Exhaust gas

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Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline, petrol, biodiesel blends,[1] 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.


  • 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. A 2013 study by MIT indicates that 53,000 early deaths occur per year in the United States alone because of vehicle emissions.[2] According to another study from the same university, traffic fumes alone cause the death of 5,000 people every year just in the United Kingdom.[3]


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, and particulate matter (mostly soot).

Exhaust gas temperature[edit]


Cold engines[edit]

Steam from tailpipe of cold car


  • 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 more so 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,[4] including electric heating, thermal battery, chemical reaction preheating, flame heating and superinsulation.

Passenger car emissions summary[edit]

U.S. Environmental Protection Agency estimates of average passenger car emissions in the United States for April 2000[5]
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 415 grams/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


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 engines[edit]

Spark-ignition engines[edit]


Combustion-engine exhaust gases[7]
All figures are approximate
 % of total
Compound Petrol Diesel
nitrogen 71 67
carbon dioxide 14 12
water vapor 13 11
oxygen 10
Trace elements[citation needed] < 0.6 ~ 0.3
nitrogen oxides < 0.25 < 0.15
carbon monoxide 1 - 2 < 0.045
particulate matter < 0.045
hydrocarbons < 0.25 < 0.03
sulfur dioxide possible traces < 0.03


Nitromethane additive[edit]


Diesel engines[edit]

See Diesel exhaust; Soot.

Gas-turbine engines[edit]


Jet engines and rocket engines[edit]



Other types[edit]

From burning coal[edit]

Steam engines[edit]


Main motor vehicle emissions[edit]




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

Volatile organic compounds[edit]

Non-road equipment is mostly gasoline and diesel stations.[12]



Ozone is beneficial in the upper atmosphere,[14] but at ground level ozone irritates the respiratory system, causing coughing, choking, and reduced lung capacity.[15] It also has many bad effects throughout the ecosystem.[16]

Carbon monoxide (CO)[edit]

MOPITT satellite computer image of carbon monoxide March 2010

Carbon monoxide poisoning is the most common type of fatal air poisoning in many countries.[17] 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 2011, 52% of carbon monoxide emissions were created by mobile vehicles in the U.S.[18]

Hazardous air pollutants (toxics)[edit]


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.[21] A 2011 UK study estimates 90 deaths per year due to passenger vehicle PM.[22] 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).[23]

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 according to the vast majority of the scientific community is causing climate change.[24] Motor vehicles are calculated to generate about 20% of the European Union's man-made CO2 emissions, with passenger cars contributing about 12%.[25] 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.[26]

Water vapour[edit]

Vehicle exhaust contains much water vapour.

Water recovery[edit]

There has been research into ways that troops in deserts can recover drinkable water from their vehicles' exhaust gases.[27]

Pollution reduction[edit]


One of the advantages claimed for advanced steam technology engines is 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.

Health studies[edit]

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.[30] A World Health Organization study found that diesel fumes cause an increase in lung cancer.[31]

Localised effects[edit]


See also[edit]

Automobile exhaust


  2. ^ Caiazzo, Fabio; Ashok, Akshay; Waitz, Ian A.; Yim, Steve H.L.; Barrett, Steven R.H. (November 2013). "Air pollution and early deaths in the United States. Part I: Quantifying the impact of major sectors in 2005". Atmospheric Environment. Elsevier. 79: 198–208. Bibcode:2013AtmEn..79..198C. doi:10.1016/j.atmosenv.2013.05.081. Retrieved 25 October 2013. 
  3. ^ Roland Pease. "Traffic pollution kills 5,000 a year in UK, says study". BBC News. 
  5. ^ "Average Annual Emissions and Fuel Consumption for Passenger Cars and Light Trucks" (PDF). Transportation and Air Quality. United States Environmental Protection Agency. 
  8. ^ turbofast.com
  14. ^ http://www.epa.gov/ozone/basicinfo.html
  15. ^ http://www.epa.gov/airquality/ozonepollution/health.html
  16. ^ http://www.epa.gov/groundlevelozone/ecosystem.html
  17. ^ Omaye ST. (2002). "Metabolic modulation of carbon monoxide toxicity". Toxicology. 180 (2): 139–150. doi:10.1016/S0300-483X(02)00387-6. PMID 12324190. 
  20. ^ "Air Toxics from Motor Vehicles" (PDF). Transportation and Air Quality. United States Environmental Protection Agency. 
  22. ^ Mazzi, Eric A.; Dowlatabadi, Hadi (2007). "Air Quality Impacts of Climate Mitigation: UK Policy and Passenger Vehicle Choice". Environmental Science & Technology. 41 (2): 387. doi:10.1021/es060517w. 
  27. ^ Google search
  28. ^ EPA Plain English Guide to the Clean Air Act

External links[edit]