STRUCTURED SURFACES USED FOR IMPROVING TRIBOLOGICAL BEHAVIOUR FOR THE PISTON RING CYLINDER LINER FRICTION COUPLES IN INTERNAL COMBUSTION ENGINES

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1 77 Paper present at International Conference on Diagnosis and Prediction in Mechanical Engineering Systems (DIPRE 07) October 2007, Galati, Romania STRUCTURED SURFACES USED FOR IMPROVING TRIBOLOGICAL BEHAVIOUR FOR THE PISTON RING CYLINDER LINER FRICTION COUPLES IN INTERNAL COMBUSTION ENGINES Gabriel Arin MIHALCEA, Leonard MIHĂESCU S.C. ICTCM S.A. Bucureşti, Romania ABSTRACT Structured surfaces means the achievement of a rate, or controlled microgeometry for one or both friction surfaces. These surface profiles are used to guarantee lubrication functions of transport, hoarding and spread of the oil and to collect the wear products for maintaining the uprightness of the friction couple surfaces. The present article will present the use of these surfaces for improving tribological properties of the piston-ring cylinder liner friction couples in combustion engines. It will be presented the improvement of the friction and wear properties and the evolution of the combustion engines properties by the use of these kinds of surfaces. KEYWORDS: Structured surfaces, piston-ring, cylinder liner, boundary lubrication. 1. INTRODUCTION A new concept in the field of tribology has been the development of contact surfaces designed for improved lubrication capability. This can vary from simple control of the surface roughness of two mating surfaces, to the more complex formation of regular structures which serve the double purpose of retaining lubricant and trapping debris particles. These surface textures are in the form of protrusions (positive asperities) or cavities (negative asperities) of known size and geometry. The benefits of such surface textures include lower friction torque, higher load capacity and lower operating temperatures, [3]. Friction of the piston and piston rings system accounts for the greatest portion, about 50-60%, of the total mechanical loss in engines. Mechanical and thermo dynamical losses, wear and emissions caused by lubrication oil combustion are principally influenced by the tribological behaviour of the piston assembly group. The trend to compact engi-nes with high power densities and raised thermo mechanical loads increases the importance of this tribological system and requires new approaches. One promising possibility is the utilization of liner surfaces with special machined microstructures. The objective of this paper is, to describe a comparison between conventional liner surface and a structured liner regarding tribological behaviour. Therefore another objective is to presents the feasibility of surface structure designed to reduce frictional losses in energy transmission components, cylinder liner ring contacts. The structure patterns must be protected by thin films or coatings and appropriate lubricating chemistry over a broad range of operated conditions. The results show a better tribological behaviour of the deterministic laser structured liner surface in comparison to the conventional stochastic plateau honing surface [2].

2 78 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 2. OF PISTON ASSEMBLY GROUP A tribological system consists of the elements basic body, contact body lubricant and surrounding. The tribological system piston assembly group of reciprocating engines the single elements will be represented by cylinder liner, piston and piston rings, the lubrication oil and the crankcase air, respectively cylinder filling, as it looks in figure 1. The changing piston velocity or gliding velocity between the friction partner piston rings and cylinder liner leads to mixed friction and hydrodynamic friction. The border between these friction phenomena is not a fixed point and can be presented by the Stribeck s curve. The parameters that influence the course of the Stribeck curve are the operation parameters, the material, lubricant and surface. In the system of piston assembly group the surface structure of the cylinder liner has an important effect on fuel consumption, wear and emissions caused by lubrication oil. Fig. 1. Tribosystem piston assembly group. Fig. 2. Pressure generation in the lubrication [4, 5]. Fig. 3. Surface topographies for cylinder liners [2, 5]. In the areas of low piston velocity (top dead centre, TDC, and bottom dead centre, BDC, and at very low engine speed) it mainly can be observed mixed friction. Between the points TDC and BDC and at high engine speed the friction becomes hydrodynamic. The necessary convergent gap for building up the hydrodynamic pressure will be generated by the crowned surface of the piston ring. The generation of pressure also can be carried out by a temporary squeeze effect. This squeeze effect takes place in the dead centres, while the piston velocity is zero. The preloaded piston ring presses the lubrication oil out of the gap, as it is shown in figure 2 [4, 5]. In the areas of mixed friction the friction partners have direct contact that causes wear. Because of the tribological processes in reciprocating engines, wear of the cylinder liner principally can be observed at the area of TDC. In view to the power losses caused due to friction, that directly increase fuel consumption, the friction in the areas of dead centres can be neglected because of the very low gliding velocity. 3. SURFCES TOPOGRAPHIES FOR CYLINDER LINERS The surface of cylinder liners for IC engines usually is created by a horning process. At the honing process then tool is leaded in a rotating and translating motion. This kinematics generates a cross structure. On the basis of the channel shape depression the oil will adhere outstanding at the surface. The depression in the whole surface represents a so called communication system. This system facilitates the entry of oil into the combustion chamber by the oscillating motion of the piston, and that causes the emission of unburned hydrocarbons and particulate matter. To counteract this process and to ensure a good sealing effect until now the piston rings have been combustion chamber by the oscillating motion of the piston, and that causes the emission of unburned hydrocarbons and particulate matter. To counteract this process and to ensure a good sealing effect until now the piston rings have been preloading strongly. But that causes high friction losses and higher wear rates. Functional areas with good gliding properties and low friction forces respectively, are characterised by

3 79 smooth surfaces without open cavities. However, these surfaces do not ensure the requisite adhesion of the oil. Until now, for the generation of conventional stochastic surfaces by the honing process it always was necessary to make compromises. New deterministic surface structures that are generated by a modified honing process and a laser based material treatment represent a promising approach for reducing both friction losses and wear without an increase of lubrication oil, as it is shown in figures 3 [2, 5], 4 [6] and 5 [6]. With the help of experimental investigations at different engines it will be evaluated the lubrication and wear behaviour of the new structure surface. One of the critical issues in fabricating the new surface features was the effect of the slope of the bottom of the feature. In bearing applications, this typically was called the B/D ratio (bearing length over the diameter of the bearing) to estimate the lift force. The concept was borrowed and applied, in paper [1], to the design in terms of the length over the depth of the slope fabricated at the bottom of the surface features. Another important factor was the size of the surface feature over the contact area, [3]. In The final goal of these new surface structures is to optimise the tribosystems. This procedure can be used for a wide range of the friction couple components macro geometry. An important abrasive and adhesive wear reducing is obtained by surface structuration. Surfaces topography allows lubricant access in the friction couple. The remain friction in an optimised tribosystem (load, sliding speed, temperature and time period) must be fluid friction, hydrodynamic lubrication. We see here the importance of the lubrication parameters, especially the viscosity and the EP additives. The structured surfaces allows lubrication retention in the dimples performed, as it is presented in figure 5 [6], and they assure a long time lubrication boundary lubrication conditions, the edges of the surface features served as rough spots, increasing the friction; therefore, the optimum number of the features might be much smaller than for the high- speed, low-load regime. In paper [1] some experiments were conducted to investigate this effect. The authors of the paper [1] fabricated several surface features; the micrographs of three patterns: A3, B2 and C4 are shown in figure 6 [1, 2]. The densities of the three patterns are kept at 5%. Pattern A has a reversed wedge, pattern B has a symmetrical wedge and pattern C has the classic wedge orientation. Fig. 4. Tribological system and different surfaces structures [6]. Fig. 5. Lubricant acting in the dimpled lubrication pockets [6].

4 80 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI Figure 7 [1] shows that classical wedge orientation has the highest friction reduction capacity, reducing the average friction coefficient of the untextured (UNT) from 0.09 to about 0.05, a 44% decrease for a steel on steel contact. Another significant point is that friction starts at 0.09, which is the low end of the boundary lubrication regime; the surface structuring features lower it to the hydrodynamic regime even though the contact is still operating at the boundary lubrication regime, as evidenced by the wear of the contact [2]. This engine is used as a multi cylinder engine for propulsion of heavy commercial trucks. Piston, piston rings, cylinder liner and cylinder head correspond to the series production to a large extent. On this condition the tribological boundary conditions are similar to that of series engine. For reducing vibration phenomena and the effect of the measuring systems the aggregate is equipped with a device to compensate the forces caused by masses of inertia completely. Fig. 7. Comparison of friction coefficients of the three patterns shown above under boundary conditions [1]. Fig. 6. Three wedge orientation patterns [1, 2]. 4. EXPERIMENTAL FRICTION AND FILM MESUREMENTS 4.1. Tested Engines The investigation for measuring the friction force of piston assembly group and oil film thickness between the top piston ring and the liner were carried out, in the paper [5], at a direct injecting single cylinder diesel engine from Daimler Chrysler with a 128 mm bore and a 1.8 litres displacement. Figure 8 [5] shows the essential data of the test engine and a cross-section of the piston assembly group, the cylinder head and the used piston rings. Fig. 8. Cross-section and data of the test engine [5]. The piston casted of aluminium has a central placed piston hollow. The first piston ring is working in a specially manufactured groove for reducing flank wear. The top ring is a trapezium ring with asymmetric crowned profile. The middle ring is a minute ring with an inside angle. The bottom ring is a commercial oil scarper ring with a tube spring. All the tests are carried by using a lubrication oil of the 10W-40 SAE-class

5 81 viscosity, that consists of a synthetically based oil and that is suitable for gasoline engines and heavily loaded diesel engines. In the paper [6] as a testing engine was used AVL FM 528 single cylinder diesel engine, studding the influence of the structured surfaces on the particles emissions of this engine. The AVL FM 528 engine is presented in figure 9 [6] and has the follows technical data: bore diameter: 85 mm; stroke: 92 mm; rated speed: 5000 rpm; greatest compression pressure: 130 bar; compression ratio: 21; greatest moment at 2500 rpm: 35 Nm; power at rpm: 9 kw. other. Every single MPC has a width of 40 to 60 µm and a depth of 10 to 25 µm. Figure 10 [5] shows a schematic top view and the characteristic values of the surface topogra-phies. The field of MPC of the laser-structured liner is restricted to the TDC-area, where high thermo-chemical loads occur. Figure 9 also shows the real structure of one MPC drawn to a larger scale. Fig. 10. Tested liner surface topographies [5]. Fig. 9. The tested AVL FM 528 single cylinder diesel engine [6] Tested Liner Surface Topographies The investigations for measuring friction and oil film thickness are carried out by using a standard honing structure (3-stage plateau honing) as well as a smooth honing structure with a superposed area of micro pressure chambers (MPC) that are produced by a laser based surface treatment. The MPC have a length of 3 mm and a vertical and horizontal distance of 2 mm to each The basic roughness and the characteristic values (Abbott-curve) of the standard liner surface are much higher than the values of the laser- structured liner. On the other side, the MPC ensure the adhesion of the necessary amount of lubricating oil. According to this the deterministic surface promises to be more wearresistant and features less friction losses. The structure of such a laser honing machine is based on a classic honing machine using a laser-head, with some characteristics like the following ones, as they are presented in paper [6]: wave-length of the Nd YAG-Laser, λ=1.06 µm, CW-power: 60W; pulse of 200 ns, at 30 kv, with 10 khz fervency; acting power 3.08x10 8 W/cm 2, for a focus diameter of 0.1 mm; optic-head: ± 0.1 mm focus, 19 mm diameter at 130 mm distance; guidance system NC, for rotation and advance movement; compressed air jet for caring out the processed material. Fig. 11. Different laser-honing structures, after [6].

6 82 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI The liner surfaces were laser-processed, as it is presented in paper [6], with the characteristics: cross-structure presented in figure 11 [6]; distance between channels: 600 µm; width of one channel: 30 µm; the angle of the channels: 30º Friction Force Measuring System The principle of the friction force measurement is the floating liner. Figure 12 [5] presents the schematic principle, realized in 1979 for the first time. This special system facilitates investigations under high loads and cylinder peak pressures to evaluate the effect on the tribological system exactly over a big area of the engine map. For measuring the friction forces of the piston assembly group the cylinder liner, which is a central component of the measuring system, is loaded axially by to piezoelectric pick-ups (7) and radial by a hydrostatic 4-chamber bearing (5). The friction forces transmitted from the piston and the piston rings (6) to the liner (4) during oscillating motion are measured directly by the force pick-ups that are placed in direction of the crank-shaft axis. For measuring both positive and negative forces it is necessary to preload the force pick-ups. forces due to the cylinder pressures in comparison to the friction forces. The sealing is realized by a temperaturestable viton-o-ring (2) and the dynamic calibration of the measuring system is carried out by a specially developed device Friction Force Measuring Results The course of the friction force of the piston and the piston rings depending of the crank angle at an engine speed of 600 rpm and a boost pressure of 1 bar are exemplary presented in figure 13 [5]. The figure shows a comparison between the standard honing surface and the laser-structured honing surface under dependency of indicated mean effective pressures of 2 bars and 12 bars. Fig. 13. Friction forces of the piston assembly group. Comparison of the standard honing surface and laser structured surface [5]. Fig. 12. Friction force measuring system [5]. A gas-balance sealing of the combustion chamber, that is adapted to big pressures, permits measuring the friction forces independent of the cylinder pressure. The compensation of the gas forces is realized by providing two areas, which are located at the top of the liner and have equal size, with the same pressure. The compensation is mandatory because of the significantly higher High friction forces can be measured near TDC, whereas the maximum forces occur in the working stroke because of the highest cylinder pressures and surface temperatures (low viscosity of lubricant). The level of these both parameters is different for different mean effective pressures. That leads to varying friction forces. In the area between top dead centres the total friction force increases due to a high rate of hydrodynamic friction that is caused by the high piston velocity at mid stroke. The comparison of both surfaces topography shows a significant reduction of the friction force particular the areas that are characterised by high thermo mechanical loads. This fact is presumably caused by the smooth surface as well as the MPC of the laserstructured cylinder liner. All investigated operating points within the engine map show the same behaviour.

7 83 The use of the smooth laser-structured cylinder liner surface leads to a reduction of mechanical losses of the piston assembly group. The absolute difference of friction mean effective pressure (fmep) between the investigated surface topographies is near constant over the engine speed, as it is presented in figure 14 [5]. At low engine speeds fmep can be halved. At higher loads the share of piston assembly group at total friction losses increases, but the mechanical efficiency also increases. So, the biggest influence on the total engine efficiency by using new deterministic surfaces can be expected at low engine map areas. Fig. 15. Inductive sensor and sensor positions within the piston ring [5] Oil Film Measuring Results Fig. 14. Friction mean effective pressure depending on the engine speed [5] Oil Film Measuring System For measuring the oil film thickness and the distance between piston ring and cylinder respectively, an inductive sensor with high resolution was developed that can be placed directly in the piston ring. Because of the inductive method only a distance can be measured. Statements about the extent of oil filling are not possible, but the behaviour (curve depending on the crank angle) of the measured distance can give information about the oil filling within the gap. Eight sensors are regularly distributed over the circumference of the piston ring, as it is shown in figure 15. The ring is able to move within the ring groove radial and axial, but it is protected against screwing. That facilitates the measurement of the distances in direction of the crank shaft axis, orthogonal to this direction and among these four points. The sensor cables are leaded over special bores in the horizontally divided piston and over a cable gear to the measuring amplifiers. By measuring the ohms resistance of the sensor it is also possible to get information about the average temperature and the thermal load of the piston ring respectively. The use of the oil film thickness measuring system described permits the investigation of the lubrication within the gap between top piston ring and cylinder liner under motored and fire operation over a large area of engine map. Figure 16 shows the influence of engine load on the oil film thickness, presented by the signal of single sensor at an engine speed of 800 rpm. On the basis of the quantitative and qualitative course of the gap distance the following interpretations can be made. With increasing loads the combustion chamber temperature increases and causes decreasing oil viscosity. In combination with the higher pressure load on the piston ring it leads to sinking distances between ring and liner. It could be observed that a very high load in combination with high temperatures mixed friction appears, that is characterized by direct contact between the components piston ring and cylinder liner. Fig. 16. Oil film thickness of one sensor depending on crank angle and load [5]. A direct comparison of the oil thickness curve for the standard honing surfaces is shown in figure 17. The mean distances averaged over the circumference of the piston ring are shown at an engine speed of 800 rpm and an indicated mean effective pressure of 2 bars. The area

8 84 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI of the laser structure in the bottom diagram is coloured grey. The expected crowned course of the oil film thickness between the dead centres according to the piston velocity can only be recognised to some extent. That speaks for a predominant partly filled gap between the ring and the liner, so that the development of a hydrodynamic pressure is limited because of lack of oil or too big gaps. The comparison shows the influence of the micro pressure chambers on the oil film thickness significantly. When the top piston ring is running over the laser structure the oil film thickness increases abruptly. That leads to a reduction of friction forces because of the sinking shear angle in the lubrication gap. The micro pressure chambers presumably cause a better local adhesion of oil at the liner surface and cause an improvement of the hydrodynamic lubrication between ring and cylinder liner. The oil film thickness reaches its maximum a short time before TDC and sinks directly in the TDC because of the squeeze-effect. At the charging top dead centre (0 to 720 degrees crank angle) this effect is more distinct as a result of the lower pressure level in comparison to the ignition TDC. The smooth finishing of the cylinder liners was made by a conventional 2-stage honing process as well as by a special smoothing honing treatment that leads to a fine plateau-like surface roughness. Added to that the surface was superposed with a saucer-shaped laserstructure in the area of top dead centre, as it is seen in figure 18. The laser-pockets have a distance of about 1800 µm, a diameter of approximately 200 µm and a depth of 10 to 20 µm. To detect the changes in the liner topography caused by the engine operation, several engines with the same operation programme but different engine running time were investigated. The programme contained different load levels to record a wide range of real engine loads and their effect on the liner surface. The engines have been operated at different engine speeds at full load and a varying part loads. For both surface topographies the maximum engine running time was 250 hours. Fig. 18 Laser-structured liner surface of the used 5-cylinder gasoline engine [5] Surface and Wear Measurements Fig. 17. Comparison of oil film thickness between standard honing surface and laser structured honing surface [5]. 5. MEASUREMENTS OF WEAR 5.1. Surface Topographies for the Tested Engin To document the wear of the standard-surface and laser structured cylinder liners, there have been carried out further investigations by using the 3- dimensional surface measurement technique. The used engine for these investigations was the Volkswagen 5-cylinder gasoline V-engine with a total displacement of about 2.3 litres. For measuring the surface topography, a tactile 3- dimensional surface measuring system from Somicronic company was used. To ensure a statistic representative message a measuring area of 4 mm 2 was chosen. The lateral distance between two measuring points was in x- direction 1 µm and in y-direction 4 µm. To separate function relevant roughness from the long-wave surface parts the Gaussian regression filtering was used for signal processing. This filtering offers the reproduction of roughness without displacing the surface topography, peaks core and groove areas. The function orientated evaluation of cylinder liners can be effected on the basis of the Abbott curve that describes the growing material share from the roughness peaks to the deepest groove. In contrast to ISO the area borders for the material share will be given. According to the recommendations of [5] for horning cylinder surfaces the core area is characterised within a material share between 10 and 80%.

9 85 The functional behaviour of a technical surface can be described by the areas peak, core and grooves. While engine running in the peak area it wears out very strongly. Fig. 20 Wear volume of standard and laser structured honing surface depending on engine running time [5]. Fig. 19. Characteristic values of standard and laser structured honing surface depending on engine running time [5]. The core area represents the permanent load bearing capacity of the surface, and the groove area is characterising the oil volume necessary for sufficient lubrication. Figure 19 presents a comparison of the investigated surface topographies in terms of the characteristic value of the Abbott-curve depending on the engine running time. Considering the groove areas, the laser structure offers significant more volume to keep oil then the standard honing structure. It also can be shown for both topographies that the groove area almost doesn t depend on the engine running time. The characteristic value Rpk of the conventional honing structure changes clearly over the engine running time. Due to the smooth-out honing process the run in wear will be anticipated. After 250 hours engine operation the characteristic curves become very similar. The combination of oil storing ability (high Rvkvalue) realised by the laser based micro pressure chambers and the smooth surface topography with high load bearing capacity (low Rk-value) leads to an outstanding functional and tribological behaviour of the cylinder liner. That the laser-structured honing surface offers further benefits can be shown by reduction of oil consumption of about 30 to 50% in comparison to the standard honing surface. In consequence the emission of unburned hydrocarbons and particulate matter, the emissions caused by lubrication oil will decrease. Calculating the eroded material volume to evaluate the wear of cylinder liner, the difference between both investigated liner topographies will become more significant. To determine the wear volume the material filled volume above a plane of 80% material share was calculated. The difference to the as new surface corresponds to the eroded volume caused by contacting piston ring and cylinder liner. The wear volume determined, shown in figure 20, presents a clearly reduced wear of the laser structured surface of almost 25% in comparison to the standard honing surface.

10 86 THE ANNALS OF UNIVERSITY DUNĂREA DE JOS OF GALAŢI 6. OIL CONSUMPTION AND PARTICULATES EMISSIONS Finishing honing was realised all over the active part of the cylinder liner, as it is presented in paper [6]. The roughness was R z =1 µm (R a =0.1 µm). In opposition with the standard cylinder liners it was obtained for the liners prepared by laser-honing general particulates emissions 20 to 25% lower and oil particulates emissions 25 to 30% lower. At an experiment on a 5-cylinder (2.9 litres) diesel engine, testing 6 engines about 330 hours, it has been observed oil consumption reducing from 40 g/h, for classical liner engine to 15 g/h for laser-honing liner engine. For the three liner variants presented in figure 11 [6] it is reported in figure 21 [6] the combusted oil, function of time of testing. Table in the figure 22 [6] shows the structure characteristics of the three studied variants. An advantage of closed pockets structure is the reducing of the wear, as it is shown in figure 23 [6]; the wear reduces from 5 µm to 2.5 µm for the piston rings and form 3 µm to 1.5 µm for cylinder liner. Fig. 21. Oil consumption for a 5-cylinder diesel engine [6]. Varia nt Distance between channels Depth of the channel Width of the channels Angle Roughness R z parameter µm 7 9 µm µm µm µm 7 9 µm µm 30 0 approx. 2.0 µm 3 Pocket structure Depth Length 2 mm 2 mm R z = 1 µm Fig. 22. Surfaces characteristics for 5-cylinder diesel engine [6]. Fig. 23. Wear of piston ring cylinder liner friction coupe for a 5-cylinder diesel engine.

11 87 7. CONCLUSIONS The use of the new type of topographies for cylinder liners in internal combustion engines offers a better functional and tribological behaviour. Apart from a significant reduction in friction losses (reduced fuel consumption), a clear decrease of cylinder wear can be noticed, and the thermodynamic losses in form of blow-by, do not alter. The consumption of lubrication oil is also decreasing. That leads to a reduced emission of unburned hydrocarbons and particulate matter. Because of the lower wear the time before overhaul (TBO) of internal combustion engine can be extended, that also leading to considerably reduced costs. The deterministic surfaces consist of a very smooth basic structure with low roughness and a micro-pressure structure that is superposed to the basic surface and produced by a laser-based material treatment. While the smooth basic structure offers good prerequisites for low friction, the micro pressure chambers ensure the adhesion of sufficient lubrication oil on the cylinder. In addition, in the laser-structured area a significant increase of oil film thickness between piston ring and cylinder liner can be noticed. That leads to a bigger share of hydrodynamic friction and lower cylinder wear due to mixed friction. Probably there will be a growth of the area of machine elements using structured surfaces. The main problem remains in computing the boundary lubrication regime using this kind of surfaces. Another problem is the tribological research and the detection of the most important friction characteristics for many variants of surface geometry and establishing some optimum variants. REFERENCES 1. Hsu, S., Wang, X., Ives, L.K., Zhang, H., Lyang, Y., Ying, C., Eberhardt, J.J., Johnson, D.R., 2005, D. An Integrated Surface Modification of Engineering Materials for Heavy Vehicle Application, Heavy Vehicle Propulsion Materials, FY Progress Report. 2. Ronen, A., Klingermann, A., Klingerman, Y., Etsion, I., Different appoches for analysis of friction in surface textured reciprocated components, 3. Bolander, N.W., Sadeghi, F., Gerber, G.R., 2005, Piston ring friction reduction through surface modification ASME Internal Combustion Engine Division, Fall Technical Conference September 11-14, 2005, Ottawa, Canada, Proceedings of ICEF Andersson, P., Tamminen, J., Sandström, C.E., 2002, Piston ring tribology, A literature survey, VTT Research Notes 2178, ESPOO. 5. Golloch, R., Merker, G.P., Kessen, U., Brinkmann, S., 2005, Functional Proprieties of Microstructured Cylinder Liner Surfaces for Internal Combustion Engines, Tribotest Journal 11-4, June, (11) Klink, U., Flores, G., 2000, Laser Strukturieren von Kolbenlaubahnen, The XII th International Colloquium Tribology 2000+, Ianuarie 11 13, Technische Akademie Esslingen, Germany.