Analysis of Thrust Bearing Impact on Friction Losses in Automotive Turbochargers

The importance of automotive turbocharger performance is continuously increasing. However, further gains in efficiency are becoming progressively difficult to achieve. The bearing friction losses impact the overall efficiency of the turbocharger and accordingly the understanding of bearing systems and their characteristics is essential for future improvements. In this work, a detailed analysis on the mechanical losses occurring in the bearing system of automotive turbochargers is presented. Friction losses have been measured experimentally on a special test bench up to rotational speeds of n_TC = 130,000 1/min. Special interest was given to the thrust bearing characteristics and its contribution to the total friction losses. For this, the experiments were split into three parts: first, friction power was determined as a function of turbocharger speed at zero externally applied thrust load. Second, external thrust load up to 40 N was applied onto the turbocharger bearing at fixed rotational speeds of n_TC = 40,000, 80,000, and 120,000 1/min. Increasing thrust load was observed to result in increasing friction losses amounting to a maximum of 32%. At last, a specially prepared turbocharger center section with deactivated thrust bearing was investigated. A comparison of these results with the measurement of the conventional bearing system under thrust-free conditions allowed separating journal and thrust bearing losses. The contribution of the thrust bearing to the overall bearing losses appeared to be as high as 38%. Furthermore, a modeling approach for estimating the friction power of both fully floating journal bearing as well as thrust bearing is illustrated. This theoretical model is shown to predict friction losses reasonably well compared to the experimental results.