Building 3-D model of diesel injector used in agriculture verified by injection rate of each hole and simulation on internal flow characteristics

Abstract: Agriculture-used diesel engine is widely used because of its simple structure, low engine cost and popularity. Because of the offset of the injector position to the cylinders' center axis in a two-valve diesel engine, the angles between the nozzle hole axis and the needle axis are different, which hence enhances efficient distribution of the mixture. It was observed from investigations that the injection rate of each nozzle hole of the injector was different but smaller for the nozzle hole with the higher angle (between the nozzle hole axis and the needle axis). The injection process is important to the spray process, mixture formation and combustion. The internal flow characteristics of the injector hole is the boundary condition for the spray, combustion and so on, which play a crucial role in improving the spray quality, optimizing the combustion process and decreasing the pollutant emissions. In the present study, a three-dimension model of valve covered orifice (VCO) injector with 5 holes used in a two-valve diesel was established. The simulation of internal cavitation and velocity distributions of each hole in the VCO injector was based on the two-fluid model and the cavitation model. Because of the needle movement and the fluctuations of the injection pressure, the internal flows in the nozzle holes were unsteady. The internal transient flow could be technically reflected by the moving mesh. The simulated and measured fuel injection rates and cyclical fuel injection quantity of each nozzle hole were compared and analyzed. Experimental validation showed that their differences were under limits, and the relative error of the cyclical fuel injection quantity per cycle of each hole between the simulated and experimental value was less than 5%, which proved that such model could be used to study the transient flow characteristics and the influences on angle between each nozzle hole axis and needle axis of the nozzle. Comparison and analysis were done, and the results showed that there were significant differences in fuel flow characteristics and cavitation among nozzle holes, which were variable during the injection process. Firstly, the continuous changing of injection pressure destabilized the internal cavitation of each hole, which influenced the injection rate at the nozzle outlet in cam angle at maximum needle lift. The increasing injection pressure resulted in the bubble's collapse, which made the effective flow area increase and the injection rate decrease, and vice versa. During the initial part of injection, the internal cavitations of the 5 holes were different and did not progress to the outlet of the nozzle holes. This extension in length of the internal cavitation did not affect the injection rate of each hole. Secondly, the bigger internal cavitation zone of the holes moved to the center with the increase in the angle between each nozzle hole axis and needle axis of the nozzles. This increased the flow velocity at the center of the holes, which enhanced the spray characteristics. The results obtained indicate that the spray characteristics and the injection rate should be comprehensively considered when designing and installing the two-valve multi-hole nozzle to ensure the optimum mixture formations, the combustion optimization and the reduction of emissions.