The exhaust manifold is connected to the engine cylinder block. The exhaust of each cylinder is concentrated and introduced into the exhaust manifold, with divergent pipes. The requirements for it are mainly to reduce exhaust resistance as much as possible and avoid interference between the cylinders. When the exhaust gas is excessively concentrated, there will be mutual interference between the cylinders, that is, when one cylinder exhausts, it happens to encounter the exhaust gas from other cylinders that is not exhausted. In this way, the resistance of the exhaust gas is increased, and the output power of the engine is reduced. The solution is to make the exhaust of each cylinder as far as possible, one branch for each cylinder, or one branch for two cylinders, and make each branch as long as possible and independently shape to reduce the mutual influence of the gas in different tubes.
At present, the exhaust manifolds commonly used are divided into cast iron manifolds and stainless steel manifolds in terms of materials and processing technology.
The exhaust manifold material must not only have good high temperature performance, but also have good casting performance. Therefore, the exhaust manifold material must have the following characteristics. Good high temperature oxidation resistance The exhaust manifold works for a long time in the high temperature cycling alternating state. The oxidation resistance of the material at high temperature directly affects the service life of the exhaust manifold. Ordinary cast iron obviously cannot meet the requirements, and alloying elements need to be added to the material to improve the high temperature oxidation resistance of the material.
Stable microstructure In the range of room temperature to working temperature, the material should not undergo phase change as much as possible or minimize phase change. Because the phase change will cause the volume change, which will cause internal stress or deformation, affecting the performance and life of the product. Therefore, the base material is preferably a stable ferrite or austenite structure. The damage of cast iron parts working under high temperature conditions mainly manifests as corrosion under high temperature conditions. After the constituent phases in the structure are oxidized (such as graphite carbon), the volume of the oxide is larger than the original volume, causing irreversible expansion of the casting. Compared with flaky, worm-like, and spherical graphite morphologies, cast iron with spheroidal graphite has the best high temperature resistance. The reason is that during the solidification of cast iron, flaky graphite grows as the leading phase. The graphite in the eutectic group forms a continuous branched three-dimensional form. At high temperatures, when oxygen invades the metal, the graphite is oxidized to form a microscopic channel to accelerate the oxidation process. When spheroidal graphite is nucleated, it grows by a certain size and is surrounded by a matrix. It exists as an isolated sphere. After the graphite sphere is oxidized, it does not form a channel, which weakens the further progress of oxidation. Therefore, the high temperature oxidation resistance of ductile iron Better than other forms of graphite, and the oxidized holes have less effect on the high temperature strength of cast iron than other forms of graphite, and vermicular ink is in between.
Low thermal expansion coefficient A small thermal expansion coefficient is conducive to reducing the thermal stress and thermal deformation of the exhaust manifold, and is conducive to improving the performance and service life of the product.
Excellent high temperature strength It must meet the necessary strength requirements when the product is used at high temperatures.
Good process performance and low cost There are many types of heat-resistant and high-temperature-resistant metal materials, but because of the complex shape of the exhaust manifold, the materials used to manufacture the exhaust manifold must have good processability, and its cost must meet the needs of mass production in the automotive industry.