Failure Analysis of a Exhaust Valve in a Bus Engine

Both of the exhaust and intake valves are very important components of a diesel engine which are used to control the entrance and exhaust of gases in internal combustion engines. During opening and closing, these valves are subjected to high temperature, spring force and high pressure (up to 15 Mpa) inside the cylinder. Various factors, including mechanical fatigue at high temperatures, thermal fatigue, reduction their mechanical properties such as hardness and yield strength due to heating, wear caused by impacts, especially at the seat and the guide sections, corrosion, excessive heating and accumulation of soot on the valve can be caused to failure of the valve. However, the valves are mainly failured by fatigue. The temperature of internal portions of intake valve and exhust valve are 550 °C and 800-900 °C, respectively. Therefore, these parts are subjected to thermal loading and chemical corrosion at high temperatures. In this research, the failure causes of a bus exhaust valve were studied using chemical analysis, macroscopic and microscopic investigations by optical and scanning electron microscopes and hardness test. The results of chemical analysis and hardness was in accordance with the relevant standard, However, results of macroscopic and microscopic investigations Were indicative of formation of microstructural defects due to increased temperature. Therefore,it was found that the probable cause of failur is the improper performance of the engine which results in excessive rising of temperature and finally causes to failure of valve.