The physical origin of injector coking in diesel engines was clarified and the most critical design parameters and operating variables for the occurrence of the phenomenon were identified. Injector fouling was shown to be affected by many factors, such as injector temperature, nozzle configuration, hole diameter and conicity, fuel composition. In particular, minute quantities of Zn, which can be added to the fuel, were verified to sensibly catalyze the growth of the undesired deposits. Optical and Scanning Electron Microscope (SEM) analyses were conducted both outside and inside different injectors and four locations were identified as main sites for the deposits. Furthermore, different typologies of coking, such as dry and wet coking, were identified and discussed. Energy Dispersive X-ray (EDX) spectroscopy images of the deposits on the spray hole walls revealed a significant presence of Zn when the fuel was doped with such an element. An extensive campaign of experimental tests was carried out at the engine test bed with different nozzle setups in order to evaluate performance deteriorations, in terms of power output, fuel consumption and emissions, after distinct ageing processes. The effects of both Zn concentration in the fuel and ageing time on the losses of engine performance were separately essessed. A previously developed combustion multi-zone diagnostic model was applied to prove the consistency between experimental in-cylinder pressure time hystories and engine-out emissions. Finally injection rate time histories were measured at the hydraulic test rig under different working conditions for both new and aged injectors. The experimental changes in the EVI profiles subsequent to fouling were analyzed and related to the corresponding variations in emissions and engine power, which resulted at the engine test bench.

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