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The demands on internal combustion engines for low emissions and fuel consumption are increasing year by year. On the other hand, engines to be used in motorcycles need to provide high output and quick response to meet user desire. In order to realize low fuel consumption while keeping high performance, it is necessary to properly understand cyclic variations during combustion as well as the influence of the injection system on fuel control during transient periods. The current paper reports on the results of a study in the influence of port fuel injection on combustion stability in a small displacement motorcycle engine, using both a series of experiments and CFD. The parameters of the injection systems under study are: (1) injection targeted area, (2) injection timing, and (3) fuel droplet size. The results of the current study show that injection aimed at the upstream wall yielded the best combustion stability.

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.