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Design analysis of a fuel injector prototype with the aid of finite element models Post, Adrian John

Abstract

A fuel injector utilising High Pressure Direct injection technology is being developed by Westport Innovations Ltd. It is designed to allow heavy-duty diesel engines to run primarily on natural gas and yet still retain the inherent advantages of diesel engines. The operation of the injector is fairly complex and the cost of prototyping is high. Fatigue cracks were detected in injector components subjected to engine cycle testing. To better understand the stresses in the injector and decrease the stress levels, several finite element models of the different components are created for analysis. Using submodelling techniques, the fine details of the design are examined at important sections of the injector. Alternative design choices are examined by creating alternative submodels with different hole intersection configurations and comparing the results to the original submodel. The results of this analysis indicate that more significant changes than altering the hole configuration are necessary if the stress levels are to be decreased. The diesel and gas needle components are studied in a separate analysis in which contact elements are utilised to model the interaction with each other and the injector tip. The sealing surface at the contact between the components is predicted to be smaller than was previously assumed. Decreasing the relative angle between the contact surfaces results in an increase in the sealing surface. The significance of thermal stresses as a cause of the fatigue failures is shown to be minor. A transient heat transfer analysis of the injector tip is performed. This predicts very small thermal variations once steady state had been reached. The end of the tip is a critical area as the injection holes act as a stress concentration and the repercussions of failure of this section are great as this section of the injector is exposed to the engine cylinder. Different design alternatives for this particular section are modelled and compared to assess the relative merits of each configuration. Finally, an investigation is conducted into the effect of pressurising the spring bore of the injector. Under the current conditions of the model, the deformation caused by this pressurisation will be unacceptable as there will no longer be sufficient clearance for the plungers in the cage and check block components of the injector.

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