Effect of Leaks on Tailpipe Sampling Systems 2005-01-0682
The on-going regulatory emphasis on reducing vehicle emission levels and increasing fuel economy has resulting in numerous efforts to improve the technology used to obtain these measurements. For example, raw exhaust flow meters are being implemented to provide improved tailpipe (i.e. modal) emission measurements as part of the vehicle development process. In addition, new sampling system technologies (e.g. partial flow dilution systems) are being implemented that use either synthetic air or catalyzed/dried ambient air to dilute exhaust samples.
With these new technologies, however, often comes an increased sensitivity to leakage from both the test equipment and from the hoses/connectors used to route the vehicle's exhaust gas to the test equipment. In the case of the exhaust flow meter, a leak upstream of the measurement zone will provide an inaccurate flow reading. With an emissions sampling system, an exhaust leak will result in inaccurate emissions and fuel economy readings. This is particularly true with sampling systems that do not dilute the exhaust gas with ambient air.
In all cases, the magnitude of the inaccuracy caused by leakage depends on several related test parameters. These parameters and the standard techniques used to
prevent exhaust leaks on a test cell are discussed in detail in this paper. We focus on typical problem areas for leaks, including clamps, flanges, gaskets, and unions. Figure 1 shows the typical connections used between a vehicle's tailpipe and the exhaust sampling system. It should be emphasized that leaks in the vehicle's exhaust system will cause similar problems, however, our investigation focuses on leaks in the connections between the vehicle and test equipment and in the test equipment itself.
To detect leaks prior to testing a vehicle, two simple leak check procedures are explained, as well as the equipment required for the leak checks. Finally, data is presented that makes obvious the significance that leaks have on fuel economy and emission measurements for tailpipe sampling systems.