An exhaust valve is a component of a gasoline engine that helps to control the flow of air and fuel into the engine. This is important because it helps to prevent the engine from becoming overloaded and from blowing smoke or flames out of the exhaust pipe.
An exhaust valve is a small (about 3mm) circular disk that is attached to the cylinder head. It is located next to a huge pipe called the exhaust manifold. The exhaust manifold is fairly large and has several smaller pipes coming out of it in order to increase the pressure inside the engine and allow it to efficiently move hot air out of the engine.
The entire purpose of exhaust system components is to help control how much air and fuel enter an engine by regulating how much hot air leaves each cylinder (this means that more air will enter each cylinder if there are more open exhaust valves than closed ones) to help control what comes out of your car's exhaust pipe.
There are three types of valves: air control valve, exhaust valve, and intake valve. As you can see, each valve has a job to do. The air control valve controls the amount of air going into each cylinder by opening and closing the exhaust valves when needed.
The discharging pressure in an engine is fairly high because the combustion takes place at extremely high temperatures (over 1000 degrees. In order for that pressure to be safely allowed to escape from the cylinder, it must be controlled. The valves on the exhaust side of the engine open or close as needed to allow more or less hot air to come out of each cylinder and increase or decrease how much fuel is burned per cycle. Their only purpose is to help balance heat inside and outside the engine.