Investigation of the influence of pulsating boundary conditions generated by an internal combustion engine on compressor operating near surge

Authors

Dominik Paul  - Hochschule RheinMain, Germany
Werner Eißler - Hochschule RheinMain, Germany
Tobias Haeckel - Technische Universität Darmstadt, Germany

Abstract

Due to the extreme increase in computing power, modern numerical development methods are becoming more and more important. Nevertheless, there is still no substitute for experimental studies. This becomes especially clear when investigating operating points that exhibit transient flow phenomena or instabilities. Such operating conditions are present at low mass flow rates near surge. It is well known that unsteady numerical calculations of the whole compressor geometry and the whole piping system are necessary to investigate those phenomena qualitatively and quantitatively. For an industrial design process, such simulations are not useful and affordable. Furthermore, quasi-steady-state boundary conditions at the inlet and outlet are used in most of the investigations. In addition to such simulations, experimental investigations on the hot gas test rig represent another way of determining compressor behaviour at low mass flow rates. These types of studies are the experimental standard. In most cases, the compressor is also examined under steady-state boundary conditions. In a real application, the centrifugal compressor of a turbocharger operates under pulsating boundary conditions. Due to the opening and closing of the engine inlet valves and the associated pressure fluctuations, the compressor experiences a pulsating backpressure. These pulsations influence compressor performance as well as shift the position of the surge onset. For this reason, it is extremely important for the design of centrifugal compressors to take this influence into account. In this paper, the influence of pulsating boundary conditions, generated by an internal combustion engine, on the position of the surge margin has been investigated. For this purpose, a methodology is presented on how to determine the surge limit in real engine operation. This method includes the experimental setup, application of the motor maps, as well as the determination method of the surge limit at different compressor speeds. To validate this methodology, the volume between the compressor outlet and the engine has been modified to verify its known influence on the position of the surge line. Transient pressure measurement technology has been used to detect instabilities during the operation of the combustion engine in the fired mode. Subsequently, the results were compared with the experimental investigations on a hot gas test rig. Special attention was paid to the location of the surge line and the comparison of the resulting frequencies at low mass flows. Among other things, it was possible to show which oscillations belong to the engine operation and which come from the compressor wheel. Furthermore, the position of the surge margin has shifted due to the engine operation to lower mass flow rates. The experimental results will later on being used to define boundary conditions for unsteady numerical simulations and to compare the results of the simulations.