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Hydraulic layout effects on multijet C.R. injection system performance and design criteria to remove the pressure wave induced disturbances on sequential injection shots


Autore: M. Baratta, A.E. Catania, A. Ferrari

Collana: CA - 61 - Perugia 2006

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 current pulses to the solenoid is one of the most important design targets. Pressure
oscillations triggered by the nozzle closure after each injection event induce disturbances in the amount of fuel injected duringsubsequent injections. This causes a remarkable dispersion in the fuel mass 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 Multijet 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 exercise on induced pressure oscillation frequency and amplitude was investigated and a physical explanation of cause-effect relationship was obtained by energetics considerations, based on 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 support of a previously developed simple zerodimensional 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 applying both optimized injector inlet-pipe sizes and oscillation damping gauged orifices at the rail outlet were thoroughly investigated.

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