The exhaust manifold is connected to the engine cylinder block, collecting the exhaust gases from each cylinder and directing them into the exhaust manifold with divergent pipelines.
The exhaust manifold is connected to the engine cylinder block, collecting the exhaust gases from each cylinder and directing them into the exhaust manifold, with divergent pipelines. The main requirements for it are to minimize exhaust resistance and avoid mutual interference between cylinders. When the exhaust is excessively concentrated, there will be mutual interference between the cylinders, that is, when one cylinder exhausts, it happens to come into contact with the uncleaned exhaust gas from other cylinders. This will increase the resistance of the exhaust, thereby reducing the output power of the engine. The solution is to separate the exhaust gases of each cylinder as much as possible, with one branch for each cylinder or one branch for two cylinders, and to make each branch as long and independent as possible to reduce the mutual influence of gases in different pipes.
Brief introduction
The exhaust manifold is a divergent pipeline connected to the engine cylinder block, which concentrates the exhaust gases from each cylinder and directs them into the exhaust manifold. The main requirements for it are to minimize exhaust resistance and avoid mutual interference between cylinders. When the exhaust is excessively concentrated, there will be mutual interference between the cylinders, that is, when one cylinder exhausts, it happens to come into contact with the uncleaned exhaust gas from other cylinders. This will increase the resistance of the exhaust, thereby reducing the output power of the engine. The solution is to separate the exhaust gases of each cylinder as much as possible, with one branch for each cylinder or one branch for two cylinders, and to make each branch as long and shaped as possible to reduce the mutual influence of gases in different pipes. In order to reduce exhaust resistance, some racing cars use stainless steel pipes to manufacture exhaust manifolds.
The exhaust manifold should take into account engine power performance, engine fuel economy, emission standards, engine cost, matching vehicle front cabin layout, and temperature field.
The commonly used exhaust manifolds are divided into two types based on materials and processing technology: cast iron manifolds and stainless steel manifolds
Characteristics and requirements of exhaust manifold materials
Early car engines had low power per unit weight, low fuel combustion efficiency, and exhaust gas temperatures not exceeding 500 ℃. With the improvement of automobile engine efficiency, the exhaust temperature has increased to 600-650 ℃. In recent years, developed countries have continuously improved their automobile exhaust emission standards, and the application of catalytic technology and worm gear turbocharging technology has significantly increased the working temperature of exhaust manifolds, reaching over 750 ℃. As engine performance continues to improve, the operating temperature of the exhaust manifold will also increase. At the same time, with the advancement of engine technology, the structure of exhaust manifolds has become more complex. In addition, working under cyclic alternating temperature conditions requires exhaust manifold materials to not only have good high-temperature performance, but also good casting performance. Therefore, the exhaust manifold material must have the following characteristics.
Good high-temperature antioxidant performance
The exhaust manifold operates in a high-temperature cyclic alternating state for a long time, and the anti-oxidation performance of materials at high temperatures directly affects the service life of the exhaust manifold. Ordinary cast iron obviously cannot meet the requirements, and alloy elements need to be added to the material to improve its high-temperature oxidation resistance.
Stable microstructure
Within the range of room temperature to operating temperature, the material should minimize or avoid phase transitions as much as possible. Because phase transition can cause changes in volume, resulting in internal stress or deformation, which affects the performance and lifespan of the product. Therefore, it is best for the matrix material to have a stable ferrite or austenite structure. The failure mode of cast iron parts working under high temperature conditions is mainly manifested as corrosion under high temperature conditions. After the oxidation of the constituent phases in the structure (such as graphite carbon), the volume of the oxide is greater than the original volume, causing irreversible expansion of the casting.
Compared with the three graphite forms of flake, worm, and ball, the cast iron with spherical graphite has the best high temperature resistance. The reason is that during the solidification process of cast iron, flake graphite grows as the leading phase. At the end of eutectic solidification, the graphite in each eutectic cluster forms a continuous branched three-dimensional state. At high temperatures, when oxygen invades the interior of the metal, the graphite forms a microchannel after oxidation, accelerating the oxidation process. When spherical graphite nucleates, it grows to a certain size and is surrounded by the matrix as an isolated ball. After the graphite ball is oxidized, no channel is formed, which weakens the further oxidation process. Therefore, the high-temperature oxidation resistance of ductile iron is better than other forms of graphite, and the influence of oxidized pores on the high-temperature strength of cast iron is smaller than that of other forms of graphite. Creep graphite is in between the two.
Small coefficient of thermal expansion
A small coefficient of thermal expansion is beneficial for reducing the thermal stress and deformation of the exhaust manifold, and improving the performance and service life of the product.
Excellent high-temperature strength
Must meet the necessary strength requirements for the product when used at high temperatures.
Good process performance and low cost
There are many types of heat-resistant and high-temperature resistant metal materials, but due to the complex shape of exhaust manifolds, the materials used to manufacture exhaust manifolds must have good processability, and their cost must meet the requirements of mass production in the automotive industry.