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Exhausts & Catalytic Converters
The purpose of the exhaust system is to deaden the sound made by the internal combustion engine, reduce the temperature of the exhaust products and generally control the emissions that come out of your vehicle.
The noise reduction is achieved by a number of silencers placed along the pipe. These consist of a standard or stainless steel casing containing an array of baffles and other sound-insulating materials. Many systems also utilise smaller resonator units as well to achieve optimum quietness.
All modern vehicles also have at least one catalytic convertor in the exhaust system. This is a smaller box, not unlike a silencer in physical appearance. Most cars have three-way catalytic converters. "Three-way" refers to the three regulated emissions it helps to reduce. Carbon monoxide, VOCs and NOx molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium.
On fuel injected vehicles there is also an oxygen sensor mounted upstream of the catalytic converter. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is using fuel efficiently, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and carbon dioxide. If the oxygen sensor is blocked then incorrect readings will be sent and the car will burn excess petrol as a result.
Recent legislation means that the amount of harmful emissions that your car produces have to fall within certain parameters for the car to successfully pass the MOT test.
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