Compressed natural gas is natural gas that has been heated and compressed to take up a smaller volume while retaining large amounts of potential energy stored in the molecules. It has been used for a wide variety of functions, from heating buildings to providing power for trains to also powering car engines. It can function similarly to diesel or gasoline in cars.

This is what it was used for during the peak of the oil crisis in many areas of the globe. Historically, New Zealand had a sizable percentage of their cars and buses fitted with CNG-fueled engines during the oil crisis. However, once the crisis passed and gas declined, a trend back to gasoline fueled cars commenced. Now that oil prices have increased, and, as a result, gasoline and diesel prices have also increased, there is a worldwide trend towards using compressed natural gas whenever possible.

Places such as Canada, which have a large amount of natural gas, tend to use more compressed natural gas to fuel their autos. Pakistan has the most CNG gas stations in the world and the largest amount of cars that use compressed natural gas. South American countries such as Brazil and Argentina also have many cars made and retrofitted to use compressed natural gas in their engines. Taxis are one form of transportation that is rapidly being switched over to CNG. In New Jersey, companies like Earthtech a waste and recycling company have converted their trucks to CNG. Earthtech now is offering CNG consulting services to corporations, municipalities, and the federal government.

In the United States, federal tax credits are allotted to anyone who retrofits their current vehicular engine or buys a car already containing an engine that can use compressed natural gas. As a result, the use of compressed natural gas has risen quickly. This is partly in response the price of oil, which is historically very high. However, only 34 states have CNG filling stations, although this number is expected to rise because the price of oil is going to remain prohibitively expensive.

There are even trains that are beginning to use compressed natural gas. The NAPA Valley Wine Train had its diesel engine changed to a retrofitted engine that could use compressed natural gas. All around the world, a historical leap from diesel and gasoline to compressed natural gas is occurring and will likely continue to occur.