A comparison of the relative effects of fuel composition and ignition energy on the early stages of combustion in a natural gas spark ignition engineusing simulation

Spark ignition engines operating on natural gas (NG) show advantages over p etrol and diesel alternatives in terms of precatalyst emissions. However, s uch engines are affected much more than their petrol counterparts by the lo ng combustion delay of the main fuel component, methane. The sensitivity of combustion delay to fuel/diluent chemistry means that this issue is exagge rated by the presence in the NG of variable concentrations of higher hydroc arbons and of inorganic components, arising from the worldwide variability of NG or from the use of exhaust gas recirculation. The present article reports on a study using a large-scale three-dimensiona l simulation, incorporating computational fluid dynamics (CFD) and fully de tailed chemical kinetics, of the early stages of NG combustion in a spark i gnition engine. The focus is on comparing the relative influences of gas co mposition (i.e. the ethane content of the NG, and carbon dioxide in the oxi dant/diluent) with the effects of ignition source location and energy on th e early progress of the combustion. The results of the studies show that, a lthough the presence of ethane enhances the combustion propagation rate and reduces delay time, and carbon dioxide has the reverse effect, the influen ces of the characteristics (energy and location) of the ignition source dom inate over quite large changes in mixture composition. These findings have practical ramifications for the development of strategies for reducing comb ustion delay time in NG-fuelled engines.