Research On Residual Life And Crack Fault Detection Of CNG Engine With Cracked Crankshaft

With the development trend of high speed and high power of internal combustion engine, the working condition of the crankshaft is becoming more and more severe. Due to the combined action of periodic impact and torsional loads, the crankshaft is prone to crack because of bending and torsion fatigue. Crankshaft cracks will bring great security risks; if we can detect the crack fault effectively, then we can find cracks timely. So we can estimate the residual life of the crack propagation accurately; and repair or replace the crank shaft in time. Therefore, the research on the residual life prediction and crack detection of crankshaft with crack can effectively avoid the casualties and reduce the economic loss.Researching the literatures of domestic and foreign, it is an urgent problem to find the fault source of the crankshaft with crack accurately and timely maintenance the crankshaft, this paper determines to make crank and connecting rod mechanism of Dongfeng NQ140BN5 engine as the carrier, we study the remaining life of cracked crankshaft three-dimensional fatigue crack expansion and crack fault detect. The main contents are as follows:(1) Make rigid flexible coupling dynamic analysis of crank and connecting rod mechanism, obtained that when the crankshaft speed is 2400r/min, each crank pin load distribution rules and determine the fourth crankpin bearing the maximum load that is 115840N. According to the investigation of crankshaft fracture, it is found that the starting position of crack usually appears in the transition corner. Therefore make it as the starting position of crack.(2) Based on the calculation formula of crankshaft stress intensity factor, the effect of the ratio between long and short axes of the ellipse crack on the stress intensity factor is calculated, Under the condition of considering the influence of different crack depth and the ratio between long and short axes of crack, calculating the stress intensity factor of the crankshaft with crack. According to the obtained stress intensity factor, fit the shape factor expression which is more suitable for the crankshaft.(3) Using the FRANC3D V6.0 software to simulate crankshaft crack propagation and find when the crankshaft crack depth expand to 21.21mm achieving to fracture toughness, and the crankshaft fatigue crack propagation life is 5.95 x 106 times, the result consistent with experimental results of a literature. Verify the feasibility and accuracy of simulation results.(4) Conduct free modal analysis of crankshaft with crack, obtained the influence of crack depth, position and number on the natural frequency and vibration mode of crankshaft. According to its influence rule, put forward the method to diagnosing the crack when the crankshaft is returned to the factory to repair or leave the factory. Then, make the dynamic analysis of crank and connecting rod mechanism, and get the influence of crack depth on the free end displacement of crankshaft longitudinal vibration. It is found that with the increase of the crack depth, the free end axial vibration amplitude of the crankshaft is increased. Put forward the on line detection method of crankshaft crack fault according to this rule.(5) The transition fillet of crankshaft has been optimized and designed, and get a ladder type crankshaft fillet; the simulation shows that the ladder type fillet can effectively slow down the stress concentration of the crankshaft over the corner. Unlike the heavy cutting type crankshaft transition fillet structure, which need to chamfer in the shaft neck and so can avoid the reduction of shaft strength.