoke in the exhaust gases.

Over cooling

More common on older vessels with separate fuel valve water cooling systems. When the injector is over cooled, the tip of the injector falls below the condensation temperature and acid corrosion due to the sulphur in the fuel oil occurs. This can severely corrode the injector tip, causing the spray pattern to be affected.

Nozzle (needle/seat) leakage

This fault will produce carbon trumpets as the dribble of fuel burns close to the tip and the carbon deposits remain. The formation of the trumpets will have a progressive affect by influencing the spray pattern of the fuel, and this can be detected in the increased exhaust gas temps and smoke levels.

Nozzle leakage can sometimes be identified by a seat defect, i.e. the seat is no longer narrow in appearance, and is caused by:-

· Insufficient cooling,

· Dirt within the fuel damaging/abrading the seating area,

· Excessive needle valve hammering, due to excessive time in service, excessive needle lift or spring force.

Weak spring

This will cause the injector to open and close at a lower pressure. Thus the size of the fuel droplets will increase during these injection periods.

Increased droplet size at the start of combustion will decrease the maximum cylinder pressure (late combustion), whilst increased droplet size at the end of combustion will increase the exhaust temperature and smoke (afterburning).

Causes of a weak spring are usually metal fatigue, due to an excessive number of operations.

Slack needle

Slight leakage between the needle valve and its body is required to provide lubrication of the moving parts. However excess leakage due to a slack needle will allow a greater quantity, and larger size of fuel particle to pass between the valve and body.

The quantity of leakage should not influence injector performance unless excessive, but dirt particles between the needle and body can increase friction and make the needle action sluggish.

The cause of a slack needle is usually poor filtration of the fuel causing wear between needle and body.

Poor atomisation

This will increase the size of the fuel droplets, which will increase the time required for combustion. Thus engine noise, exhaust smoke, exhaust temperatures, etc will increase. Poor atomisation can be caused by low injection pressure (fuel pump wear), high fuel viscosity and nozzle hole obstruction such as carbon trumpets.

Poor penetration

This will reduce the mixing which occurs between the fuel and air, and will increase the over-rich areas in the centre area of the cylinder. Thus only following combustion in the centre area will the expanding gases move the fuel charge into the air rich outer ring of the cylinder where the greatest mass of air is present.

This will increase the time required for combustion as the fuel/air mixture is not correct in many areas, and hence afterburning, exhaust temps, and smoke will increase.

Causes of poor penetration is reduced injection pressure, and nozzle hole blockage such as trumpets or sac deposits.

Over penetration

This will occur when the air density within the cylinder is reduced, or with over-size holes. The liquid stream travels too far into the cylinder, so that a high level of liquid impingement on the liner wall takes place. This will remove the liner lubrication, and once burning will greatly increase the liner wall temperature, and its thermal stress.

If this over penetration is caused by prolonged low power operations, then “slow speed” nozzles should be fitted.

Slow steaming nozzles can be used when regular and prolonged engine operation is required between 20-50% power.

The nozzle hole diameter is reduced to

1. Reduce the penetration that will occur into the less dense cylinder air

2. Keep the atomisation level and injection pressure sufficient, as mass flow rate is reduced.

If the engine is operated for long period on low levels of power/speed with `normal' size injector nozzles, then the atomisation will reduce, thus engine noise, mechanical loading, exhaust smoke, exhaust temps, and fuel consumption will increase.

FUEL PUMP CAMS

The profile of the fuel cam is dictated by the following requirements:

1. Need to avoid sudden accelerations, which increases shock loading and the motive force required.

2. Smooth deceleration, to avoid the plunger leaving the cam profile at the end of injection, or bouncing. As bouncing speed is proportional to

then increasing the nose radius will reduce k and so increase the permissible speed without bouncing. However increasing this radius also reduces the effective stroke of the cam.

3. The level of max fuel pressure required, dictated by the speed of fuel pump plunger. This can be at constant velocity, or slightly rising/falling before spill ports open.

4. The fuelling rate of the engine, depends on pump stroke/bore size, engine speed, and effective nozzle area.

5. Rapid decrease in fuel pressure when spill ports open, this needs sufficient sized spill ports and slowing plunger speed.

6. The amount of dwell angle provided at the toe of the cam profile may take into consideration some of the following:

· checking plunger clearance

· ensuring that on 4/S engines the fuel injection will take place near to exhaust valve opening timing should the engine inadvertently run in the opposite direction

· to phase spill and filling sequences, such that pressure fluctuations in the fuel filling line and pump inlet chamber are reduced

· to combine the fuel and exhaust cam profiles on a single 2/S engine camshaft, for reversible engines

Thus from basic cam profile, modifications may be carried out to produce the best profile, which probably will be a compromise of the various factors involved. Considering the above, it is highly unlikely that a symmetrical harmonic motion unit such as a crank or eccentric would be effective.

DOUBLE SKIN FUEL LINES

The main cause of engine room fires for many years is the ignition of oil on hot surfaces escaping from pressurised lines.

In order to reduce this risk;

1. All hot surfaces should be protected. Although this should be easily carried out, repetitive maintenance on machinery can often reduce the quality of the fit of the protective devices, or shields used.

2. Pressurised oil lines should be encased. It is difficult to protect all lines, as that would mean all transfer lines would need protecting which is uneconomical. However an outer protective sheath should cover vulnerable lines that are exposed to vibration, and are close to the hot surfaces. In fact it is Class regulations that ALL fuel injection lines are protected in this way for UMS operated ships.

Although the Class or Flag requirements do not state that an alarm system has to be fitted, it is a prudent option, as if a fuel injection line fails, the quantity injected into the cylinder will fall, and there will be an imbalance of cylinder developed powers. Various methods have been used to indicate failure of the main fuel line:

A. Detection of the pressure between the fuel line and the outer sheath. This pressure should obviously be zero when no leakage occurs. A pressure sensor is fitted to monitor this pressure, and when it rises the alarm system will either lift the fuel pump concerned, or activate the leakage alarm. There is a small vent on this void space to prevent any small leakage at the connections or joints falsely activated this alarm. Note that the sheath can not accept high pressures, so that a vent valve is often fitted which will prevent the sheath pressure rising above 1 bar. This method was fitted to many MAN B&W GF series engines.

B. Monitoring the quantity of liquid leaking from the void space between the sheaths. Under normal conditions no leakage occurs, but should a fault exist then the leak off fluid is collected in a small tank. Once the quantity of liquid exceeds a pre-determined limit, an alarm will be sounded via the activation of a float switch. To prevent this alarm occurring due to a build-up of liquid, rather than a definite leak, there is often a small leak-off hole fitted to the tank. This system is shown on the diagram overleaf.

C. Monitoring of the cylinder exhaust temperatures will also indicate when a fuel line has failed, although this is not this system's primary function. It may occur that even though a fuel line has partially failed, the sensitivity of the exhaust temperature monitor may not be sufficient to provide an alarm.

The MCA have published MIN 25 on the Failure of Engine Low Pressure Fuel Systems. This investigation was in response to the frequent failures and consequent fires on-board due to fuel leakage, often from low pressure rather than high pressure fuel lines.

The main contributing factors of this leakage was the effects of high pressure pulses (up to 60-80 bar) caused by the spill operation at the end of the fuel injection, coupled with the effects of frequent dismantling and re-assembly for maintenance.

Some of the recommendations are listed:

· More stringent controls on the installation, inspection and maintenance of low pressure fuel systems

· ISM assessors should consider whether the ship’s SMS should contain procedures to identify possible causes of pipe failure, and protection of hot surfaces

· Pipe work supports and retaining devices to be checked every 6 months

· Flexible pipes to be closely examined, and care taken when tightening to avoid twisting. Recommends that these pipes are pressure tested every five years or even renewed.

· Engine builders to limit the pressure pulses to under 16 bar. Only pressure accumulators fitted to the fuel pump have proved effective in limiting the pressure pulses.

· All copper and aluminium-brass piping should be heat treated to reduce work hardening effects

· Spray and defector plates to be correctly fitted after dis-assembly

· Change in pipe design to eliminate compression fittings and copper/brass pipework

· Remote isolation valves should be fitted in the event of fuel leakage as suggested in M Notice 1456

· Knowledge of the fuel systems and the problems of pressure pulses should be included in the training and examination of Engineering Officers.