The prediction capability of an advanced Common Rail (CR) piezoelectric injection-system mathematical model has been upgraded and applied to the analysis of transient flows inside injectors. The predicted time histories of the sac-, delivery- and control-chamber pressures, of the pilot- and needle-valve lifts, and of the mass flow-rates through the Z and A holes, as well as through the bypass, have been analyzed to explain the differences in performance between piezoelectric and solenoid injectors. The objective was to study the influence exerted by the different hydraulic and mechanical setups in order to assess the effective benefits that could be obtained from the replacement of the solenoid driving system with the piezoelectric one. Then, the upgraded mathematical model has been applied to the design of new-concept injectors. First, specific attention was given to the effects that variations in the peak current values could have on the injected flow-rate. Furthermore, numerical simulations were performed to comprehend the effects that remarkable increments in the injector internal accumulation volume could have on injection system performance. A prototype of the new-concept injector with a small integrated accumulator (Minirail) has also been tested on the hydraulic rig. (C) 2012 Elsevier Ltd. All rights reserved.