Abstract
Within a broader national project aimed at the hybridization of a standard city car (the 998 cc Mitsubishi-derived gasoline engine of the Smart W451), our team tackled the problem of improving the supercharger performance and response. The originally conceived design innovation was that of eliminating the mechanical connection between the compressor and the turbine. In the course of the study, it turned out that it is also possible to modify both components to extract extra power from the engine and to use it to recharge the battery pack. This required a redesign of both compressor and turbine. 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 us 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 nozzle guide vanes (NGV) and rotor blades shape were deemed necessary, while a more substantial modification was in order for the compressor. Fully 3-D computational fluid dynamics simulations of the rotating machines were 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 specific fuel consumption for the original engine. The results presented in this paper demonstrate that the efficiency of the compressor is noticeably improved for steady operation at all three operating points, and that its choking characteristics have been improved, while its surge line has not been appreciably affected. The net energy recovery was also calculated and demonstrated interesting returns in terms of storable energy in the battery pack.
Highlights
The increasing concern about CO2 emissions led to stricter regulations on the exhaust limitation of internal combustion engines (ICE)
It is intuitively clear that the most influential performance governing feature in a tandem compressor configuration is the interaction between the inducer boundary layer and the exducer blades, and it is clear that in general the stage efficiency increases when the inducer boundary layer falls in the pressure side of the exducer, there seems to be no general agreement on how the circumferential and axial separation between inducer outlet and exducer inlet affect performance (Figure 19)
The latter configuration displays a flatter characteristic, maintains a reasonably high efficiency, especially in the high-flow region, attains a higher compression ratio at all flowrates, delays chocking and expands operation in the stall region, albeit at a nonnegligible efficiency loss. These results indicate that a tandem compressor must be designed as such, and that a simple” refurbishing” of an existing compressor does not lead to a real performance optimization
Summary
The increasing concern about CO2 emissions led to stricter regulations on the exhaust limitation of internal combustion engines (ICE). The three solutions are not equivalent in terms of feasibility, cost, and time-to-market, and only the first one is based on a sufficiently mature technology to be immediately implemented without substantial modifications to the production and assembly lines Within this scenario, the Italian Ministry of Research launched a series of R&D projects aimed at a better definition of short-term modifications to existing gasoline ICE. Since downsized engines are not palatable for the average European customer, it is necessary to increase the power density of the ICE: turbocharging is an industrially mature measure introduced in large-series commercial power units already in the 1970s [4] It has been continuously improved -first by testing it on racing prototypes and “Gran Turismo” competition cars and by its limited introduction into the high-end, high-performance luxury sports car niche—until the advance in electronic control units made its adoption possible on large series commercial vehicles
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