Measurement of Brake-specific NO x Emissions using Zirconia Sensors for In-use, On-board Heavy-duty Vehicle Applications

Emissions tests for heavy-duty diesel-fueled engines and vehicles are normally performed using engine dynamometers and chassis dynamometers, respectively, with laboratory grade gaseous concentration measurement analyzers and supporting test equipment. However, a considerable effort has been recently expended on developing in-use, on-board tools to measure brake-specific emissions from heavy-duty vehicles with the highest degree of accuracy and precision. This alternative testing methodology would supplement the emissions data that is collected from engine and chassis dynamometer tests. The on-board emissions testing methodology entails actively recording emissions and vehicle operating parameters (engine speed and load, vehicle speed etc.) from vehicles while they are operating on the road. This paper focuses on in-use measurements of NOx with zirconium oxide sensors and other portable NOx detectors. A zirconia sensor along with a NOx converter was used in the MEMS for the measurement of NOx, with an electrochemical NO cell as a QC/QA device. Brake-specific mass emissions of NOx were reported by the MEMS within 5% of laboratory results integrated over an FTP cycle in an engine test cell. While the zirconium sensor had T₉₀ response times of 5 seconds or less, yielded accurate measurement and had no detectable effects from vibration or orientation bias, it did not provide a 100% response to NO₂. Consequently, a NOx converter was employed to convert NO₂ to NO in order to obtain a NOx measurement. The NOx converter was placed upstream of a thermoelectric chiller to prevent loss of the water-soluble NO₂ before being converted to NO. NOx concentrations may be measured more accurately by improving the conversion efficiency of the NOx converter. The NOx converter efficiency was found to decrease as NO₂ concentration increased. Thus, the percentage of NO₂ measured depends on engine operating conditions. Durability tests were not conducted on the zirconia sensors as part of this program. This paper presents results from an extensive evaluation of the zirconia sensor for NOx emissions measurements.