Common Rail without Accumulator: Development, Theoretical-Experimental Analysis and Performance Enhancement at DI-HCCI Level of a New Generation FIS 2007-01-1258

An innovative hydraulic layout for Common Rail (C.R.) fuel injection systems was proposed and realized. The rail was replaced by a high-pressure pipe junction to have faster dynamic system response during engine transients, smaller pressure induced stresses and sensibly reduced production costs. Compared to a commercial rail, whose inside volume ranges from 20 to 40 cm³, such a junction provided a hydraulic capacitance of about 2 cm³ and had the main function of connecting the pump delivery to the electroinjector feeding pipes. In the design of the novel FIS layout, the choice of high-pressure pipe dimensions was critical for system performance optimization. Injector supplying pipes with length and inner diameter out of the actual production range were selected and applied, for stabilizing the system pressure level during an injection event and reduce pressure wave oscillations.

The new injection system was realized and subjected to experimentation under engine-like working conditions on a high performance Moehwald-Bosch MEP2000-CA4000 test bench. The injection performance of the new system was shown to be similar to those of a commercial C.R., for a single injection. Besides, for multiple injections, the innovative layout dynamics was substantially improved by a reduced dependence of the overall injected fuel amount on dwell-time (DT) in sequential injection events. Hydraulic layout solutions preventing the occurrence of resonance flow phenomena among injectors were used to minimize fluid dynamic interference among injectors. The results confirmed that the rail capacitance was not a key parameter in pressure wave disturbance attenuation, whereas injector inlet-pipe sizes did have a considerable effect on these.

In addition, a modification of latest solenoid-generation commercial Multijet electroinjector was realized so as to sensibly reduce the limits of DT for fusion-free sequential injection shots, thus enhancing such component at HCCI combustion application level. The design of the injector based on an integrated experimental-theoretical analysis of system dynamics, was carried out using a previously developed C.R. numerical model. Experimental results on injector characteristics were discussed.