Jae-Kwon Choi’s research while affiliated with Electric Power Research Institute and other places

This paper reports on the theoretical analysis of mixed lubrication for the piston ring. The analytical model is presented by using the average flow and asperity contact model. The cyclic variations of the nominal minimum oil film thickness are obtained by numerical iterative method. The total friction is calculated by using the hydrodynamic and asperity contact theory. The effects of the roughness height, pattern, and engine speed on the nominal minimum film thickness, friction force, and frictional power losses are investigated. As the roughness height increases, the nominal oil film thickness and total friction force increase. Also, the effect of the surface roughness on the boundary friction is dominant at low engine speed and high asperity height. The longitudinal roughness pattern shows lower mean oil film pressure and thinner oil film thickness compared to the case of the isotropic and transverse roughness patterns.

The aim of this paper is to study the thermal effects on minimum oil film thickness in a connecting-rod bearing. A comparison between the theoretical and experimental results of minimum oil film thickness in a connecting-rod bearing is presented. The transient energy equation is solved by adapting the adiabatic boundary condition for considering the thermal effects. The minimum oil film thicknesses are measured by the total capacitance method. The minimum oil film thickness over an engine cycle estimated with consideration of thermal effects is closer to that measures than that estimated by using the iso-viscous lubrication theory.

The capacitance technique was used to measure the minimum oil film thickness in engine bearing and the central oil film thickness between cam and tappet. This method is based on the measurement of total capacitance of oil film. For the measurement of the oil film thickness between cam and tappet, two surfaces were assumed to be flat and parallel within the Hertzian region and all the measured capacitance originated from this region. Shear rates from the measured minimum oil film thickness are over 10$^{6}$ sec$^{-1}$ in the greater part in both two cases. The minimum oil film thickness in engine bearing is larger than the surface roughness. Between cam and tappet it is mostly smaller than the surface roughness. In spite of the awkward restriction of the reliability of measured oil film thickness, it was known that the capacitance technique makes it possible to measure the oil film thickness in elastohydrodynamic and mixed lubrication regimes as well as in hydrodynamic regime. Therefore, it is also possible to classify the lubrication regimes based on the oil film thickness.

The minimum oil film thicknesses(MOFT) in the connecting-rod bearings of a 1.5 liter, L-4, gasoline engine are measured up to 5500 rpm and calculated to study the dynamically loaded engine bearings. Short bearing approximation and Mobility methods are used for theoretical analysis of oil film characteristics. Also cylinder pressure, crank-pin surface temperature and bearing temperature are measured and used as the input data of theoretical analysis. The MOFT are measured by the total capacitance method(TCM). To improve the reliability of the test results, a reasonable determination method of bearing clearance is introduced and used, and the effects of cavitation and aeration on the test results are neglected. Also the crankshaft is grounded by means of a slip ring. A scissor type linkage system was developed to measure the MOFT and bearing temperature. The effects of engine speed, load and oil viscosity on the measured and calculated minimum oil film thicknesses are investigated at 1500 to 5500 rpm. From the comparison between the measured and calculated MOFT, it is found that a qualitative similarity exists between them, but in all cases, the measured MOFT are smaller than those calculated.

The minimum oil film thicknesses (MOFT) in the crankshaft main bearings of a 1.5 liter, L-4, gasoline engine are measured and calculated to study the dynamically loaded engine bearing. The MOFT are measured simultaneously at each of the five main bearings using the total capacitance method(TCM). To improve the reliability of the TCM, a reasonable determination method of bearing clearance is introduced and the effects of bearing cavitation and aeration on the test results are analyzed. Also the crankshaft is grounded by means of a slip ring instead of the friction contact method to improve the test precision. The calculation is based on the model of statically determinate beam, short bearing approximation and Mobility method From the comparison between the measured and calculated MOFT curves, it is found that a qualitative similarity exists between them, but in all cases, measured MOFT are smaller than that of calculated The crankshaft vibration and the imbalance of the load distribution between the engine bearings have important influence upon the MOFT curve. So. it is found that the calculation result from the model of the statically determinate beam has a limitation in predicting bearing performance.