The use of carbon dioxide as a tracer in the determination of particle number emissions from heavy-duty diesel vehicles

In this paper, we have analysed the particle number and CO2 concentration data from four previous dynamometer studies, each consisting of a number of heavy-duty vehicles of different age and operating on different diesel fuel blends. Emission rates were calculated for four operational modes corresponding to 0%, 25%, 50% and 100% of the maximum rated engine power. In a given mode the calculated CO2 emission rates were approximately the same for all vehicles, but the calculated particle number emission rates varied considerably between vehicles. Using concentrations measured directly from the dilution tunnel, we found that the ratio of diluted particle number concentration, to diluted CO2 concentration (P/C ratio) was directly proportional to the calculated undiluted particle number emission concentration, with a high degree of correlation. These observations suggest that the P/C ratio within the diluted sample is a good indicator of the particle number emission concentration for the undiluted exhaust. This was confirmed by grouping the vehicles according to age whence the newest age group, expected to have the cleanest emissions, always showed the lowest P/C ratio. An additional series of experiments were conducted on-road with a light duty diesel vehicle, at speeds ranging from 40 to 100 km h(-1). The diluted exhaust emissions were collected in a large bag housed in a trailer attached to the back of the vehicle. Various dilution ratios were achieved by sampling over a range of distances directly behind the vehicle tail-pipe outlet. As expected, the particle number concentration in the bag, for different distances and therefore different dilution ratios, showed a definite relationship to the dilution ratio, however the P/C ratio was independent of dilution for dilution ratios in the range 20-110. Based on the results of the dynamometer and on-road studies, it is suggested that the P/C ratio may be used as a viable method for the rapid identification of high particle number emitting vehicles as they drive past on a road. The technique has the added advantage that it is independent of the position of the sampling point in relation to the emission plume. (c) 2005 Elsevier Ltd. All rights reserved.