No abstract

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 nTC = 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 nTC = 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.

No abstract

Heat transfer effects play a significant role in assessing the performance of automotive turbochargers. Thermal effects are becoming increasingly relevant due to reduced machine sizes and increased exhaust gas temperatures. In this work, a study of the individual energy flows is conducted by simulation of a complete turbocharger comprising compressor (dC = 51 mm), turbine, and bearing housing using conjugate heat transfer. Special focus is given to the analysis of the various heat flows occurring in the machine aiming to identify the major heat transfer paths and their sensitivity with respect to varying operating conditions. Cooling of the bearing housing is shown to be a powerful thermal isolator mitigating the heat transferred to the compressor by up to 60%. Moreover, the rotating speed largely dictates the amount of heat transfer in the compressor and the direction of the heat flow: Whereas at low speeds (22% of max. speed), 117 W are introduced into the fluid and 338 W are being discharged from the fluid at maximum speed. At high speed operation, the heat transfer is shown to be insignificant compared to the aerodynamic work. At low speeds, however, it can reach up to 35% of the aerodynamic work. While the turbine inlet temperature largely governs the overall heat that is lost from the exhaust gas passing the turbine (from 630 W at 300 °C up to 3.72 kW at 1050 °C), only a minor effect on the compressor heat transfer is detected.