Experimental Analysis of Transient Flow Phenomena in Multi-Jet Common-Rail Systems

A detailed experimental analysis of the pressure wave propagation phenomena and fuel-injection system dynamics in a last-generation C.R Multijet system was carried out on a high performance new test bench Moehwald-Bosch MEP2000-CA4000 under real engine simulated conditions. Specific attention was paid to the wave propagation induced pressure oscillations as well as to their relationships with system control parameters and multiple-injection performance. In particular the authors aim at illustrating how the geometrical features of each component of the injection-system hydraulic layout influence the aforementioned wave motion providing a key for design optimization in order to obtain efficient multiple injection performance. A parametric analysis on the whole injection system performance when multiple injections are executed in an injection cycle has been carried out, taking the most important geometric features, i.e. pipe lengths, pipe diameters, and rail volume, as test parameters. A simple zero-dimensional model has been developed to work out the natural frequencies of the injection system when hydraulic layout is modified. Such a model was shown to be capable of correctly predicting the main frequencies of the hydraulic circuit and their dependence on the geometrical parameters. The good agreement between the outcome of this simple model and the experimental data also substantiated the reliable authors’ interpretation of the cause and effect main relations underlying the complex flow phenomena occurring in the system. The hydraulic circuit frequencies are compared to the natural frequencies of the mobile elements present in the electro-injector to individuate mechanical resonance conditions. Furthermore the energizing times, which produce hydraulic resonance, i.e. high-amplitude pressure oscillations, are determined: these solenoid excitation durations represent critical values to be employed when multiple injections are performed. A concrete layout solution to minimize the problem of fluid dynamics interaction between injectors, that is disturbances produced by one injector on the others, is provided and justified. Finally a study about the possibility of severely reducing the rail volume has been leaded in order to obtain a system with a more prompt dynamic response during engine speed transients and with condensed overall dimensions.