CNG is compressed natural gas. Natural gas is the same gas that you cook with and use to heat your homes. Natural gas is a mixture containing about 75% methane (CH4), 15% ethane (C2H6), and 5% other hydrocarbons, such as propane (C3H8) and butane (C4H10). Methane is a colorless, odorless gas with a wide distribution in nature. At room temperature, methane is less dense than air, so it rises and dissipates. Burning methane in the presence of oxygen produces carbon dioxide and water.
In your car, CNG is stored in cylinders under pressure. CNG cylinders are filled to a pressure of 3000 to 3600 pounds per square inch (psi), which is about the same pressure that SCUBA tanks are commonly filled. Just like SCUBA tanks, CNG tanks must be handled properly and be periodically inspected. CNG cylinders are robust and are rated well above working pressures, are equipped with pressure relief devices (PRDs) that prevent over-fill, and are vented to the outside of the vehicle. CNG storage cylinders may be located between the frame rails and/or behind the rear axle. Light-duty CNG vehicles often have tanks behind the back seat or in the trunk/pickup bed. CNG tanks may be constructed of steel or aluminum or composite materials such as carbon fiber.
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