Experimental Study of the Coking Phenomenon Effects on Spray Characteristics of the High Pressure Diesel Injector

The novel diesel engines with advanced fuel injection systems are equipped with solenoid injectors comprising multiple small nozzle orifices which makes considerable improvement in fuel spray characteristics and engine performance along with providing high pressure fuel injection system. On the other hand, poor fuel quality, impurities and heavy metal elements in the diesel fuel, and high temperature medium in the diesel engines combustion chamber lead to remarkable deposits formation in the small holes of the nozzle. In addition, it results in partial or complete nozzle hole obstruction which is called injector nozzle coking having detrimental effects on discharged spray ideal behavior and proper engine performance. In this work, the analysis of coking phenomenon influences on diesel spray macroscopic characteristics have been done. Initially, the coked injectors with different time operation and deposit amounts are prepared under experimental and specific operating conditions. Then, the images recorded from the spatial and temporal evolution of a diesel spray in various injection and chamber pressures, are processed through the extended code in MATLAB software in order to analyze discharged fuel spray characteristics. The SCHLIEREN Imaging Method with high speed camera has been utilized in a CVC (constant volume chamber) without combustion. Non-Destructive Electron Microscopy Method of SEM (Scanning Electron Microscope) imaging was utilized in order to analyze sediments quantity and construction changes during injector working in the real engine conditions. The results show that, sediments occupy 20, 40, 75 and 90% of the total hole opening surface, respectively in the injectors with 300, 700, 800 and 900 hours operating time. By increasing the injector operation time and accumulated sediment amount on the nozzle, the discharged injector spray exhibits a more inappropriate behavior. Moreover, The Results revealed that coking has considerable effects on the spray tip penetration at low injection pressures. As injection pressure increases, the decreasing rate of the penetration length alleviates gently. In other words, at high injection pressures (1500 bar and higher) the penetration length has minor drop compared with non-utilized injectors even at 900 hours operating time, but the spray projected area can be reduced up to 28% in high chamber pressures.