Development of an electronic sensor for engine exhaust particulate measurements
A new electronic sensor has been developed to measure the time-resolved concentration of carbonaceous particulate matter (PM) emitted in engine exhaust. The sensor is approximately the size of a standard automotive spark-plug or lambda sensor and can be mounted directly in the engine exhaust. It consists of a pair of closely spaced electrically isolated electrodes that protrude into the exhaust flow. One electrode is given a voltage bias of approximately1000 V while the other is the signal electrode. The sensor is capable of providing cycle-resolved feedback on the carbonaceous PM concentration in the exhaust to the engine control unit (ECU), thereby enabling real-time control of engine operating parameters. The sensor was tested in exhaust flows from a single cylinder diesel engine and from a steady-state acetylene diffusion flame in a flow tunnel. Steady-state engine measurements were made over a range of speed and load conditions while transient measurements were performed during engine start-ups and accelerations. The sensor response was compared to an opacity meter, gravimetric filter measurements, an aerodynamic particle sizer and a light scattering nephelometer. The output of the sensor compared well with exhaust opacity for the transient measurements and to filter mass loading for the steady-state tests. Various parameters that affect the performance of the electronic sensor were studied. Parameters considered included sensor electrode length, diameter, electrode spacing, applied bias voltage, bulk flow velocity across the sensor electrodes, and the concentration of carbonaceous particulate matter in engine exhaust. The sensor signal varied linearly with the carbon mass concentration in the exhaust, the applied bias voltage and electrode length; it varied exponentially with the flow velocity, and had an inverse power dependence on the spacing between the electrodes. Electrode diameter did not have a significant effect on the sensor signal. A correlation was developed to predict the sensor signal under any engine operating condition and values of these parameters. Based on the experimental data a hypothesis explaining the physical mechanism governing sensor behavior, regarding the charge transport between the two electrodes of the sensor, was proposed. A 1-D drift-diffusion model was developed to support this hypothesis.