Exhaust Emissions

Most vehicles are powered by an internal combustion engine, either a gasoline/ethanol engine or a diesel engine, while others may use natural gas or propane among other fuels. While all of these engines essentially produce the same types of emissions, this article will detail the emissions produced by gasoline/ethanol engines.

Burning Hydrocarbons

Primary Emissions

Gasoline, ethanol, diesel, natural gas and propane are all examples of hydrocarbon (HC) fuels. A hydrocarbon fuel is a compound consisting of hydrogen and carbon, hence its name. Our atmosphere consists of about 21% oxygen (O2), 78% nitrogen (N2), 1% water vapor (H2O), and very small amounts of other gases like carbon dioxide (CO2) and argon(Ar). Igniting the air/fuel mixture in the engine causes a chemical reaction between the hydrocarbon compound and the oxygen in the air. This reaction breaks the bond between the carbon and the hydrogen, and combines these separately with the oxygen. The primary compounds formed in this reaction are CO2 and H2O. You see the water vapor (H2O) produced by your engine as steam when you start your car on a cold morning. As the hot water vapor leaves the tailpipe, it condenses as fog onto the cold air molecules. Once your engine warms, the water vapor leaving the tailpipe takes too long to cool and condense, so it dissipates and cannot be seen.

Other Emissions

In an ideal environment, with ideal mixtures of oxygen and HC fuel, these would be the only emissions. Unfortunately, other harmful emissions are produced due to incomplete combustion as well as excessive combustion temperature causing the nitrogen in the air to react with the oxygen. Carbon Monoxide (CO) and unburned hydrocarbons in the exhaust result from incomplete combustion which may be caused by low engine compression, improper air/fuel mixture, or weak ignition spark among other causes. Oxides of nitrogen (NOx) are produced when combustion temperatures exceed about 2,500o F.

Harmful Effects

Carbon Dioxide (CO2)

While an engine would ideally produce CO2 and H2O, CO2 is still a harmful emission. Most people today have heard about carbon dioxide as a greenhouse gas, meaning that it traps energy from the sun within our atmosphere, and is largely attributed to global warming. The reason that CO2 is of such great concern today despite it being the least dangerous of the harmful emissions produced, is the large quantity that is produced. Even with modern vehicles burning cleaner than ever before, this means that even more CO2 is produced.

Unburned Hydrocarbons (HC)

Unburned hydrocarbons are a very dangerous emission and actually have a much higher greenhouse effect than CO2, but since they are produced at much lower levels by modern vehicles are of far less concern today. HC is a major contributor of smog, and can cause serious health problems.

Carbon Monoxide (HC)

Carbon monoxide is an extremely dangerous gas that blocks the body's ability to absorb oxygen, and can cause death. Carbon monoxide is colorless and odorless. Vehicles and other equipment using internal combustion engines or any other type of combustion should never be used inside garages or other enclosed spaces, and exhaust systems should be inspected for leaks to avoid carbon monoxide asphyxiation.

Oxides of Nitrogen (NOx)

Oxides of nitrogen are the primary cause of brown smog and acid rain. They can cause a variety of health problems, but on a brighter note the brown smog they produce can make for impressive sunsets.


Harmful exhaust emissions are an unfortunate by-product of our vehicles and other fuel-burning equipment. Check out future articles for information on how these emissions are being controlled and reduced on modern vehicles, as well as other information regarding the science behind combustion and air pollution.

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