Quantification of folded metal in cylinder bores through surface relocation

In the development of internal combustion engines, many attempts have been made to numerically quantify a defect known as folded metal in cylinder bores, due to the detrimental effects these defects cause in terms of friction and wear. Despite recent advances in topography analysis, some questions still remain in terms of what is really considered a folded metal. In this work, in addition to the proposition of a method for the quantification of folded metal, a sensitivity study was conducted to evaluate how critical the criteria for groove definition are in folded metal analysis. The present study also analyzes how metal folds can vary for different heights before and after engine tests and how wear mechanisms can affect folded metal during running-in. To this end, cylinders from an engine, as new and after 100 h dynamometer test were analyzed in terms of folded metal by using the relocation technique. The results show that the amount of folded metal can be strongly reduced after engine test if running-in has occurred under conditions that favor the removal of material without substantial amounts of plastic deformation. Otherwise, folded metal can be formed during engine operation. The present work also shows that the use of inadequate threshold values for folded metal detection may mask the results giving the false impression that folded metal is not removed after engine test. Finally, a mechanism for folded metal formation based on the plastic deformation of the matrix induced by the presence of near-surface cracks at graphite tips is also suggested.