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Tribology Transactions, 48: 583-588, 2005 C Society of Tribologists and Lubrication Engineers Copyright ISSN: 1040-2004 print / 1547-357X online DOI: 10.1080/05698190500313544

Experimental Investigation of Partial Laser Surface Texturing for Piston-Ring Friction Reduction G. RYK, Y. KLIGERMAN, I. ETSION and A. SHINKARENKO Department of Mechanical Engineering Technion Haifa 32000, Israel

An experimental study is presented to evaluate the effect of partial laser surface texturing (LST) on friction reduction in piston rings. In a previous study, 30% friction reduction was obtained with full LST where the full width of the piston ring is textured with a very large number of microdimples that act individually as microhydrodynamic bearings. In partial LST, only a portion of the piston-ring width is textured with high dimple density, producing a “collective” effect of the dimples that provides an equivalent converging clearance even with nominally parallel mating surfaces. Experimental results obtained with flat and parallel test specimens with partial LST are presented, confirming a previously published theoretical model and the advantage of partial over full LST. Friction reduction by LST with actual production-crowned piston rings and cylinder liner segments is not straightforward and needs further investigation.

Laser surface texturing (LST) has emerged in recent years as a potential new technology to reduce friction in mechanical components (Ryk, et al. (7); Etsion, et al. (8); Kligerman and Etsion (9); Ronen, et al. (10)). Ronen, et al. (10) developed a theoretical model for a piston/cylinder system with LST piston rings. The authors studied the potential use of piston-ring microsurface structure in the form of spherical microdimples to reduce the friction between rings and cylinder liner where the entire ring face in contact with the cylinder liner was textured. It was demonstrated that a significant hydrodynamic effect can be generated by this surface texturing even with nominally parallel mating surfaces. The time variation of the clearance between the piston ring and cylinder liner and the friction force for any given operating conditions were obtained by simultaneously solving the Reynolds equation and a dynamic equation of the ring radial motion. The main parameters of the problem were identified as the area density of the dimples, dimple diameter, and dimple depth. An optimum value of the microdimple depth over diameter ratio was found, which yields a minimum friction force. It was found that a friction reduction of 30% and even more is feasible with a textured ring surface. The model prediction was experimentally verified by Ryk, et al. (7). Two LST modes are available to reduce the friction losses and improve tribological performance of mechanical components. The first one is the full-width LST mode, which is based on an individual dimple effect (local cavitation in each dimple; e.g., Etsion, et al., (8)). The second mode is a partial LST, which is based on a socalled collective effect of the dimples that provides an equivalent converging clearance between nominally parallel mating surfaces (similar to the “inlet roughness” concept of Tonder (11)). This collective effect of the partial LST was demonstrated by Etsion and Halperin (12) for high-pressure hydrostatic mechanical seals, and by Brizmer, et al. (13) for parallel thrust bearings. It was shown theoretically by Brizmer, et al. (13), and verified experimentally by Etsion, et al. (14), that partial LST significantly increases the load-carrying capacity compared to full LST. More recently, Kligerman, et al. (15) developed an analytical model of partial LST flat piston rings. They found that the friction for the optimum partial LST piston rings is significantly lower than that for the corresponding optimum full LST rings. The difference varies from about 30% reduction for narrow rings to about 55% reduction in wide rings. The main purpose

KEY WORDS Automotive; Friction; Piston Rings; Surface Texturing; Testing

INTRODUCTION Friction loss in an internal combustion engine (ICE) is an important factor in determining fuel economy and performance of the vehicle. Approximately 50% of the friction losses in an ICE are due to the piston/cylinder system, of which 70 to 80% comes from the piston rings (e.g., Nakada (1); Knoll and Peeken (2); Mitsuru and Yasukazu (3); Takiguchi, et al. (4)). Typically the total ICE frictional losses are responsible for about 25% of the fuel consumption. Proper lubrication and surface texture are key issues in reducing friction in a piston/cylinder system and, hence, have received a great deal of attention in the relevant literature. Surface texturing as a means for enhancing tribological properties of mechanical components has been well known for many years. Perhaps the most familiar and earliest commercial application of surface texturing in engines is that of cylinder liner honing (Jeng (5); Willis (6)). Presented at the STLE Annual Meeting in Las Vegas, Nevada May 15-19, 2005 Manuscript approved August 12, 2005 Review led by Gray Barber

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of the present article is to examine experimentally the finding of Kligerman, et al. (15) regarding the potential benefit of using partial instead of full LST for friction reduction in piston rings.

TEST-RIG DESCRIPTION A special test rig was designed to provide linear reciprocating sliding motion simulating the case of piston ring and cylinder liner. It allows study of the effect of laser texturing on friction under full and starved lubrication conditions. A detailed description of the test rig is presented in the literature (Ryk, et al. (7)). The main structural features of the test rig are shown in Figs. 1 and 2. Electric motor 1 drives crank mechanism 2, which ensures reciprocal motion of a planar plate or cylinder liner segment 3 along the two linear bearing guides 9, fixed on a common basis and isolated from the laboratory floor by special damping pads. Self-alignment holder mechanism 4 (see details in Fig. 2) ensures alignment of piston-ring simulation specimens 8 with respect to

Fig. 1—Reciprocating test rig.

Fig. 2—Self-alignment holder mechanism.

the counterpart planar plate simulating cylinder liner segment 3. It also allows the application of a normal load Fe on holder 4 as well as feeding of lubricant to the contact zone via oil conduit 6, and leading out the wires of thermocouples 5 that are embedded

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nar plate 2 is made of cast iron (2.5–3% C, 1.5–2.5% Si, 0.6% Mn). The operating normal load Fe is applied to the specimen’s holder by means of accurate weights. Fully formulated engine oil Ultra40 (equivalent of SAE 40) with a viscosity index of 95 is used and the test rig is provided with a heat source for heating the friction zone and maintaining a selected ambient temperature to simulate the lubricant viscosity conditions in an ICE as closely as possible.

TEST PROCEDURE

Fig. 3—Schematic of the test.

in the test specimens to measure their face temperature. Proximity probe 7 provides an online measurement of the wear of the two rubbing surfaces, 3 and 8. A special device consisting of two elastic beams 11 was designed to measure the friction force. These beams enable the displacement of arm 13, which deflects due to the friction force acting between the rubbing surfaces. Proximity probe 12 registers the time variations of this deflection corresponding to variations in the friction force. The reciprocating speed measurement is realized with optical gauge 14. A schematic of the test for the flat and parallel rubbing surfaces is presented in Fig. 3. In this case, steel specimen 1 with two chrome-coated flat surfaces, each having a width of 3 mm and length of 11 mm, is fixed in the special holder. Reciprocating pla-

According to the analysis of Kligerman, et al. (15), the textured portion of the partial LST can be located symmetrically at the center of the ring width, symmetrically at both ends of the ring, or at an arbitrary distance from the ring center (see Fig. 4). Because the piston-ring reciprocal motion is symmetric it makes sense to apply the texturing symmetrically. The analysis of Kligerman, et al. (15) showed that the position of the textured portion has little effect on the friction force. Hence, for the present experimental study, the partial LST was applied symmetrically at both ends of the flat specimens (see Figs. 3 and 4(b)). The parameters that could be varied in the present study are operating normal load Fe , sliding speed U, ambient temperature, oil feeding rate, and the parameters of the laser texturing. The friction tests were carried out with several values of normal load Fe corresponding to a contact pressure range from 0.1 to 0.5 MPa. The lower contact pressure represents typical values caused by the ring’s own elasticity in a real engine. The higher contact pressures represent the additional average gas pressure acting on the back of the ring, which for a medium-power gasoline

Fig. 4—Different locations of the textured zone: (a) symmetrically in the center, (b) symmetrically at both ends, and (c) arbitrarily at a distance d from the ring center.

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engine is about 0.2 MPa. It is important to emphasize that the actual external normal load Fe acting on the piston ring varies with time (crankshaft angle) along with the gas pressure change in the combustion chamber. However, since the average friction force is only slightly affected (less than 15%; see Ronen, et al. (16)) by the combustion pressure spike, the external normal load in the present experiments was maintained constant during the reciprocation of the specimens. The average friction over one reciprocating cycle was evaluated by on-line integration of the absolute values of the instantaneous measured friction force. The resolution of these measurements was 0.1 N and the accuracy was 5%. The average friction force was used to evaluate the efficiency of the partial LST. The stroke in the tests was 100 mm and the angular velocity of the crank was varied in each test from 500 to 1200 rpm. The heater was tuned to provide an ambient temperature of 63–65◦ C. The temperature in the friction zone was higher and increased with increasing speed and normal loading. The dynamic viscosity of the oil corresponding to the aforementioned temperature range was 38.4–33.0 mPas, respectively. The surface roughness of both the plain and textured surfaces (area between the dimples) was Ra = 0.04 µm (note that Kligerman, et al.’s model (15) assumes smooth surfaces). A running-in procedure (see details of Ryk, et al. (7)) was performed prior to each test to ensure full conformity of the mating surfaces. Laser texturing was applied following the completion of the running-in of the specimens. The self-alignment holder mechanism, with its kinematic mounting, ensured precise alignment when the textured specimens were reinstalled. All tests started at 500 rpm followed by step increments of 100 rpm each, up to the maximum speed of 1200 rpm. It took between 5 and 7 min for the surface temperature to stabilize at each speed level. After reaching the stable temperature, friction measurement was taken at each speed level. A personal computer accomplished data acquisition and reduction, thus enabling on-line calculation of the average friction force over one cycle of revolution at every crank speed.

RESULTS AND DISCUSSION Three series of tests were carried out to study the benefit of partial LST in friction reduction of the textured specimens. The first of them consists of untextured specimens to establish a reference, the second utilized full LST specimens for comparison, and the third was performed with partial LST specimens. The LST parameters were selected based on the optimum results from the models of the full and partial LST of Ronen, et al. (10) and Kligerman, et al. (15), respectively, and the experience gained in previous tests (Ryk, et al. (7)). These parameters were 78 µm dimple diameter, 9 µm dimple depth, and 10% area density for full LST, and 75 µm dimple diameter, 7 µm dimple depth, and 50% area density for partial LST. It was shown by Kligerman, et al. (15) that in partial LST an optimum textured portion (the ratio Bp /W∗ in Fig. 4) of 0.6 holds for a wide range of LST parameters and operating conditions of the piston-ring/cylinder system simulation. Hence, a textured portion of 0.6 was applied to all the partial LST specimens symmetrically at their ends (see Fig. 4(b)). The results of the average friction force versus crank angular velocity are shown in Figs. 5–7 for the reference untextured case and for the two modes of full and partial LST cases. As can be seen, the average friction increases with speed and load in all three cases as would be expected (see also Ronen, et al. (16), (17)). Clearly the LST has a substantial effect on friction reduction compared to the untextured reference case. The average friction obtained with the full LST is about 40 to 45% lower than in the reference case at low speeds around 500 rpm, and 23 to 35% lower at higher speeds around 1200 rpm. These percentage differences between the average friction in the untextured and full LST cases are almost independent of the external normal load and slightly decrease with increasing angular velocity. The results in Figs. 5–7 clearly show the additional reduction in friction that can be obtained with partial LST over that of the full LST case, as was predicted by Kligerman, et al. (15). This additional reduction varies from 12 to 29% depending on the load and speed. In the present tests, the maximum benefit of the partial

Fig. 5—Time-averaged friction force vs. crank angular velocity for external normal pressure 0.1 MPa.

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LST was obtained with the combination of lowest speed and highest load. Table 1 summarizes the percentage gain in friction reduction with partial LST compared to full LST at the three load levels and at the extremes of the angular velocity range. As can be seen from Table 1, at the lowest speed of 500 rpm the behavior is very consistent, showing improving partial LST performance with increasing external loading. The behavior is somewhat random at the highest speed of 1200 rpm but still shows at least 12% gain with the partial LST. The different behavior at 1200 rpm could be attributed to the fact that above 900 rpm the vibration level of the test rig starts to increase and above 1200 rpm it reaches a high enough level to prohibit testing in this speed range. Hence, the friction measurements at 1200 rpm can be considered less reliable than at the 500 to 900 rpm range.

A comparison was made between the theoretical prediction of the model of Kligerman, et al. (15) and the current experimental results for full LST at the medium external loading of 0.3 MPa (see Fig. 6). The dashed line in Fig. 6 presents the theoretical results and, as can be seen, these are in good agreement with the experimental results in the angular velocity range from 500 to 900 rpm. At velocities above 900 rpm, this agreement deteriorates with increasing speeds, probably due to the increasing level of test-rig vibrations mentioned earlier. Finally, some preliminary tests were performed with production piston rings and cylinder liner segments that replaced the planar surfaces shown in Fig. 3 (for more details see Ryk, et al. (7)). The tested piston rings had a cross section of the “barrel” shape for their face rather than a “cylindrical”-shaped face. With


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TABLE 1—PERCENTAGE GAIN LST COMPARED TO FULL LST

500 rpm 1200 rpm

IN

FRICTION REDUCTION

WITH

PARTIAL

ACKNOWLEDGMENT Partial support by the Argonne National Laboratory, the Israel Ministry of National Infrastructure, and the Japan Technion Society Research Fund is gratefully acknowledged.

0.1 MPa

0.3 MPa

0.5 MPa

12% 19%

23% 12%

29% 16%

REFERENCES these partial LST barrel-shaped rings the reduction in friction was much less than with the flat specimens. Some real engine tests that were performed with partial LST barrel-shaped rings also show very little friction reduction at low speeds below 2000 rpm. Above 2000 rpm, this little benefit of the partial LST vanished completely. It seems that the barrel shape, which presumably was arrived at by trial-and-error experience over many years, is not a good candidate for partial LST. The crowning of the ring face by itself provides a strong hydrodynamic effect that masks the weaker hydrodynamic effect of the surface texturing, especially at high speeds. Hence, a more appropriate comparison should be made between the performance of optimum untextured barrel shape and optimum partial LST cylindrical shape rings.

CONCLUSION A previously developed model for the effect of partial LST of piston rings was evaluated on a reciprocating test rig by measuring the friction force between parallel flat faces simulating flat piston rings and cylinder liner. The results were compared with a reference untextured case as well as with a full LST case. It was found that, within the speed limitation of the test rig, a friction reduction of up to about 25% can be obtained with partial LST compared to full LST. This is an additional improvement over the ∼40% friction reduction obtained with the full LST compared to the untextured case. Some preliminary rig and real engine tests with production piston rings and cylinder liners did not show the same amount of friction reduction. These tests were, however, performed with barrel-shaped piston rings and not with conformal cylindrical rings. Further investigation is required to compare the performance of optimum partial LST cylindrical shape with that of the optimum more popular untextured barrel-shaped piston rings.

(1) Nakada, M. (1994), “Trends in Engine Technology and Tribology,” Tribol. Int., 27, pp 3-8. (2) Knoll, G. D. and Peeken, H. J. (1982), “Hydrodynamic Lubrication of Piston Skirts,” ASME J. Lubr. Technol., 104, pp 504-509. (3) Mitsuru, H. and Yasukazu, B. (1987), “A Study of Piston Friction Force in an Internal Combustion Engine,” ASLE Trans., 30, pp 444-451. (4) Takiguchi, M., Machida, K. and Furuhama, S. (1988), “Piston Friction Force of a Small High Speed Gasoline Engine,” ASME J. Tribol., 110, pp 112-118. (5) Jeng, Y. R. (1996), “Impact of Plateaued Surfaces on Tribological Performance,” Tribol. Trans., 39, pp 354-361. (6) Willis, E. (1986), “Surface Finish in Relation to Cylinder Liners,” Wear, 109, pp 351-366. (7) Ryk, G., Kligerman, Y. and Etsion, I. (2002), “Experimental Investigation of Laser Surface Texturing for Reciprocating Automotive Components,” Tribol. Trans., 45, pp 444-449. (8) Etsion, I., Kligerman, Y. and Halperin, G. (1999), “Analytical and Experimental Investigation of Laser-Textured Mechanical Seal Faces,” Tribol. Trans., 42, pp 511-516. (9) Kligerman, Y. and Etsion, I. (2001), “Analysis of the Hydrodynamic Effects in a Surface Textured Circumferential Gas Seals,” Tribol. Trans., 44, pp 472478. (10) Ronen, A., Etsion, I. and Kligerman, Y. (2001), “Friction Reducing Surface Texturing in Reciprocating Automotive Components,” Tribol. Trans., 44, pp 359-366. (11) Tonder, K. (2001), “Inlet Roughness Tribodevices: Dynamic Coefficients and Leakage,” Tribol. Int., 34, pp 847-852. (12) Etsion, I. and Halperin, G. (2002), “A Laser Surface Textured Hydrostatic Mechanical Seal,” Tribol. Trans., 45, pp 430-434. (13) Brizmer, V., Kligerman, Y. and Etsion, I. (2003), “A Laser Surface Textured Parallel Thrust Bearing,” Tribol. Trans., 46, pp 397-403. (14) Etsion, I., Halperin, G. Brizmer, V. and Kligerman, Y. (2004), “Experimental investigation of Laser Surface Textured Parallel Thrust Bearings,” Tribol. Lett., 17, pp 295-300. (15) Kligerman, Y., Etsion, I. and Shinkarenko, A. (2005), “Improving Tribological Performance of Piston Rings by Partial Surface Texturing,” ASME J. Tribol., 127, pp 632-638 (16) Ronen, A., Kligerman, Y. and Etsion, I. (2001), “Different Approaches for Analysis of Friction in Surface Textured Reciprocating Components,” in Proc. of the 2ndWorld Tribology Congress, Vienna, Session 44-1, 368, p 486. (17) Ronen, A., Kligerman Y., Ryk, G. and Etsion, I. (2000), “Improving Tribological Properties of Automotive Mechanical Components by Laser Surface Texturing,” in Proc. of the 2000 Global Powertrain Congress, 16, Ed. Roessler, D. M., GMC, Detroit, MI, pp 54-61.