Development of a reduced chemical kinetic model of n-heptane - NG blend for homogeneous charge compression ignition engine combustion

ABSTRACT: As the transportation technologies move forward and the need of travelling becomes more important, the mankind is facing two major challenges, namely, emission of green-house gases and excessive fuel consumption. The homogeneous charge compression ignition (HCCI) engines have the well known benefits of emitting very low amounts of NOx and soot, while producing higher efficiencies compared to the conventional engines. Computational modeling is a useful tool for engine design and optimization. The full chemical kinetic mechanisms to simulate the fuel oxidation consist of hundreds or thousands of species and reactions. Utilizing such a detailed mechanism requires extremely long computational time. In order to facilitate practical simulations, reduced mechanisms of smaller sizes are necessary. A threestage reduction process is proposed in this research. The performance of the proposed method is investigated by producing reduced mechanisms of n-heptane fuel. This work is performed by using a validated single zone 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, once again DRGEP is applied to the mechanism for further reduction. This combination of methods successfully reduced the comprehensive Curran's nheptane mechanism (561 species and 2539 reactions) to a reduced mechanism with only 118 species and 330 reactions, while maintaining small errors (less than 2 percent) compared to the detailed mechanism in predicting selected representative parameters. The simulation time required for calculation is decreased from about 601 minutes when the detailed mechanism is used to 8 minutes by applying reduced mechanisms to the model. Also, a reduced mechanism for a fuel blend of natural-gas and n-heptane is proposed. The approach is validated for the prediction of ignition timing in the HCCI combustion engine. A two-stage reduction process is used to produce two reduced mechanisms of existing detailed mechanisms for natural-gas and n-heptane fuels. The combination of the generated reduced mechanisms is used to develop a reaction mechanism for a fuel blend of natural-gas/n-heptane. Then, the genetic algorithm is used for optimization of reaction rate constants in the newly generated mechanism. The proposed mechanism includes only 41 species and 109 reactions. Simulation results agree well with the experimental results under various operating conditions, while maintaining small errors (less than 2 degrees) in predicting ignition timing. Furthermore, effect of heat transfer through the boundaries in HCCI combustion simulation in generating reduced mechanism from the detailed mechanism is also investigated. A two-stage reduction process is used to produce reduced mechanisms of existing detailed GRI-Mech. 3.0 mechanism. Small differences observed in the developed reduced mechanisms for HCCI combustion model with considering heat transfer and in the adiabatic condition. Keywords: HCCI engine, Ignition timing, Reduced mechanism, DRGEP, CSP, PCA, Blended fuel …………………………………………………………………………………………………………………………