The Study of Crank train Friction Characteristics for Friction Loss Reduction in Light-duty Diesel Engine

Abstract

In this study, measurements and calculations of friction energy loss in the crank train of a light-duty vehicle diesel engine are presented. The main objectives of the study are to estimate the friction loss of each engine component and to demonstrate the validity of our Matlab-based friction modeling method. Friction measurement results obtained via a strip-down method are utilized to separate the entirety of engine friction loss into individual component groups. The FMEP (friction mean effective pressure) of each component of the crank train was determined from the engine motoring torque via the strip-down method. The results from experimental measurements and calculations are compared to verify our friction model. The results from the Matlab friction model are compared with various reference study results. The model considers the lubrication of crank train components using the solution of the Reynolds equations. To solve the Reynolds equation of each component lubrication, the behavior of each component is considered. The friction modeling method is used to analyze the friction characteristics of piston rings, skirts, big end bearings of connecting rods, and the main journal bearing of the crankshaft with various engine operating conditions. The engine in-cylinder pressures were used to investigate the effect of engine load variation on engine friction. The friction of each component of the crank train increased with engine load due to combustion gas pressure loading in cylinder. The crank train components predominantly operate in hydrodynamic lubrication regime. In the case of the piston first compression ring, the friction of boundary lubrication regime was high during the expansion stroke. In addition, the Crankshaft and Piston & Con-rod friction measurements were carried out at various temperature to investigate the effect of temperature operating conditions on friction characteristics. For each experimental measurement, the coolant, oil main gallery, and oil pan temperature were controlled at 30±2℃, 50±2℃, 70±2℃, and 90±2℃, respectively. The FMEP of crank train components decreased with the operating temperature due to higher oil viscosity at lower oil temperature. The FMEP of the crank train increased with engine speed. These phenomena indicate that the hydrodynamic lubrication regime is dominant. To utilize the friction model for engine friction reduction, the impact of geometric changes to the piston assembly components on friction characteristics was investigated. Each case of piston ring height of 1.0 .mm and crown height of 10 μm has a minimum value of average power loss. In the case of piston skirt friction, a piston skirt profile of 0mm and piston pin offset of 1.3mm has the minimum value of average power loss. As a result of analyzing this phenomenon, the proper minimum film thickness is found which reduces the friction of the piston assembly. When the oil film is thick, it corresponds to the fluid lubrication area. The thicker the oil, the higher the friction. However, when the oil film thickness is too thin, it corresponds to the boundary lubrication area and friction increases due to asperity contact friction. Also, the friction loss is affected by the frictional force and the piston speed. Even though the frictional force is large near the top dead center and the bottom dead center, the friction loss is not greatly influenced by the frictional force due to the low piston velocity. On the contrary, the friction loss is small when the friction force is small in the middle of the piston stroke (when the piston velocity is high).