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Turbocharger compressors are limited in their operating range at low mass flows by compressor surge, thus restricting internal combustion engine operation at low engine speeds and high mean effective pressures. Since the exact location of the surge line in the compressor map depends on the whole gas exchange system, a safety margin towards surge must be provided. Accurate early surge detection could reduce this margin. During surge, the compressor outlet pressure fluctuates periodically. The Hurst exponent of the compressor outlet pressure is applied in this paper as an indicator to evaluate how close to the surge limit the compressor operates. It is a measure of the time-series memory that approaches zero for anti-persistence of the time series. That is, a Hurst exponent close to zero means a high statistical preference that a high value is followed by a low value, as during surge.

The challenging problem of noise generation and propagation in automotive turbocharging systems is of real interest from both scientific and practical points of view. Robust and fast steady-state fluid flow calculations, complemented by acoustic analogies can represent valuable tools to be used for a quick assessment of the problem during e.g. design phase, and a starting point for more in-depth future unsteady calculations. Thus, as a part of the initial phase of a long-term project, a steady-state Reynolds Averaged Navier-Stokes (RANS) flow analysis is carried out for a specific automotive turbocharger compressor geometry. Acoustic data are extracted by means of aeroacoustics models available within the framework of the STAR-CCM+ solver (i.e. Curle and Proudman acoustic analogies, respectively).

The compressor surge line of automotive turbochargers can limit the low-end torque of an engine. In order to determine how close the compressor operates to its surge limit, the Hurst exponent of the pressure signal has recently been proposed as a criterion. The Hurst exponent quantifies the fractal properties of a time series and its long-term memory. This paper evaluates the outcome of applying Hurst exponent based criterion on time-resolved pressure signals, measured simultaneously at different locations in the compression system. Experiments were performed using a truck-sized turbocharger on a cold gas stand at the University of Cincinnati. The pressure sensors were flush-mounted at different circumferential positions at the inlet of the compressor, in the diffuser and volute, as well as downstream of the compressor.