Content of this work was the design and investigation of a novel exhaust aftertreatment concept for NOx removal avoiding a second operating supply item as well as the development of a statistical model for mathematical description. Target was the development of fundamentals for an operating strategy in the vehicle.
Therefore, a two-stage DeNOx system for catalytic NOx removal was applied consisting of a NOx storage catalyst (NSC) and a downstream SCR catalyst (SCR). Main procedural principal is the onboard generation of the reducing agent ammonia for the SCR reaction during the regeneration phase of the NOx storage catalyst. For compliance with future emission limits for particulate matter, a diesel particulate filter (DPF) has to be added to the DeNOx system.
The experimental studies were based on laboratory experiments with synthetic exhaust gas and real exhaust investigations on an engine test bench.
Initially, the laboratory experiments were carried out on the individual catalyst units NOx storage catalyst and SCR catalyst to identify the relevant actuating variables as well as for the selection of suitable catalytic materials. Fundamental experiments on NOx storage catalyst focused on the ammonia formation. The experiments on the SCR catalyst concentrated on the transient NOx conversion characteristics under lean rich operation. Based on these findings, a complete system was characterised. Via design of experiments (DoE), the primary actuating variables were varied systematically and the catalytic efficiency of the complete system was determined in lean rich alternation under laboratory conditions close to reality. Thus, numerous synergetic effects between NOx storage catalyst and SCR catalyst and the reaction products respectively could be demonstrated.
A statistical model describing the system behavior of the combined exhaust aftertreatment system was developed by adaption of polynomial smoothing functions to the experimental data. For validation of the statistical model, real exhaust experiments on an engine test bench for commercial vehicles were carried out and compared to model results. Using the statistical model, operating strategies for application in commercial vehicles were developed and optimised, considering parameters such as NOx and HC conversion, ammonia emissions and additional fuel consumption.
It can be concluded that the combined catalytic system "NSC-Advanced" is considerably superior to a NOx storage catalyst only system regarding NOx conversion, additional fuel consumption, required NOx storage catalyst volume and secondary emissions. Thereby, numerous disadvantages which have lead to the preference of urea-based SCR systems so far, can be overcome. Therefore, the "NSC-advanced" technology represents an attractive solution for future diesel emission reduction of commercial vehicles. Further development is needed in the range of NH3 storage capacity of the SCR catalyst at elevated temperatures as well as concerning thermal durability of the catalysts.