There are millions of cars on the road in China, and each one is a source of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.Â
To solve those problems, cities, states and the federal government create clean-air laws that restrict the amount of pollution that cars can produce. Over the years, automakers have made many refinements to car engines and fuel systems to keep up with these laws. One of these changes came about in 1975 with an interesting device called a catalytic converter. Catalytic converters typically consist of a ceramic or metal honeycombed monolith substrate that carries precious metal catalysts. The job of the catalytic converter is to convert harmful pollutants into less harmful emissions before they ever leave the car’s exhaust system. The coated substrate is wrapped in an intumescent mat that expands when heated, securing and insulating the substrate which is packaged in a stainless steel shell and fitted into the engine exhaust system.
As exhaust gases pass over the catalysts, they promote chemical reactions that convert pollutants into harmless gases and water. Hydrocarbons combine with oxygen to become carbon dioxide; oxides of nitrogen react with carbon monoxide to produce nitrogen and carbon dioxide; and with hydrogen to produce nitrogen and water vapour.
Catalytic converters are amazingly simple devices, so it is incredible to see how big an impact they have. In this article, you will learn which pollutants are produced by an engine and how a catalytic converter deals with each of these pollutants to help reduce vehicle emissionsHow the Catalytic Converter works
There are millions of cars on the road that are potential sources of air pollution. In a major effort to reduce vehicle emissions, carmakers have developed an interesting device called a catalytic converter, which treats the exhaust before it leaves the car and removes a lot of the pollution.
Pollutants Produced the Engine
In order to reduce emissions, modern cars have been designed to carefully control the amount of fuel they burn. The goal is to keep the air-to-fuel ratio very close to the “stoichiometric” point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. [For petrol, the stoichiometric ratio is about 14.7 to 1, meaning that for each pound of fuel, 14.7 pounds of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be "lean" (an air-to-fuel ratio higher than 14.7); and other times the mixture can be "rich" (an air-to-fuel ratio lower than 14.7).]
The main emissions of a car engine are:
Nitrogen gas: Air is 78 percent nitrogen gas, and most of this passes right through the car engine.
Carbon Dioxide: This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.
Water vapor: This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.
These emissions are mostly benign (although carbon dioxide emissions are believed to contribute to global warming), however because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines:
Carbon monoxide: A poisonous gas that is colorless and odorless.
Hydrocarbons or volatile organic compounds (VOC’s): Produced mostly from unburned fuel that evaporates. Sunlight breaks these down to form oxidants, which react with oxides of nitrogen to cause ground level ozone, a major component of pollution.
Oxides of nitrogen: Contributes to smog and acid rain, and also causes irritation to human mucus membranes.
These are the three main regulated emissions, and also the ones that catalytic converters are designed to reduce.
How Catalytic Converters Reduce Pollution
Most modern cars are equipped with three-way catalytic converters. “Three-way” refers to the three regulated emissions it helps to reduce – carbon monoxide, unburnt hydrocarbons and nitrogen oxide molecules. The converter uses two different types of catalysts, a reduction catalyst and an oxidization catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of the catalyst to the exhaust stream, while also minimising the amount of catalyst required (they are very expensive).
Most modern cars are equipped with three-way catalytic converters. This refers to the three regulated emissions it helps to reduce :Â Â Â
The Reduction Catalyst
The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help reduce the nitrogen oxide emissions. When such molecules come in contact with 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.
Â Â Â Â Â Â Â Â Â Â Â 2NO => N2 + O2 or 2NO2 => N2 + 2O2Â
The Oxidization Catalyst
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.
Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 2CO + O2 => 2CO2Â
Â The Control System
The third stage is a control system that monitors the exhaust stream, and uses this information to control the fuel injection system. There is a heated oxygen sensor (also called a Lambda Sensor) mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter. This sensor tells the EEC-V PCM how much oxygen is in the exhaust. The EEC-V can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the EEC-V to ensure that the engine is running at close to the stoichiometric point, while also making sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and carbon monoxide.