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Combustion Visualization and Experimental Study on Multi-Point Micro-Flame Ignited (MFI) Hybrid Lean-Burn Combustion in 4-Stroke Gasoline Engines
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on September 15, 2020 by SAE International in United States
Lean-burn combustion is an effective method for increasing the thermal efficiency of gasoline engines combusted the mixture with stoichiometric fuel-air ratio, but leads to an unacceptable level of high cyclic variability before reaching ultra-low NOx emissions emitted from conventional gasoline engines. Multi-point micro-flame ignited (MFI) hybrid combustion was proposed to overcome the 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 fast propagation of multi-point flame occurred in the central zone of cylinder in the double-peak type. However, the multiple flame sites occur around the cylinder, and then gradually propagate and form 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 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 excess air coefficient from 1.5 to 2.7. The maximum indicated thermal efficiency reaches around 42% at 1500 rpm engine speed and 7 bar IMEPn.