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Fundamental aspects of Common Rail (CR) fuel-injection-system dynamics

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A rather complete mathematical model for a common-rail injection-system dynamics numerical simulation was developed to support experimentation, layout, and control design, as well as performance optimization. The thermofluid dynamics of the hydraulic-system components, including rail, connecting pipes, and injectors was modeled in conjunction with the solenoid-circuit electromagnetics and the mechanics of mobile elements. One-dimensional flow equations in conservation form were used to simulate wave propagation phenomena throughout the high-pressure connecting pipes, including the feeding pipe of the injector nozzle. In order to simulate the temperature variations due to the fuel compressibility, the energy equation was used in addition to mass conservation and momentum balance equations. Besides, the possible cavitation phenomenon effects on the mass flow rate through the injector bleed orifice and the nozzle holes were taken into account. A simple model of the electromagnetic driving circuit was used to predict the temporal distribution of the force acting on the pilot-valve anchor. It was based on the experimental time histories of the current through the solenoid and of the associated voltage that is provided by the electronic control unit to the solenoid. The numerical code was validated through the comparison of the prediction results with experimental data, that is, pressure, injected flow rate, and needle lift time histories, taken on a high performance test bench Moehwald-Bosch MEP2000-CA4000. The novel injection-system mathematical model was applied to the analysis of transient flows through the hydraulic circuit of a commercial multijet second-generation common-rail system, paying specific attention to the wave propagation phenomena, to their dependence on solenoid energizing time and rail pressure, as well as to their effects on system performance. In particular, an insight was also given into the model capability of accurately predicting the wave dynamics effects on the rate and mass of fuel injected when the dwell time between two consecutive injections is varied.

An experimental-theoretical study has been carried out on high-pressure volumetric radial-piston pumps for diesel fuel injection systems. The dependence of the pump inducted flow rate on speed and load was investigated and the characteristic curve of the pump cooling-lubrication circuit was derived. The head-capacity curves were determined for different types of pumps at different revolution speeds and compared with the injector flow requirements in order to evaluate the pressure-control strategy efficiency. An insight into the ageing effects on the pump performance was also provided. Furthermore, the dynamic pump behavior was investigated, with specific reference to the flow-rate ripple at the delivery port. A general analytical expression has been derived for the volumetric efficiency. Furthermore, a specific procedure has been developed and applied to the experimental evaluation of the fuel leakages in pressure control valve (PCV) integrated pumps. Finally, the mechanical-hydraulic efficiency of the pump has experimentally been assessed as a function of head and speed in order to obtain a reliable evaluation of the pump shaft power and torque at different working conditions.

Cavitation is the transition from a liquid to a vapour phase, due to a drop in pressure to the level of the vapour tension of the fluid. Two kinds of cavitation have been reviewed here: acoustic cavitation and hydrodynamic cavitation. As acoustic cavitation in engineering systems is related to the propagation of waves through a region subjected to liquid vaporization, the available expressions of the sound speed are discussed. One of the main effects of hydrodynamic cavitation in the nozzles and orifices of hydraulic power systems is a reduction in flow permeability. Different discharge coefficient formulae are analysed in this paper: the Reynolds number and the cavitation number result to be the key fluid dynamical parameters for liquid and cavitating flows, respectively. The latest advances in the characterization of different cavitation regimes in a nozzle, as the cavitation number reduces, are presented. The physical cause of choked flows is explained, and an analogy between cavitation and supersonic aerodynamic flows is proposed. The main approaches to cavitation modelling in hydraulic power systems are also reviewed: these are divided into homogeneous-mixture and twophase models. The homogeneous-mixture models are further subdivided into barotropic and baroclinic models. The advantages and disadvantages of an implementation of the complete Rayleigh-Plesset equation are examined.

In “multijet” common rail (CR) diesel injection systems, when two consecutive injection current pulses approach each other, a merging of the two injections into a single one can occur. Such an “injection fusion” causes an undesired excessive amount of injected fuel, worsening both fuel consumption and particulate emissions. In order to avoid this phenomenon, lower limits to the dwell-time values are introduced in the control unit maps by a conservatively overestimated threshold, which reduces the flexibility of multiple-injection management. The injection fusion occurrence is mainly related to the time delay between the electrical signal to the solenoid and the nozzle opening and closure. The dwell-time fusion threshold was found to strongly decrease particularly with the nozzle closure delay. A functional dependence of the nozzle opening and closure delays on the solenoid energizing time and nominal rail pressure was experimentally assessed, and the injection temporal duration was correlated to the energizing time and rail pressure. A multijet CR injection-system mathematical model that was previously developed, including thermodynamics of liquids, fluid dynamics, mechanics of subsystems, 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, and mass flow rates through Z and A holes were obtained and analyzed to highlight the dependence of nozzle opening and closure delays on injector geometric features, physical variables, and valve dynamics. The nozzle closure delay was shown to strongly depend on the needle dynamics. Parametric numerical tests were carried out to identify configurations useful for minimizing the nozzle closure delay. Based on the results of these tests, a modified version of a commercial electroinjector was built, so as to achieve effectively lower nozzle closure delays and very close sequential injections without any fusion between them.

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