Numerical-Experimental Study and Solutions to Reduce the Dwell-Time Threshold for Fusion-Free Consecutive Injections in a Multijet Solenoid-Type CR System

In “multijet” common rail (CR) diesel injection systems, when two consecutive injection current pulses approach each other, a merging of the two injections into a single one can occur. Such an “injection fusion” causes an undesired excessive amount of injected fuel, worsening both fuel consumption and particulate emissions. In order to avoid this phenomenon, lower limits to the dwell-time values are introduced in the control unit maps by a conservatively overestimated threshold, which reduces the flexibility of multiple-injection management. The injection fusion occurrence is mainly related to the time delay between the electrical signal to the solenoid and the nozzle opening and closure. The dwell-time fusion threshold was found to strongly decrease particularly with the nozzle closure delay. A functional dependence of the nozzle opening and closure delays on the solenoid energizing time and nominal rail pressure was experimentally assessed, and the injection temporal duration was correlated to the energizing time and rail pressure. A multijet CR injection-system mathematical model that was previously developed, including thermodynamics of liquids, fluid dynamics, mechanics of subsystems, and electromagnetism equations, was applied to better understand the cause and effect relationships for nozzle opening and closure delays. In particular, numerical results on the time histories of delivery- and control-chamber pressures, pilot- and needle-valve lifts, and mass flow rates through $Z$ and $A$ holes were obtained and analyzed to highlight the dependence of nozzle opening and closure delays on injector geometric features, physical variables, and valve dynamics. The nozzle closure delay was shown to strongly depend on the needle dynamics. Parametric numerical tests were carried out to identify configurations useful for minimizing the nozzle closure delay. Based on the results of these tests, a modified version of a commercial electroinjector was built, so as to achieve effectively lower nozzle closure delays and very close sequential injections without any fusion between them.