Parameters Affecting Direct Vehicle Exhaust Flow Measurement

As SULEV emission regulations approach, the bag mini-diluter (BMD) technology is gaining acceptance as a replacement for the existing constant volume sampler (CVS) for SULEV exhaust emission measurement and certification. The heart of the BMD system is the direct vehicle exhaust (DVE) flow measurement system. Due to the transient nature of vehicle exhaust during a standard FTP emission test cycle, the DVE must be capable of rapid and accurate response in order to track these varying exhaust flow rates. The DVE must also be robust enough to accurately measure flow rate despite variations in exhaust gas composition, pulsation effects, and rapid changes in both exhaust temperature and pressure.

One of the primary DVE systems used on BMDs is the E-Flow, an ultrasonic flow meter manufactured by Flow Technologies, Inc. Based on over five years of production testing at DaimlerChrysler emission testing facilities, the unit has demonstrated good potential in its ability to accurately track vehicle exhaust flow rates under varying temperature and pressure conditions. However, in a three-shift production test environment, several operating limitations and reliability issues were identified, requiring significant modifications to both the original hardware and software designs. This paper discusses the problems encountered due to flow path configurations and the resolutions adopted that led to improved and satisfactory results. Factors influencing flow measurement will also be discussed, such as the upstream and downstream flow configurations, sensor head degradation and sensor response.