A review of soot sensors considered for on-board diagnostics application

If emissions control systems and engine control devices experience a failure, the engine system may malfunction causing exhaust emissions to exceed the standard levels of the regulated emissions. Thus, the functional status of each of all primary control devices needs to be monitored for detecting possible failures, so that the proper action can be taken to warn the vehicle operator. This article reviews a soot sensor currently developed for on-board diagnostics to monitor a diesel particulate filter and detect the failure. Original equipment manufacturers have selected a resistive soot sensor for the on-board diagnostics application because of its commercial feasibility in terms of functionality and cost. The sensor accumulates soot particles in exhaust gas on the sensing element. Structure, design, and function of the cumulative sensor including the signal processing techniques are described. Since the sensor signal contains noises and fluctuations, the post-diesel particulate filter soot concentration is repeatedly measured over 100 times and averaged in order to ensure the measurement accuracy, so that the diesel particulate filter failures can be detected. In addition to the resistive soot sensor, this article reviews the status of currently developed continuous soot sensors such as an electrostatic sensor, an electrical charging sensor, and a radio frequency–based sensor. These sensors have major issues of packaging and cost to be resolved prior to application to production vehicles. This review article is based primarily on scientific literatures most of which were published a few years ago, but the development of new soot sensor concepts has been continuing in the industry. Three new soot sensor concepts all of which are under development are also reported.