Glossary: Exhaust manifold
Combustion of fuel produces nitrogen, carbon monoxide, carbon dioxide, water vapor, particulate matter and other hydrocarbons. These by-products have to be collected and let off some place else.
An exhaust manifold does exactly that by collecting the exhaust gases from the cylinders and passing it on to the one common pipe called the exhaust pipe.
The manifold should also be as light and compact as possible. Materials used for exhaust systems need to be corrosion resistant and have to withstand the heat and moisture produced by the engine. Thus the exhaust manifold is usually made of cast iron or tubular steel which can combat both pretty well. In-line cylinder engines use one exhaust manifold for all the cylinders. V type cylinder engines use of two exhaust manifolds, one for each bank of cylinders.
It is a common practice to coat the exhaust manifold with a special material. The coating material sometimes appears to be red in colour. There are many reasons for coating the manifold. The first reason being protection from corrosion as mentioned previously. The coating also acts as a thermal barrier, thus keeping heat generated by the exhaust gases within the manifold. There are a number of advantages by doing this. First, by keeping heat within the manifold, the exhaust gas velocity is going to be more(forcing it out quicker) which reduces the back pressure on the engine. Next, the surface temperature of the manifold is reduced. This means that the heat radiated by the manifold under the hood or into the engine compartment is lesser. This reduces the underhood temperature which, again, reduces the temperature of near by parts, such as the alternator and the starter motor. It also reduces the temperature of the air that is drawn through to the engine as the intake duct is placed under the hood and lower air temperature means better engine performance. There are a variety of coatings that can be used to coat the exhaust manifold.
Oxygen sensors and heat riser valves are sometimes mounted on the exhaust manifold. Poorly designed exhaust manifolds may affect engine performance as they can prove to be a major source of back pressure. The exhaust gases from one cylinder build up pressure in the manifold which affects the next cylinder that uses the manifold. Engine power is considerably wasted in the whole process.
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