Cycle-resolved numerical and experimental analyses were carried out to study the dynamics of an automotive diesel injection system with a distributor-type pump during transient control phases. The investigation had the following main objectives: to assess a flexible and efficient simulation program of high-pressure fuel-injection systems in unsteady operating conditions; to examine the system response to rapid load or speed variations, regardless of the control device which may cause them. The program, based on a fast implicit numerical algorithm with second-order accuracy, was an extension of the computational code NAIS which had been previously developed and validated for stationary operating conditions. The experiments were made on a test bench usually used by industry to evaluate diesel injection equipment.
A nonlinear system numerical model with two degrees of freedom, developed for the pump speed-governor, was set up and included in a library containing a variety of system component models so as to give a modular structure to the NAIS code. Comparison between computed and measured static characteristics of the governor was made in order to assess the model of this system component.
Experimental tests were carried out by rapidly moving the pump control lever from its idle position to the full-load position at two constant pump speeds, in the low and high-speed range, respectively. During these transient operations pressure time-histories were gauged and simulated cycle-by-cycle in the pumping chamber and at the distribution-pipe ends. In-cycle fuel-injected quantities were also measured and computed. Results were compared and discussed.
The dynamic system behavior under assigned transient load or speed phases was then theoretically analyzed by computing local-pressure, needle-lift and injection-rate time histories. Besides, experimental steady-state distributions of the injection rate and injection pressure, that is the pressure in the sac chamber, were compared to the simulation results during an acceleration phase at full load.