The most widely used fuels for automotive engines is the liquid fuel gasoline, Gasoline is a hydrocarbon (HC) made-up mostly of hydrogen (H) and carbon (C). Hydrocarbon is known as a fossil fuels also.
Burning gasoline and if prefect combustion occurred, all the gasoline in the combustion chamber would burn completely, however; complete or perfect combustion does not occur. Some gasoline vapor does not burn it leave the cylinder as part of the exhaust gas along with the carbonmono oxide (CO) and nitrogen oxides (Nox). here is the equation that shows what happens during combustion in an engine cylinder;
HC+N+O2 = CO2 + H2O + CO +NOx + HC
Combustion begins with the hydrocarbon fuel burning in the presence of nitrogen (N) and oxygen (O2). This heats the trapped air, causing it to expend and do work on the piston. Notice that HC appears on both sides of the equation, this means that some of the gasoline does not burn, it leaves the cylinder as gasoline vapor in the exhaust gases.
The right side of the equation also shows the other compounds that forms during combustion. In addition to HC the exhaust gas always contains some carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxides (NOx) and water vapor (H2O).
Carbon monoxide result from incomplete combustion, it forms when one atom of the carbon unites with one atom of oxygen instead of two as in complete combustion, this lack of oxygen prevent the formation of carbon dioxide. Carbon monoxide is a poisonous gas that can cause death, enough CO can accumulate from a running engine in a closed one-car garage in only three minutes to kill any one in the garage.
Smog is a form of “dirty air” that result from combustion, one cause of smog is the reaction that occurs between HC and nitrogen oxides in the presence of sunlight. Preventing the formation of nitrogen oxides in the engine helps preventing smog.
The nitrogen oxides are created by high combustion temperatures when the temperature is above 25000 F the heat causes some of the nitrogen and oxygen in the air to unite and form nitrogen oxides.
Water vapor and carbon dioxide are harmless gases, however carbonmono oxide (CO) hydrocarbon (HC) and nitrogen oxides are air pollutants. They pollute the air we breath, breathing pollutant air is bad for humans and animals. Exposure to polluted air is also bad for plant and trees.
Most but not all, automotive emission are the result of combustion, there are four source of air pollutants from the vehicle, these are the engine crankcase, air cleaner or carburetor, fuel tank and tail pipe the amount of the pollutants from each source is controlled by the vehicle emission controls.
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