NGV is the acronym for Natural Gas for Vehicles. The fuel that used in vehicles is Compressed Natural Gas (CNG) and CNG is the term that most commonly describes this type of fuel system. People convert their vehicles to natural gas for reasons similar to auto propane. In many ways, the technologies are very similar but because the fuels have different storage requirements, the components are different too. Natural does not liquefy easily and so must be stored as a gas at high pressures.
The cost of CNG components are much expensive than LPG components because of the much higher pressures involved. A vehicle conversion for CNG could cost upwards of $7000 compared with a comparable propane conversion costing around $5000. The cost of CNG fuel must be much less than the cost of propane for the conversion to have any reasonable payback. Utah, for example, has extremely low prices for CNG (reportedly less that $1/GGE of CNG).
Government legislation is the other driving force for CNG conversions. Some states (like New York), are encouraging the use of CNG in fleet vehicles (like taxis) in their downtown cores for environmental reasons (see NYSERDA Alternative Fuel Vehicle Program). These vehicles (or vehicle conversions) must also be CARB (rather than EPA) certified on CNG, which restricts the allowable vehicles to a handful that have been CARB-certified.
On a mass basis, natural gas has more energy than the same amount of propane and even more than gasoline. Because it is a gas and not a liquid, it is difficult to store enough CNG to give a driving range comparable to gasoline. Natural Gas is typically stored at 2400 to 3600 psi in an automotive CNG fuel tank and even a fairly large tank might only hold the equivalent of 3 or 4 gallons of gasoline. For example, a 3000 psi cylinder with an internal capacity of 68 litres (18 US Gallons) holds the equivalent of 20.2 litres (5.3 US Gallons) of gasoline. A Dynetek Industries Q068 (68 litre) cylinder would have a diameter of 327 mm (12.9") and a length of 1090 mm (42.9") and would weigh 25 kg (55 lb).
For modern 4-cylinder vehicles being used for commuting, a fill every day or every other day would be required depending upon the distance driven. Daily stops at a CNG filling station might be inconvenient for those commuters used to getting 600+ km from a tank of gasoline. Commuters in large urban centers can sometimes find refueling stations relatively easily. Unfortunately, these refueling stations are generally not plentiful enough to cause price competition and consumers end up paying too much for their fuel. Often, the installed cost of the CNG refuelling station is so expensive that only the gas utilities can afford them.
The other option is to install a Vehicle Refueling Appliance (VRA) in their home. FuelMaker is one manufacturer of VRAs and they have a residential VRA they call Phill. Their commercial VRA is their only product available for residential installations. With the VRA, the commuter simply plugs his car into the natural gas compressor to have it filled overnight, ready for the next day's commute. However, Phill is better suited to the Honda Civic GX and its 8.03 GGE @ 3600psi CNG cylinder, which would take about 16 hours to completely refuel. It would be impossible for Phill to refuel the much larger CNG tank(s) required the daily operation of a full-size pickup truck. It would also be impossible to use this vehicle to commute with its maximum range of 225-250 miles if you used Phill to refuel at home and Honda suggests a maximum of 100 miles/day to allow for home refuelling. Therefore, you are limited to a 50 mile daily commute and you may not use enough fuel have an economic payback on the added cost of the CNG system (Phill costs over $6000 installed!) without government incentives.
T. Boone Pickens (and his Pickens Plan) makes a lot of sense. The increased use of natural gas for vehicles is an excellent way to reduce foreign oil consumption in the USA but the USA faces a chicken and egg situation with this strategy. Although natural gas is readily available across the USA, CNG stations for refueling vehicles are not. Increased demand for CNG would drive the construction of CNG stations but there is not point in converting your vehicle (or buying a new CNG vehicle) if CNG is not already readily available. Increased CNG use by fleets can help to increase the number of CNG stations but the added cost of a CARB/EPA-certified vehicle is a definite factor in the payback economics.