Explain why incomplete combustion happens in car engines.
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The term complete combustion generally is used in connection with the burning of hydrocarbons. Combustion is the process of burning that occurs when fuel, oxygen, and heat are simultaneously present. The result of complete combustion is the release of energy, carbon dioxide, and water vapor. If the hydrocarbon contains sulfur, sulfur dioxide also will be present. On the flip side, incomplete combustion results in some of the carbon atoms combining with only one oxygen atom to form carbon monoxide and other potentially harmful byproducts.
Usually, the combustion process is triggered by heating a hydrocarbon above its ignition temperature in an oxygen-rich environment. When the compound is heated, the chemical bonds of the hydrocarbon are split. The elements of the hydrocarbon then combine with the oxygen to form oxygen-containing compounds known as oxides. This rearrangement of hydrocarbon elements into oxides is accompanied by release of energy and heat.
Complete combustion occurs when the fuel and oxygen are in the perfect combination, or ratio, to completely burn the fuel. This condition also is referred to as stoichiometric or zero excess air combustion. On the other hand, incomplete combustion may leave some of the fuel unused.
Propane combustion serves as an example of a commonly burned hydrocarbon in household use. Usually, propane combustion will occur when the gas in the air mixture is between 2.2 percent and 9.6 percent. This range is referred to as propane’s limits of flammability. A properly functioning propane appliance producing an ideal burn will give off a blue flame and should present no danger of carbon monoxide poisoning, which is a deadly byproduct of incomplete propane combustion.
Incomplete combustion of propane occurs when the mix ratio is higher or lower than the ideal ratio, but still occurs within the limits of flammability. If the ratio of propane to air is less than the ideal ratio, a lean burn will occur, as evidenced by flames that appear to lift away from the burner or go out. A rich burn occurs when the ratio of propane to air is greater than the ideal ratio, and can be recognized by larger flames that are yellow rather than blue. Incomplete combustion of propane or other hydrocarbons typically will result in carbon monoxide release, an extremely serious environmental and health hazard for humans as well as for most animals.
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