Influence of injector nozzle design and cavitation on coking phenomenon

For several years, the development of combustion and injection systems has been focused on drastically reducing pollutant emissions while preserving the high fuel efficiency which characterizes Diesel engines. In the mean time, the industrial robustness and the customer reliability had to be secured for worldwide applications. Within this working frame, project investigations have shown the importance of injector nozzle characteristics as a potential to greatly improve the fuel spray quality and thereby reduce engine-out emissions. Design parameters like hole diameter, hydro grinding, conicity or inlet hole radius have shown a direct influence on the internal hydraulic flow, allowing a better trade off between high performances and emissions; at the same time, induced cavitation has been identified not only for its well known influence on the discharge coefficient but also for its key role versus coking phenomena which especially appear with small hole diameters. A study using an innovative high pressure flow bench, 3D-CFD hydraulic simulation including cavitation effects and engine tests, has been carried out to analyze the relation between these geometrical parameters, cavitation and coking. A new criterion based on a “cavitation intensity” is proposed in this paper to evaluate the possible coking problems and at the same time find the best balance between power density and reduced pollutant emissions. The new test bench and the followed methodology are described in detail with an example on a middle size engine.