Improving Direct Vehicle Exhaust Flow Measurement

Measuring vehicle exhaust volumetric flow rate accurately and precisely is critical in calculating the correct vehicle modal and bag mini-diluter exhaust emission constituent masses. It is also instrumental in engine calibration practices. Currently, DaimlerChrysler's Emission and Certification Lab in Auburn Hills, Michigan utilizes constant volume sampling bag systems to certify vehicles but the automotive technological trend is heading toward the bag mini-diluter for greater precision at low emission levels. The bag mini-diluters, as well as the modal sampling system, used extensively in vehicle development testing, rely on exhaust flow rate measurement by means of a direct vehicle exhaust flow meter named E-Flow. The E-Flow has few limitations such as flow profile instability at low idle flow rates and reaction to resonating pressure waves in the exhaust system. A previous study by the authors showed that for very low flow rates such as idle on small displacement engine vehicles, the flow is not fully developed and the entry length requirement upstream of the measuring section on the E-Flow must be extremely long and is thereby impractical to install on the E-Flow in the test cell. An alternative approach was investigated to achieve an equivalent benefit of the long entry length by utilizing a flow-straightening element to be placed upstream of the measuring spool piece where the ultrasonic probes are located. A second study was also made to investigate the effect of using a dual set of four ultrasonic probes instead of the traditional single set with two probes resident on the E-Flow. To quantify the effect of these E-Flow modifications on exhaust flow rate variability and accuracy, two vehicles with different engine displacement were run with EPA Hot 74 test cycles on a single-roll 48″ electric chassis dynamometer. The flow straightening element and the four ultrasonic probes were tested independently with the E-Flow and under identical test conditions. The exhaust flow measurement stability is evaluated and analyzed statistically under idle, cruise, acceleration and deceleration modes.