Hydraulic Circuit Design Keys to Remove the Dependence of the Injected Fuel Amount on Dwell Time in Multijet C.R. Systems

In Multijet Common Rail (C.R.) systems, the capability to manage multiple injections with full flexibility in the choice of the dwell time (DT) between consecutive solenoid current pulses is one of the most relevant design targets. Pressure oscillations triggered by the nozzle closure after each injection event induce disturbances in the amount of fuel injected during subsequent injections. This causes a remarkable dispersion in the mass of fuel delivered by each injection shot when DT is varied. The present works aims at investigating hydraulic circuit design keys to improve multiple injection performance of C.R. systems, by virtually removing the dependence of the injected fuel amount on DT. A Multi-Jet C.R. of the latest solenoid-type generation was experimentally tested at engine-like operating conditions on a high performance test bench. The considerable influence that the injector supplying pipe can exert on induced pressure oscillation frequency and amplitude was widely investigated and a physical explanation of cause-effect relationships was found by energetics considerations, starting from experimental tests. An optimization study was carried out to identify the best geometrical configurations of the injector supplying pipes so as to minimize pressure oscillations. The analysis was carried out with the aid of a previously developed simple zero-dimensional model, allowing the evaluation of pressure wave frequencies as functions of main system geometric data. Purposely designed orifices were introduced into the rail-pipe connectors or at the injector inlet, so as to damp pressure oscillations. Their effects on injection system performance were experimentally analyzed. Hydraulic circuit solutions that apply both optimized injector inlet-pipe sizes and oscillation damping orifices at the rail outlet were thoroughly investigated. Finally, the influence of the rail volume on pressure wave dynamics was studied to evaluate the possibility of severely reducing the rail capacitance. This would lead to a system, not only with reduced overall dimensions, but also with a prompter dynamic response during engine transients.