Oil Film Thickness Simulations and Measurements of a Main Bearing in a High Speed DI Engine Using Reduced Engine Speed

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Brook Owens
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1 Oil Film Thickness Simulations and Measurements of a Main Bearing in a High Speed DI Engine Using Reduced Engine Speed J. Tamminen, C-E. Sandström Helsinki University of Technology, Internal Combustion Engine Laboratory ABSTRACT Engine noise, specific fuel oil consumption, and engine durability can be influenced by reduction of mean piston speed. When simultaneously reducing the engine speed and remaining the rated power and maximum torque at the original level, the load of the engine crankshaft bearings will increase and the oil film thickness of the bearings will decrease. Both engine tests and bearing simulations have been performed in order to study the influence of the engine speed reduction to the minimum oil film thickness of the main bearing in a high-speed heavy duty DI diesel engine. The results of the simulations and the engine tests, and their comparison are presented in this paper. INTRODUCTION The objective of this study is to simulate and measure the oil film thickness in a main bearing of a DI diesel engine when using engine speed lower than in normal application. The results are used to compare and evaluate the effect of the engine speed at constant engine load to the oil film thickness of the main bearing. The present work is a part of the project Tribology of Internal Combustion Engines, which is a part of the Finnish National Technology Agency Tekes research programme ProMotor. This study of main bearing tribology was carried out at Helsinki University of Technology, Internal Combustion Engine Laboratory. EXPERIMENTAL SETUP AND SIMULATION SOFTWARE EXPERIMENTAL SETUP Engine description A six-cylinder direct-injection Sisudiesel 634 DSBIE diesel engine was used as an experimental engine. The technical data of the is listed in Table 1. Table 1. Test engine specification. Type Sisudiesel 634 DSBIE Design 4-stroke, direct-injection, water-cooled, provided with inter-cooler Number of cylinders 6 Firing order Cylinder bore 18 mm Stroke 134 mm Swept volume 7,4 dm 3 Compression ratio 16,5 Rated speed 22 1/min Mean piston speed at 9,8m/s 22 RPM Fuel injection pump Bosch PES6P Turbocharger Schwitzer S2B Main bearing Features related to the main bearing are presented in Table 2 and schematics of the bearing instrumentation are presented in Figure 1. Table 2. Main bearing characteristics. Diameter of bearing 91 mm housing (Bearing inner (85,7 mm) diameter) Bearing width 32 mm Bearing thickness Bearing material: -bearing metal -bearing shell -bearing surface Oil supply to the bearing Average surface pressure (at rated speed and load) Oil flow to the bearing Oil pressure 2,965 mm Lead bronze Steel, C1 Lead-indium plate Oil groove in the upper half of the bearing, oil supply diagonally to the groove About 3 MPa 2,8 l/min,2,4 MPa

2 Figure 1. Instrumentation of the main bearing. The oil film thickness in the main bearing was measured with two eddy current displacement sensors. The temperature of the main bearing was measured with two K-type thermocouples and using the temperature compensation signal coming from the pressure sensor. SIMULATION SOFTWARE The journal bearing simulation software ORBIT version 1.3, developed by Ricardo Software, was used for the bearing computation. A rigid bearing model was adapted in the simulations. Bearing forces were calculated regarding only the two cylinders adjacent to the bearing, as this was considered sufficient. The bearing properties are according to Table 2. Furthermore, the finite volume method, a mesh size of 11x11 nodes, and a crank angle increment of one degree crank angle were used as simulation parameters. EXPERIMENTAL PROGRAM Engine speed and load In the tests, the maximum brake mean effective pressure (p e ) of maximum torque (T max ) and rated power (P nom ) of Sisudiesel 62 standard and reduced speed engine versions were experimentally simulated with 634 DSBIE engine. Also the original maximum brake mean effective pressures of the standard 62 engine at the engine speeds corresponding to the T max and P nom of the reduced speed engine version, were run in the test engine. This experimental simulation was done since the 634 DSBIE engine was already instrumented for the measurements. The tested engine speed and load values are presented in Table 3. Table 3. Engine loads and speeds used in the tests. 1 (P nom ) 2 3 (P nom ) 4 (T max ) 5 6 (T max ) 62 Standard Standard Low speed Standard Standard Low speed P e kw n 1/min M Nm p e bar DSBIE P e kw n 1/min M Nm Measurement point 1 stands for the rated power of the standard 62 engine rated speed 22 1/min. Measurement point 2 stands for the maximum brake mean effective pressure of the standard 62 engine at 18 1/min. Measurement point 3 stands for the rated power of the reduced speed engine version rated speed 18 1/min. Measurement point 4 stands for the maximum torque of the standard 62 engine 14 1/min. Measurement point 5 stands for the maximum brake mean effective pressure of the standard 62 engine at 11 1/min. Measurement point 6 stands for the maximum torque of the reduced speed engine version 11 1/min. The cylinder pressure values measured in the engine test runs were used to calculate the loads for the computer simulations. Oil The tests were run using lubricating oil of SAE viscosity grade 15W4. RESULTS CYLINDER PRESSURE The cylinder pressure at different loading points measured from 634 DSBIE is presented in Figure 2. The measured cylinder pressures of the 634DSBIE engine were lower than those of the 62 engine version. This is mainly due to the difference in the swept displacement of these two engines. The maximum cylinder pressure in the 62 engine versions is around 13 bar. OIL FILM THICKNESS The simulated and measured curves of the minimum oil film thickness related to the rated power (measurement points 1, 2 and 3 in Table 3) are presented in Figure 3 and the curves of the minimum oil film thickness related

3 to the maximum torque (points 4, 5 and 6) are presented in Figure 4. Some numerical values of the measured and simulated results are collected in Table 4. Cylinder pressure 12 1 pcyl [bar] rpm, 7.6 bar 18 rpm, 9.4 bar 18 rpm, 1.bar 14 rpm, 1.3 bar 11 rpm, 1.1 bar 11 rpm, 12.4 bar Figure 2. Cylinder pressures of different test points. In the legend text the first number, e.g. 22 rpm, stands for the engine speed and the second number, e.g. 7.6 bar, for the brake mean effective pressure. MOFT related to the rated power [mm] _7.6_calc 18_9.4_calc 18_1_calc 22_7.6_meas 18_9.4_meas 18_1_meas Figure 3. Minimum oil film thickness related to the rated power. In the legend text the first number, e.g. 18, stands for the engine speed in revolutions/minute and the second number, e.g. 9.4, for the value of the brake mean effective pressure in bars. The calculated values are marked with calc and the measured values with meas.

4 MOFT related to the maximum torque [mm] _1.3_calc 11_1.1_calc 11_12.4_calc 14_1.3_meas 11_1.1_meas 11_12.4_meas Figure 4. Minimum oil film thickness related to the maximum torque. In the legend text the first number stands for the engine speed in revolutions/minute and the second number for the value of the brake mean effective pressure in bars. The calculated values are marked with calc and the measured values with meas. Table 4. Measured and calculated results. n 1/min P kw BMEP bar Pcyl, max. bar MOFT, measured µm Timing CA Location (bearing) BA MOFT, calculated µm Timing CA Location (bearing) BA MOFT_ average, measured µm MOFT_average, calculated µm Bearing temperature, measured C POFP, calculated bar Timing CA Location (bearing) BA IMEP bar Pind kw Pf kw ηmek % HDPL_average, calculated W BLPL_average, calculated W HDPL + BLPL, calculated KW n - engine speed IMEP - indicated mean effective pressure P - engine power Pind - indicated power BMEP- brake mean effective pressure Pf -friction power Pcyl. - cylinder pressure ηmek - mechanical efficiency MOFT - minimum oil film thickness HDPL - hydrodynamic power loss POFP - peak oil film pressure BLPL - boundary lubrication power loss OBSERVATIONS AND CONCLUSION Due to the measurement inaccuracy and simplifications used in computer simulations, both the measured and the simulated results of the oil film thickness should be considered as trend setting values. The results are also affected by the difference in the maximum cylinder pressure (2-3 bar) between the and the 62 engine versions.

5 The difference between the values of calculated and measured oil film thickness varies throughout the engine working cycle (Figures 3 and 4). However, the measured and simulated values of minimum oil film thickness at their lowest are almost even (Table 4). Table 4 shows, further, that the mechanical efficiency does increase with lower engine speed, when remaining the rated power and maximum torque. In this case, it seems that the lowest minimum oil film thickness of the main bearing decreases less than 1 µm from the original value, on an average about.5 µm, when the engine speed of the maximum torque or the rated speed is reduced. Compared to the values mentioned in literature [Taylor, 1993] and [Affenzeller, 1996], the lowest minimum oil film thickness (2-3 µm) at lowered engine speeds does not appear to be critical. Presumably the higher cylinder pressure of the original 62 engine applications is likely to decrease the oil film thickness on some degree more than in this test case. ACKNOWLEDGMENTS The authors wish to thank the participating companies and institutions National Technology Agency Tekes, Wärtsilä Corporation, Sisu Diesel Oy, Fortum Oil & Gas Oy and Volvo Technological Development Corporation - for their financial and technical support on the project, as well as the colleagues and project co-workers for their support in the work. REFERENCES 1. Affenzeller J. Gläser H. Die Verbrennungskraftmaschine Neue Folge: Lagerung und Schmierung von Verbrennungsmotoren. Springer-Verlag. Wien New York p. 2. Taylor C. M. (Editor). Engine Tribology (Tribology Series: 26). Elsevier Science Publishers B. V. Amsterdam p.

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