With a growing demand for high-power diesel engines, a key issue in engine development is to create efficient methods for developing highly durable cylinder heads, without having to repeat trial-and-error testing. Especially, it was difficult to accurately predict the occurrence and origin of cracks on the surfaces of cylinder heads in hot and cold cycle engine operation.
This paper describes a thermal fatigue evaluation method developed by analyzing areas around the glow plug hole where cracks often occur during hot and cold cycle engine operation. To reveal the conditions of edges from which cracks were formed under engine durability tests, we used two procedures. One was estimating local temperature of edge areas based on material hardness determination, in order to compensate for the accuracy of the thermal analysis. The other was analyzing the strain amplitudes on the cylinder head surface using computer simulation. This new approach was adopted to enable endurance reliability assessment of the cylinder head surface including edges, because it had been difficult to precisely simulate edge temperature (most of the preceding studies therefore focused on flat areas). In addition, a criterion for evaluating the thermal fatigue of cylinder head material was established, giving consideration to the actual-engine durability test mode. The validity of the criterion and its design method were verified by testing the durability of a prototype HSDI engine designed with this method.