Abstract
The detailed combustion kinetic processes of a high-pressure direct injection (HPDI) natural gas (NG) marine engine was investigated in the present work. A postprocessing code was employed to visualize the characteristic reactions that determine the combustion process. To evaluate the effect of mixture stratification on the combustion process, various NG injection timings were employed and four representative combustion periods were selected, including the timings when 1% (CA1), 5% (CA5), 10% (CA10), and 50% (CA50) of the total fuel energy are released. A higher heat release rate (HRR) was generated with a more advanced NG start of injection (SOI) timing, which, however, had limited effects on the main representative exothermic reactions (REXRs) within the high heat release (HHR) region and the dominant formation reactions of CH2O and OH, which are known as the indicators of low heat release (LHR) and HHR, respectively. Besides, for different NG SOI timing simulation, the consumption of CH4 was all dominated by reactions H + CH4 = CH3 + H2 and OH + CH4 = CH3 + H2O and the dominated REXR of the LHR region was reaction CH3 + O2 = CH3O2. Furthermore, with an advanced NG injection timing, significant changes were observed for the reaction paths of CH2, CH2O, and HCO from the premixed-combustion phase (CA10) to the mixing-controlled combustion phase (CA50). The results of the present study are able to provide a theoretical fundamental for the practical control of the HPDI NG marine engine.