Emissions legislation for internal combustion engines has been tightened yet further, demanding a reduction of more than 90% with respect to untreated emissions. In conjunction with measurements concerning the engine and the power system, λ = 1 regulation, and 3-way catalytic converter, it is possible to reduce the pollutants CO, HC and NOx by means of catalytic oxidation. Within the first few minutes after a cold start of the engine the temperature of the catalytic converter is below its specified “light-off” temperature. The injection of air into the exhaust manifold causes an after-burning of the flue gas. So this leads to a more quickly raising of the temperature of the catalytic converter and a better reduction of the pollutants CO and HC in the cold start phase of the combustion engine. The secondary air is compressed by a centrifugal blower which is directly driven by a 13-Volt DC motor. To reduce the power consumption for the electric drive and to minimize the construction volume of the machine, the blower has been redesigned by means of commercial CAx-methods [1]. First the single components of the turbomachine, the impeller, the return channel and the volute have been designed for the required operational characteristics. After an optimization process for these single components the complete machine consisting of a two stage rotor, a return channel between the impellers and a volute at the end of the machine has been investigated numerical by a commercial 3D Navier-Stokes solver. The differences between the computations of the single components compared to the according computations for the whole machine are shown as well. Beside the calculation of the flow field in the desired design point an investigation of the blower characteristic was accomplished. To validate the numerical results a prototype by the method of rapid prototyping was generated. The measured overall characteristic curves of the machine have been compared to the calculated ones. The differences are discussed in detail with the aid of pressure measurements at several locations in the machine.

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