This paper describes experimental research aimed at developing an on-board smoke sensor for diesel engines. The sensor element was similar to a conventional spark plug. Electrical heating of the insulator was used to prevent carbon fouling from the diesel soot. The sensing element created sparks within the exhaust pipe and changes in smoke levels were detected through analysis of the voltage levels of the sparks. The system was tested in a heavy duty diesel engine equipped with Exhaust Gas Recirculation (EGR)) and compared with reference measurements of the Filter Smoke Number (FSN). The experiments showed good sensitivity to step changes in smoke levels (accomplished by varying EGR levels) at smoke levels below 0.5 FSN. However, the sensor suffered from temperature induced signal drift and was unstable under some circumstances. The use of a spark plug with a smaller electrode tip diameter improved the signal stability. It is proposed that measurement and control of the electrode temperature will be necessary to control the signal drift.

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