Tribiological Analysis of Engine Valve Train Performance Considering Effects of Lubricant Formulation

Abstract

Fuel economy, reduction in power losses and control on emissions are the key drivers of the automotive industry. Development of new technologies and lubricant formulation is being pursued relentlessly to improve the engine performance. This in turn demands comprehensive experimental work based on reliable and accurate measurement system to analyze the effectiveness of these technologies. In roller follower valve train configuration, the power losses and deterioration of mating surfaces of cam and roller is largely governed by the sliding of rollers and lubrication conditions at cam/roller interface whereas the different operating conditions, lubricant rheology/chemistry can play an extremely important role in this context. In this research project, an advanced real production gasoline engine having end pivoted roller finger follower valve train has been instrumented, for the very first time, by employing the recently developed techniques based on advanced sensor technology to measure the rollers tribological behavior and oil film thickness at cam/roller interface under realistic environment. An elaborated experimental research work comprising of series of tests has been undertaken to investigate the effects of different operating conditions, oil rheology, lubricant chemistry and low viscosity oil on these important parameters. A new flexible test rig has been designed and developed whereas a high speed synchronized data acquisition system has been employed to hunt for the vital information. A numerical approach based on the lubrication and friction modeling is also the part of this research work to understand the tribological characteristics of the valve train and to predict various important tribological parameters. The experimental results showed that due to shear drag it was not necessary for roller rotational speed to increase with camshaft speed. The lubricant viscosity played a key role in the roller sliding at lower temperatures however at higher oil temperatures negative slip was also observed indicating that component inertia and internal friction have a role to play in roller slip. Relatively, higher magnitude of roller sliding was observed for mineral oil as compared to synthetic oil having almost same viscosity while operating under similar conditions. Good lubrication conditions were also observed at cam/roller interface due to dominance of rolling motion. Increase of roller sliding with corresponding rise in oil film thickness was recorded. A good agreement between the theoretical predictions and experimental evidences was also found. It is strongly believed that the obtained realistic data will provide greater flexibility in validating the predictive mathematical models on the valve trains and will be extremely beneficial for the engine designers and lubricant formulators in their ongoing efforts to improve the engine tribological efficiency.