Abstract
The particulate filter in the exhaust system of automotive combustion engines is one of the most important components for protection of our environment. Therefore, it is in the focus of legislation to include PM-filter monitoring in the on board diagnostics regulations. Within OBD, there are several malfunctions that are required to be detected prior to emissions exceeding defined tailpipe levels, e.g. prior to emissions exceeding 2.0 times the standard. PM-filter monitoring is currently based on the differential pressure sensor signal and cannot support such a threshold. In order to achieve the current (US) or proposed (EU) emission thresholds for PM filter monitoring, manufacturers believe PM sensors are necessary.
Continental Automotive decided to work on a type of sensor, where isolated electrodes on a sensor surface measure the conductivity of soot, deposited on the sensor surface. At a predefined threshold, the soot deposits will be burnt off, to regenerate the sensor. Although this sensor type is robust and inexpensive, the signals provided, are difficult to use. The sensor output is current slope and the frequency of sensor regeneration events. The current slope provides a real time signal, but the correlation between current, number of soot bridges and electrical resistance of the soot require extensive calculations. More easy to use, is the frequency of sensor regeneration. Due to the predefined regeneration thresholds, soot is collected repeatedly to approximately the same thickness of soot layer. Therefore, this signal provides a kind of integrated data. The disadvantage of this type of signal is, that the lower the PM concentration, the more delayed is the signal, because it takes a longer period of time to collect soot up to the regeneration threshold.
The according PM sensor comprises the exhaust gas probe and an electronic module connected to the probe by short wires.
The electronic module provides control of sensor temperature, regeneration, current measurement, signal conditioning and processing and a CAN interface to the engine control unit. For calculation of PM concentration, the sensor signal is compensated for exhaust gas conditions (e.g. temperature, flow) which are available in the engine control unit. The engine control unit will calculate the PM filter efficiency by correlating the measured data with the calculated engine out emission.
The presentation illustrates, how the signals of the Continental Automotive PM Sensor processed with functions provided by AVL allow for meeting the proposed PM filter OBD requirements, and for reliable detection of exceeding the OBD thresholds, especially within the FTP and NEDC driving cycles.
Keywords: PM Sensor, OBD, PM Filter, Filter Efficiency, Exhaust Gas