Erweiterung der kennfeldbasierten Verdichtermodellierung für sequentielle Aufladesysteme

  • Enhancement of the state-of-the-art map-based compressor modelling for sequential boosting systems

Schloßhauer, Adrian; Pischinger, Stefan (Thesis advisor); Wirsum, Manfred (Thesis advisor)

Aachen : RWTH Aachen University (2021)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2021

Abstract

The overall goal of this thesis is to extend the map-based compressor modeling, for example for the layout of sequential boosting systems using 1D engine process simulation. The need for research is initially demonstrated with a mixed-sequential boosting concept. Due to serial arranged compressors, low pressure ratios occur in both compressors during stationary full load operation. The conventional methods for stationary compressor measurements on a hot gas test bench do not cover the operating range at low compressor pressure ratios. With new measuring methods the examined operating range is extended to zero speed and pressure ratios of less than one (ΠC < 1). The evaluation of the operating ranges in which power input or power output, i.e. turbine operation, occurs provides a fundamental understanding of the processes in the compressor. The findings of the extended measurements show that the usual map-based compressor modelling approach based on isentropic efficiencies is not sufficient to describe the system behavior at ΠC < 1. An extended modelling approach, as well as a new compressor model based on the newly introduced corrected compressor torque, allows a predictive 1D engine process simulation. Extended measurements are not always available. Different methods from literature to extrapolate the compressor map to low pressure ratios are investigated. However, these show weaknesses in the prediction of choking mass flow rates. A detailed analysis by means of 3D-CFD compressor simulations allows the localization of the choking cross-sections of the compressor in engine-relevant operating areas. At high circumferential speeds, choking occurs at the impeller inlet, whereas at medium circumferential speeds choking can be observed at the impeller outlet. At the lowest circumferential speeds choking occurs in the junction of the compressor volute. Based on these findings, a 0D/1D-modelling approach for extrapolation of the compressor map is presented. The model allows the calculation of flow conditions along the flow path in the compressor based on a few geometric dimensions as well as empirical loss models. The parameters of the loss models are adjusted so that the model matching the measured data in the conventionally measured operating range. The model shows a good prediction accuracy of the choking mass flow rate in relation to the extended measured data. Finally, the extended compressor modelling is validated with measurements on the engine test bench. Compared to state of the art compressor modelling, the new modelling approach shows clear advantages in predicting pressure losses for engine operation points at low compressor pressure ratios.

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