AAAR 35th Annual Conference October 17 - October 21, 2016 Oregon Convention Center Portland, Oregon, USA
Abstract View
Optimizing a Photoacoustic Soot Sensor for the Measurement of Ultra-low Soot Concentrations in Exhaust from Combustion Engines
MICHAEL ARNDT, Wolfgang Schindler, Herbert Reingruber, Klaus Harms, Stefan Riedler, AVL List GmbH
Abstract Number: 281 Working Group: Instrumentation and Methods
Abstract Photoacoustic soot sensors are capable of measuring very low concentrations of soot by using the light absorption of the black carbon in order to generate an acoustical signal. In real engine exhaust the chemical composition of the gas can influence the precision of the measurement by changing the acoustic properties of the gas.
This effect can be reduced by proper dilution of the sample, but in many applications this might not be desirable. Conducting a zeroing with filtered exhaust eliminates most disturbances caused by cross-sensitivities and acoustical effects. However, this method adds complexity to the application and only yields perfect results when the exhaust composition stays constant.
In order to overcome these issues a novel method for real-time compensation of the mentioned distortions was developed. It makes use of the fact that changes in the sonic speed manifest as a shift in the resonance frequency of the photoacoustic resonator system. The laser light source is modulated at the characteristic resonance frequency of the cell in order to obtain best signal amplification. The modulation frequency of the laser gets adjusted in real-time in order to keep the system at the resonance maximum. This is done by analyzing the phase angle of the standing acoustical wave. If the signal intensity is not high enough to generate a clean signal, an additional acoustical wave at a different frequency is induced by an internal loudspeaker. This additional signal is analyzed in parallel to the main signal. Thereby a correction of the modulation frequency is calculated in real-time. The processed measurement signal only consists of the main frequency, any other acoustical waves are removed by synchronous demodulation.
The accuracy of the recent version of a widely used photoacoustic instrument, the AVL Micro Soot Sensor plus, could be improved significantly by applying this method.