Abstract
Thermal shocks are reasons for high temperature gradients occurring in materials of engine components and sets, what in turn makes for high total stresses, even at lack of mechanical loads that accelerate damage of the engine components. With reference to heterogeneous elements like bimetals and from materials with covers, temperature gradients will be considerably greater and will represent what is due to different material properties.
Experimental test results of thermal shocks for heavy-duty pistons of combustion engines are presented in the paper. The range and the level of the thermal loads were diverse and dependent on the working engine conditions, as well as the temperature ranges that were adapted for real conditions in which combustion engine pistons work. Temperature measurement results on the crown and the skirt of the piston during the Diesel engine operating under different conditions are presented in the paper. Temperatures on the piston surface in the area of combustion chamber change in an every working cycle and these changes grow less with frequency (engine speed) increasing.
Measured temperature differences on the operating engine were greatest in the areas where greatest piston temperatures appeared. The differences of the piston temperature also depended on the operating engine conditions and the piston material. The researches of the thermal shocks were realized on a special testing device. The device is automatic, controlled by the computer that makes possible the realization of the cycle of research of thermal shocks in the necessary range of temperature. The test results showed that in the following cycles of the thermal shocks stable changes of dimensions of the piston took place. The piston areas around the combustion chamber of direct-injection heavy-duty Diesel engines are especially exposed to the negative effects of thermal shocks.
Main factors affecting the resistance of the piston materials to thermal shocks are presented in the paper, therefore the essential parameters influencing the resistance of materials to thermal shocks are the coefficients of thermal expansion and the Young's modules. The resistance to the piston thermal shocks made up from composite materials is especially essential for heavy-duty Diesel engines.
In order to reduce the thermal loads on the piston and the negative effects of thermal shocks, the novel construction based design was proposed, where during the time when the thermal loads are the most severe, the mechanical loads are isolated from the inertial loads and the inertial loads are non existent. Besides the reduction of the thermal loads additional gains in overall efficiency were obtained as the result of the combustion process with the piston stopped. That system forms the part of United States Patent No. 6,481.393 B1.
Keywords: piston composite alloys, thermal shocks, thermal fatigue damage, dual heat dissipation path, piston with cold centre