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. With injection aimed at the intake valve face, combustion stability deteriorated depending on injection timing and coolant temperature. Even when using an injector with good atomizing properties, combustion stability could not significantly be improved. Comparing CFD results with experimental result, it is shown that the cyclic variations of NMEP can be correlated with the mixture distribution in the cylinder. CFD results also shows that wall film quantity which can be expected to affect the transient behavior of the engine is relatively high with injection aimed at the upstream wall and low with injection aimed at the intake valve face.