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

Abstract

Spark ignition engines operating on natural gas (NG) show advantages over petrol and diesel alternatives in terms of precatalyst emissions. However, such engines are affected much more than their petrol counterparts by the long combustion delay of the main fuel component, methane. The sensitivity of combustion delay to fuel/diluent chemistry means that this issue is exaggerated by the presence in the NG of variable concentrations of higher hydrocarbons 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-dimensional simulation, incorporating computational fluid dynamics (CFD) and fully detailed chemical kinetics, of the early stages of NG combustion in a spark ignition engine. The focus is on comparing the relative influences of gas composition (i.e. the ethane content of the NG, and carbon dioxide in the oxidant/diluent) with the effects of ignition source location and energy on the early progress of the combustion. The results of the studies show that, although the presence of ethane enhances the combustion propagation rate and reduces delay time, and carbon dioxide has the reverse effect, the influences of the characteristics (energy and location) of the ignition source dominate over quite large changes in mixture composition. These findings have practical ramifications for the development of strategies for reducing combustion delay time in NG-fuelled engines.