Parameter Estimation of a DOC from Engine Rig Experiments

Licentiatavhandling, 2013

In this thesis methods of parameter estimation of a Diesel Oxidation Catalyst (DOC) from engine rig experiments were investigated. The investigation did not only include methods of parameter fitting to experimental data but a large effort was also put into catalyst modeling and experimental design. Several different catalyst configurations were used with varying Pt loading, washcoat thickness and volume. To further expand the experimental space, engine operating points were chosen with a wide variation in variables (inlet conditions) and both transient and stationary operation was used. A catalyst model was developed where the catalyst washcoat was discretized as tanks in series both radially and axially and for parameter estimation a traditional gradient search method was used. Four different modeling approaches were used for parameter tuning where the most successful one tuned kinetic parameters as well as internal mass transfer parameters. It was also shown that it is of high importance that the kinetic model used has an intrinsic structure when the catalyst model separates mass transport and kinetics and when several catalyst configurations are used. A new method was evaluated where sensitivity analysis and data selection was used as a part of the parameter estimation. This methodology renders better statistical properties and should improve the parameter tuning when using gradient search methods. Furthermore, a reduced computational cost could be achieved by using only the most relevant data points during parameter tuning. The evaluated method did neither result in an improved fit to measurement data nor reduce the time for parameter tuning compared to a reference case. Adjustment of an unbalanced weighting of the residuals for the different components was identified as the most important factor for a future improvement of the method but more transient experimental data was also suggested as a possible improvement. A method of creating fast transients in concentration for a full scale engine rig system was presented and evaluated. The method included an engine rig where an SCR with urea injection and a DOC with bypass possibility were situated between the test object, which also was a DOC, and the engine. By controlling urea injection and DOC bypass a wide range of exhaust compositions, not possible by only controlling the engine, could be achieved which will improve the possibilities for parameter estimation for the modeling of the DOC in future studies.