Soot sensors may be used in engine emissions applications, e.g. for on-board diagnostics (OBD). A sensor of this type may be used to detect and measure particulate matter build-up, e.g. soot concentration, in an engine exhaust gas. In diesel engines in particular, it is desirable to have the lowest possible soot particle concentration when exhaust gas is released into the environment. To monitor the operating status of the internal combustion engine, it is expedient for this purpose to position a soot sensor in the exhaust system associated with the internal combustion engine. The soot sensor may be positioned upstream or downstream from a diesel particulate filter (DPF). If it is positioned downstream from the DPF, function monitoring of the DPF may also be performed using the soot sensor. When the (DPF) fails, the soot sensor may detect excessive soot in engine exhaust and alert the vehicle engine control unit (ECU).
Soot sensors may be relatively simple resistive devices. FIG. 1 is a schematic top view of one known configuration of a soot sensor having an on-board heater element, and FIG. 2 is a schematic bottom view of the soot sensor of FIG. 1. The sensor 100 may include a non-conductive substrate 102 defining a first surface 104 and a second surface 106 opposite the first surface 104. A sense element 108 is formed on the first surface 104 of the substrate 102, and includes a conductive material defining a first electrode 110 and a separate second electrode 112. The conductive material may be a precious metal selected to withstand high temperatures, and the first 110 and second 112 electrodes may be electrically separate from each other to establish an open circuit therebetween.
As shown, the first and second electrodes 110, 112 may be configured with inter-digitized “fingers” that maximize a perimeter between the first and second electrodes 110, 112. The first electrode 110 defines a first set of fingers 114 and the second electrode 112 defines a separate second set of fingers 116. In operation, when soot (not shown) from exhaust lands on the sensing element 108, carbon in the soot electrically connects the first and second electrodes 110, 112, effectively lowering the resistance therebetween. The resistance between the electrodes is measured as an indication of the amount of soot present.
FIG. 3 is an enlarged sectional view of the soot sensor of FIGS. 1 and 2 taken along line 3-3. As shown in FIGS. 2 and 3, in some applications, the sensor 100 will also have an on-board heater element 118 implemented on the second surface 106 of the substrate 102. The on-board heater element 118 is configured to heat the soot sensor 100 through resistive heating. For example, it may be desirable to clean off soot that has collected on the first and/or second surfaces 104, 106 of the substrate 102. The on-board heater element 118, which may include a platinum trace with a known resistance, may be activated, heating the sensor element 108 to a relatively high temperature, e.g. 650° C., thereby causing any accumulated soot particles to incinerate.
A soot sensor of the type described above is susceptible to breakdown under the conditions existing in the exhaust system. The electrodes are directly subjected to exhaust gas flow, wherein certain exhaust materials may lead to corrosion of the electrodes and/or contamination of the sensor surface, which may have an interfering effect on soot accumulation measurement. Additionally, the sense element of current soot sensors lacks diagnostic functions capable of sensing a break in the sense element traces. Moreover, on-board heaters included in current soot sensors have difficulty reaching high temperatures required to sufficiently incinerate accumulated soot during high flow conditions.