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.
