Exhaust gas or flue gas is emitted as a result of the combustion of
fuels such as natural gas, gasoline, petrol, biodiesel blends,
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:
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. 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.
Exhaust gas temperature
3 Cold engines
4 Passenger car emissions summary
5.1 Internal-combustion engines
5.1.1 Spark-ignition engines
5.1.2 Diesel engines
5.1.3 Gas-turbine engines
5.1.4 Jet engines and rocket engines
5.2 Other types
5.2.1 From burning coal
5.2.2 Steam engines
6 Main motor vehicle emissions
6.2 Volatile organic compounds
Carbon monoxide (CO)
6.5 Hazardous air pollutants (toxics)
Particulate matter (PM10 and PM2.5)
Carbon dioxide (CO2)
6.8 Water vapour
6.8.1 Water recovery
7 Pollution reduction
8 Health studies
9 Localised effects
10 See also
12 External links
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
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).
Steam from tailpipe of cold car
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 vaporize 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
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, including electric heating, thermal battery,
chemical reaction preheating, flame heating and superinsulation.
Passenger car emissions summary
U.S. Environmental Protection Agency estimates of average passenger
car emissions in the United States for April 2000
Annual pollution emitted
2.80 grams/mile (1.75 g/km)
77.1 pounds (35.0 kg)
20.9 grams/mile (13.06 g/km)
575 pounds (261 kg)
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
European emission standards EURO III as it was
applied on October 2000
In 2000, the
United States Environmental Protection Agency
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)
Annual pollution emitted
NMOG (Volatile organic compounds)
0.075 grams/mile (0.046 g/km)
2.1 pounds (0.95 kg)
3.4 grams/mile (2.1 g/km)
94 pounds (43 kg)
0.05 grams/mile (0.0305 g/km)
1.4 pounds (0.64 kg)
0.015 grams/mile (0.0092 g/km)
0.41 pounds (0.19 kg)
See also: Automobile emissions control
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
All figures are approximate
% of total
Trace elements
1 - 2
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
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.
Diesel exhaust and Soot
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
United States Air Force Academy in Colorado Springs,
In jet engines and rocket engines, exhaust from propelling nozzles
which in some applications shows shock diamonds.
From burning coal
Flue gas emissions from fossil fuel combustion
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 emissions
New York City
New York City as viewed from the World Trade Center in 1988
Mono-nitrogen oxides NO and NO2 (whether produced this way or
naturally by lightning) react with ammonia, 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 NO species
increases the risk of lung cancer and colorectal cancer. and
inhalation of such particles may cause or worsen respiratory diseases
such as emphysema and bronchitis and heart disease.
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.
The resulting nitric acid may be washed into soil, where it becomes
nitrate, which is useful to growing plants.
Volatile organic compounds
Non-road equipment is mostly gasoline and diesel stations.
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. 27% of VOC emissions are from solvents which are used in
the manufacturer of paints and paint thinners and other uses.
Ozone is beneficial in the upper atmosphere, but at ground level
ozone irritates the respiratory system, causing coughing, choking, and
reduced lung capacity. It also has many negative effects
throughout the ecosystem.
Carbon monoxide (CO)
MOPITT satellite computer image of carbon monoxide March 2010
Carbon monoxide poisoning is the most common type of fatal air
poisoning in many countries.
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.
Hazardous air pollutants (toxics)
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
Particulate matter (PM10 and PM2.5)
The health effects of inhaling airborne particulate matter have been
widely studied in humans and animals and include asthma, lung cancer,
cardiovascular issues, premature death. Because of the
size of the particles, they can penetrate the deepest part of the
lungs. A 2011 UK study estimates 90 deaths per year due to
passenger vehicle PM. 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).
Carbon dioxide (CO2)
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. Motor
vehicles are calculated to generate about 20% of the European Union's
man-made CO2 emissions, with passenger cars contributing about
European emission standards
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.
Vehicle exhaust contains much water vapour.
There has been research into ways that troops in deserts can recover
drinkable water from their vehicles' exhaust gases.
See also: Air suction valve
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. 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 they produce smaller quantities of toxic pollutants (e.g. oxides
of nitrogen) than petrol and diesel engines of the same
power. They produce larger quantities of carbon
dioxide but less carbon monoxide due to more efficient combustion.
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. A World Health
Organization study found that diesel fumes cause an increase in lung
California Air Resources Board
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.
Air pollution#Most polluted cities
Atmospheric dispersion modeling
Clean Air Act
European emission standards
Mobile source air pollution
Motor vehicle emissions and pregnancy
United States emission standards
Vehicle emissions control
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About diesel exhaust:
U.S. Department of Labor Occupational Safety & Health
Administration: Safety and Health Topics: Diesel Exhaust
Partial List of Chemicals Associated with Diesel Exhaust
Diesel Exhaust Particulates: Reasonably Anticipated to Be A Human
Scientific Study of Harmful Effects of Diesel Exhaust: Acute
Inflammatory Responses in the Airways and Peripheral Blood After
Short-Term Exposure to Diesel Exhaust in Healthy Human Volunteers
Diesel exhaust: what you need to know
Lead Replacement Petrol
Compressed natural gas
Racing fuel (Tetraethyllead)
Methyl tert-butyl ether
Automatic transmission fluid
Windshield washer fluid
Pay at the pump