Development of a Reduced Mechanism for <i>n</i>-Heptane Fuel in HCCI Combustion Engines by Applying Combined Reduction Methods

Abstract

Employing comprehensive chemical kinetics mechanisms in predictive models results in the high demand for simulation time, which makes the use of these mechanisms questionable. Consequently, reduced mechanisms of smaller sizes are needed. The objective of this study is to produce reduced mechanisms of <i>n</i>-heptane fuel, by utilizing a three-stage reduction process. This work is performed using a validated single-zone homogeneous charge compression ignition (HCCI) combustion model. To remove unimportant species at the first stage, the directed relation graph with error propagation (DRGEP) is applied. In the second stage, the computational singular perturbation (CSP) method is used to eliminate insignificant reactions. In the third stage, because of the change in the net production and consumption rate of species as a result of utilizing the second stage and its effect on direct interaction coefficients calculated with DRGEP method, once again DRGEP is applied to the mechanism for further reduction. Peak pressure, maximum heat release, and CA50 have been selected as representative parameters for evaluating the performance of the reduced mechanism. For the generated reduced mechanism at each reduction step, these parameters are calculated and the deviations from the corresponding value obtained by applying detailed mechanism to the model are evaluated until user-specified error tolerances are exceeded. This combination of methods successfully reduced the comprehensive Curran’s <i>n</i>-heptane mechanism (561 species and 2539 reactions) to a reduced mechanism with only 118 species and 330 reactions, while maintaining small errors (<2%), compared to the detailed mechanism. The simulation time required for calculation by applying reduced mechanisms is decreased from ∼601 min to 8 min, in comparison to the detailed mechanism. In addition to matching pressure, temperature, and heat-release-rate traces, the mass fraction of some important species calculated from the reduced mechanism closely agree with the results obtained from the detailed mechanism. The reduced mechanism is included as Supporting Information with this article