Exhaust gas

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Exhaust gas is flue gas which occurs as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel, fuel oil or coal. It is discharged into the atmosphere through an exhaust pipe or flue gas stack.


Although the largest part of most combustion gases is relatively harmless nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except with hydrogen as fuel), a relatively small part of it is undesirable noxious or toxic substances, such as carbon monoxide (CO), hydrocarbons, nitrogen oxides (NOx), partly unburnt fuel, and particulate matter.

Exhaust gas from an industrial plant


Spark-ignition engines

Main article: Automobile emissions control

Exhaust gas from an internal combustion engine whose fuel includes nitromethane, contains nitric acid vapour, which when inhaled causes a muscular reaction making it impossible to breathe, and people exposed to it should wear a gasmask.[1]

Diesel engines

Diesel Particulate Matter is the main article about diesel exhaust.

Here, conditions in the engine are different from in a spark-ignition engine, because power is controlled by controlling the fuel supply directly, not by controlling the air supply. As a result, when the engine is running at low power, there is enough oxygen present to burn the fuel, and diesel engines only make significant amounts of carbon monoxide when running under load.


Gas-turbine engines and jet engines

From burning coal

Steam engines

In steam engine terminology the exhaust is steam that is now so low in pressure that it no longer can to do useful work.


Pollution reduction

Emission standards focus on reducing pollutants contained in the exhaust gases from vehicles as well as from industrial flue gas stacks and other air pollution exhaust sources in various large-scale industrial facilities such as petroleum refineries, natural gas processing plants, petrochemical plants and chemical production plants.[1][2]

One of the advantages claimed for advanced steam technology engines is that that they produce smaller quantities of toxic pollutants (e.g. oxides of nitrogen) than petrol and diesel engines of the same power. However, there is a downside – they produce larger quantities of carbon dioxide.

See also

Automobile exhaust

External links


  1. EPA Plain English Guide to the Clean Air Act
  2. US EPA Publication AP 42, Fifth Edition, Compilation of Air Pollutant Emission Factors

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