The performance and emission characteristics of the compression ignition engines are largely governed by the fuel atomization and air mixing, processes which in turn are strongly influenced by the flow dynamics inside the injector nozzle. This is controlled by dynamic (injection pressure, needle lift, etc.) and geometrical factors (orifice conicity, hydro grinding, etc.). Moreover, the modern diesel fuel injection systems are susceptible of deposits formation that can occur in different locations, e.g. in nozzle spray-holes and inside the injector body.
The present paper describes the results of a research project aimed at studying the impact of injector coking on diesel spray formation for three injectors with different flow numbers. The characterization of the injection process has been carried out in terms of fuel injection rate as well as spatial and temporal fuel distribution in a quiescent chamber in non evaporative conditions. Three different injection pressures have been adopted namely 40, 80, and 160 MPa with 1.0, 0.6, and 0.3 ms solenoid energizing time. A Bosch second generation common rail solenoid-driven fuel injection system has been used. Digital processing of the spray images, captured at different instant from the start of injection, has enabled the characterization of the fuel dispersion in terms of tip penetration and cone angle. Different behaviors of the injected jets have been measured for the diverse nozzles at early time from the start of injection. Comparisons of the results before and after the coking process of the injectors have been carried out for each nozzle type.
No sensible effects have been found on the injection rate shapes due to the coking process at parity of the nozzle flow number. A slight reduction of the total amount of injected fuel has been noted with decreasing of the injection durations (0.6 and 1.0 ms). Moreover repetitive anomalies in the jet-to-jet propagations have been highlighted at early time from the SOI for high and medium flow rates nozzles for both injectors new and coking.