1. Scope--A subcommittee within SAE ISTC Division 35 has written this report to provide automotive engineers and designers a basic understanding of the design considerations and high temperature material availability for exhaust manifold use. It is hoped that it will constitute a concise reference of the important characteristics of selected cast and wrought ferrous materials available for this application, as well as methods employed for manufacturing. The different types of manifolds used in current engine designs are discussed, along with their range of applicability. Finally, a general description of mechanical, chemical, and thermophysical properties of commonly-used alloys is provided, along with discussions on the importance of such properties.

1.1 Background--Figure 1 provides a diagram of a typical fabricated exhaust manifold, in this case for one side of an eight-cylinder engine. Cast versions are similar in geometry. In simple terms, it provides a means of containing exhaust gases generated from each cylinder within the engine block, combining the volume, and passing the gas on to the catalytic converter.

Operating demands on exhaust manifolds, as with many other elevated temperature engine components, have increased significantly over the past decade. There are numerous reasons why this has occurred, including the usually-cited reasons of tighter emissions requirements, improved fuel efficiencies, and design toward higher specific engine power (kW/kg), with a cumulative end-effect yielding higher exhaust gas temperatures. Techniques used to meet emissions requirements, such as the addition of air injection systems and the use of controlled variations in air-fuel ratios, have changed overall hydrocarbon levels, and, under certain conditions, have increased the emissivity of the exhaust gas, further raising the manifold inner wall temperature. This has led to much higher elevated temperature strength, creep, and fatigue demands on exhaust manifold alloys. Radioactive heat shields that are now used to protect underhood electronics from high temperatures further exacerbate the issue by reflecting otherwise lost heat back on to the manifold.

Such thermal demands lead to reduced alloy strength simply from the higher temperatures, but perhaps more importantly higher internal stresses can also develop from the higher thermal gradients via thermal expansion mismatch considerations in the cylinder head - manifold interface. The cumulative effect then becomes higher temperatures in combination with higher cyclic stresses. Thermal fatigue, a condition in which time-dependent stress variations occur directly as a result of thermal expansion mismatch and mechanical constraint, becomes an important issue. Distortion, gas blow-by, and cracking of metal components result. To avoid such problems, designers have had to examine stronger alloys and employ alternate mechanical designs.

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