Experimental Validation of a Common-Rail Injector Model in the Whole Operation Field

Abstract

The present study deals with the definition of an accurate mathematical model of a production common-rail-type injector for automotive diesel engines. The mathematical model defined in a previous work was refined, accounting for a broader range of effects on injector performance, thus allowing a more strict validation of the model predictions against experimental data. The geometry of the control-valve holes, crucial in determining the actual discharge coefficient, was accurately evaluated by means of silicone molds. The moving mechanical components of the injector, such as control valve, needle, and pressure rod were modeled using the mass–spring–damper scheme. The axial deformation under pressure of needle, pressure rod, nozzle and injector body was modeled. This effect was found to also affect the control valve device operation, and was properly accounted for. The model obtained was implemented in Simulink; the ordinary differential equations were solved by means of the numerical differentiation formulas implicit scheme of second-order accuracy, while the partial differential equations were integrated using the finite-difference Lax–Friedrichs method. In order to obtain sufficient data for validating the model in its entire operation field, two separate sets of tests were carried out. In the first analysis, a constant reference pressure was imposed in the rail, and the injector energizing time was progressively increased from values relative to small pilot injections to values characteristic of large main injections. The injected volume per stroke was measured by means of a mean delivery meter (EMI) device. During the second set of tests, the injector was mounted on a flow rate meter (EVI) device so as to measure the injection law. Electric current flowing through the injector solenoid, oil pressure in the common rail, and at the injector inlet, needle, and control valve lift were also gauged and recorded. The good agreement between numerical and experimental results allowed the use of the model to gain greater insight into the mechanisms and phenomena that regulate injector behavior. The nozzle hole discharge coefficient dependence upon time and needle lift was discussed, and the trends were presented in several working conditions. The flow in the control volume holes was studied, in order to determine whether cavitation occurs or not, giving an answer to a long disputed topic. Finally, the effects of injector deformation caused by fuel pressure on performance were investigated.