Calibration and Validation of a Diesel Oxidation Catalyst Model: from Synthetic Gas Testing to Driving Cycle Applications 2011-01-1244

To meet future stringent emission regulations such as Euro6, the design and control of diesel exhaust after-treatment systems will become more complex in order to ensure their optimum operation over time. Moreover, because of the strong pressure for CO₂ emissions reduction, the average exhaust temperature is expected to decrease, posing significant challenges on exhaust after-treatment. Diesel Oxidation Catalysts (DOCs) are already widely used to reduce CO and hydrocarbons (HC) from diesel engine emissions. In addition, DOC is also used to control the NO₂/NOx ratio and to generate the exothermic reactions necessary for the thermal regeneration of Diesel Particulate Filter (DPF) and NOx Storage and Reduction catalysts (NSR). The expected temperature decrease of diesel exhaust will adversely affect the CO and unburned hydrocarbons (UHC) conversion efficiency of the catalysts. Therefore, the development cost for the design and control of new DOCs is increasing. To select the best quality product at affordable price, the authors have evaluated exhaust simulation as an additional development tool.

In this study, a multidimensional exhaust modeling tool based on physical background was used. The model is able to solve heat and mass transfer equations under transient conditions. "Apparent" reaction rates and Langmuir-Hinshelwood rate expressions are used for the description of the chemical reactions. The model includes appropriate HC storage models applicable to DOC with storage capacity.

In principle, exhaust model calibration can be performed using engine bench experimental data under steady state and transient conditions with full scale catalysts. However, with this approach it is indeed difficult to identify chemical mechanisms and to develop a standard calibration procedure. This paper proposes a new DOC model calibration methodology based primarily on synthetic gas tests with small scale catalysts. The main target of this methodology is to define a process to transfer the information obtained from synthetic gas tests to full-scale tests with real exhaust gas, which is not straightforward due to the complex hydrocarbon speciation. In the present work, the hydrocarbon speciation and the chemical reaction parameters of the heavy diesel exhaust hydrocarbons are investigated using well-controlled experiments with small-scale samples fed by real exhaust gas. The main target of this approach is to be able to use this calibration under real exhaust gases under driving cycle application without any modification. This methodology is expected to be also applicable to a wide range of DOC technologies.

In this paper, the calibration methodology is presented step by step. First, the adsorption mechanisms of heavy hydrocarbons (HHC) were calibrated using storage and release curves at constant flow and at different temperatures. Then, standard light-off tests were performed using different gas compositions in order to obtain the CO and HC oxidation rates. Finally, the small-scale catalyst tests with real diesel exhaust were performed. The resulting model calibration was then validated via full scale engine tests under steady state and under New European Driving Cycle (NEDC) conditions. In this study, experimental CO, HC and NO conversion efficiencies were well predicted. Finally this methodology was successfully applied to a new DOC formulation to monitor the impact of the exhaust temperature reduction on the CO and HC conversion efficiency.