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A. Schröder, M. Kupnik, P. O´Leary, E. Benes, M. Gröschl:
"Fast flow measurements in hot pulsating gases with capacitance ultrasonic transducers";
in: "Proc. 2nd Congress of Alps-Adria Acoustics Association and 1st Congress of Acoustical Society of Croatia", Acoustical Society of Croatia, Zagreb/Croatia, 2005, ISBN: 953-95097-0-x, S. 375 - 380.

Up to now, application of the advantageous ultrasonic transit-time flowmeter (UFM) measuring principle in
exhaust gas trains of combustion engines was limited by the temperature-induced depolarisation effect and by the
pronounced resonant behaviour of the piezoelectric transducers normally used. This problem can be overcome by using
high-temperature resistant capacitance transducers. This type of transducer is designed particularly for a UFM and meets
two main requirements not fulfilled by common piezoelectric transducers. Firstly, a special construction based on an
oxidised and patterned silicon backplate combined with a metallic membrane enables transducer operation at elevated gas
temperatures of up to 600 °C. Secondly, the geometry and material parameters were chosen to obtain a broad-band device
allowing for high signal slew rates and pulse repetition rates. As proven by measurements in a test bed environment, this
new transducer suits for internal combustion engine exhaust flow measurements at any position between the catalytic
converter and the end of the exhaust pipe. Preliminary results for the exhaust mass flow (up to 160 kg/h) of a typical
automotive engine measured with these novel transducers are given and compared to the mass flow calculated from fuel
consumption and air/fuel ratio (λ).
Key words: mass flow measurement, capacitance ultrasonic transducer, high temperature gas flow

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