The exhaust manifold collects the exhaust gas from the engine and sends it to the catalyst converter. Recently due to increasing the engine power, the exhaust manifold is exposed to a higher temperature environment and it experiences severer thermal load and more often fatigue failure. The great deals of effort are requested for the exhaust manifold to be safe from the thermal fatigue failure. In this study the characterizing equations for the fatigue behaviors are derived by simplifying the complex loading conditions and by neglecting the product quality uncertainties selectively.
From the characterizing equations, the effective thermal fatigue damage index $(N_{TFDI})$ is developed for designers to evaluate easily the fatigue performance of the exhaust manifold. $N_{TFDI}$ is composed of temperature profiles, material characteristics and geometric shapes of the exhaust manifold. Through a number of parametric studies for $N_{TFDI}$, the effect of each geometric element is examined in this study on the fatigue life: the ratio of the inter-inlet distance versus the length of exhaust pipe, temperature distribution and outer diameter of pipe have a large influence on $N_{TFDI}$ and the thickness of the pipe and flange has a little influence. Guidelines for the exhaust manifold design are established in this study.
