Comparing Single-Step and Multi-Step Chemistry Using The Laminar and Turbulent Characteristic Time Combustion Model In Two Diesel Engines

Three-dimensional diesel engine combustion simulations with single-step chemistry have been compared with two-step and three-step chemistry by means of the Laminar and Turbulent Characteristic Time Combustion model using the Star-CD program. The second reaction describes the oxidation of CO and the third reaction describes the combustion of H₂. The comparisons have been performed for two heavy-duty diesel engines. The two-step chemistry was investigated for a purely kinetically controlled, for a mixing limited and for a combination of kinetically and mixing limited oxidation. For the latter case, two different descriptions of the laminar reaction rates were also tested. The best agreement with the experimental cylinder pressure has been achieved with the three-step mechanism but the differences with respect to the two-step and single-step reactions were small. Also, the maximum and mean gas temperatures, and hence the NO_x, were not noticeably affected by the different choices of chemistry. In contrast, the net soot production was sensitive to the chemistry used. Finally, a study of the drop discretization resulted in the fact that approximately 20 000 parcels or 1E+6 parcels per second were sufficient to achieve drop independent results.