Loading conditions of pistons of internal combustion engines and causes of crack formation on combustion chamber edge

Abstract

In order to develop the methods for assessment of durability of pistons of tractor diesel engines, it is necessary to conduct the research of causes of failures in their operation, in particular crack formation on the edge of combustion chamber. Tractor engines operate in transient regimes due to periodic changes of control organ position and resisting moment when tractor performs agricultural, logging and other works. In transient regimes, the stress-strain state of piston varies in time, which leads to the formation of fatigue cracks on the edge of combustion chamber. The paper reviews the studies on regularities of change in the rate of propagation of fatigue cracks on the edge of combustion chamber of tractor diesel engine. The peak values of heat flows passing through the heat-absorbing surface of piston head in transient regimes exceed by 2.5 times the same values in steady-state regimes. According to the researches, for the 4ChN 11/12.5 tractor diesel engine stress intensity on the edge of combustion chamber caused by temperature gradients equals to 92-98 MPa, and exceeds more than by 2 times the stresses caused by gas pressure forces in cylinder (38-42 MPa). With the increase of engine forcing, the edge of combustion chamber becomes a local concentrator of thermal stresses, that leads to elastic-plastic deformations of cyclical pattern and microdefects. Stresses appearing on the edge of combustion chamber under rapid change of loading modes typically exceed the values of yield stress of piston material. This leads to the appearance of plastic deformations on the edge of combustion chamber. The analysis of causes of crack formation on the edge of combustion chamber of piston of a tractor diesel engine shows that when assessing the durability of piston, the whole range of indicators and parameters of the engine should be taken into account.