Low NOx stratified combustion of a compressed natural gas with an over-expansion injector

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The present study aimed at revealing the feasibility of utilizing over-expansion behavior of supersonic gas flow. It would generate lower pressure area at the fuel jet than the ambient air, the ambient air was expected to be sucked into the fuel jet, and the richer zone in the central region of the fuel jet would become leaner resulting in lower combustion temperature and lower NOx formation. On the basis of the estimation of this behavior on the pressure ratio and the mach number plane, the shock wave pattern was predicted under the condition of high pressure and temperature ambient realized by a rapid compression machine. When the reduced compression ratio of 5.8 in the rapid compression machine was applied, slight increase of combustion efficiency and reduction of CO were observed, which suggested the effect of formation of over-expansion condition. However, little variation of NOx level was detected. Thus the over-expansion effect was realized at lower compression ratio and the ext … More ent is not so remarkable.For introducing ambient air into the fuel jet at subsonic condition, utilization of air entrainment with turbulent mixing, so-called Bunsen burner mechanism, was also tested. Under atmospheric condition with steady-state test rig experiment, the phenomenon of air introduction was successfully observed. However, in the experiment of rapid compression machine, little effect of such air introduction was obtained. Thus it was also shown that the air introduction is not so effective enough to reduce NOx emission.In terms of application to engine combustion, effect of injector positioning was examined since it is normally limited to cylinder head. Comparison study of injector positioning, side and central injection was carried out. With the central injection, combustion efficiency was increased at higher equivalence ratio condition but it deteriorated much at leaner condition. This would be generated by the better mixing of central injection due to the interaction between the fuel jet and the combustion chamber wall. Thus the importance of fuel injector positioning was clearly shown and the need of the future strategy of injection parameter determination was suggested. Less