Automatic Chemistry Mechanism Reduction of Hydrocarbon Fuels for HCCI Engines Based on DRGEP and PCA Methods with Error Control

The chemical kinetics of hydrocarbon fuels determines the combustion characteristics and pollutant emissions of homogeneous charge compression ignition (HCCI) engines. Including comprehensive chemical mechanisms in HCCI engine models provides accurate predictive results that can be used to improve engine designs. However, a large number of simulations are usually required to optimize an HCCI engine, and the use of comprehensive chemical mechanisms is prohibitive. Furthermore, an increased demand for surrogate fuels that better represent real fuels has resulted in further increases in the size of chemical mechanisms as the carbon number of surrogate fuel species and the number of fuel components considered increases. Consequently, reduced mechanisms of smaller sizes, which are able to represent their corresponding comprehensive mechanisms over a wide range of conditions are necessary. This paper presents an approach that fully automates the process of reducing comprehensive chemical mechanisms of fuels for HCCI engines. The approach is based on the directed relation graph with error propagation (DRGEP) and principal component analysis (PCA) methods. In the first stage, the DRGEP method is used to efficiently remove redundant species. This is followed by the use of the PCA method to further remove insignificant reactions and species. During the entire process, the performance of the reduced mechanism is monitored to ensure that the generated mechanism satisfies user-specified error tolerances. In the present study three comprehensive mechanisms that include n-heptane, <i>iso</i>-octane, and methyl decanoate (MD) were investigated. The proposed approach successfully reduced the comprehensive mechanisms of n-heptane (561 species and 2539 reactions), <i>iso</i>-octane (857 species and 3606 reactions), and MD (2878 species and 8555 reactions) to reduced mechanisms with sizes of 140 species and 491 reactions, 195 species and 647 reactions, and 435 species and 1098 reactions, respectively, while maintaining small errors compared to the full mechanisms

Kinetic and Numerical Study on the Effects of Di-<i>tert</i>-butyl Peroxide Additive on the Reactivity of Methanol and Ethanol

A numerical investigation was conducted to study the effects of di-<i>tert</i>-butyl peroxide (DTBP) additive on the reactivity of methanol and ethanol fuels. First, a reduced primary reference fuel (PRF)–​​methanol–​​ethanol–​​DTBP mechanism was proposed to simulate the homogeneous charge compression ignition (HCCI) combustion processes of PRF and alcohol–​​DTBP fuel mixtures. By linking through the combustion phasing of HCCI operation with the PRF fuels, effective PRF number maps were generated for the alcohol–​DTBP fuels. The agreement between experimental and simulation results was reasonably good. Both the experiments and simulations showed that DTBP enhances the fuel reactivity of the alcohols and that the rate of reactivity enhancement decreases with increasing DTBP percentage. The reasons for the enhancement of reactivity by DTBP addition to both methanol and ethanol fuels were then explored kinetically. It was found that both thermal and chemical effects contribute to the reactivity enhancement, and this can be attributed to the heat released in the DTBP decomposition process, the reactive radicals generated through the CH₃ → CH₃O₂ → CH₃O₂H → OH pathway, and the reaction pathway of fuel + CH₃O₂ → CH₃O₂H → OH. The major reason for the different response of DTBP between methanol and ethanol was found to be the higher DTBP content in methanol–​DTBP mixtures for the same operating conditions, and this was further confirmed by the fact that the effects of DTBP addition on methanol and ethanol reactivity were quite similar if the same absolute DTBP mass was added to these two alcohols

Kinetic and Numerical Study on the Effects of Di-<i>tert</i>-butyl Peroxide Additive on the Reactivity of Methanol and Ethanol

A numerical investigation was conducted to study the effects of di-<i>tert</i>-butyl peroxide (DTBP) additive on the reactivity of methanol and ethanol fuels. First, a reduced primary reference fuel (PRF)–​​methanol–​​ethanol–​​DTBP mechanism was proposed to simulate the homogeneous charge compression ignition (HCCI) combustion processes of PRF and alcohol–​​DTBP fuel mixtures. By linking through the combustion phasing of HCCI operation with the PRF fuels, effective PRF number maps were generated for the alcohol–​DTBP fuels. The agreement between experimental and simulation results was reasonably good. Both the experiments and simulations showed that DTBP enhances the fuel reactivity of the alcohols and that the rate of reactivity enhancement decreases with increasing DTBP percentage. The reasons for the enhancement of reactivity by DTBP addition to both methanol and ethanol fuels were then explored kinetically. It was found that both thermal and chemical effects contribute to the reactivity enhancement, and this can be attributed to the heat released in the DTBP decomposition process, the reactive radicals generated through the CH₃ → CH₃O₂ → CH₃O₂H → OH pathway, and the reaction pathway of fuel + CH₃O₂ → CH₃O₂H → OH. The major reason for the different response of DTBP between methanol and ethanol was found to be the higher DTBP content in methanol–​DTBP mixtures for the same operating conditions, and this was further confirmed by the fact that the effects of DTBP addition on methanol and ethanol reactivity were quite similar if the same absolute DTBP mass was added to these two alcohols

Construction of Skeletal Oxidation Mechanisms for the Saturated Fatty Acid Methyl Esters from Methyl Butanoate to Methyl Palmitate

A series of skeletal oxidation mechanisms for the saturated fatty acid methyl esters (FAMEs) from methyl butanoate to methyl palmitate were developed on the basis of a decoupling methodology with special emphasis on engine-relevant conditions from low to high temperatures at high pressures. When detailed H₂/CO/C₁, reduced C₂–C₃, and skeletal C₄–C_<i>n</i> submechanisms are introduced, the final mechanism consists of 42 species and around 135 reactions for each methyl ester. Both the high-temperature reactions of the methyl ester moiety and the low-temperature reactions of the aliphatic chain of the ester are included in the mechanism. The skeletal mechanisms were verified against experimental data in shock tubes, jet-stirred reactors, flow reactors, and premixed and opposite flames over the temperatures from 500 to 1700 K at pressures of 1–50 atm from fuel-lean to fuel-rich mixtures. An overall satisfactory agreement between the measurements and computational results was achieved for all of the saturated methyl esters, especially for the large saturated methyl esters with a long aliphatic main chain. The results also indicate that the ignition delay time and the consumption of reactants could be reproduced by employing a skeletal C₄–C_<i>n</i> submechanism reasonably well. In addition, the evolution of major products and flame propagation and extinction characteristics were satisfactorily reproduced because the detailed H₂/CO/C₁ mechanism was used. The compact size makes it easy to integrate the mechanism into multi-dimensional computational fluid dynamics (CFD) simulation

Construction of Skeletal Oxidation Mechanisms for the Saturated Fatty Acid Methyl Esters from Methyl Butanoate to Methyl Palmitate

A series of skeletal oxidation mechanisms for the saturated fatty acid methyl esters (FAMEs) from methyl butanoate to methyl palmitate were developed on the basis of a decoupling methodology with special emphasis on engine-relevant conditions from low to high temperatures at high pressures. When detailed H₂/CO/C₁, reduced C₂–C₃, and skeletal C₄–C_<i>n</i> submechanisms are introduced, the final mechanism consists of 42 species and around 135 reactions for each methyl ester. Both the high-temperature reactions of the methyl ester moiety and the low-temperature reactions of the aliphatic chain of the ester are included in the mechanism. The skeletal mechanisms were verified against experimental data in shock tubes, jet-stirred reactors, flow reactors, and premixed and opposite flames over the temperatures from 500 to 1700 K at pressures of 1–50 atm from fuel-lean to fuel-rich mixtures. An overall satisfactory agreement between the measurements and computational results was achieved for all of the saturated methyl esters, especially for the large saturated methyl esters with a long aliphatic main chain. The results also indicate that the ignition delay time and the consumption of reactants could be reproduced by employing a skeletal C₄–C_<i>n</i> submechanism reasonably well. In addition, the evolution of major products and flame propagation and extinction characteristics were satisfactorily reproduced because the detailed H₂/CO/C₁ mechanism was used. The compact size makes it easy to integrate the mechanism into multi-dimensional computational fluid dynamics (CFD) simulation