Development of a Spark Discharge PM Sensor for Measurement of Engine-Out Soot Emissions

Filter paper methods are well recognized as an effective means of measuring soot emissions from diesel engines. However, these methods provide an average soot value over a relatively long time period, rather than a real-time signal. Real-time measurements of engine-out soot emissions that could track changes in soot levels during transient operating conditions would be useful for the optimization of engine control strategies such as exhaust gas recirculation. This paper presents experimental results obtained using a real-time PM sensor based upon a spark discharge measuring principle. Like traditional filter paper devices, it is sensitive to the carbon or soot component of the particulate matter emitted by diesel engines. The sensor was tested on a turbocharged diesel engine, and compared with reference measurements of Filter Smoke Number (FSN) from an AVL 415s smokemeter. Improvements to the sensor made it possible to measure soot levels at FSN values over 3.5, while retaining good sensitivity below FSN values of 0.1. The sensor signal showed a high correlation with the reference FSN measurements. This correlation was used to develop a signal processing technique so the sensor provided a real-time signal for predicted FSN. Conversion of the FSN values to mass concentration values (using published techniques for the reference instrument) indicated that the output of the spark discharge soot sensor was nearly linear with mass concentration over a substantial portion of the measuring range. The sensor showed a response time of under 2 seconds to step changes in FSN levels.