Development and Testing of an Aerosol Sensor for direct Measurements in Engine Exhaust

Summary: An experimental setup of a condensation nuclei counter (CNC) operating at “high temperature” > 150 °C (HT-CNC) for application on raw automotive exhaust gas was designed, built and tested. To enable a loss-free long-term operation that is immune to thermal degradation of the working fluid, a special operation mode was developed, taking advantage of a special mixed-flow geometry. By mixing a controlled fraction of the concentrated exhaust gas with an inert gas stream pre-saturated with the working fluid, a long-chained aliphatic hydrocarbon. This prototype was used for the fundamental proof of principle of an HT-CNC and various detailed studies. These studies include material tests of wick materials and a detailed examination of nucleation at HT-operation, investigating particles from ambient air, a GRIMM 7860 WOx-generator and a Jing 5201 miniCAST. The grown droplets were counted using specially developed fiber-optically coupled counting. Motivation and results: CNCs are established for measuring particle number (PN) concentrations in science and industry. For measuring PN in automotive exhaust, e.g. to assure the compliance to legislative limits, extensive preconditioning must be applied on the exhaust gas, which is also regulated by EU [1]. This preconditioning is until now an indefinite source of particle losses. To minimize the effort of aerosol preconditioning and eliminate possible condensation artefacts, HT-CNCs are of great interest for these measurements [2] since due to the high operational temperature volatile particle formation is prevented. Our research shows that key elements to realize such a long-term stably operable sensor are a suitable selection of wick and working fluid and a special operation mode to prevent thermal degradation of the working fluid. Tests with particles from ambient air and the GRIMM 7860 WOx-generator proved heterogeneous nucleation in all tested wicking materials [3]. The behaviour shows a strong dependence to the wick used since it has a big influence on the (super-)saturation of the working fluid in the carrier gas. Further it turned out that under normal operation the speed of degradation of the working fluid differs also from one used wick to another. Full prevention of the degradation of the working fluid was obtained by a special operation mode taking advantage of the mixed-flow geometry for all used material combinations, as you can see in figure 1 [3]. The design also enables an inherently regulated ratio between the carrier aerosol and the sample aerosol by operating the HTCNC with a defined lower pressure relating to the sample line and guidance the flow across a capillary, as shown in figure 2. In Table 1 the pressure ratios for two different dilution ratios are given. The operation was tested with different particle sources and a reference CPC with the result that the comparison of the particle count to a TSI 3775 using soot from a Jing 5201 miniCAST showed a good agreement.