Axial Flow Turbine Concept for Conventional and e-Turbocharging

Abstract

Engine downsizing has established itself as one of the most successful strategies to reduce fuel consumption and pollutant emissions in the automotive field. In this regard, a major role is played by the turbocharging, allowing to increase engine power density, and so reducing engine size and weight. However, the need for turbocharging imposes some issues to be solved. In the attempt of mitigating turbo lag and poor low-end torque, many solutions have been presented in the open literature so far, such as: low inertia turbine wheels and variable geometry turbines; or even more complex concepts such as twin turbo and electrically assisted turbochargers. None of them appears as definitive, though. As possible way of reducing turbine rotor inertia, and so the turbo lag, also the change of turbine layout has been investigated, and it revealed itself as viable option, leading to the use of mixed-flow turbines. Only recently, the use of axial-flow turbines, with the aim of reducing rotor inertia, has been proposed as well. The current paper documents a case study involving the design of unconventional axial-flow turbocharger turbines for a 1.6 liters S.I. light-duty automotive engine. The goal of the work is to improve engine transient performance, while guaranteeing the same level of boost pressure with respect to the baseline case, i.e. engine equipped with radial-flow turbine. To do so, two possible proposals are investigated: a "conventional" turbocharger concept, say turbine and compressor mechanically coupled, is compared against an advanced turbocharging concept, say turbine and compressor electrically coupled. A single-stage axial flow turbine is employed, for both cases, to extract energy from the exhaust gas. Ad hoc preliminary turbine design tools are developed, accounting for both design point and offdesign performance. Turbocharger-engine matching is subsequently verified by means of a 1D engine model. Finally, results are used to derive guidelines for unconventional turbocharging turbine design.

Design and CFD Analysis of a Radial-Inflow Turbine for Small Scale ORC Applications

“…The preliminary design is accomplished by means of an in-house code implemented in Matlab environment, which performs the preliminary sizing of volute (treated as isentropic), stator, vaneless interspace and rotor, whose global structure is similar to the code presented by the authors in Ref. [17] for preliminary design of an axial-flow turbine. However, some additional design variables were introduced to account for the change in turbine layout, as well as some others were replaced.…”