Importance of light alignment of on-road remote sensing technology on diesel emission measurements

On-road remote sensing (RS) technology provides a rapid, cost-effective and non-intrusive tool to test vehicle emissions, which is gaining significant attention worldwide in recent years. RS systems are usually placed at a fixed height to test the emissions of all the passing vehicles. However, the tailpipe height varies among vehicles which may greatly affect the accuracy of RS tests. In addition, a key assumption of RS measurements is that the ratios of pollutants over carbon dioxides are constant for a given plume. So far, however, there is a lack of experimental investigation on these effects. Therefore, this study is conducted to investigate the effect of beam alignment of an RS system on the diesel emission tests. The experiments were performed using a diesel engine bench, whose emissions were tested simultaneously by a Horiba MEXA-7500 gas analyzer (serving as the comparison baseline) and a BLH-5000 on-road RS system. The experimental results show that in the three-dimensional space behind the exhaust tailpipe, the more accurate area for remote sensing technology tests along the direction of the measured light (x) is in the area 6m from the RS light source and receiver. In the direction of the exhaust discharge plume along the exhaust pipe (y), the test results do not vary much throughout the test range of 0 to 6m. In the height direction (z), when the height of the tested light is 0.3m, the area where the remote sensing technique is accurate is within the range of 0.2m to 0.4m. The relative concentration of CO tested by RS in this region has an error rate of only ± 5% with the CO concentration measured by the exhaust gas analyzer. Similarly, this indicator for HC is ± 10%, which is also smaller. Although this indicator is larger for NO, at 100%, the NO test error in this region is also the smallest in the entire test area.