The Reliability Improvement of a Conventional Cast Iron Exhaust Manifold for a Small Size Gasoline Engine

This paper describes a challenging phase of the development of a cast iron exhaust manifold for a FIAT FIRE 16v engine. On these engine family, a close-coupled catalyst directly connected to the manifold is used. Conventional cast iron could be considered an obsolete material to realize modern car engine exhaust manifolds, but it’s still considerable as a profitable technology due to relatively low processing and part costs. Moreover, due to actual exhaust emission regulation, the engine calibration determinates higher temperature values and gradients on the exhaust system than in the past. Consequently thermal fatique life, weight and manifold wall thickness limits become some of the main concerns for the engine designer when conventional cast iron is considered. To improve the actual cast iron manifold, according to the engine program timeline, a simplified, fast approach to thermal fatigue life improvement has been used, involving only in-house resources to achieve a further cost-time reduction in the definition and implementation of the necessary design changes. By means of Infrared Thermography, the distribution of the external surface of the exhaust manifold was acquired, on the engine bench, in-firing conditions, during specific accelerated thermal fatigue test cycle. The 2D surface temperature distribution was properly elaborated to obtain a 3D temperature distribution and a thermostructural FE Analysis has been developed to predict the critical areas of the component, considering the variation of the relevant material properties with temperature. The evaluation of the regions where thermal crack initiation and propagation were expected has been done considering the stress distributions in the high-temperature phase and during the cool-down phase of the test cycle. The comparison with the cracked manifold returned from the road test has shown a good agreement with the first results obtained from the analysis. For the optimisation, the same temperature acquisition and analysis were performed considering the manifold, made of the same material, used for a similar application for which no failures occurred during tests, obtaining information about the safe combination of stress and temperature level for the specific spheroid cast iron used. Those data were applied to validate the improved design that passed the following endurance tests, with only a minor acceptable impact in terms of weight, costs and development time.