10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
Abstract View
Using an Electrostatic Sensor to Measure Real Time PM Levels in Engine Exhaust
MATTI MARICQ, David Bilby, Ford Motor Company
Abstract Number: 1006 Working Group: Instrumentation
Abstract Combustion engines are under pressure to meet increasingly stringent particulate matter emissions standards world wide. For many engine technologies, such as diesel engines, the only way to meet these standards is via exhaust particulate filters. In turn, these devices require on-board diagnostics to ensure their proper function over the full useful life of the vehicle. Identifying a low cost sensor technology capable of robust operation in an engine exhaust pipe is a difficult challenge. The currently used sensors operate by recording a current increase as soot deposits on the sensor element and produces carbon bridges across a pair of electrodes. This technology provides a qualitative means to assess diesel particulate filter (DPF) performance, but as the in-use emissions allowances are brought closer to the regulatory limits they lack the necessary sensitivity and time resolution.
A potential alternative sensor concept is based on a simple electrostatic trap. It is predicated on the intriguing physics of electrically charged soot particles in an electric field. A bipolar electrical charge arises naturally on soot particles as a result of combustion chemistry. As the electric field draws the charged soot agglomerates to the electrodes they grow into elongated dendritic structures normal to the electrode. Above a critical height, these dendrites fragment and carry a large electric field induced charge between the electrodes, easily producing currents of tens of nanoamperes. This current is proportional to the soot concentration; hence, in principle it provides a real time means to monitor soot levels in engine exhaust.
However, this electrostatic sensor responds not just to soot level, but also to the flow rate through the sensor. Flow through the sensor occurs due to the pressure drop across the sensor tip; sensor flow rate changes in response to exhaust pipe flow. The dendrites are not exactly normal to the electrodes; rather they increasingly tilt as a function of sensor flow. Increases in flow rate tilts the dendrites and causes sensor dead time while the dendrites grow to reach their new critical length. Conversely, decreases in flow cause a large spike in current from the release of fragments from the suddenly more upright dendrites.
In spite of this flow induced interference we show that a useful measure of PM emissions can be obtained from the cumulative sensor signal. Namely, the flow effects approximately cancel out over time; the current bursts during flow decreases are compensated by the dead time during flow increases. This talk will present data demonstrating the fragment breakoff mechanism for sensor operation, the flow effects on sensor response and vehicle based comparisons between sensor response and photo-acoustic measurements of soot mass emissions.