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Mixture formation is one of the most important factors for the combustion in the spark ignition engine with port fuel injection. The relation between combustion and mixture quality, however, is not quantitatively well established. In this study, the connection of combustion and mixture formation was explored with various measurement techniques. Borescopes were used in order to investigate the flame propagation in the combustion chamber and behavior of spray and fuel film on the wall in the intake port. For the purpose of investigation on the effect of mixture formation, various port fuel injection systems and parameters were tested and compared: direction, timing, and size of droplet. An SI engine for small vehicle was used under condition of 4 000 rpm. The investigation by images obtained has shown that inhomogeneity of mixture causes low combustion stability, especially due to direct introduction of fuel droplets into the combustion chamber.

This paper reports visualization of behavior of spray, wall film, and initial flame propagation in an SI engine with port fuel injection system for motorcycle in order to directly investigate their influences on combustion and relations among them. Borescopes were used to visualize the flame propagation in the combustion chamber and wall film in the intake port. Various injection systems and injection parameters were tested: injection direction, timing, and size of droplets to investigate the effect of mixture formation. It is concluded that combustion stability under low load condition is greatly influenced by mixture inhomogeneity in the combustion chamber whose evidence is the luminous emission. It is caused by direct induction of considerable amount of liquid fuel with large size of droplets into combustion chamber or too inhomogeneous mixture in the intake port.