Effect of Engine Operating Conditions and Lubricant Rheology on the Distribution of Losses in an Internal Combustion Engine

Abstract

With new legislation coming into place for the reduction in tail-pipe emissions, the OEMs are in constant pressure to meet these demands and have invested heavily in the development of new technologies. OEMs have asked lubricant and additive companies to contribute in meeting these new challenges by developing new products to improve fuel economy and reduce emissions. Modern low viscosity lubricants with new chemistries have been developed to improve fuel consumption. However, more work is needed to formulate compatible lubricants for new materials and engine technologies. In the field of internal combustion engines, researchers and scientists are working constantly on new technologies such as downsized engines, homogeneous charge compression ignition, the use of biofuel, new engine component materials, etc., to improve vehicle performance and emissions. Mathematical models are widely used in the automotive and lubricants industry to understand and study the effect of different lubricants and engine component materials on engine performance. Engine tests are carried out to evaluate lubricants under realistic conditions but they are expensive and time consuming. Therefore, bench tests are used to screen potential lubricant formulations so that only the most promising formulations go forward for engine testing. This reduces the expense dramatically. Engine tests do give a better picture of the lubricants performance but it does lack detailed tribological understanding as crankcase oil has to lubricant all parts of the engines, which do operate under different tribological conditions. Oil in an engine experiences all modes of lubrication regimes from boundary to hydrodynamic. The three main tribological components responsible for the frictional losses in an engine are the piston assembly, valve train, and bearings. There are two main types of frictional losses associated with these parts: shear loss and metal to metal friction. Thick oil in an engine will reduce the boundary friction but will increase shear losses whereas thin oil will reduce shear friction but will increase boundary friction and wear. This paper describes how engine operating conditions affect the distribution of power loss at component level. This study was carried out under realistic fired conditions using a single cylinder Ricardo Hydra gasoline engine. Piston assembly friction was measured using indicated mean effective pressure method and the valve train friction was measured using specially designed camshaft pulleys. Total engine friction was measured using pressure-volume diagram and brake torque measurements, whereas engine bearing friction was measured indirectly by subtracting the components from total engine friction. The tests were carried out under fired conditions and have shown changes in the distribution of component frictional losses at various engine speeds, lubricant temperatures, and type of lubricants. It was revealed that under certain engine operating conditions the difference in total engine friction loss was found to be small but major changes in the contribution at component level were observed.