Vehicle modal emissions measurement - techniques and issues

The measurement of vehicle modal emissions is technically challenging owing to the major issue of determining exhaust-gas mass flowrate and ensuring that it is synchronous with the corresponding 'slug' of gas to be measured. This is also extended to the simultaneous measurement of pre- and post-catalyst emissions to determine small passive NOx conversion efficiencies. Although only really evident for passive NOx conversion efficiencies where the magnitude of catalyst performance is low in comparison to HC and CO, a misalignment between these measuring points of between +/-2 s will cause the resulting NOx conversion efficiency to lie anywhere between 0 per cent and 20 per cent. Further alignment issues arise when the CO2 tracer method is used for determining exhaust-gas volume flowrates. The sensitivity of time-alignment along with techniques and associated issues concerned with modal gas-flow measurement is presented in this paper.