Numerical and experimental analysis of unsteady cavitating nozzle flow and diesel spray characteristics in a high-pressure piezo-driven injector

摘要

The diesel injector, which is generally used in high-pressure common rail fuel injection system, is driven by the solenoid coil energy for needle's movement. Its main disadvantage in diesel engine application is high power consumption, high power loss through solenoid coil and relative needle response's problem. In this study, as a new method driven by piezoelectric energy, a prototype piezo-driven injector has been designed and fabricated based on the concept of inverse piezo-electric effect. Firstly, the dynamic characteristics of the hydraulic component by using the AMESim code have been investigated together with the inside nozzle flow by a fully transient simulation with cavitation model using a VOF method. The numerical calculation has been performed to simulate the cavitating flow of 3- dimensional real size single hole nozzle along the injection time. Secondly, we investigated the piezo-electric characteristics for high-pressure injection (130MPa) to evaluate the potential of new control capability of a piezodriven injector by experimental approach. To effect of piezo-driven injection processes on the diesel spray structure in high pressure two injectors, namely a solenoid-driven injector and a piezo-driven injector, we also investigated the injection rate and spray characteristics in a constant volume chamber pressurized by nitrogen gas using the back diffusion light illumination method for high-speed temporal photography. The obtained research results show that the input voltage exerted on the piezo stack is the dominant factor than the hydraulic force generated by the constant injection pressure through the analysis model of the piezo actuator. And the piezo-driven injector has short injection delay and a faster spray development and produces higher injection velocity than solenoid-driven injector. It also has a better spray tip penetration due to higher fuel momentum. The spray characteristics is sensitive to induced voltage in piezo-driven injector. A faster spray development by altering the injection rate shape can be adjusted in piezo-driven injector according to induced voltage. Also we could predict the degree of cavitation's generation inside nozzle for faster needle response in a piezo-driven injector This predicted simulation results with the higher injection velocity and faster onset of cavitation are reflected to spray development, in agreement with the experimental spray images.