Diesel Injector Coking: Optical-Chemical Analysis of Deposits and Influence on Injected Flow-Rate, Fuel Spray and Engine Performance

The physical origin of injector coking in diesel engines has been clarified and the most critical design parameters and operating variables pertaining to the occurrence of the phenomenon have been identified. Fouling has been shown to be affected by many factors, such as injector temperature, nozzle configuration, hole diameter and conicity as well as fuel composition. Optical and scanning electron microscope (SEM) analyses have been conducted both inside and outside injectors of different type and four locations have been identified as the main deposition sites. Furthermore, different coking typologies, i.e., dry and wet coking, have been assessed and discussed. Energy Dispersive X-ray (EDX) spectroscopy images of the deposits on the spray hole walls have revealed that minute quantities of Zn catalyze the coking reactions to a great extent. Significant quantities of Zn have also been found in the injector deposits. An extensive experimental test campaign has been carried out at the engine test bench with different nozzle setups in order to evaluate performance deterioration after different ageing procedures. The effects of both the Zn concentration in the fuel and running time have been assessed separately on the fouling rate. Injection rate time histories have been acquired 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 have been analyzed and related to the corresponding variations in engine power measured at the engine test bench. A previously developed combustion multi-zone diagnostic model has also been applied to gain a further insight into the cause and effect relationships between the experimental in-cylinder pressure time histories and engine-out emissions.