Fundamental study on lean operation limit of super lean-burn spark ignition engines: MIE transition and limit prediction

Abstract

Fuel-lean combustion is challenging because of the difficulty in successful ignition-to-flame propagation transition in intense turbulence conditions. This study aims to elucidate the governing factor of fuel dependence on the lean limit through fundamental ignition experiments and numerical simulations. Previous scaling analysis has reported strong correlations between lean engine operation limit and Minimum Ignition Energy (MIE) transitions. Additionally, the temporal evolution of turbulent intensity in the engine cylinder plotted on Peters diagram suggested that the flame kernel growth occurs only in relatively weak turbulent intensity, , the condition under which is lower than the MIE transition. To investigate the behavior of flame kernel growth in the vicinity of the MIE transition condition, we conducted ignition experiments under both laminar and turbulent conditions utilizing a constant volume chamber with counter-rotating fans. Flame initiation was achieved by spark discharge at various turbulent intensities. The results showed notable distinctions in flame kernel growth processes between below and above the MIE transition condition. For MIE transition, flame kernel development is governed by molecular transports showing an apparent Lewis number effect, whereas for MIE transition, the effect seems to disappear. Subsequently, experiments and numerical simulations on spherically propagating flames in quiescent mixtures with various blended fuels were conducted. The results indicated that fuels facilitating rapid flame kernel growth generally exhibited leaner engine operation limits, regardless of engine specifications. The present study successfully demonstrated that the fuels suitable for lean combustion could be predicted by investigation of spherically propagating flames in quiescent mixtures.