An Investigation of Mixture Formation Processes During Start-Up of a Natural Gas Powered SI Engine

The mixture formation processes of methane and air in an optical access engine operating steadily at 200 RPM have been explored in order to study charge inhomogeneity in a natural gas powered spark ignition engine during transient engine cranking. Planar Laser Induced Fluorescence has been used to create fuel/air equivalence ratio maps as a function of injection timing for various image planes at intervals throughout the intake and compression strokes. The work has been done using a Honda VTEC-E engine head that features port injection, four valves per cylinder, a pentroof style combustion chamber for the generation of tumble motion, and one nearly deactivated intake valve to generate swirl motion at low engine speeds in order to enhance mixing. Experimental results show that, shortly before ignition, the dependency of the mixture inhomogeneity on the fuel injection timing is greatly reduced (as compared to idle operation at 700 RPM) due to the effects of fuel carryover between cycles. This is caused by the minimal intake manifold pressure drop that is observed during engine start-up and constrained by the placement of the fuel injector, which has been optimized for high-speed operation with the use of both intake valves. Back flow of the fuel/air charge into the intake port during the start of compression while the active intake valve is still partially open is also believed to have a significant effect.