Capacitive soot sensor for diesel exhausts

Abstract

Particulate matter (PM) emissions of automotive exhausts are considered to cause serious health problems. Diesel particulate filters (DPF) are in serial use to trap soot from the exhaust. For on-board diagnostics, soot sensors are applied downstream of a DPF. They follow a conductometric principle: The current signal for a constant voltage applied between two interdigital electrodes (IDE) increases with increasing soot deposition after an initial blind time, at which the sensors show no signal. In the present study, a thin insulation layer – manufactured by the aerosol-deposition-method (ADM) – covers the electrode area of a conductometric device and forms a capacitive device. During soot deposition on the surface, the sensor capacitance increases as calculated by FEM simulations. The higher the soot concentration, the faster is the capacitance increase. By applying a dc voltage between the electrodes during soot collection, the sensor performance can be further improved. Compared to the conductometric device with a similar IDE geometry, a reduced blind time at the same dc voltage is achieved. Even more, the capacitive sensor can be operated at higher collection voltages, since the electrodes are insulated towards each other. This allows for a further increase of the applied dc voltage, which reduces even more the blind time. Measurements in an engine dynamometer showed that the blind time can be reduced by a factor of three, with a high potential for a further reduction by technological means.