Materials for High-Pressure Fuel Injection Systems

Abstract

The high-level goal of this multi-year effort was to facilitate the Advanced Combustion Engine goal of 20% improvement (compared to 2009 baseline) of commercial engine efficiency by 2015. A sub-goal is to increase the reliability of diesel fuel injectors by investigating modelbased scenarios that cannot be achieved by empirical, trial and error methodologies alone. During this three-year project, ORNL developed the methodology to evaluate origins and to record the initiation and propagation of fatigue cracks emanating from holes that were electrodischarge machined (EDM), the method used to form spray holes in fuel injector tips. Both x-ray and neutron-based methods for measuring residual stress at four different research facilities were evaluated to determine which, if any, was most applicable to the fuel injector tip geometry. Owing to the shape and small volumes of material involved in the sack area, residual stress data could only be obtained in the walls of the nozzle a few millimeters back from the tip, and there was a hint of only a small compressive stress. This result was consistent with prior studies by Caterpillar. Residual stress studies were suspended after the second year, reserving the possibility of pursuing this in the future, if and when methodology suitable for injector sacks becomes available. The smooth specimen fatigue behavior of current fuel injector steel materials was evaluated and displayed a dual mode initiation behavior. At high stresses, cracks started at machining flaws in the surface; however, below a critical threshold stress of approximately 800 MPa, cracks initiated in the bulk microstructure, below the surface. This suggests that for the next generation for high-pressure fuel injector nozzles, it becomes increasingly important to control the machining and finishing processes, especially if the stress in the tip approaches or exceeds that threshold level. Fatigue tests were also conducted using EDM notches in the gage sections. Compared to the smooth specimens, EDM notching led to a severe reduction in total fatigue life. A reduction in fatigue life of nearly four orders of magnitude can occur at an EDM notch the approximate size of fuel injector spray holes. Consequently, the initiation and propagation behavior of cracks from small spray holes is relevant for generation of design quality data for the next generation diesel fuel injection devices. This is especially true since the current design methodologies usually rely on the less conservative smooth specimen fatigue testing results, and since different materials can have varying levels of notch fatigue resistance.