How to Redesign a Compressor Stage for a High-Performance Electric Supercharger in a Heavily Downsized Engine

Recently, automotive companies have shown a large investment and push toward downsizing automotive gasoline and diesel fueled engines. The drive to downsize engines aims to conform with legislation and trends in lowering CO2 output.

For example, the European Union’s target by 2021 is a 38% reduction in current CO2 vehicular output. Though downsized engines reduce CO2 output, performance is not necessarily affected. This is attributed to the addition of an electric supercharger and or turbocharging components, allowing increased efficiency and improved performance even in transient engine operation. Thus, making downsized engines is a highly competitive option in the marketplace in the short and medium term future. A crucial factor in the success of the downsizing initiative for automotive manufacturers is the effective pairing of electric turbocharging to the engine.

Turbochargers themselves are not a new concept; automotive manufacturers have gained performance and fuel economy with the incorporation of such systems in the past. The traditional drawback of such systems has always been the associated lag, a result of the direct mechanical coupling of the compressor to an exhaust gas driven turbine. New innovations in electric turbochargers have essentially overcome that hurdle by decoupling the turbine and compressor with means of an electric system rather than a mechanical one. Even with improvement in transient performance, a turbocharger’s effectiveness is still very much attributed to its aerodynamic performance.

Currently, what is underemphasized in literature is the aerodynamic design process of the compressor blades themselves. Although automotive manufactures have a significant wealth of expertise in this area, the issue of downsizing has forced a reduction in the size of the compressor blades; ultimately, designers are facing unfamiliar territory. This is made more difficult by the competitive market nature and a cost adverse climate which emphasize rapid design. Conventionally speaking, turbochargers are made based on considerable prior experience and plenty of hours of computational iterative simulations.

Recently, Diener et al. (Diener, O., Spuy, S., Backstr€om, T., and Hildebrandt, T., 2006, “Multi-Disciplinary Optimization of a Mixed-Flow Compressor Impeller,” ASME Paper No. GT2016-57008.) optimized a mixed flow compressor impeller mainly through the modifications of the meridional housing using the genetic algorithm. Their computational fluid dynamics (CFD) predictions reported that the optimized impeller has improved both pressure ratio and efficiency while the surge margin was not as good as the baseline design.

Perrone et al. (Perrone, A., Ratto, L., Ricci, G., Satta, F., and Zunino, P., 2006, “Multi-Disciplinary Optimization of a Centrifugal Compressor for Micro-Turbine Applications,” ASME Paper No. GT2016-57278.) conducted a study of multidisciplinary optimization of a centrifugal compressor for microturbine applications. In their work, both meridional channel and blade profiles were parametrized directly and optimized using design of experiments technique. The flow field analysis showed that their optimized impeller reduced the shock strength and tip leakage flows, which resulted in improved pressure ratio, efficiency, and choke margin. However, there was no improvement of surge margin found in their paper either.

An alternate process is to approach the design by first specifying a particular blade loading distribution for either compressor or turbine blades. Specifying the distribution has the effect of dictating the overall flow behavior, which in turn provides blade geometries which closely match desired performance parameters. This process, which is often referred to as the inverse design method, has already proven effective in a variety of industries
including the automotive industry.

In this paper, we present a novel three-dimensional (3D) inverse design of a centrifugal compressor stage used in an electronic supercharger. An impeller wheel was first designed using the 3D inviscid code TURBOdesign1; following that, a two-dimensional (2D) inverse code TURBOdesign Volute was used for the volute design. The CFD analysis has been performed at design and off design speeds to construct a complete compressor performance map, which will be compared with the predicted baseline compressor map. It will be shown that the redesigned compressor characteristics are significantly improved. This redesigned compressor stage has also been prototyped and tested at Aeristech’s test stand. The experimental measurements have very good agreement with CFD predictions and also have confirmed the performances are much better than the baseline design in terms of pressure ratio, efficiency, and operating range at every operating speed.

To discover more about the design targets, the three-dimensional design of the impeller wheel and its analysis, the design of the volute, the computational fluid dynamics analysis for the complete compressor stage and the experimental validations, you can from now on download your own copy of the publication below.