Within a broader national project aimed at the hybridization of a standard city car (the 899 cc Mitsubishi-derived gasoline engine of the Smart W451), our team tackled the problem of improving the supercharger performance and response. The design concept is that of eliminating the mechanical connection between the compressor and the turbine and to possibly modify both components to extract extra power from the engine and to use it to recharge the battery pack.
First, the initial configuration was analyzed on the basis of the design data provided by the manufacturer. Then, a preliminary performance assessment of the turbocharged engine allowed to identify three “typical” operating points that could be used to properly redesign the turbomachinery. It was decided to maintain the radial configuration for both turbine and compressor, but to redesign the latter by adding an inducer. For the turbine, only minor modifications to the blades shape were deemed necessary.
A 3-D CFD simulation of the rotating machines was performed to assess their performance at three operating points: the kick-in point of the original turbo (2000 rpm), the maximum power regime (5500 rpm) and an intermediate point (3500 rpm) close to the minimum sfc for the original engine.
The results demonstrate that the efficiency of the compressor is noticeably improved for steady operation at all three points, that its choking characteristics have been improved and that the surge line has not been appreciably affected. For the turbine, the efficiency displayed only little improvement with respect to the original configuration, but the introduction of variable nozzle guide vanes led to a much more stable performance.. The net energy recovery was also calculated, and demonstrated interesting returns in terms of storable energy in the battery pack.