INFLUENCE OF FUEL QUALITY ON DIESEL INJECTOR FAILURES

Transcription

1 INFLUENCE OF FUEL QUALITY ON DIESEL INJECTOR FAILURES Author: AJ von Wielligh Pr Eng BSc Eng (Mechanical) Co Authors NDL Burger Pr Eng M Eng (Mechanical) PL de Vaal Pr Eng PhD (Chemical) Staff members of the Faculty of Engineering at the University of Pretoria, Republic of South Africa. Paper delivered at the Fifth International Colloquium Fuels on 13 January 2005 in Ostfildern Stuttgart, Germany. SUMMARY In order to achieve higher output, lower fuel consumption as well lower air pollution, several changes had to be made to the combustion and injection systems of modern Diesel engines. The sulphur content of the fuel was lowered, the number of orifices in injector tips increased and the injection pressures were drastically stepped up, requiring reduced clearances between moving parts. This lead to higher demands on the fuel, in the areas of Lubricity and Contamination. Poor fuel quality and handling practices are no longer acceptable. A large number of engine failures have recently occurred in South Africa on these diesel engines, which can be directly blamed on the quality of the fuel used. The failure process started with injector failure, often leading to engine destruction. Case studies show clear distinction between injector failures due to Contamination and Poor Lubricity. In South Africa where dust is a general problem, better handling procedures will have to be adopted, as the 2 micron filters on new engines do not prevent injector failures. 1 INTRODUCTION In order to meet the demands of industry, the manufacturers of diesel engines had to drastically redevelop the modern diesel engine. These demands cover a wide range of aspects. The most important of these are the following: Higher output Smaller engine size Higher efficiency Lower air pollution Longer engine life Longer service intervals To achieve these demands, the modern diesel engine had to be redeveloped in different areas. The main area of development was the injection and combustion system. The system is electronically controlled to ensure more exact control over the amount of fuel, the point of injection and the rate of injection. The injectors, although electronically controlled are nowadays operating at substantially higher pressures and with a bigger number of orifices. They are discharging the fuel into the combustion chamber at extremely high pressures, much higher than before and this is done in order to make the fuel droplets smaller, which result in more complete, faster and more efficient combustion. To achieve these higher pressures, the clearances and tolerances on injection components are much smaller than in the past and this places a demand on lubricating 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 these demands are not met, the engine will not perform as designed and premature failure will result. During the regular investigation of engine failures, it was found that a large proportion of engines failed due to the seizing of the piston in the cylinder liner, or the failing of crankshaft bearings due to lubricating oil being diluted by fuel. Several cases were encountered where the piston crown started melting, or the sides of the piston skirt and the piston crown seized onto the cylinder liner. 1

2 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 related problems, which in turn was caused by injector failures. The injector failures in turn, were caused by poor quality fuel. 2 BACKGROUND OF COMBUSTION IN A DIESEL ENGINE In order to understand the problem around poor combustion it is necessary to understand the combustion process in the diesel engine. In order to understand the mechanism and importance of the fine droplets, the mechanism of the injector must be briefly discussed. Figures 2, 3 and 4 show the typical layout of an injector, which is commonly used in diesel engines. Figure 2 shows the more conventional system, whereas Figure 3 shows the electronically controlled type of unit injector. Figure 4 shows a Common Rail Injector. 2.1 Diesel engine injection principles and operation The piston of a diesel engine, fits tightly in the cylinder to provide high compression, in order to cause ignition of the injector fuel. The fuel is delivered in an exactly metered quantity, which is metered by the injection system. This fuel is delivered to the cylinders at very high pressure and is broken up into a very fine spray with droplet diameters of a few microns. This is achieved by forcing the fuel through very small orifices at extremely high pressures. In the modern diesel engine, the tendency is to increase the number of these orifices and thereby making them smaller as well as increasing the pressure. In some of the modern diesel engines the injection pressure is in the range of as much as 200 MPa. This is typically 10 times higher than the older generation of engines. Figure 2: Conventional injector The result of the higher pressure and smaller orifice sizes, is that the droplets are significantly smaller and the exposed area is therefore significantly bigger. This results in a smoother and faster and more complete combustion. The typical required spray pattern is indicated in Figure 1 below. Figure 3: Electronic unit type injector Figure 4: Common rail injector Figure 1: Spray pattern The injector consists in essence of a needle with a sharp point, which acts as a seal 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 cylindrical shaped 2

3 section, which fits very tightly into a passage in the body of the tip. This needle is forced down onto the seat by means of a spring force, which is adjustable during setting up of the injector and thereafter stays unchanged. High pressure fuel is then delivered from the pump and the high pressure acts on the shoulder of the cylindrical section, causing a force which opposes the spring force. When the hydraulic force is able to overcome the spring force, the needle is lifted and displaced 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 tip and 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 again stronger and able to quickly push the needle back and to close the orifices. Figure 5 P = Cylinder Pressure L = Needle lift P i = Injector Pressure It must be kept in mind, that the hydraulic force is provided by the engine power and can usually build up very high forces, due to the pressures involved. The spring 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 lift the needle, but the needle might be sluggish and slow in the return back to the seat. It could also mean that the pressure of the needle end onto the seat is not enough to properly seal off the fuel. This causes the fuel to leak out. This is then referred to as dripping of the fuel from the nozzle. In the hot air caused by compression, the fuel starts burning and due to the heat release, the pressure rises. The piston is then forced down, to produce the power of the engine. It must be kept in mind that the injection is not an instantaneous happening. The injection process has a certain duration and the more fuel that has to be injected, the longer the process takes. Some injection processes consist of an initial pilot injection to start the flame burning, with a secondary higher volume injection where the majority of the fuel is delivered. The injection process starts when the needle lifts off the seat and opens the orifices for the fuel to be squirted out under very high pressure. Figure 5, indicates the cylinder pressure, the needle lift, as well as injector pressure. Figure 6, is a photograph indicating the combustion process from about 7º Before Top Dead Centre, (BTDC) to 30º After Top Dead Centre (ATDC). From these photographs, it can be seen that the biggest flame is present just after 13º, which is typically the position namely between 15º and 20º (ATDC), when the biggest pressure is needed on the piston crown. Figure 6: Stages of combustion (Internal Combustion Engine Fundamentals Heywood.) 2.2 SPRAY PATTERN REQUIREMENTS The injected fuel is broken up into a very fine spray consisting of micro fine droplets. The combustion process starts by the oxidising of the fuel droplets from the surface of these droplets. It must be kept in mind that the smaller the droplets the bigger the specific area. This means that the combustion takes place faster and more efficiently, with smaller size of droplets. There is therefore a tendency, to use smaller droplets and finer spray in the high injection pressure modern diesel engine. When combustion takes place efficiently and properly, the droplets burn out completely, before they reach the cylinder liner. The fuel is normally sprayed into the combustion chamber, which is housed inside the piston crown. As the piston moves downwards in its power stroke, the spray protrudes further into the volume of the combustion chamber, but it should be burnt out before any droplet reaches the cylinder walls. However, when the spray pattern generated by the injector, is not as described above, the droplets become bigger and therefore take longer to burn out, or a jet of fuel is emitted from the orifice, instead of the very fine spray. The following photographs, Figure 7 (good) and Figure 8, (poor), indicate good and poor spray patterns. 3

4 the piston. The following photograph, Figure 9, shows such a piston, which is in the initial stages of seizing. Seizing Figure 7: Good Spray pattern Figure 9 : Seizing due to jet of fuel (see Figure 8) Dripping from nozzle Figure 8: Poor Spray pattern (Jets) Another possibility is that due to poor closure of the needle on the seat in the injector, 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, is no longer present and this eventually results in melting of the piston crown, which is shown in photograph Figure 10 below. 2.3 CONSEQUENCES OF POOR SPRAY PATTERNS Melting When a poor spray pattern exist, as described above, the following actions usually take place: 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 of lubricating oil is washed away. This leads to dry rubbing of the piston and piston ring on the cylinder wall. Due to the absence of the lubricating film, the friction coefficient rises and excessive heat is developed. Damage to the surfaces, leading to eventual seizing of the piston usually results. In some cases accelerated wear can also take place. In the initial stages of piston failures, the position where the jet of fuel is directed onto the cylinder wall can clearly be seen on the piston crown and on the piston side where seizing starts. The damage usually starts above the top piston rings and then gradually works downwards, towards the skirt of Figure 10 Melting due to Dripping from nozzle The damage shown above, is different to the damage due to Heat Seizure, where the damage starts on the four quarter positions on the skirt. Figure 11 and 12 shows typical heat seizure damage. 4

5 in the crank case. In such cases the piston does not necessarily seize in the cylinder liner. Due to the fact that the lubricating oil dilutes in the crank case, the viscosity drops substantially 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. Several cases where investigated, where the wear pattern on the ring, almost resembled the same situation as where dust was inhaled by the engine. The ring corners are usually sharp and spectrographic oil analysis indicates high iron content in the lubricating oil. The Silicon content of the oil is however then usually normal, which indicates the absence of excessive dust. The rapid wear process, then leads to poor sealing of the piston ring. The blow-by of the combustion gasses past the piston ring increases. Figure 11: Heat Seizure The following photographs, Figure 13 and Figure 14, show the main bearings of a Diesel engine which has rubbed on the crank journal. The block was damaged when a connecting rod was pushed through the side, due to a big end bearing failing. Figure 13 Figure 12: Heat seizure Fuel dilution of lubricant In cases where the spray pattern is such, that the droplets are bigger than they should be, the fuel does not burn out completely in time. The drops of fuel then reaches the cylinder wall and thins down the lubricating film. The fact that these droplets 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 of the lubricating oil Figure 14 5

6 3 REQUIRED PROPERTIES OF FUEL As mentioned above, it is important to remember that the needle must move freely up and down the passage under the influence of either hydraulic or spring forces. It must furthermore be kept in mind, that no lubrication other than the fuel is available for the movement of the needle and for any part in the whole injection system. There are therefore certain demands that are placed on the fuel to provide proper functioning of the injection equipment. Apart from the requirements for proper combustion, the most important physical requirements for good quality diesel fuel are the following: 3.1 Good lubricity The fuel must have enough lubricity properties, in order to provide lubrication to all the moving parts in the whole injection system. The conditions in the injectors are however very severe and therefore special lubrication properties are required in this 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 provide additional lubricity additives to improve the fuel quality. However with the demand on a cleaner environment, Sulphur levels had to be brought down. Presently the South African limit for Sulphur is 0,3% (3000 ppm) and lately fuel with typically 0,05% (500 ppm) Sulphur, also became available. Worldwide there is a strong movement to lower the Sulphur content to 10 ppm and even lower. Experience has indicated that whenever Sulphur levels in the fuel is reduced below 0,3% (3000 ppm), the inherent lubricity properties of the fuel is not sufficient to lubricate all the components, especially the injection equipment of Modern Engines. The process that removes the Sulphur, also removes the constituents in the diesel that provide Lubricity. Problems in the needle area then start appearing. This is caused by a lack of lubrication, between the moving needle and the stationery passage of the injector tip. Water in the fuel also seriously affects the Lubricity. In the case of synthetically produced diesel and other fuels like kerosene, the Sulphur content is very low and these fuels do not have inherent lurbricity properties. It is therefore necessary to add additives to the fuel to render the fuel acceptable to a diesel engine. The purpose of these additives is to provide lubricity properties, as well as some cleaning and detergent properties to the fuel nozzle. 3.2 Cleanliness of fuel Due to the extremely small clearances between the moving parts in the injection system and especially in the injectors, lately as little as about 1 micron, the fuel has to be cleaner than before. The small particles present in the fuel, can enter the small space between the needle and the barrel of the injector tip and this can cause jamming and damage to the needle. For this reason, very fine filtration of the 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 of recent injector failures, this may still be inadequate. 3.3 Tests on Diesel fuel In order to control the properties of lubricity and cleanliness, tests are done on fuel to determine the lubricity properties and the cleanliness levels. The following typical tests are done, in order to determine the lubricity and cleanliness of the fuel Lubricity a) The Optimol SRV machine. The layout of the Optimol SRV machine is shown in Figure 15: Figure 15 The SRV machine consists of a steel disc, which is mounted on a load cell, which can measure forces reacting on the disc. A loaded steel ball, is rubbed on the top of 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 a holder, which can be oscillated and a load is applied to the ball. The frequency of movement, the load, and the amplitude of movement can be adjusted and controlled by a computer. The temperature can also be maintained at virtually any required level. Typical test values are: Frequency: 50 Hz Amplitude: 1 mm 6

7 Temp.: 110 C Load step duration 1 minute The operation of the machine is such that the set-up is done and the temperature is raised to the required level, typically 110ºC. The machine is then started, while the ball and the disc is covered with a film of the fluid that has to be tested, for instance diesel fuel. The fuel then provides a lubricating film between the sliding ball and the stationary disc. Due to the fact that all the forces are known, the computer 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 N and the process carried on. Whenever the friction coefficient reaches a value of 0,3, it is considered that the film between the ball and the disc is penetrated and that solid to solid contact takes place. The loading force acting on the ball is constantly measured and at the point of failure, it is recorded. It has been found by a large number of practical tests, that a good quality diesel fuel, should be able to withstand a load of 700 N, before the breakthrough friction value appears. When levels lower than 700 N is obtained, the fuel usually causes injector failures Results of Lubricity Tests The following graphs in Figure 17 and Figure 18, obtained from the SRV machine, indicates a fuel with good lubricity properties, as well as a fuel with poor lubricity properties. Figure 17: Good Lubricity b) The High Frequency Reciprocating Rig (HFRR) 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 disc which is kept stationary and a small ball which is rubbed across the disc. The difference is that in this case the load is kept constant at typically 2 N. The frequency 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 60ºC and the test is typically run for 2 hours. After the period of 2 hours, the ball is removed and the flat area, which is worn onto the rounded ball is measured under a microscope. The diameter of the flat face of the ball is normally the indication of the lubricity quality of the fuel. A good quality fuel must provide a flat area smaller than 400 or 460 microns, depending on the specification. Fig 16 shows such a wear scar on a test ball. Figure 16: Wear scar on Ball Figure 18: Poor Lubiricity Cleanliness The cleanliness level of the fuel can only be determined by filtration or by particle counting of the contaminants of the fuel. It is customary to filter the fuel samples before lubricity tests are carried out. In the case of the Tribology Laboratory at the University of Pretoria, the fuel is typically filtered through a 0,8 micron filter before the test is done. The residue on the filter paper can be analysed. A particle counter can also be used to determine the cleanliness levels. The graphs in Figure 19 and Figure 20, show fuel where the fuel was deliberately contaminated with very fine dust and then put through a 2 micron filter. The samples of fuel were taken before the filter and after the filter. It can clearly be seen on the graphs, that the contamination caused a very spiky line of friction coefficient, but that there is a significant improvement after filtration. It is however necessary, to note that even after the 2 micron filtration, the 7

8 spikes are still present, although to a lesser degree. The particles passing this filter can still do damage to the most modern injector. The fuel must therefore be kept clean all the way along the supply chain. 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 their exact positions. When the engine was reassembled for the second time, the cylinder heads were interchanged and the front cylinder head was placed at the back, while the back cylinder head was placed at the front position. The engine was again started up and tested on the dynamometer and within 1 hour the engine seized again, now on the number 5 piston. The two pistons that were involved are shown in photographs Figure 21 and Figure 22 below. Figure 19 Figure 21 Figure 20 4 DAMAGE TO INJECTORS DUE TO POOR LUBRICITY AND DUE TO CONTAMINATED FUEL 4.1 Lubricity Two recent case studies provided excellent examples of injectors that failed. In one 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 engine seized, on the number 2 cylinder, while still on the dynamometer. Figure 22 It can be seen that the failure pattern of these two pistons is exactly the same. It was then realized that the number 2 injector was the cause. The same injector was initially in position 2 and later in position 5. This injector was then tested, opened and inspected. The injector shown in Figure 8, is the injector in question. It was found that the needle had score marks on the shank of the needle. The needle next to a 2 cent coin is shown in photograph Figure 23 and the damage to the needle is shown under the microscope in photograph Figure 24. 8

9 smoke was emitted from the exhausts when the engines failed. The injectors of both engines were removed. The 12 injector tips were then inspected to determine the cause of the failure. 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 Figure 26. Damage Figure 23 Figure 26 The wear pattern was very peculiar, in that a narrow band of wear was visible at the front end of the shank of the injector needle. The damage can be seen with the normal eye and this damage is shown in photograph Figure 27. Figure 24: Score marks on shank of needle Wear scar The fuel was tested on the SRV machine at the Tribology Laboratory at University of Pretoria, and the results are shown in Figure 25. Fuel that caused Injector Failure Ul timate Loa d Tes t: R un a t 110 deg re es C, f re quen cy 50 H z, st ro ke len gth 1mm w ith 50 N increments of 1 m in duration each. Break-through coefficient set at Friction Coefficient Load (N) 0 Test fuel (Batem an test2) Reference fuel Test load Reference load Figure 25 It can be seen that the fuel sample did not meet the required 700N on the SRV machine but only a load of 350 N was sustained. 0 Figure 27 Under the microscope the damage can more clearly be seen and some of the damage is shown in photographs Figure 28 and Figure Contamination During the investigation of the failure of two six cylinder engines from the same farmer, it was found that both engines started to run unevenly. White 9

10 Figure 28 : Band of Damage Enlarge 40X Figure 30 Inspection of the fuel handling practices on the farm revealed the following: Figure 29 : Damage Enlarge X60 The farm on which the engine problems occurred was then visited. Fuel samples were taken from the Bulk Tank, the Tractor tank as well as the return line from the Common Rail system. The samples were filtered through a 3 micron Micropore filter, then through a 1.2 micron filter paper and finally through a 0.22 filter paper. The filter patches after the filtering, are shown in Figure 30. It can be seen that the fuel was received in a slightly contaminated state, but that most of the contamination took place between the Bulk tank and the Tractor tank. The effect of the 2 micron onboard filter can also be seen. Figure 31 Tanker trailer used to transport fuel from bulk tank to tractor working on lands. 10

11 Figure 32 : Unprotected breather on tank Figure 34 : Poor lubricity failure Figure 33 : Unprotected filling spout 4.3 Comparison When the appearance of the injector needles that failed due to lubricity and the ones that failed due to contamination are compared, it is quite clear that there is a distinct difference in the characteristic damage to the injector needles. This can clearly be seen in photographs Figure 34 (Lubricity) and Figure 35 (Contamination). Figure 35 : Contamination Failure The effect of both these types of failure, is that the needle becomes sticky in his movement and that bigger droplets are emitted and in some cases jets of fuel are emitted 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 Figure 36 shows such a needle under the microscope, indicating the rough sealing surface which is no more in a position to seal properly, especially at the higher injection pressures. 11

12 The result of these combustion disturbances can be traced back to injector failure. The injector failed due to the fact that the needle became sticky in its movement and did not close properly. The result of this stickiness of the needle is that the droplets sizes increased dramatically. In some cases the fuel was emitted as a jet of fuel, from the injector tip orifices. The cause of the injector needle deterioration is either fuel with poor lubricity properties, or fuel that is contaminated by hard dust particles. Both these situations lead to damage of the needle, which then results in the stickiness of the needle. Figure 36: Damaged needle seat From the above, the importance of good quality clean fuel can clearly be seen. 5 CONCLUSION Several engine failures occurred in the recent past due to combustion disturbances. The engines failed either due to the fact that the piston was damaged and then finally seized, or the piston melted away. In the third case, the lubricating oil was diluted and the bearings on the crankshaft were damaged. It has been proven that the fuel can be analysed by means of testing systems such as the SRV and HFRR machines. It is therefore now possible to determine the quality of fuel in the laboratory by means of the SRV Machine or HFRR and by means of filtration and thereby ensuring that engine 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. 12