Investigation of the effect of double ignition on the combustion processes in a 2-valves gasoline engine through 3D simulation

In this study, the ability of 3D engine computation to take into account the effects of double spark ignition in a small size 2-valves gasoline engine has been investigated. The KIVA-II GSM code has been used, in combination with the AKTIM spark ignition model and the ECFM premixed combustion model. Comparisons of the combustion processes using one or two spark plugs have been made at partial load, using different EGR concentrations and spark timings. A careful post-processing of the flame surface fields leads to a complete understanding of the balance between gain and losses relative to the double ignition strategy. For a low EGR concentration, IMEP gains expected for the double ignition appear to be rather weak, partly due to an early merging of the two flame fronts. Thus, despite radically different instantaneous combustion scenarios, the resulting motoring work remains close to the single spark case. Thermal losses analysis brings further insight into this gains and losses balance. However, at higher EGR concentration and earlier corresponding spark timings, gains linked with double ignition become higher, because the presence of a second spark plug corrects a weakness in the development of the flame front issued from the serial spark plug, keeping the global combustion speed to an acceptable level. NOx formation for the different strategies has also been analyzed and linked to the local flame propagation processes and burnt gases temperatures.