Exhaust gas

Exhaust gas
A diesel-powered truck emits a large amount of exhaust gas while starting its engine.

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.

It often disperses downwind in a pattern called an exhaust plume.



The largest part of most combustion gas is nitrogen (N2), water vapor (H2O) (except with pure-carbon fuels), and carbon dioxide (CO2) (except for fuels without carbon); these are not toxic or noxious (although carbon dioxide is generally recognized as a greenhouse gas that contributes to global warming). A relatively small part of combustion gas is undesirable noxious or toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (properly indicated as CxHy, but typically shown simply as "HC" on emissions-test slips) from unburnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, Ozone (O3), and particulate matter (mostly soot).


Spark-ignition engines

In spark-ignition engines 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 gas mask.[1]

Diesel engines

See Diesel exhaust.

Gas-turbine engines

  • In aircraft gas turbine engines, "exhaust gas temperature" (EGT) is a primary measure of engine health. Typically the EGT is compared with a primary engine power indication called "engine pressure ratio" (EPR). For example: at full power EPR there will be a maximum permitted EGT limit. Once an engine reaches a stage in its life where it reaches this EGT limit, the engine will require specific maintenance in order to rectify the problem. The amount the EGT is below the EGT limit is called EGT margin. The EGT margin of an engine will be greatest when the engine is new, or has been overhauled. For most airlines, this information is also monitored remotely by the airline maintenance department by means of ACARS.

Jet engines and rocket engines

What looks like exhaust from jet engines, is actually contrail. (Jet flying over the United States Air Force Academy in Colorado Springs, Colorado).

In jet engines and rocket engines, exhaust from propelling nozzles which in some applications shows shock diamonds.[citation needed]

From burning coal

Steam engines

In steam engine terminology the exhaust is steam that is now so low in pressure that it can no longer 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.[2][3]

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.[citation needed] They produce larger quantities of carbon dioxide but less carbon monoxide due to more efficient combustion.

See also

Automobile exhaust


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

External links

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