Compressed natural gas (CNG) is essentially a methane gas mixture that has been compressed to a high pressure. Natural gas is typically compressed to 200 bar or 250 bar for use in vehicles.
At standard temperature and pressure, natural gas has a density of around 0.7 kg/m³ to 0.9 kg/m³ depending on the composition. This increases to around 180 kg/m³ (at 200 bar) and to around 215 kg/m³ (at 250 bar) for a typical methane-rich gas composition.
CNG is generally stored in high-pressure cylinders of varying type and construction. Many common cylinders are made from steel, whilst other cylinders are made from lightweight materials such as aluminium and composites.
In recent years, natural gas has becoming an increasing attractive fuel, not least because of the economic savings that be made in comparison to other fuels but it is generally regarded as being much more environmentally-friendly.