Review of turbocharger mapping and 1D modelling inaccuracies with specific focus on two-stage systems

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 increasing torque and pedal response of small displacement engines. In two stage serial turbocharges, a high pressure (HP) and a low pressure (LP) turbocharger are connected by a series of ducts. The former can increase charge pressure for low air mass flow typical of low engine speed. The latter has a bigger size and can cooperate with higher mass flows. In serial configuration, 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. By-pass valves and waste-gated turbines are often included in two stage boosting systems in order to regulate turbochargers operations. One-dimensional modelling approaches are considered for investigating the integration of boosting systems with internal combustion engines. In this scenario, turbocharger behaviour are input in the powertrain models through previously measured compressor and turbine maps in turbocharger gas stands. However, this procedure does not capture all the effects that occur on engine application such as heat transfer, friction and flow motion that influence the turbochargers operations. This is of particular importance for two stage serial turbochargers where the LP compressor may induce a swirling motion to the flow at the entry of the HP compressor. In addition, flow non-uniformities caused by bends between the two compressors can make the HP compressor perform differently. In this paper, a review of the available literature containing approaches to quantify the effects of heat transfer on turbocharger efficiency and the flow influence in the prediction of two stage serial turbochargers performance is explored.