Particulate matter may be present in the exhaust of some engines, diesel engines for example. Vehicle tailpipe emissions may be reduced by installing particulate filters and particulate matter sensors in vehicle exhaust systems. One particular type of particulate sensor senses particulate matter by electrodes separated by a dielectric gap. When particulate matter bridges the gap between electrodes, the resistance and conductance of the particulate matter sensor is changed.
The above-mentioned sensor may under some conditions experience degradation. For example, a wire connecting the particulate matter sensor to an engine controller may become disconnected or holes in a protective shield may become partially plugged. In such circumstances it may be desirable to determine whether or not the particulate matter sensor is operating as desired. However, it may be difficult to establish sensor degradation because the particulate sensor exhibits a high resistance when there is little particulate matter between the positive and negative electrodes. Thus, the particulate sensor output may appear to look like an open circuit. Therefore, it may be difficult to distinguish a properly operating particulate sensor from an open circuit. One embodiment of the present description includes a method for detecting a degraded particulate matter sensor, comprising: starting an engine when a temperature of said engine is less than a threshold temperature; and indicating degradation of said particulate matter sensor in response to an output of said particulate matter sensor being below a threshold, said output related to water vapor in exhaust gases generated by said engine during said starting of said engine.
Thus, by recognizing that the output of a particulate matter sensor may be affected by water vapor contained in engine exhaust gases, the inventors herein have developed a way to diagnose a resistive particulate sensor that exhibits characteristics of an open circuit when the particulate sensor is operating properly in the presence of little particulate matter. For example, the particulate sensor can be monitored during an engine start when water in the exhaust may cause a change in the state of the output of the particulate sensor. If the output of the particulate sensor does not increase above a threshold level, it may be judged that the particulate matter sensor is degraded.
The present description may provide several advantages. Specifically, the approach may improve engine emissions by allowing degraded sensors to be replaced before engine emissions degrade. Further, special hardware for monitoring or stimulating the particulate sensor may not be required because the method may be performed with the hardware that controls the particulate heater and senses the particulate sensor output. As a result, system cost may be reduced.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.