Assessment of Novel In-Line Particulate Matter Sensor With Respect to OBD and Emissions Control Applications

Upcoming emissions legislations and On-Board-Diagnostics (OBD) system requirements, both in the US and Europe, impose new challenges for particulate matter (PM) control strategies, especially with regard to upcoming EURO-VI regulations that are expected to limit the particulate number (PN) emissions. Indeed, the US-EPA Heavy-Duty-OBD regulations already require monitoring of diesel particulate filters (DPF) in 2010 for at least one engine series, with extension to all engine series by 2013. Due to the current absence of reliable in-line PM sensors to monitor DPF filtration efficiencies, manufacturers adopted alternative methods based on pressure drop measurements and semi-empirical models that require extensive calibration efforts; hence, driving upward the development costs. In order to meet upcoming OBD requirements and reduce unnecessary DPF regeneration frequencies, so as to minimize fuel consumption penalties, reliable sensors need to be integrated into the aftertreatment control environment. These next-generation sensors must be capable of performing actual real-time PM concentration measurements on a continuous basis within the exhaust stream. The primary objective of this study was to assess the capabilities, limitations and sensitivity of a newly developed inline PM sensor, with regard to its future application for OBD monitoring or control strategies of PM filter systems. The operation of the so called Pegasor Particulate Sensor (PPS) [1] is based on the escaping current principle. The instrument is capable of performing continuous PM measurements, directly from the exhaust stack, while providing a real-time signal with a resolution of 100 Hz. The sensor’s output signal can be calibrated to either measure the concentration of mass, surface or number of exhaust particles. Designed as a flow-through device, the PPS has a tungsten corona wire imposing an equal charge on particles that is subsequently measured from the outflowing particles via a built-in electrometer. The system does not involve collection or contact with particles in the exhaust stream, which is especially advantageous for long-term stability and operation without frequent maintenance; hence, best suited for in-use application. A comprehensive test matrix was developed to gain a more pronounced understanding of the sensor’s measurement technology by comparing it to other proven aerosol instruments, namely the Ultrafine Condensation Particle Counter (UCPC), Engine Exhaust Particle Sizer (EEPS™) spectrometer, Tapered Element Oscillating Microbalance (TEOM), and gravimetric PM. Test results demonstrated a stable and repeatable response over consecutive European Transient Cycle (ETC) and Federal Test Procedure (FTP) cycles, as well as over idle and constant load operation with coefficients of variation below 2%, which is a prerequisite for OBD algorithm implementation.