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Exhaust Gases 101

Gases Cars Produce

The Basics

Gasoline engines are fueled with two chemical compounds, gasoline and air. Gasoline is made of two elements, hydrogen (H2) and carbon (C). In the form of gasoline, these two chemicals combine to make what we call a hydrocarbon (HC). Air is mostly made of two elements, oxygen (O2) and nitrogen (N2). 

If gasoline engines burned their fuel as efficiently as possible, they would produce three by-products: water vapor (H2O), carbon dioxide (CO2) and nitrogen (N2). 

Chemistry of Exhaust Gases

For the most part, none of these chemical compounds are harmful; however, environmentalists argue that excessive amounts of carbon dioxide promote the formation of the greenhouse effect. Nonetheless, H2O, CO2 and N2 are the most desirable by-products of combustion, and automotive engineers strive to create emission control systems that allow a vehicle to produce only these three chemical groups. 

Unfortunately, engines do not run perfectly, and as a result, they also produce three by-products commonly referred to as the "terrible trio" of automotive pollutants. This trio includes the following:

  • Carbon monoxide (CO) – An odorless, tasteless, poisonous gas, carbon monoxide can cause a variety of health problems and even death. Many urban areas experience critically high levels of carbon monoxide, especially during the cold winter months when engines take longer to warm up and run cleanly

  • Unburned hydrocarbons (HC) – Responsible for causing a variety of respiratory problems, unburned hydrocarbons can also cause crop damage and promote the formation of smog

  • Oxides of nitrogen (NOX) – Like unburned hydrocarbons, oxides of nitrogen cause respiratory problems and promote the formation of smog

Dealing with These Gases

The catalytic converter in your vehicle's exhaust system is designed to deal with the "terrible trio" of automotive pollutants. To learn more about the catalytic converter, check out this article: Evolution of a Catalytic Converter


The content contained in this article is for informational purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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