diesel injector failure

7/30/2019 Diesel Injector Failure

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DIESEL INJECTOR FAILURES AND THECONSEQUENCES - CAUSED BY FUEL

QUALITY CASE STUDIES

Author: AJ von Wielligh Pr Eng BSc Eng (Mechanical)NDL Burger Pr Eng M Eng (Mechanical)PL de Vaal Pr Eng PhD (Chemical)

Staff members of the Department of Mechanical and Aeronautical Engineering andChemical Engineering, University of Pretoria.

Paper delivered at the International Conference of the South African Institute ofTribology - 24 March 2004.

Awarded the Engen prize as one of the two Best Local Papers


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1. ABSTRACT

The modern diesel engine has been developed from older generations to satisfythe requirements of modern day operation. These developments took place inthe injection system, combustion, piston design, breathing, lubrication, etc.

In order to achieve higher outputs, lower fuel consumptions as well lower airpollution, several changes had to made to the combustion and injection system ofengines. The sulphur content of the fuel was lowered in order to improve airpollution properties, and the number of orifices in injector tips increased, while theinjection pressures were stepped up. This was done in order to achieve morecomplete combustion at a faster rate and thereby decrease hydro-carbons inexhaust. The lower sulphur content of the fuel, in some cases necessitated theaddition of additives to fuel, in order to improve the lubricity properties of the fuel.Due to the higher pressures required in the injection system, clearances betweenmoving parts had to be reduced and this placed another demand on the fuel,namely that of cleanliness.

A large number of engine failures have recently occurred on these modern dieselengines, which can be directly blamed on the quality of the fuel used. Because ofpoor lubricity of the fuel, as well as some particle contamination, injectors failedprematurely, leading to poor combustion and subsequent damage to the engine.Several failures were investigated and eventually tests were conducted on the

lubricity and particle contamination of the fuels used in these engines. The testsproved that whenever the lubricity of the fuel is lower than an accepted norm,injector failures occur and engine failure follow. Several cases were also studiedwhere particle contamination of the fuel occurred and this in turn led to injectorfailure and subsequent engine damage.

The fuel supply situation in South Africa is such, that it is very often difficult topinpoint the source of the poor quality of the fuel. It is accepted that fuel leavethe refineries within acceptable quality specifications, but along the supply chain,dilution with other fuels occur and particle contamination is picked up. This paperdeals with a comparison between the damage to the injectors caused by fuel withpoor lubricity and fuel with good lubricity but contaminated by dust particles.


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2. Introduction

In order to meet the demands of industry, the manufacturers of diesel engineshad to drastically redevelop the modern diesel engine. These demands cover awide range of aspects. The most important of these are the following:

1. Higher output2. Smaller engine size3. Higher efficiency4. Lower air pollution5. Longer engine life6. Longer service intervals

To achieve these demands, the modern diesel engine had to be redeveloped indifferent areas. The main area of development was the injection and combustionsystem. The system is electronically controlled to ensure more exact control overthe amount of fuel, the point of injection and the rate of injection.

The injectors, although electronically controlled are nowadays operating atsubstantially higher pressures and with a bigger number of orifices. They aredischarging the fuel into the combustion chamber at extremely high pressures,much higher than before and this is done in order to make the fuel dropletssmaller, which result in more complete, faster and more efficient combustion.

To achieve these higher pressures, the clearances and tolerances on injectioncomponents are much smaller than in the past and this places a demand onlubricating these components as well as keeping them clean.

The modern diesel engine therefore places a much higher demand on the

cleanliness and quality of the fuel that it uses, than its predecessors. If thesedemands are not met, the engine will not perform as designed and prematurefailure will result.

During the regular investigation of engine failures, it was found that a largeproportion of engines failed due to the seizing of the piston in the cylinder liner, orthe failing of crankshaft bearings due to lubricating oil being diluted by fuel.

Several cases were encountered where the piston crown started melting, or thesides of the piston skirt and the piston crown seized onto the cylinder liner.Several cases were also encountered where bearings failed on the crankshaft,usually big-end bearings.

The root cause of these failures could be traced back to combustion relatedproblems, which in turn was caused by injector failures. The injector failures inturn, were caused by poor quality fuel.


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3. BACKGROUND OF COMBUSTION IN A DIESEL ENGINE

In order to understand the problem around poor combustion it is necessary tounderstand the combustion process in the diesel engine.

Diesel engine injection principles and operation

The piston of a diesel engine, fits tightly in the cylinder to provide highcompression, in order to cause ignition of the injector fuel. The fuel is delivered inan exactly metered quantity, which is metered by the injection system. This fuelis delivered to the cylinders at very high pressure and is broken up into a very finespray with droplets diameters of a few microns.

This is achieved by forcing the fuel through very small orifices at extremely highpressures. In the modern diesel engine, the tendency is to increase the numbersof these orifices and thereby making them smaller as well as increasing thepressure. In some of the modern diesel engines the injection pressure is in therange of as much as 200 MPa. This is typically 10 times higher than the oldergeneration of engines.

The result of the higher pressure and smaller orifice sizes is that the droplets aresignificantly smaller and the exposed area is therefore significantly bigger. Thisresults in a smoother and faster and more complete combustion. The typicalspray pattern is indicated in Fig 1 and Fig 2 below.

Figure 1 Figure 2

In order to understand the mechanism and importance of the fine droplets, themechanism of the injector must be briefly discussed.

Figures 3, 4 and 4a show the typical layout of an injector, which is commonlyused in diesel engines. Figure 3 shows the more conventional system, whereasfigure 4 shows the electronically controlled type of injector. Fig 4a shows aCommon Rail Injector.


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Figure 3 Conventional

Fig 4 Electronic Unit

Fig 4 a Common Rail

The injector consists in essence of a needle with a sharp point, which acts as aseal and securely seals off the orifices at the seat at the end of the needle tip.

The needle has a thinner section, which ends up on the seat. At the back end,there is a larger barrel shaped section, which fits very tightly into a passage in thebody of the tip. This needle is forced down onto the seat by means of a springforce, which is adjustable during setting up of the injector and thereafter staysunchanged.

High pressure fuel is then delivered from the pump and the high pressure acts onthe shoulder of the barrel, causing a force which apposes the spring force. Whenthe hydraulic force is able to overcome the spring force, the needle is lifted anddisplaced and therefore the sharp end of the needle is pulled away from the seat.This enables the high pressure fuel to flow out through the holes of the needle tipand to be squirted into the combustion chamber. The flowing out of the fuel,

causes the pressure to drop in this chamber and the spring force is then againstronger and able to quickly push the needle back and to close the orifices.

It must be kept in mind, that the hydraulic force is provided by the engine powerand can usually build up very high forces, due to the pressures involved. Thespring force is however limited due to the inherent force, to which it is adjusted.This means that when the needle starts getting sticky, the hydraulic force will liftthe needle, but the needle might be sluggish and slow in the return back to theseat. It could also mean that the pressure of the needle end onto the seat is not


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enough to properly seal off the fuel. This causes the fuel to leak out. This is thenreferred to as dripping of the fuel from the nozzle.

In the hot air caused by compression, the fuel starts burning and due to the heatrelease, the pressure rises. The piston is then forced down, to produce the powerof the engine. It must be kept in mind that the injection is not an instantaneoushappening. The injection process has a certain duration and the more fuel that

has to be injected, the longer the process takes. Some injection processesconsist of an initial pilot injection to start the flame burning, with a secondaryhigher volume injection where the majority of the fuel is delivered. The injectionprocess starts when the needle lifts off the seat and opens the orifices for the fuelto be squirted out under very high pressure. Figure 5, indicates the cylinderpressure, the needle lift, as well as injector pressure. Figure 6, is a photographindicating the combustion process from about 7 Before Top Dead Centre,(BTDC) to 30 After Top Dead Centre (ATDC). From these photographs, it canbe seen that the biggest flame is present just after 13, which is typically theposition namely between 15 and 20 (ATDC), when the biggest pressure isneeded on the piston crown.

P = CylinderPressureL = Needle liftPi = Injector Pressure

Fig 5

Fig 6 From ( Internal Combustion Engines by JB Heywood)


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SPRAY PATTERN REQUIREMENTS

The injected fuel is broken up into a very fine spray consisting of micro finedroplets. The combustion process starts by the oxidising of the fuel droplets fromthe surface of these droplets. It must be kept in mind that the smaller the dropletsthe bigger the specific area. This means that the combustion takes place fasterand more efficiently, with smaller size of droplets. There is therefore a tendency,

to use smaller droplets and finer spray in the high injection pressure moderndiesel engine.

When combustion takes place efficiently and properly, the droplets burn outcompletely, before they reach the cylinder liner. The fuel is normally sprayed intothe combustion chamber, which is housed inside the piston crown. As the pistonmoves downwards in its power stroke, the spray protrudes further into the volumeof the combustion chamber, but it should be burnt out before any droplet reachesthe cylinder walls.

However, when the spray pattern generated by the injector, is not as describedabove, the droplets become bigger and therefore take longer to burn out, or a jetof fuel is emitted from the orifice, instead of the very fine spray.

The following photographs, Fig 7 (good) and Fig 8, (poor), indicate good and poorspray patterns.

Fig 7 Good Fig 8 Poor

3.3 CONSEQUENCES OF POOR SPRAY PATTERNS

When a poor spray pattern exist, as described above, the following actionsusually take place:

3.3.1. Washing away of the oil film on the cylinder wall

Whenever a jet of diesel fuel is directed onto the cylinder wall, the thin film oflubricating oil is washed away. This leads to dry rubbing of the piston and pistonring on the cylinder wall. Due to the absence of the lubricating film, the friction


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coefficient rises and excessive heat is developed. Damage to the surfaces,leading to eventual seizing of the piston usually results. In some casesaccelerated wear can also take place. In the initial stages of piston failures, theposition where the jet of fuel is directed onto the cylinder wall can clearly be seenon the piston crown and on the piston side where seizing starts. The damageusually starts above the top piston rings and then gradually works downwards,towards the skirt of the piston. The following photograph, Fig 9, shows such a

piston, which is in the initial stages of seizing.

Fig 9

3.3.2 Dripping from nozzle

Another possibility is that due to poor closure of the needle on the seat in theinjector, fuel might drip from the injector tip and wet the surface of the piston

crown. This then results in combustion taking place directly on the piston crown.The protecting stationary gas layer, which normally protects the piston material, isno longer present and this eventually results in melting of the piston crown, whichis shown in photograph Fig 10 below.

Fig 10

Seizing

Melting


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3.3.3 Fuel dilution of lubricant

In cases where the spray pattern is such, that the droplets are bigger than theyshould be, the fuel does not burn out completely in time. The drops of fuel thenreaches the cylinder wall and thins down the lubricating film. The fact that thesedroplets are spread over a wide area, means that the washing away is not

localized. This usually results in dilution of the oil film and subsequent dilution ofthe lubricating oil in the crank case. In such cases the piston does notnecessarily seize in the cylinder liner.

Due to the fact that the lubricating oil dilutes in the crank case, the viscosity dropssubstantially and the oil looses it ability to carry the heavy loads of the engine.This results in bearing failure, usually the big-end bearings of the crankshaft.

Rapid wear of the piston rings can also take place in such an instance. Severalcases where investigated, where the wear pattern on the ring, almost resembledthe same situation as where dust was inhaled by the engine. The ring cornersare usually sharp and spectrographic oil analysis indicates high iron content in thelubricating oil. The Silicon content of the oil is however then usually normal,which indicates the absence of excessive dust. The rapid wear process, thenleads to poor sealing of the piston ring. The blow-by of the combustion gassespast the piston ring increases.

The following photographs, Fig 11 and Fig12, shows the main bearings of aDiesel engine which has rubbed and the block, which was damaged when aconnecting rod was pushed through the side, due to a big end bearing failing.

Fig 11 Wiped Bearings Fig 12 Block Damage

4. REQUIRED PROPERTIES OF FUEL

As mentioned above, it is important to remember that the needle must movefreely up and down the passage under the influence of either hydraulic or springforces. It must furthermore be kept in mind, that no lubrication other than the fuelis available for the movement of the needle and for any part in the whole injection


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system. There are therefore certain demands that are placed on the fuel toprovide proper functioning of the injection equipment. Apart from therequirements for proper combustion, the most important physical requirements forgood quality diesel fuel are the following:

Good lubricity

The fuel must have enough lubricity properties, in order to provide lubrication toall the moving parts in the whole injection system. The conditions in the injectorsare however very severe and therefore special lubrication properties is required inthis area. This is even more important in the modern diesel engine.

In the past, fuel refined from petroleum contained approximately 0,5% Sulphur.The Sulphur and other compounds present in the fuel had lubrication properties.In the older generation engines, it was usually never necessary to provideadditional lubricity additives to improve the fuel quality.

However with the demand on a cleaner environment, Sulphur levels had to bebrought down. Presently the South African limit for Sulphur is 0,3% and latelyfuel with typically 0,05% Sulphur is also available. Experience has indicated thatwhenever Sulphur levels in the fuel drops below 0,3%, the inherent lubricityproperties of the fuel is not sufficient to lubricate all the components, especiallythe injection equipment of Modern Engines. Problems in the needle area thenstart appearing. This is caused by a lack of lubrication, between the movingneedle and the stationery passage of the injector tip. Water in the fuel alsoseriously affects the Lubricity.

In the case of synthetically produced diesel and other fuels like kerosene, theSulphur content is very low and these fuels do not have inherent lurbricityproperties. It is therefore necessary to add additives to the fuel to render the fuel

acceptable in a diesel engine. The purpose of these additives is to providelubricity properties, as well as some cleaning and detergent properties to the fuelnozzle.

Cleanliness of fuel

Due to the extremely small clearances between the moving parts in the injectionsystem and especially in the injectors, lately as little as about 1 micron, the fuelhas to be cleaner than before. The small particles present in the fuel, can enterthe small space between the needle and the barrel of the injector tip and this cancause jamming and damage to the needle. For this reason, very fine filtration ofthe fuel is lately being carried out on modern diesel engines. Several new

engines are now equipped with fuel filters of 2 micron capability. In the light ofrecent injector failures, this may still be inadequate.

4.3 Tests on Diesel fuel

In order to control the properties of lubricity and cleanliness, tests are done onfuel to determine the lubricity properties and the cleanliness levels. The followingtypical tests are done, in order to determine the lubricity and cleanliness of thefuel.


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4.3.1 Lubricity

a) The Optimol SRV machine.

The layout of the Optimol SRV machine is shown in Fig 13:

Fig 13

The SRV machine consists of a steel disc, which is mounted on a load cell, whichcan measure forces reacting on the disc. A loaded steel ball, is rubbed on the topof the disc, in an off set manner. Due to the loading of the steel ball on the disc,friction forces are created between the two bodies. The ball is clamped in aholder, which can be oscillated and a load is applied to the ball. The frequency ofmovement, the load, and the amplitude of movement can be adjusted andcontrolled by a computer. The temperature can also be maintained at virtually

any required level. Typical test values are:

Frequency: 50 Hz

Amplitude: 1 mm

Temp.: 110 C

Load step duration 1 minuteThe operation of the machine is such that the set-up is done and the temperatureis raised to the required level, typically 110C. The machine is then started, whilethe ball and the disc is covered with a film of the fluid that has to be tested, forinstance diesel fuel. The fuel then provides a lubricating film between the slidingball and the stationary disc. Due to the fact that all the forces are known, thecomputer calculates the friction coefficient, which at any point in time exist

between the ball the disc. After a 1 minute period, the load is increased by 50 Nand the process carried on. Whenever the friction coefficient reaches a value of0,3, it is considered that the film between the ball and the disc is penetrated andthat solid to solid contact takes place. The loading force acting on the ball isconstantly measured and at the point of failure, it is recorded. It has been foundby a large number of practical tests, that a good quality diesel fuel, should be ableto withstand a load of 700 N, before the high friction value appears. When levelslower than 700 N is obtained, the fuel usually causes injector failures.


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b) The High Frequency Reciprocating Rig (HFRR) (as per SANS 342 (2006)

Although the SRV machine described above is strictly speaking also an HFRR,the term HFRR is normally reserved for a machine consisting of a circular discwhich is kept stationary and a small ball which is rubbed across the disc. Thedifference is that in this case the load is kept constant at typically 20 N. Thefrequency is the same as that of the SRV namely 50Hz and the stroke length is

1mm, which is also a typical stroke of the SRV machine.

In the case of the normal HFRR machine, the temperature is kept at 60C and thetest is typically run for 75 min. After the period of 75 min, the ball is removed andthe flat area, which is worn onto the rounded ball is measured under amicroscope. The diameter of the flat face of the ball is normally the indication ofthe lubricity quality of the fuel. A good quality fuel must provide a flat area smallerthan 400 or 460 microns, depending on the specification. Fig 14 shows such awear scar on a test ball.

Fig 14 Wear Scar

4.3.1.1 Results of Lubricity Tests

The following graphs in Fig 15 and Fig 16, obtained from the SRVmachine, indicates a fuel with good lubricity properties, as well as a fuelwith poor lubricity properties.

Fig 15 Good Fig 16 Poor


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4.3.2 Cleanliness

The cleanliness level of the fuel can only be determined by filtration or by particlecounting of the contaminants of the fuel. It is customary to filter the fuel samplesbefore lubricity tests are carried out. In the case of the Tribology Laboratory atthe University of Pretoria, the fuel is typically filtered through a 0,4 micron filter

before the test is done. The residue on the filter paper can be analysed. Aparticle counter can also be used to determine the cleanliness levels.

The graphs in Fig17 and Fig18, show fuel where the fuel was deliberatelycontaminated with very fine dust and then put through a 2 micron filter. Thesamples of fuel were taken before the filter and after the filter. It can clearly beseen on the graphs, that the contamination caused a very spiky line of frictioncoefficient, but that there is a significant improvement after filtration. It is howevernecessary, to note that even after the 2 micron filtration, the spikes are stillpresent, although to a lesser degree. The particles passing this filter can still dodamage to the most modern injector. The fuel must therefore be kept clean allthe way along the supply chain.

Fig 17 Fig 18

5. DAMAGE TO INJECTORS DUE TO POOR LUBRICITY AND DUE TOCONTAMINATED FUEL

5.1 Lubricity

Two recent case studies provided excellent examples of injectors that failed. Inone case the injector failed due to poor lubricity and in the other case due to

contaminated fuel.

In case study one, the engine was completely overhauled and new cylinder liners,pistons, bearings, injector tips, etc. fitted.

The engine was then tested on a dynamometer and within 1 hour the engineseized, on the number 2 cylinder.


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The engine was stripped and the piston and liner was removed and replaced.The injectors were not removed from the two cylinder heads and remained in theirexact positions. When the engine was re-assembled for the second time, thecylinder heads were interchanged and the front cylinder head was placed at theback, while the back cylinder head was placed at the front position. The enginewas again started up and tested on the dynamometer and within 1 hour theengine seized again, now on the number 5 piston. The two pistons that were

involved is shown in photographs Fig 19 and Fig 20.

Fig 19 Fig 20

It can be seen that the failure pattern of these two pistons are exactly the same.It was then realized that the number 2 injector was the cause. The same injectorwas initially in position 2 and later in position 5. This injector was then openedand inspected. It was found that the needle had score marks on the shank of theneedle. The needle next to a 2 cent coin is shown in photograph Fig 21 and the

damage to the needle is shown under the microscope in photograph Fig 22.

Fig 21 Fig 22


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The fuel was tested on the SRV machine at the Tribology Laboratory at Universityof Pretoria, and the results are shown in figure 23.

Fig 23

It can be seen that the fuel sample did not meet the required 700N on the SRVmachine but only a load of 350 N was sustained.

5.2 Contamination

During the investigation of two six cylinder engine failures in the same area, itwas found that both engines started to run unevenly. White smoke was emittedfrom the exhausts when the engines failed. The injectors of both engines wereremoved. The 12 injector tips were then inspected to determine the cause of thefailure.

It was found, that both sets of injectors suffered from exactly the same damage.Some of the needles of these injectors are shown in photograph Fig 24.

Fig 24

Damage


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The wear pattern was very peculiar, in that a narrow band of wear was visible atthe front end of the shank of the injector needle. The damage can be seen withthe normal eye and this damage is shown in photograph Fig 25.

Fig 25

Under the microscope the damage can more clearly be seen and some of thedamage is shown in photographs Fig 26 and Fig 27

.

Fig 26 (50X) Fig 27 (60X)

The farms on which the engine problems occurred were then visited. Fuelsamples were taken from the Bulk Tank , the Tractor tank as well as the returnline from the Common Rail system. The samples were filtered through a 3 micronMicropore filter, then through a 1.2 micron filter paper and finally through a 0.22filter paper. The filter patches after the filtering are shown in Fig 28 . It can beseen that the fuel was received in a slightly contaminated state, but that most of

Wear Scar


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the contamination took place between the Bulk tank and the Tractor tank. Theeffect of the 2 micron onboard filter can also be seen.

Fig 28

5.3 Comparison

When the appearance of the injector needles that failed due to lubricity and theones that failed due to contamination are compared, it is quite clear that there is adistinct difference in the characteristic damage to the injector needles. This can

clearly be seen in photographs Fig 29 (Lubricity) and Fig 30 (Contamination).

Fig 29 ( Lubricity ) Fig 30 ( Contamination )


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The effect of both these types of failure, is that the needle becomes sticky in hismovement and that bigger droplets are emitted and in some cases jets of fuel areemitted from the injector tip. This would then result in piston failure and/or big-end bearing failure.

The effect of the stickiness of the needle is that the seat of the needle gets

damaged and Fig 31 shows such a needle under the microscope, indicating therough sealing surface which is no more in a position to seal properly.

Fig 31

From the above, the importance of good quality clean fuel can clearly beseen.

6. CONCLUSION

Several engine failures occurred in the recent past due to combustiondisturbances. The engines failed either due to the fact that the piston wasdamaged and then finally seized, or the piston melted away. In the third case, thelubricating oil was diluted and the bearings on the crankshaft were damaged.

The result of these combustion disturbances can be traced back to injectorfailure. The injector failed due to the fact that the needle became sticky in itsmovement and did not close properly. The result of this stickiness of the needleis that the droplets sizes increased dramatically. In some cases the fuel wasemitted in a jet of fuel, emitted from the injector tip orifices.

The cause of the injector needle deterioration is either fuel with poor lubricityproperties, or fuel that is contaminated by hard dust particles. Both thesesituations lead to damage of the needle, which then results in the stickiness of theneedle.

It has been proven that the fuel can be analysed by means of testing systemssuch as the SRV and HFRR machines.


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It is therefore now possible to determine the quality of fuel in the laboratory bymeans of the SRV Machine and by means of filtration and thereby ensuring thatengine failures do not occur as regularly.

It can therefore be said that:

The Modern Diesel engine is an Efficient and Dependable Work Horse, BUT

that it demands a Clean, Good Quality Fuel to survive.

diesel injector failure

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