Compressor Map Width Enhancement

Hannu Jääskeläinen

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Abstract: High pressure EGR reduces the mass flow through the turbocharger and requires a compressor with a surge line far to the left on the compressor map. To avoid the use of more expensive, dual stage turbochargers, methods have been developed to extend the width of compressor map. These methods, some of them introduced in commercial diesel engines, include passive and active casing treatment, inlet swirl control vanes and diffuser vanes.

Introduction

In selecting a turbocharger for a given application, a single turbocharger with a single compressor solution is the preferred choice for reasons of cost and complexity. With increased use of higher levels of high pressure EGR, especially in diesel engines, this can be particularly challenging. As shown in Figure 1, high levels of high pressure EGR can tax the limits of a single compressor solution [2659]. Large fractions of high pressure EGR at low load/low speed reduces the mass flow through the turbocharger and pushes the demand for a compressor with a surge line far to the left. At high load/speed conditions where EGR fractions are relatively low, demand for high rated power requires a high pressure ratio at a high mass flow. As low load/speed EGR rates increase, the difference between these two extremes can increase and require very wide compressor map widths. While multiple compressors offer one solution to this problem, the potential savings in cost and complexity of using a single compressor provides strong motivation for considering options to expand the map width of a single compressor.

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Figure 1. Critical Engine Points on Compressor Map

Compressor map wide enhancement is of course only useful if the turbine can provide sufficient power to the compressor to generate the required pressure and flow rate. In some cases, variable geometry turbines offer an expanded range needed to drive a compressor with an expanded map width.

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