Modeling Coupled Processes of CO and Soot Formation and Oxidation for Conventional and HCCI Diesel Combustion

The study of soot oxidation and CO formation in internal combustion engine applications is the subject of numerous recent investigations. Their modeling is particularly important for Diesel operating conditions. Models have been developed recently at IFP to account for the complicated kinetic processes involved in CO / soot production and oxidation. This paper presents the equations for CO formation and oxidation based on a reduced 6 step chemistry model coupled with the PSK reduced soot production and oxidation mechanism. The species are accounted for in the conservation equations. Model development is done on the framework of the ECFM3Z engine combustion model.

The global CO/soot model is first validated in a constant volume high pressure cell against LII measurements. Model parameters are adjusted and kept constant for the remaining of the simulations. Then, engine simulations are used to validate the model behavior in conventional and HCCI Diesel conditions. A parametric study based on experimental results includes: EGR (Exhaust Gas Recirculation), geometry and nozzle tip protrusion (NTP) variations at high and partial loads. Simulated pressure evolutions are compared to experimental measurements to ensure that the upstream mechanisms (injection, mixture formation and auto-ignition) are well reproduced. Then, the different post-oxidation models are tested. We compare CO and soot models alone as well as within the coupled approach against measurements. The trends and orders of magnitude are in good agreement for all cases. The limitations are due to the fast chemical times associated with the simplified mechanisms which can penalize the computation convergence. The coupled approach allows improvement of both CO and soot emissions modeling within reasonable CPU times.