A capacitance ultrasonic transducer for fast flow-measurements in hot pulsating gases [automotive applications]

A novel high-temperature resistant capacitance transducer is presented. It is designed for an ultrasonic transit-time flowmeter (UFM) and meets two main requirements not fulfilled by common piezoelectric transducers. Firstly, a special construction, based on an oxidized 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 is suitable for internal combustion engine exhaust flow measurements in 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).