Abstract
Abstract. Diesel particulate filters are emission-relevant devices of the exhaust gas aftertreatment system. They need to be monitored as a requirement of the on-board diagnosis. In order to detect a malfunction, planar sensors with interdigital electrodes on an insulating substrate can be installed downstream of the filter. During the loading phase, soot deposits onto the electrodes, but the sensor remains blind until the percolation threshold has been reached (initiation time) and the sensor current starts to flow. In order to detect small soot concentrations downstream of the filter from small defects, this initiation time needs to be as low as possible. One may reduce the initiation time by covering the interdigital electrodes with an electrically conductive layer. Using finite element method (FEM) simulations, the influence of conductivity and thickness of such a coating on the initiation time are determined. It is found that a thin, screen printable coating with a thickness of 20 μm and a conductivity in the range of 10−3 to 10−1 S m−1 may reduce the initiation time by about 40%. The FEM results were verified by a commercially available thick film resistor paste with a conductivity of 0.45 mS m−1, showing an improvement of about 40% compared to an uncoated sensor.
Highlights
Together with the constantly tightening limits of harmful exhaust gas emissions of internal combustion engines, a permanent monitoring of all emission-relevant devices is required (Europaisches Parlament, 2007; European Union, 2008)
particulate matter (PM) consists of aggregated carbon soot particles, covered with organics like hydrocarbons, and inorganic oxides (Spears, 2008; U.S Environmental Protection Agency, 2002)
Due to the small size of these particles, ranging between only a few tens and some hundreds of nanometers (Harris and Maricq, 2001), and because of the soluble organic fraction (SOF) covering the surface of these small particles (Otto et al, 1980; U.S Environmental Protection Agency, 2002), serious concerns exist about the effect of PM to human health (Geiser, 2005; Adar et al, 2010; Grahame and Schlesinger, 2010; U.S Environmental Protection Agency, 2002)
Summary
Together with the constantly tightening limits of harmful exhaust gas emissions of internal combustion engines, a permanent monitoring of all emission-relevant devices is required (Europaisches Parlament, 2007; European Union, 2008). To overcome this and to reduce the blind time, we suggest applying an electrically conductive layer that connects both electrodes and covers all the space in between them. The conductivities were adjusted in a way that the basic current of the sensor without soot load, Inosoot, was 0.1 mA, 1 mA, or 10 mA at an applied voltage of U = 30 V.
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