Thermo-mechanical fatigue damage and failure of modern high performance diesel pistons

This study resulted from an engineering failure investigation related to diesel engine piston failures which occurred during a bench dynamometer engine durability test programme. The test programme aimed at evaluating the effects of various fuel types on the durability of fuel system components in passenger car diesel engines. A number of unexpected cylinder head, turbocharger and piston failures were experienced during the course of the test programme. This study focused on the cause of the piston failures experienced during these tests.Analyses of the fractured pistons revealed that thermo-mechanical fatigue initiation occurred as a result of primary silicon phase cracking and subsequent micro-crack formation due to excessive thermo-mechanical loading. Progressive formations of such micro-cracks lead to flaws that were of sufficient magnitude to initiate propagation by high cycle fatigue mechanisms.The investigation also revealed that the excessive thermo-mechanical piston loading was caused by over-fuelling and a combination of elevated and poorly controlled post intercooler air temperature. There was no evidence to suggest that the failures were related to the test fuel formulations.