THERMAL AND KINETIC IMPACT OF CO, CO2, AND H2O ON THE POSTOXIDATION OF IC-ENGINE EXHAUST GASES

The thermal and kinetic impact of the residual species CO, CO2, and H2O on hydrocarbon (HC) oxidation chemistry was investigated numerically. The case of pure dilution by N-2 was tested against a dilutant composed of CO, CO2, and H2O in proportions corresponding to internal combustion (IC)-engine Postoxidation conditions (at the end of the expansion stroke and throughout exhaust). The impact of each residual species was tested individually, as well as in combination with others. Attention was given to the thermal impact, kinetic impact and third-body effects of each residual. In the cases of CO2 and H2O, a negative thermal C-p-effect in competition with an accelerating kinetic impact due to third-body reactions was observed. The influence of CO on HC-oxidation is restricted to its direct participation in oxidizing reactions, and its thermal impact is negligible compared to N-2. CO2 and H2O have a negative thermal impact compared to N-2, resulting from their increased heat capacities. Kinetically, they interact as collision partners with HC-oxidation through H2O2-dissociation. Generally, it was shown that the composition of a dilutant strongly impacts HC-oxidation and that the presence of CO2 and mainly H2O may lead to an acceleration of HC-oxidation, in spite of their unfavorable thermal properties compared to N-2.