The Challenge of Precise Characterizing the Specific Large-Span Flows in Urea Dosing Systems for NOx Reduction

The reduction of nitrous oxides in the exhaust gases of internal combustion engines using a urea water solution is gaining more and more importance. While maintaining the future exhaust gas emission regulations, like the Euro 6 for passenger cars and the Euro 5 for commercial vehicles, urea dosing allows the engine management to be modified to improve fuel economy as well.

The system manufacturer Robert Bosch has started early to develop the necessary dosing systems for the urea water solution. More than 300.000 Units have been delivered in 2007 for heavy duty applications. Typical dosing quantities for those systems are in the range of 0.01 l/h for passenger car systems and up to 10 l/h for commercial vehicles.

During the first years of development and application of urea dosing systems, instantaneous flow measuring devices were used, which were not operating fully satisfactory. When checking the different measuring principles on the market, the controlled gear pump measuring system of AVL-Pierburg Instruments, which was up to then only in use for fuels, seemed to have the best chances for achieving the very specific demands of the urea dosing systems. The development of a specific device was found to be necessary. The development of this specific device and its first results are described in this paper.