Dynamic Connecting Rod Bolt Self-Loosening in Internal Combustion Engines 2010-01-0692

As automobile manufacturers continue to strive toward delivering vehicles with the highest possible blend of fuel efficiency, reliability, performance and cost, new engine designs need to be as optimized as possible. A significant barrier to engine efficiency is unnecessary reciprocating and rotating mass in the internal combustion engine, which has driven the advancement of computer modeling and finite element analysis in order to optimize these components. As a consequence of this drive to reduce mass and cost, the safety factor of a component is made as low as possible. When a component is considered optimized for its environment and has a very low safety factor, any unforeseen stresses or strains may cause the part to fail or perform unexpectedly. Such stresses can be caused from engine misalignment, crank throw bending or abnormal combustion phenomenon. This is especially problematic when a supplier of one component that is involved in a dynamic assembly is not able to predict external or unexpected forces caused by other components outside of its range of responsibility.

This is believed to be the root cause for recent dynamic connecting rod bolt self-loosening issues, which caused multiple engine failures during durability testing. A thorough investigation was conducted that relates the broader issue of fastener self-loosening to the specific example of the internal combustion connecting rod bolted joint. Through empirical evidence, firing engine measurements and analysis, the theory of bolt self-loosening in the connecting rod bolted joint is presented. Additionally, a new machine has been developed that accurately simulates connecting rod bolt self-loosening in a manner similar to the Junkers fastener vibration machine. This paper will also address fixes that can be made with regards to the fastener itself.