The cyclic thermal stress and plastic strain in the combustion chamber of a diesel engine due to the cyclic heating and cooling when starting and stopping the engine is probably the most severe example of thermal strain-governed low-cycle fatiguein commonly available cast products. Two computer models have been created to study this problem: I) a model of the bridge between exhaust valves of a cylinder head of a four-valve-per-cylinder engine was used to simulate the transient heat flow regime;and 2) a model was used to explore the thermal stress and strain by considering the high temperature creep leading to stress relaxation. This was used to study the fatigue performance of materials in a simulated cyclic heating and cooling environment ofthe diesel engine combustion chamber. In this model, an empirical formula, the Coffin -Manson relationship, was employed to predict the low-cycle fatigue life of the materials from the calculated thermal plastic strain amplitude.The results of the simulation indicated that extended running of the engine for times up to an hour or so lead to high temperature creep, thus widening both the thermal stress and plastic strain hysteresis loops, and so reducing the fatigue life. However, this is a relatively small effect, which, at longer running times is expected to have a progressively reduced effect. The temperature fluctuation at the hot surface of the engine components due to the cyclic heating from the combustion gas was also foundto be negligible at about 5-6℃, the fluctuation depth being about 1 mm.The really important factor affecting the fatigue life was found to be the temperature of the hot face of the cylinder head; a reduction of this temperature for gray iron, from 500C to 316C, increases the thermal fatigue life by a factor of about 11times; a reduction of 30℃ for Al alloy A356 may double its fatigue times. The surface operating temperature is linearly related to the effective heat resistance of the component and is, therefore, greatly improved by reducing the wall thickness, but does not change greatly with differences in thermal conductivity, resulting in an aluminum alloy being negligibly better in this respect than gray cast irons or even some steels.
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