PORTO @ Archivio Istituzionale della Ricerca

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A fully predictive one-dimensional model of a Common Rail injection apparatus for diesel passenger cars is presented and discussed. The apparatus includes high-pressure pump, high-pressure pipes, injectors, rail and a fuel-metering valve that is used to control the rail pressure level. A methodology for separately assessing the accuracy of the single submodels of the components is developed and proposed. The complete model of the injection system is finally validated by means of a comparison with experimental high-pressure and injected flow-rate time histories. The predictive model is applied to examine the fluid dynamics of the injection system during either steady-state or transient operations. The influence of the pump delivered flow-rate on the rail-pressure time history and on the injection performance is analysed for different energizing times and nominal rail pressure values. A comparison between fuel metering valve and pressure-control-valve strategies is also carried out in terms of rail pressure and pump delivered flow-rate time histories. The advantages and the weak points of each strategy are highlighted: the fuel metering valve control is more efficient, due to the absence of a throttling of high-pressure fuel, but leads to poorer dynamic response during nominal rail pressure transients. Even though the injection performance is the same under steady-state working conditions, significant differences between the two strategies can occur during engine transients.

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