Experimental studies on the dual-fuel sequential combustion and emission simulation

Abstract

This study theoretically proposes and experimentally demonstrates the simultaneous reductions of NOx and soot using a novel combustion mode, (dual-fuel sequential combustion) DFSC, on a single-cylinder engine. DFSC introduces a well-mixed, lean fuel/air mixture into the cylinder by injecting high-cetane number fuel (n-heptane was used in this study) at the intake port followed by the direct injection of a high-octane number fuel (ethanol, iso-propanol, 1-butanol and iso-octane were used in this study) near the top dead center (TDC). Four fuel combinations (n-heptane/iso-octane, n-heptane/ethanol, n-heptane/iso-propanol, and n-heptane/1-butanol) were operated with the DFSC mode in this study, and the ignition mechanism and combustion processes of each fuel combination were investigated. Three types of burn modes of directly injected fuels were observed: active atmosphere-dominated ignition, active-thermal atmosphere-dominated ignition, and thermal atmosphere-dominated ignition. The ignition timings of the directly injected fuels were strongly dependent on the premixed n-heptane concentration and were slightly influenced by the charge cooling effects. In the present testing conditions, the n-heptane/ethanol DFSC showed almost smokeless and ultra-low NOx emissions at the maximum indicated (mean effective pressure) IMEP of 0.76 MPa. To increase understanding of the formation mechanisms of the HC and CO emissions in this combustion mode, three-dimensional (computational fluid dynamics) CFD coupled with reduced chemical kinetic mechanisms was used to simulate the n-heptane/iso-octane DFSC mode. Preliminary results show that the HC emissions originate primarily from the core region of the spray jet, where the fuel injection tip was unable to fully oxidize and burn. The CO emissions produce primarily at the boundary layer of the cylinder wall, piston ring crevice region, and combustion chamber bowl.