| Summary: | Two-stage sequential turbocharging systems are adopted in automotive powertrains, in order to enhance torque level and transient response of the internal combustion engine. High pressure (HP) and low pressure (LP) turbochargers are placed in sequence, with exhaust gasses expanding through LP and HP turbines; while, air is compressed by LP and HP compressors, respectively, before being forced into the engine intake. In order to maximise performance and flexibility of the internal combustion engine, regulating valves are implemented into the turbocharging system. In order to investigate the interaction between the boosting system and the internal combustion engines, one-dimensional (1D) model simulations are considered a fundamental tool. In this scenario, turbomachinery performances are imposed into the model through compressor and turbine maps, to analyse the complete power unit. The adoption of standard gas-stand maps would be omitting the flow non-uniformity effects on a turbomachine due to bends and the presence of heat transfer in turbochargers could influence enthalpy level at boundaries. In addition, specific to two-stage turbocharging systems, the high degree of swirling flows and temperatures at the inlet of the HP compressor induced by the upstream turbomachine can influence significantly the performance of the two-stage system. Therefore, it has been fundamental to perform this study on the advanced characterisation of turbochargers, focussing on the modelling two-stage turbocharging systems. In order to perform the research study, a new engine gas-stand facility has been developed, mapping turbochargers compressors under steady flows. In addition, in the unsteady layout, the introduction of pressure pulsations at the compressor under real engine pulsations has shown a significant shift of the surge line towards lower mass flows. In order to quantify the effects occurring in a sequential two-stage turbocharging system, an equivalent mapping approach has been developed, aiming to the improvement of accuracy in performance characterisation, accounting for inter-stage flows and heat transfers effects. Equivalent two-stage compressor maps have differed from the stand-alone mapping approach below 23Krpm due to poor accuracy of stand-alone maps at low compressor speeds. On the turbines side, maximum differences of 10% between equivalent and stand-alone maps persist in turbine efficiency, due to turbine isentropic power.
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