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This work investigates the mixture formation process in a SI engine with air-assisted port fuel injection. The combination of this injection method with low flow velocities and wall temperatures, typical of engine starting and warm-up conditions, results in the build-up of a fuel film along the walls of the port. This in turn results in increased fuel consumption and high unburned hydrocarbon emissions. This investigation concluded that with the application of air-assisted injectors, the mixture formation process could be improved with a resulting reduction of the fuel-film build-up in the intake ports. Comparative measurements of the drop size distribution from conventional and air-assisted injectors showed that even very small amounts of air resulted in better fuel atomization. An experimental analysis of the effect of unsteady air flow on spray dispersion and fuel film creation characteristics of air supported injectors was undertaken for a straight test section.

The study presented in this two part paper was focused on the influence of primary mixture formation on engine running behavior covering the areas combustion and raw emissions. Two different concepts for primary fuel atomization were utilized and compared, the standard production injector and a flash boiling injector. The spray generated by the flash boiling injector was characterized by a significant reduction in droplet size and a partial direct vaporization during the injection process by preheating the fuel inside the injector. In this study special emphasis was put on the transient process of engine start between typical cooling water temperatures of -7°C and 85°C. Various measurements and visualization techniques were applied to investigate the mixture preparation, the deposition of liquid fuel on the walls, the start of combustion, and in-cylinder and engine-out UHC emissions.

The intention of the study presented in this two part paper is to investigate the influence oalf primary mixture formation on engine running behavior, covering the areas of combustion and raw emissions. Two different concepts for primary fuel atomization were utilized and compared, the standard production injector and a flash boiling injector. The flash boiling injector is characterized by a significant reduction in droplet size and a partial direct vaporization during the injection process by preheating the fuel inside the injector. In this study special emphasis was laid on the transient process of engine start between typical cooling water temperatures of -7°C and 85°C. Various measurements and visualization techniques had been applied to investigate mixture preparation, deposition of liquid fuel on the walls, start of combustion, and in-cylinder as well as engine-out UHC emissions.