A more acceptable solution than two-stage for higher boost pressures is
the single-stage High Pressure Ratio Compressor (HPRC).
As the trends for higher power density and reduced emissions
continue, the turbocharger has to compensate by providing
higher compressor outlet pressures. This provides two challenges.
The first is how to produce an aerodynamic design that will achieve these higher
pressure ratios yet still having a usable flow range and a satisfactory efficiency.
For example, it is known that pressure ratio and efficiency can be increased
by the use of vaned diffusers, but at the expense of compressor map width. Cummins Turbo Technologies is continuously performing fundamental research to discover how to achieve an
improvement in all three parameters.
The second challenge is to find a material that will survive at the high temperatures
generated when operating at these high pressure ratios. Aluminium alloy is normally
used for compressor wheels, but all known alloys have serious deterioration in
material properties at temperatures much above 230 degrees Celsius. Aerospace
applications have long used titanium alloy as a suitable higher temperature material,
but have manufactured it by machining from solid wrought billets. This is not
a cost-effective means of production for the automotive industry. For this reason
Cummins Turbo Technologies has invested heavily in developing the technology to cast titanium compressor
wheels in high volume and at a competitive cost. Serious challenges have been
overcome to maintain dimensional accuracy, high casting quality and high yield
in the casting process and Cummins Turbo Technologies was the first automotive turbocharger manufacturer
to go into volume production of cast titanium compressor wheels in October 1997.
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