Most vehicle fuels (gasoline, diesel, natural gas, ethanol, etc.) are mixtures of hydrocarbons, compounds that contain hydrogen and carbon atoms. In a “perfect” engine, oxygen in the air would convert all of the hydrogen in fuel to water and all of the carbon in the fuel to carbon dioxide (carbon mixed with oxygen). Nitrogen in the air would remain unaffected. In reality, the combustion process is not “perfect, ” and automotive engines emit several types of pollutants:a. “Perfect” Combustion Process: FUEL (hydrocarbons) + AIR (oxygen and nitrogen) = CARBON DIOXIDE (CO2) + Water (H2O) + Nitrogen b. Typical Real-World Engine Combustion Process: FUEL (hydrocarbons) + AIR (oxygen and nitrogen) = UNBURNED or PARTIALLY BURNED HYDROCARBONS (VOCs) + NITROGEN OXIDES (NOx) + CARBON MONOXIDE (CO) + CARBON DIOXIDE (CO2) + Water (H2O) Exhaust Pollutants
- HYDROCARBONS (HC): Hydrocarbon emissions result when fuel molecules in the engine do not burn or burn only partially. Hydrocarbons react in the presence of nitrogen oxides and sunlight to form ground-level ozone, a major component of smog. Ozone can irritate the eyes, damage lungs, and aggravate respiratory problems. It is our most widespread urban air pollution problem. Some kinds of exhaust hydrocarbons are also toxic, with the potential to cause cancer.
- NITROGEN OXIDES (NOx): Under the high pressure and high temperature conditions in an engine, nitrogen and oxygen atoms in the air we breathe react to form various nitrogen oxides, collectively known as NOx. Nitrogen oxides, like hydrocarbons, are precursors to the formation of ozone. They also contribute to the formation of acid rain.
- CARBON MONOXIDE (CO): Carbon monoxide is a product of incomplete combustion and occurs when carbon in the fuel is partially oxidized rather than fully oxidized to carbon dioxide. Carbon monoxide reduces the flow of oxygen in the bloodstream and is particularly dangerous to persons with heart disease.
- CARBON DIOXIDE (CO2): Carbon dioxide does not directly impair human health, but it is considered a “greenhouse gas”. In other words, as it accumulates in the atmosphere, it is believed to trap the earth’s heat and contribute to the potential for climate change.
- HYDROCARBONS: Hydrocarbons also escape into the air through fuel evaporation. With today’s efficient exhaust emission controls and today’s clean burning gasoline formulations, evaporative losses can account for a majority of the total hydrocarbon pollution from current model cars on hot days when ozone levels are highest. Evaporative emissions occur from fuel vapors escaping from the fuel system or while the vehicle is being refueled.