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.

It is a major component of motor vehicle emissions (and from stationary internal combustion engines), which can also include:

  • Crankcase blow-by
  • Evaporation of unused gasoline

Motor vehicle emissions contribute to air pollution and are a major ingredient in the creation of smog in some large cities.[1][2][3][4] A 2013 study by MIT indicates that 53,000 early deaths occur per year in the United States alone because of vehicle emissions.[5] According to another study from the same university, traffic fumes alone cause the death of 5000 people every year just in the United Kingdom.[6]


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 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 temperatureEdit

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).

Cold enginesEdit


During the first two minutes after starting the engine of a car that has not been operated for several hours, the amount of emissions can be very high. This occurs for two main reasons:

  • Rich air-fuel ratio requirement in cold engines: When a cold engine is started, the fuel does not vaporise completely, creating higher emissions of hydrocarbons, nitrogen oxides and carbon monoxide, which diminishes only as the engine reaches operating temperature. The duration of this start-up phase has been reduced by advances in materials and technology, including computer-controlled fuel injection, shorter intake lengths, and pre-heating of fuel and/or inducted air.
  • Inefficient catalytic converter under cold conditions: Catalytic converters are very inefficient until up to their operating temperature. This time has been much reduced by moving the converter closer to the exhaust manifold and even moreso placing a small yet quick-to-heat-up converter directly at the exhaust manifold. The small converter handles the start-up emissions, which allows enough time for the larger main converter to heat up. Further improvements can be realised in many ways,[7] including electric heating, thermal battery, chemical reaction preheating, flame heating and superinsulation.

Passenger car emissions summaryEdit

U.S. Environmental Protection Agency estimates of average passenger car emissions in the United States for July 2000[8]
Component Emission Rate Annual pollution emitted
Hydrocarbons 2.80 grams/mile (1.75 g/Km) 77.1 pounds (35.0 kg)
Carbon monoxide 20.9 grams/mile (13.06 g/Km) 575 pounds (261 kg)
NOx 1.39 grams/mile (0.87 g/Km) 38.2 pounds (17.3 kg)
Carbon dioxide - greenhouse gas 0.916 pounds per mile (258 g/km) 11,450 pounds (5,190 kg)

Comparable with the European Emission Standards EURO III as it was applied on October 2000

In 2000, the United States Environmental Protection Agency began to implement more stringent emissions standards for light duty vehicles. The requirements were phased in beginning with 2004 vehicles and all new cars and light trucks were required to meet the updated standards by the end of 2007.

United States Light-Duty Vehicle, Light-Duty Truck, and Medium-Duty Passenger Vehicle—Tier 2 Exhaust Emission Standards (for Bin 5)[9]
Component Emission Rate Annual pollution emitted
NMOG (Volatile organic compounds) 0.075  grams/mile (0.046 g/Km) 2.1 pounds (0.95 kg)
Carbon Monoxide 3.4 grams/mile (2.1 g/Km) 94 pounds (43 kg)
NOX 0.05 grams/mile (0.0305 g/Km) 1.4 pounds (0.64 kg)
Formaldehyde 0.015 grams/mile (0.0092 g/Km) 0.41 pounds (0.19 kg)


Internal-combustion enginesEdit

Spark-ignition enginesEdit

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:

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.

Combustion-engine exhaust gases[10]
All figures are approximate
 % of total
Compound Petrol Diesel
nitrogen 71 67
carbon dioxide 14 12
water vapor 12 11
oxygen 10
Trace elementsScript error < 0.5 ~ 0.3
nitrogen oxides < 0.25 < 0.15
carbon monoxide 1 - 2 < 0.045
particulate matter < 0.045
hydrocarbons < 0.25 < 0.03
sulphur dioxide possible traces < 0.03
Nitromethane additiveEdit

Exhaust gas from an internal combustion engine whose fuel includes nitromethane will contain nitric acid vapour, which is corrosive, and when inhaled causes a muscular reaction making it impossible to breathe. People exposed to it should wear a gas mask.[1]

Diesel enginesEdit

See Diesel exhaust; Soot.

Gas-turbine enginesEdit

  • 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 enginesEdit

File:Jet exhaust by David Shankbone.jpg

In jet engines and rocket engines, exhaust from propelling nozzles which in some applications shows shock diamonds.Script error

Other typesEdit

From burning coalEdit

Steam enginesEdit

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

Main motor vehicle emissionsEdit



Mono-nitrogen oxides NO and NO2 (whether produced this way or naturally by lightning) react with NH3, moisture, and other compounds to form nitric acid vapor and related particles. Small particles can penetrate deeply into sensitive lung tissue and damage it, causing premature death in extreme cases. Inhalation of such particles may cause or worsen respiratory diseases such as emphysema and bronchitis. It may also aggravate existing heart disease.[1][2][3] In a 2005 U.S. EPA study the largest emissions of NOx came from on road motor vehicles, with the second largest contributor being non-road equipment which is mostly gasoline and diesel stations.[3]

The resulting nitric acid may be washed into soil, where it becomes nitrate, which is useful to growing plants.

Volatile organic compoundsEdit

File:Sources of Volatile Organic Compounds.PNG

When oxides of nitrogen (NOx) and volatile organic compounds (VOCs) react in the presence of sunlight, ground level ozone is formed, a primary ingredient in smog. A 2005 U.S. EPA report gives road vehicles as the second largest source of VOCs in the U.S. at 26% and 19% are from non road equipment which is mostly gasoline and diesel stations.[4] 27% of VOC emissions are from solvents which are used in the manufacturer of paints and paint thinners and other uses.[5]


Ozone is beneficial in the upper atmosphere,[6] but at ground level, ozone irritates the respiratory system, causing coughing, choking, and reduced lung capacity.[7] It also has many bad effects throughout the ecosystem.[8]

Carbon monoxide (CO)Edit

File:MOPITT ratio at surface.png

Carbon monoxide poisoning is the most common type of fatal air poisoning in many countries.[9] Carbon monoxide is colorless, odorless and tasteless, but highly toxic. It combines with hemoglobin to produce carboxyhemoglobin, which is ineffective for delivering oxygen to bodily tissues. In the U.S. 60% of carbon monoxide is caused by on road vehicles.[10]

Hazardous air pollutants (toxics)Edit

Chronic (long-term) exposure to benzene (C6H6) damages bone marrow. It can also cause excessive bleeding and depress the immune system, increasing the chance of infection. Benzene causes leukemia and is associated with other blood cancers and pre-cancers of the blood.[11][12]

Particulate matter (PM10 and PM2.5)Edit

The health effects of inhaling airborne particulate matter have been widely studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. Because of the size of the particles, they can penetrate the deepest part of the lungs.[13] A 2011 UK study estimates 90 deaths per year due to passenger vehicle PM.[14] In a 2006 publication, the U.S. Federal Highway Administration (FHWA) state that in 2002 about 1 per-cent of all PM10 and 2 per-cent of all PM2.5 emissions came from the exhaust of on-road motor vehicles (mostly from diesel engines).[15]

Carbon dioxide (CO2)Edit

Carbon dioxide is a greenhouse gas. Motor vehicle CO2 emissions are part of the anthropogenic contribution to the growth of CO2 concentrations in the atmosphere which is causing climate change.[16] Motor vehicles are calculated to generate about 20% of the European Union's man-made CO2 emissions, with passenger cars contributing about 12%.[17] European emission standards limit the CO2 emissions of new passenger cars and light vehicles. The European Union average new car CO2 emissions figure dropped by 5.4% in the year to the first quarter of 2010, down to 145.6 g/km.[18]

Pollution reductionEdit

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.[19][20] 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.Script error They produce larger quantities of carbon dioxide but less carbon monoxide due to more efficient combustion.

Health studiesEdit

Researchers from the University of California, Los Angeles School of Public Health say preliminary results of their statistical study of children listed in the California Cancer Registry born between 1998 and 2007 found that traffic pollution may be associated with a 5% to 15% increase in the likelihood of some cancers.[1] A World Health Organization study found that diesel fumes cause an increase in lung cancer.[2]

Localised effectsEdit

The California Air Resources Board (C.A.R.B.) found in studies that 50% or more of the air pollution (smog) in Southern California is due to car emissions.Script error

See alsoEdit

File:Automobile exhaust gas.jpg


External linksEdit

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