How Your Vehicle Works

Catalytic Converters

There are millions of cars on the road in the United States, and each one is potentially a source of air pollution. Especially in large cities, the amount of pollution that all the cars produce together can create big problems.

To solve those problems, cities, states and the federal government create clean-air laws, and many laws have been enacted that restrict the amount of pollution that cars can produce. To keep up with these laws, automakers have created many refinements to car engines and fuel systems. To help reduce the emissions further, they have developed a device called a catalytic converter, which effects the exhaust before it leaves the car and removes a lot of the pollution.

In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try 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 gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each pound of gasoline, 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, and other times the mixture can be rich.

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). But 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 - 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 smog.

Nitrogen oxides - 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.

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