Experimental and Numerical Investigation of Vapor Formation in a Fuel Rail

Recently, additional scrutiny is being placed on all vapor releases to the environment from the fuel system of an automobile. In an effort to lower the overall release of fuel vapor, a preliminary study of the vapor formation processes that occur in a low pressure supply fuel rail was undertaken. The first objective of this work was to determine the means by which fuel vapor is generated within the fuel rail, particularly during hot soak conditions. Then, using this information, the next task was to develop a computational fluid dynamics (CFD) code which would model the vapor formation in the rail. An investigation of the fuel rail material and design revealed that the probable mechanism for vapor formation is nucleate boiling from cavities in the fuel rail surface and at the o-ring connections with the fuel injectors. Therefore, an experiment was constructed to investigate the vapor formation from artificial cavities on a metallic surface and at an o-ring interface. The data collected from the experiment included the departure diameter of the vapor bubbles, the bubble frequency, and the bubble rise velocity. These values, which are used to determine the vapor generation rate, were compared to the results predicted by various correlations available in the literature. Subsequently, a CFD model was constructed of the fuel rail, using Star-CD, by incorporating the appropriate vapor generation correlations as user-defined subroutines. The experimental observations clearly demonstrated that a large amount of vapor was generated at the o-ring interface and, to a lesser degree, from the cavities in the metallic surface. A CFD model was constructed to predict the vapor generated in a fuel rail from these cavities. Existing correlations that describe nucleate boiling adequately model this generation mechanism in the fuel rail. This CFD code can be used to determine the amount of vapor formed under various hot soak conditions. An analytical means of predicting the vapor formation at the o-ring interface will have to be developed in order to complete the CFD model.