CNG has been an options as an automotive fuel since 1994 under the Central Motor Vehicle Act. NG is fossil fuel more evently distributed worldwide than crued oil. It is found in reservoirs 3000 to 15000 feet below the surface of earth, with reserves in about 90 countriesaround the world. World-proved reserves of NGat the end of 1999 wrer about 5172 trilion cubic feet (TCuf). While two-thirds of crued oil reserves are concentrated in the Middle East.

CNG is safe fuel and being lighter than air, it disperses easily into the atmosphere and does not form a sufficiently rich mixture from combustion to take place. CNG is 130 octane, which is considerably higher than 93 octane for petrol; consequently, CNG vehicle is more energy-efficient. Higher octane rating allows higher compression ratios and improved thermal efficiently, reducing carbon dioxcide.

Advantages:

 Lighter than air, easily to disperse at atmosphere.

Excellent knock-resistance thus, allows use of higher compression ratio higher than 10 compare than gasoline. Higher compression ratio, will develop higher thermal efficiency and increase power output and fuel economy.

Energy content equal to petroleum based fuels, but has lower energy content per unit volume.

Higher octane number, therefore possible to operate at higher compression ratio without knocking. Knock or detonation can be extremely damaging to an engine. Knock occurs when there is uncontrolled combustion with multiple flame fronts rather than smooth combustion proceeding along a flame front initiated at the spark plug. Knock can result from the heat produced by compression of air/fuel gas mixture in the piston. Methane has very high knock resistance but if heavier hydrocarbons in CNG, such as ethane, propane and butane, have lower knock resistance and thus reduce overall knock resistance of the fuel. 

All of these advantages, will encourage fuel saving.

The Alternative Fuels Data Center ( http://www.eere.energy.gov/webpolicies/ ) is a vast collection of information on alternative fuels and the vehicles that use them.