How Well Can mPEMS Measure Gas Phase Motor Vehicle Exhaust Emissions? 2020-01-0369

“Real world emissions” is an emerging area of focus in motor vehicle related air quality. These emissions are commonly recorded using portable emissions measurement systems (PEMS) designed for regulatory application, which are large, complex and costly. Miniature PEMS (mPEMS) is a developing technology that can significantly simplify on-board emissions measurement and potentially promote widespread use. Whereas full PEMS use analyzers to record NO_x, CO, and HCs similar to those in emissions laboratories, mPEMS tend to use electrochemical sensors and compact optical detectors for their small size and low cost. The present work provides a comprehensive evaluation of this approach. It compares measurements of NO_x, CO, CO₂ and HC emissions from five commercial mPEMS to both laboratory and full regulatory PEMS analyzers. It further examines the use of vehicle on-board diagnostics data to calculate exhaust flow, as an alternative to on-vehicle exhaust flow measurement. The evaluations include two vehicle types, gasoline direct injection and diesel, and employ the US EPA Federal Test Procedure and Worldwide Harmonized Light duty test cycles. The results show that two classes of electrochemical NO_x sensors are capable of providing high quality, second-by-second emissions data. One of these is ammonia sensitive, thus alternatively functioning as an effective NO_x + NH₃ sensor. Accurate measurement of CO and CO₂ is also possible, but the ability to record rapid transients depends on detection method. HC measurement at present is only semi-quantitative and has a limit of detection well above the range of concentrations observed during normal vehicle operation. The mPEMS NO_x, CO, and CO₂ capabilities are sufficiently comparable to laboratory instruments that they should provide useful tools to investigate real world emissions behavior of motor vehicles.