The Failure Analysis And Improvement For Diesel Engine High Pressure Fuel Pipe Joint

Nowadays the diesel engine is the main source of power in shipping field.Fuel supply system plays a significant role on diesel engine,which may have important impacts on performance and emission of diesel engine,and high pressure fuel pipe is the main part of fuel supply system.The sealing of high pressure fuel pipe ends are mainly linear sealing.The high pressure fuel pipe conical joint pressed on fuel inlet hole by threaded pipe joint and be sealed.High pressure fuel pipe conical joint is easy to be damaged because of bad working conditions.After the completion of 72 h running-in test for the first trial of high pressure oil pipe installed in the XX-type diesel engine,three pipes of sixteen were found broken in later overhaul.All the fracture occurred in the largest cross-section area of joint between pipe and injector.Through the chemical analysis,metallographic analysis and mechanical property test,the results showed that the pipes were qualified for the relevant national standards and component technical specifications.The microscopic analysis result suggested that the fracture should be ascribed to the fatigue failure.The fatigue source was located at inner wall of the cold heading pipe joint,which the most severely deformed area during the cold heading process.The further investigation indicated that the serious internal extrusion streamline was formed in this area in the cold-heading process,which was perpendicular to inner wall.Moreover,the cold heading caused the higher inner surface roughness.These conditions were supposed to facilitate the crack initiation and propagation.Commercial FEM software were applied to make a comparative analysis between stress distribution of high pressure fuel pipe joint with and without defect at inner surface.Based on this,life prediction of simulated condition were made,the result shown that defect at inner surface which is the major fatigue fracture factor can greatly reduce fatigue life of high pressure fuel pipe joint.According to the major causes of fracture,the thesis proposed the improving measure for the cold heading process of the high pressure fuel pipe production.In order to achieve the optimum measure,a series of improving schemes were promoted and compared using the finite element simulation method.Improving schemes are divided into two batches.For the first batch of schemes,compared with simulated results and actually cold heading testified the feasibility of simulation method,and main parameters influence on simulation results were analyzed.The second batch of schemes were based on the first batch of schemes,by the way of adding and modifying cold heading dies,and iterative refinements were made,finally optimum solution achieved.The optimum solution was applied to the production of the high pressure fuel pipe thereby.After the long period running-in tests,the improved pipes were overhauled and dissected.No crack initiation was found in the improved pipe,demonstrating that the improving measure is effective.