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Welcome to Brain Stuff from howstuffworks.com, where smart happens.

Marshall Brain

Hi, I’m Marshall Brain with today’s question, why doesn’t gasoline burn cleanly? Why do we end up with things like smog and ozone that pollute the skies? Gasoline is a liquid formed almost purely of carbon and hydrogen. If you could somehow burn gasoline as a vapor with a hot flame and plenty of oxygen, you would get nearly pure carbon dioxide and water as the combustion products. That’s why you can burn natural gas, LP gas, and kerosene indoors in the winter. An appropriately designed kerosene heater, for example, can be ventless.

Automobile exhaust, unfortunately, contains a lot more of the carbon dioxide and water. The most important pollutants in car exhaust include first carbon monoxide, which is a poison. Carbon monoxide is formed because combustion is incomplete in the cylinder. Not enough oxygen is available fast enough to react completely with all of the carbon available. Second is nitrogen oxides. Because of the pressure and temperature inside the cylinder, nitrogen and oxygen in the air combine in various ways we would rather it didn’t.

And third, is unburned hydrocarbons. Not all of the hydrocarbons participate in the reaction because there’s so little time available during the combustion phase. There can also be some impurities, like sulfur, in the gas to form sulfur oxides. The idea behind a catalytic converter is to try to eliminate the carbon monoxide, nitrogen oxides, and hydrocarbons by reacting them with plenty of oxygen on a platinum catalyst. However, catalytic converters aren’t perfect, either, so some of the pollutants still escape. On hot summer days, we see the effect of these pollutants in the form of smog and ozone.

Nitrogen oxides and hydrocarbons mix with air and get bombarded by ultraviolet rays and sunlight. Nitrogen dioxide releases an oxygen atom, which combines with oxygen gas to form O3 or Ozone at ground level.

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