Compressed natural gas is a fuel that can be used in place of gasoline, diesel fuel and liquefied petroleum gas (LPG). CNG combustion produces fewer undesirable gases than the aforementioned fuels. In comparison to other fuels, natural gas poses less of a threat in the event of a spill, because it is lighter than air and disperses quickly when released. Biomethane — refined biogas from anaerobic digestion or landfills — can be used.
CNG is made by compressing natural gas, which is mainly composed of methane (CH4), to less than 1% of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 20–25 MPa (2,900–3,600 psi), usually in cylindrical or spherical shapes.
CNG is used in traditional gasoline/internal combustion engine automobiles that have been modified or in vehicles specifically manufactured for CNG use, either alone (dedicated), with a segregated gasoline system to extend range (dual fuel) or in conjunction with another fuel such as diesel (bi-fuel). Natural gas vehicles are increasingly used in Iran, especially Pakistan, the Asia-Pacific region, Indian capital of Delhi, and other large cities like Ahmedabad, Mumbai, Pune, Kolkata—as well as cities such as Lucknow, Kanpur, Varanasi, etc. Its use is also increasing in South America, Europe and North America because of rising gasoline prices. In response to high fuel prices and environmental concerns, CNG is starting to be used also in tuk-tuk, pickup trucks, transit and school buses, and trains.
The cost and placement of fuel storage containers is the major barrier to wider/quicker adoption of CNG as a fuel. It is also why municipal government, public transportation vehicles were the most visible early adopters of it, as they can more quickly amortize the money invested in the new (and usually cheaper) fuel. In spite of these circumstances, the number of vehicles in the world using CNG has grown steadily (30 percent per year). Now, as a result of the industry's steady growth, the cost of such fuel storage cylinders has been brought down to a much more acceptable level. Especially for the CNG Type 1 and Type 2 cylinders, many countries are able to make reliable and cost effective cylinders for conversion need.CNG's volumetric energy density is estimated at 42 percent of that of liquefied natural gas, because it is not liquefied, and at 25 percent of that of diesel fuel.