The impact of voltage and flow on the electrostatic soot sensor and the implications for its use as a diesel particulate filter monitor

Abstract This paper takes a detailed look at the operation of the electrostatic soot sensor and its potential use to monitor motor vehicle particulate matter emissions. Charged soot particles entering the sensor are trapped and grow into dendritic soot structures aligned with the electric field. Above a critical length, the electrostatic force fractures the dendrites, producing fragments that carry sufficient charge between the electrodes to generate nanoamp level currents. While proportional to soot concentration, this current also responds to flow variations, which limits the sensor's application in areas such as motor vehicle on-board diagnostics. The present work demonstrates that this flow dependence is closely related to the sensor's response to variations in electric field. The measured response to step changes in applied voltage agrees very well with a kinetic model of electric field induced dendrite growth and fragmentation. Variations in flow induce fluctuations in sensor current as dendrites reorient in response to the drag force and find themselves above or below the critical length for fragmentation. This mechanism qualitatively fits the experimental data. In engine exhaust applications, variation of exhaust flow with engine operation interferes with time resolved soot measurement. This interference partially cancels out when averaged over time, rendering the electrostatic soot sensor a potentially viable means to distinguish diesel particulate filter performance.

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