Engine downsizing has established itself as one of the most successful strategies to reduce fuel consumption and pollutant emissions in the automotive field. To this regard, a major role is played by turbocharging, which allows an increase in engine power density, 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 a 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 to be a 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-liter spark ignition light-duty automotive engine. The goal of the work is to improve engine transient performance, while ensuring 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, namely turbine and compressor mechanically coupled, which is compared with an advanced turbocharging concept, based on turbine and compressor electrically coupled. In both cases, a single-stage axial flow turbine is employed to extract energy from the exhaust gases. Ad hoc preliminary turbine design tools are developed, accounting for both design point and off-design 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.