Analysis of methods for improvement of thermal stability of pistons of tractor diesel engines

Abstract

The article considers the applied methods for improving the thermal strength of pistons of internal combustion engines. Tractor diesels with combustion chamber in the piston have the greatest rate of temperature change at load rise and release, as well as the highest values of temperature gradients. The highest thermal loads occur in pistons with a semi-open combustion chamber. Periodic thermal loads with high value of temperature gradient lead to thermal fatigue fractures. Such fractures may begin with the appearance of cracks on the edge of combustion chamber. The presence even of a small crack on the edge of combustion chamber leads to its further growth, which can cause the destruction of piston. The main causes of cracks formation on the edge of combustion chamber are the alternating stresses induced from alternating gas pressure in cylinder during the working cycle; the low-frequency oscillations of the piston temperature arising from the changing of operation modes of engine; the high-frequency cyclical thermal oscillations caused by the temperature change of material in the surface layer of combustion chamber for each working cycle. The most common design and technology solutions improving the thermal strength of pistons are the following ones: the change of the edge radius of combustion chamber throat; the reinforcement of combustion chamber edge with more heat-resistant materials; the use of materials with high thermal stability for manufacture of pistons; the artificial heat insulation of piston or its cooling by oil. The disadvantage of use of cooling oil gallery is the acceleration of oil aging process. The alternative solution is to limit the heat supply to the walls of combustion chamber by means of heat protection coating applied by gas-flame, detonation or electric-arc way. The easiest way to improve the thermal strength of diesel piston with a semi-open combustion chamber is the design change.