Exhaust gas

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A diesel-powered truck emits an exhaust gas rich in black particulate matter when 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).

Exhaust gas temperature[edit]

Exhaust gas temperature (EGT) is important to the functioning of the catalytic converter of an internal combustion engine. It may be measured by an exhaust gas temperature gauge. EGT is also a measure of engine health in gas-turbine engines (see below).


Spark-ignition engines[edit]

In spark-ignition engines the gases resulting from combustion of the fuel and air mix are called exhaust gases. The composition varies from petrol to diesel engines, but is around these levels:

Combustion-engine exhaust gases[1]
All figures are approximate
This table DOES NOT correctly reflect graph presented on page 6 of referenced document!
The 10% oxygen for "diesel" is likely if the engine was idling, e.g. in a test rig. It is much less if the engine is running under load.
 % of total
Compound Petrol Diesel
N2 71 67
CO2 14 12
H2O 12 11
O2 10
Trace elements[citation needed] < 0.5 ~ 0.3
NOx < 0.25 < 0.15
CO 1 - 2 < 0.045
PM < 0.045
CxHy < 0.25 < 0.03
SO2 possible traces < 0.03
  • N2 = Nitrogen
  • CO2 = Carbon dioxide
  • H2O = Water
  • O2 = Oxygen
  • CxHy (or Hx or HC) = Hydrocarbons
  • CO = Carbon monoxide
  • NOx = Nitrogen oxides
  • SO2 = Sulphur dioxide
  • PM = Particulate matter

Nitromethane additive[edit]

Exhaust gas from an internal combustion engine whose fuel includes nitromethane will contain nitric acid vapour, which when inhaled causes a muscular reaction making it impossible to breathe. People exposed to it should wear a gas mask.[2]

Diesel engines[edit]

See Diesel exhaust; Soot.

Gas-turbine engines[edit]

  • 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[edit]

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[edit]

Steam engines[edit]

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[edit]

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.[3][4] However, these are often referred to as flue gases. Catalytic converters in cars intend to break down the pollution of exhaust gases using a catalyst. Scrubbers in ships intend to remove the sulfur dioxide (SO2) of marine exhaust gases. The regulations on marine sulfur dioxide emissions are tightening, however only a small number of special areas worldwide have been designated for low sulfur diesel fuel use only.

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[edit]

Automobile exhaust


  1. ^ Self-Study Programme 230: Motor Vehicle Exhaust Emissions (PDF). AUDI. April 2000. Retrieved 23 March 2012. 
  2. ^ turbofast.com
  3. ^ EPA Plain English Guide to the Clean Air Act
  4. ^ US EPA Publication AP 42, Fifth Edition, Compilation of Air Pollutant Emission Factors

External links[edit]