VARIABLE INJECTION TIMING

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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.

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 PUMPS

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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)           

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.

FUEL COMBUSTION

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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%)

COMBUSTION PERIODS OF FUEL

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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.

IGNITION QUALITY OF FUELS

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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.

FUEL CONTAMINANTS

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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.

COMPONENTS OF FUEL

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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.