Review of Turbocharger Mapping and 1D Modelling Inaccuracies with Specific Focus on Two-Stag Systems 2015-24-2523
The adoption of two stage serial turbochargers in combination with internal combustion engines can improve the overall efficiency of powertrain systems. In conjunction with the increase of engine volumetric efficiency, two stage boosting technologies are capable of improving torque and pedal response of small displacement engines. In two stage sequential systems, high pressure (HP) and low pressure (LP) turbochargers are packaged in a way that the exhaust gases access the LP turbine after exiting the HP turbine. On the induction side, fresh air is compressed sequentially by LP and HP compressors. The former is able to deliver elevated pressure ratios, but it is not able to highly compressor low flow rates of air. The latter turbo-machine can increase charge pressure at lower mass air flow and be by-passed at high rates of air flow. In fact, by-pass valves and waste-gated turbines are often included in two stage boosting systems in order to regulate operations and divert flow away from the turbocharger when necessary. By-pass valves are often external to the turbocharger and wastegates valves are incorporated in the turbine housing.
One-dimensional modelling approaches are considered fundamental to investigate interaction between boosting systems and internal combustion engines. In this scenario, turbomachinery performances are imposed into the model through compressor and turbine maps which are previously measured in gas stands as single stages. This procedure could not capture all the effects that occur in a system layout due to combination of heat transfer and motion of internal flow. This has a significant importance for defining HP compressor and LP turbine performances which could be influenced by swirling flows induced by the previous turbo-component in the series. Additionally, bends between the two compressors/turbines can reduce uniformity of flow and cause pressure drops at the same time. As in single turbochargers, heat transfer influences the boundary conditions which would influence performance predictions of the machine in the sequence. In this paper, a review of the available literature containing approaches and study to quantify the effects of heat transfer on turbocharger efficiency and flow non-uniformities on two stage serial turbochargers performance predictions is explored. Furthermore, an appropriate mapping strategy has been proposed which could minimize the cause of inaccuracies in predicting turbochargers performance in two-stage systems. Conclusively, a methodology for map integration into 1D models has been proposed.