A Study on Reaction Rate Factors of a Single-Step Combustion Mechanism for Gasoline HCCI Combustion
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Homogeneous Charge Compression Ignition (HCCI) engine has been invented to achieve the advantages of both diesel and gasoline engines. HCCI engine can run with a higher compression ratio like diesel engine in order to achieve high thermal efficiency. Meanwhile, like gasoline engine, the engine produces less NOx gas and particulate matters than diesel engine. However, there are several difficulties associated with HCCI combustion, which still require further development to the engine. The most important issues are autoignition timing and narrow operational range. In order to develop an HCCI engine, a numerical simulation can significantly save cost, time and resources over an experimental study. KIVA3V program is chosen to simulate an HCCI engine in this study. This program is compatible chemically reactive flows in complex geometries, and featured with spray dynamics which is highly suitable for internal combustion engine. The program has been developed into several versions to suite experimental conditions. It yields essential data such as temperature, pressure, heat release, species concentrations and droplets data at any particular location of the geometry. The main goal of this study is to obtain an accurate simulation for an HCCI combustion in KIVA3V program. KIVA3V provides a single-step combustion model with sets of reaction rate factors for some fuels. The fuels data and combustion model have been proven to work well with typical spark ignition and compression ignition engine. However, HCCI combustion has autoignition activity and relatively high residual gas content. The original reaction rate factors were derived from flame propagation and not accounted for high residual gas content. Thus, the reaction rate was overpredicted causing an excessively early combustion The two reaction rate factors, pre-exponential factor and activation energy, are modified separately obtain precise autoignition timing for the HCCI combustion for each. Then, the simulated in-cylinder temperature and pressure from both cases are found to match the experimental data. The modified activation energy case provided slightly more rapid heat release than the modified pre-exponential factor case.