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

In ‘Multijet’ Common Rail (C.R.) diesel injection systems, when two consecutive injection current-pulses are approached to each other, the fusion of the two injections can occur. This causes undesired excessive amount of injected fuel, which leads to worsening of particulate emissions and fuel consumption. In order to avoid such a phenomenon, lower limits to the values of dwell time are introduced in the control unit maps, by means of a conservatively overestimated threshold, limiting the flexible management of multiple injections and C.R. system capability to perform a larger number of injection shots. The reason of the injection fusion is mainly due to the time delay between the electrical signal to the solenoid and the needle lift at both valve opening and closure. In particular, the dwell-time range inside of which injection fusion occurs was shown to decrease by reducing the nozzle closure delay. Experimental tests were carried out on a high-performance Moehwald-Bosch MEP2000/CA4000 test bench for determining the functional dependence of nozzle closure and opening delays on solenoid energizing time and nominal rail pressure. Besides, a mathematical relation between the solenoid energizing time and the injection time interval was determined. A Multijet C.R. injection system mathematical model, that was previously developed, including thermodynamics of liquids, fluid dynamics, subsystem mechanics, 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, mass flow rates through Z and A holes, were obtained and analyzed in order to highlight the dependence of nozzle opening and closure delays on electro-injector internal geometric features and on the needle dynamics. For all the considered operating conditions, the model predictions were compared to the experimental injection flow-rate patterns and to the pressure data taken at the injector inlet, for assessment. The nozzle closure delay was shown to strongly depend on the needle dynamics. Parametric tests were carried out with the numerical code by changing needle and control plunger mass, needle spring preload and stiffness, maximum needle stroke, in order to identify configurations useful for minimizing the nozzle closure delay. On the basis of the indications derived from these numerical tests, a modified version of the commercial electro-injector was realized so as to achieve effectively reduced nozzle closure delays and very close sequential injections without any fusion between them.