Compressed Natural Gas (CNG) is a substitute for gasoline (petrol) or diesel fuel. It is considered to be an environmentally "clean" alternative to those fuels. It is made by compressing methane (CH4) extracted from natural gas. It is stored and distributed in hard containers, usually cylinders.
Argentina and Brazil, in the Southern Cone of Latin America, are the two countries with the largest fleets of CNG vehicles. Conversion has been facilitated by a substantial price differential with liquid fuels, locally-produced conversion equipment and a growing CNG-delivery infrastructure. A 'Blue-network' of CNG stations is being developed on the major highways of the Southern Cone (including Chile and Bolivia) to allow for long-haul transportation fuelled by CNG.

High Fuel Prices

In response to high fuel prices and environmental concerns, compressed natural gas is starting to be used in light-duty passenger vehicles and pickup trucks, medium-duty delivery trucks, and in transit and school buses.
CNG has grown into one of the major fuel sources used in car engines in Pakistan, Bangladesh and India. The use of CNG is mandated for the public transport system of New Delhi, India's capital city as well as mega city Ahmadabad in the state of Gujarat. The Delhi Transport Corporation operates the world's largest fleet of CNG buses. The government of Punjab, Pakistan, the most populous province of that country, has mandated that all public-transport vehicles will use CNG by 2007. Today, many rickshaws as well as personal vehicles in India and Bangladesh are being converted to CNG powered technology, the cost of this is in range of $400-$450,. In the mega city Dhaka there is not a single auto rikshaw is permitted from 2003 without CNG. Pakistan is the largest user of CNG in Asia, and third largest in the world. As of 2005.

According to the International Association for Natural Gas Vehicles, Pakistan has the third-largest number of natural gas vehicles. In the Middle East and Africa, Egypt is a top ten country in the world with more than 63000 CNG vehicles and 95 fueling stations nationwide. Egypt was also the first nation in Africa and the Middle East to open a public CNG fuelling station in January 1996.
During the 1970s and 1980s, CNG was commonly used in New Zealand in the wake of the oil crises, but fell into decline after petrol prices receded.
Brisbane Transport and Transperth in Australia have both adopted a policy of only purchasing CNG buses in future; the former purchasing 216 Scania L94UB and 180 MAN 18.310 models, with the latter purchasing 451 Mercedes-Benz OC500LE buses, including 58 articulated buses. Brisbane Transport has also ordered up to 30 articulated CNG buses on MAN chassis'.
In Germany, CNG-generated vehicles are expected to increase to two billion units of motor-transport by the year 2020. The cost for CNG fuels are 1/3 less than LNG fuels, in Europe.

Compatability

CNG buses are not completely compatible with existing transport infrastructure; the MBTA in Massachusetts, for instance, does not permit its CNG buses to enter its underground bus tunnels or enclosed stations. (See the "No CNG buses" sign in this image.). This is typically because bus CNG tanks are kept on top of the bus, increasing the height of the roof of the vehicle and potentially interfering with low overpasses and bus terminals as in the picture. Leaks in enclosed areas are of course a matter of concern, but CNG dissipates far more readily than other fuels and is far safer than common alternative LPG in this regard. CNG is often measured and sold in Gasoline Gallon Equivalent GGE to help American consumers when comparing to gasoline.
In emerging countries, the cost of financing a conversion may represent a significant barrier, thus requiring promotional schemes by the government or private sector. A scheme allowing the owner to recover up to 60% of the conversion cost through 'free-CNG' vouchers was implemented in the city of Santa Cruz, Bolivia, doubling the total number of CNG vehicles in less than one year. In Europe CNG is not an issue for a sustainable fuel since by 2030 only 30% of natural gas will be produced in Europe.

Technology

CNG can be used in Otto-cycle (gasoline) and modified Diesel cycle engines. Lean-burn Otto-cycle engines can achieve higher thermal efficiencies when compared with stoichiometric Otto-cycle engines at the expense of higher NOx and hydrocarbon emissions. Electronically-controlled stoichiometric engines offer the lowest emissions across the board and the highest possible power output, especially when combined with EGR, turbocharging and intercooling, and three way catalytic converters, but suffer in terms of heat rejection and fuel consumption. A suitably designed natural gas engine may have a higher output compared with a petrol engine because the octane number of natural gas is higher than that of petrol. CNG may be refueled from low-pressure ("slow-fill") or high-pressure ("fast-fill") systems. The difference lies in the cost of the station vs. the refueling time. There are also some implementations to refuel out of a residential gas line during the night, but this is forbidden in some countries.

CNG cylinders can be made of steel, aluminium, or plastic. Lightweight composite (fibre-wrapped plastic) cylinders are especially beneficial for vehicular use because they offer significant weight reductions when compared with earlier generation steel and aluminium cylinders, which leads to lower fuel consumption.
The equipment required for CNG to be delivered to an Otto-cycle engine includes a pressure regulator (a device that converts the natural gas from storage pressure to metering pressure) and a gas mixer or gas injectors (fuel metering devices). Earlier-generation CNG conversion kits featured venturi-type gas mixers that metered fuel using the Venturi effect. Often assisting the gas mixer was a metering valve actuated by a stepper motor relying on feedback from an exhaust gas oxygen sensor. Newer CNG conversion kits feature electronic multi-point gas injection, similar to petrol injection systems found in most of today's cars.

CNG compared to LNG and LPG
CNG is often confused with LNG. While both are stored forms of natural gas, the key difference is that CNG is in compressed form, while LNG is in liquified form. CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks. CNG requires a much larger volume to store the same mass of natural gas and the use of very high pressures (3000 to 4000 lbf/in² , or 205 to 275 bar).