Environmental Protection Agency EPA has stated automobile emissions are a significant and growing cause of global warming, because of their release of nitrous oxide N2O , a greenhouse gas over three hundred times more potent than carbon dioxide.
The EPA states that motor vehicles contribute approximately 8. Nitrous oxide makes up 7. An engine equipped with a three-way catalyst must run at the stoichiometric point, which means more fuel is consumed than in a lean-burn engine.
This, in turn, means relatively more CO2 emissions from the vehicle. Nevertheless, catalyst-equipped engines produce cleaner exhaust than lean-burn engines. What is an autocatalyst?
By how much do they reduce pollution? Inside this converter there are two substrates. The front, coated with the precious metal rhodium, is used to reduce NOx emissions into simple N2 and O2. This process is most effective when little O2 is present rich mixture. That is why it is located upstream of the air tube. Since a rich mixture is high in HC and CO, an air pump and tube supply additional O2 to this mixture before it enters the second substrate.
This system was not very efficient and was phased out in the early '80s, when the current three-way converter was introduced. Three-way converters have been used in vehicle emissions control systems in North America - and many other countries - since The three-way converter without air uses advanced catalyst chemistry to store and release O2, in conjunction with an O2 monitoring and control system.
This system utilizes one or more O2 sensors to oscillate the fuel mixture between lean and rich conditions.
Catalysts participate in the reactions, but are neither reactants nor products of the reaction they catalyze. In the human body, enzymes are naturally occurring catalysts responsible for many essential biochemical reactions [source: Chemicool ]. In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst.
The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as the materials are extremely expensive.
Some of the newest converters have even started to use gold mixed with the more traditional catalysts. Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce. It uses platinum and rhodium to help reduce the NOx emissions.
When an NO or NO2 molecule contacts the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:. The oxidation catalyst is the second stage of the catalytic converter. It reduces the unburned hydrocarbons and carbon monoxide by burning oxidizing them over a platinum and palladium catalyst.
This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. There are two main types of structures used in catalytic converters -- honeycomb and ceramic beads.
Most cars today use a honeycomb structure. In the next section, we'll look at the third stage of the conversion process and how you can get the most from your catalytic converter. The third stage of conversion is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system.
There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the 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 running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.
The catalytic converter does a great job at reducing the pollution, but it can still be improved substantially. One of its biggest shortcomings is that it only works at a fairly high temperature. When you start your car cold, the catalytic converter does almost nothing to reduce the pollution in your exhaust.
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