Cylinder head design is a highly challenging task for modern engines, especially for the proliferation of boosted, gasoline direct injection engines (branded EcoBoost® engines by Ford Motor Company). The high power density of these engines results in higher cylinder firing pressures and higher operating temperatures throughout the engine. In addition to the high operating stresses, cylinder heads are normally heat treated to optimize their mechanical properties; residual stresses are generated during heat treatment, which can be detrimental for high-cycle fatigue performance.
In this paper, a complete cylinder head high cycle fatigue CAE analysis procedure is demonstrated. First, the heat treatment process is simulated. The transient temperature histories during the quenching process are used to calculate the distribution of the residual stresses, followed by machining simulation, which results in a redistribution of stress. After the heat treatment, the cylinder head is assembled to the engine and subjected to the engine operation loads. The engine assembly and operation stresses are employed for the high cycle fatigue calculation. In addition to the fatigue safety factors, the finite life of the aluminum material is considered, and the high cycle fatigue damage is also calculated.
Initial calculations are performed in the nominal condition of the part, but due to manufacturing process variations, high cycle fatigue properties vary from part to part. A method to calculate the effects of high cycle fatigue properties variation and the prediction of the range of high cycle fatigue life/damage is also investigated.
The developed analysis method has been successfully used for cylinder head design to improve high cycle fatigue performance.