Combustion Visualization and Experimental Study on Multi-Point Micro-Flame Ignited (MFI) Hybrid Lean-Burn Combustion in 4-Stroke Gasoline Engines
Lean-burn combustion is an effective method for increasing the thermal efficiency of gasoline engines fueled with stoichiometric fuel-air mixture, but leads to an unacceptable level of high cyclic variability before reaching ultra-low nitrogen oxide (NOx) emissions emitted from conventional gasoline engines. Multi-point micro-flame ignited (MFI) hybrid combustion was proposed to overcome this problem, and can be can be grouped into double-peak type, ramp type and trapezoid type with very low frequency of appearance. This research investigates the micro-flame ignition stages of double-peak type and ramp type MFI combustion captured by high speed photography. The results show that large flame is formed by the fast propagation of multi-point flame occurring in the central zone of the cylinder in the double-peak type. However, the multiple flame sites occur around the cylinder, and then gradually propagate and form a large flame accelerated by the independent small flame in the ramp type. Furthermore, the combustion and emissions characteristics of ramp type MFI lean-burn combustion are experimentally investigated in a thermodynamic gasoline engine. The results show that ramp type MFI combustion can achieve a stable combustion process with high thermal efficiency and ultra-low NOx emissions when net indicated mean effective pressure increases from 2 bar to 7 bar and the excess air coefficient increases from 1.5 to 2.7. The maximum indicated thermal efficiency reaches around 42% at an engine speed of 1500 rpm and a net indicated mean effective pressure of 7 bar.