(AOL Autos) -- Regular readers of AOL Autos know that we have done a series of stories on the development and increasing popularity of cars that run -- or will eventually run -- on alternative fuels.

The mandatory use of CNG in public service vehicles began in New Delhi, India in 2000.

We've written about hybrids, clean-diesel engines, fuel-cell technology, ethanol and more.

That brings us to another entry in the auto industry's ongoing research and development of green-technology: compressed natural gas (CNG). Scientists are trying to determine which alternative fuel will best strike a balance between being environmentally friendly and commercial viability.

CNG might be the answer.

Vehicles running on CNG have actually been around since the early 1990s, but have not been a dominant force in the marketplace mostly because the infrastructure (i.e. re-fueling stations) is not yet in place to support high-volume sales of CNG-powered cars. Presently, there are only about 1,600 CNG refueling stations in the U.S, compared with up to 200,000 gas stations.

Currently there is only one CNG-powered model sold on the consumer market that is actually manufactured as a CNG-powered car. That's the Honda Civic GX, which boasts an EPA-estimated fuel economy of 24/36 miles per gasoline-gallon equivalent. AOL Autos: Best hybrids

Rich Kolodziej, president of Natural Gas Vehicles for America said there are other natural-gas-powered vehicles on the road -- about 130,000, he estimates -- but that the vast majority of those are part of commercial or transit fleets or have been converted from gasoline-powered cars.

"They're mostly fleet vehicles, like transit buses, delivery trucks, and the fleets maintained by the gas company, the electric company, etc," he said.

"We don't have hard figures about conversions, but I'd bet that about 25 percent of those 130,000 were once gasoline-powered vehicles that have been converted to CNG vehicles," Kolodziej said. "There are a growing number of companies making certified conversion systems and installing them."

NGVAmerica maintains a list of certified conversion systems on its Web site: www.ngvc.org. AOL Autos: Fuel-efficient used cars

Some of those 130,000 are also natural-gas-powered vehicles manufactured and sold by Ford, Chrysler and GM back in the early '90s, he says -- back when the U.S. manufacturers were still in the business of making CNG-powered cars. But we'll get back to that later.

But with the price of gasoline currently averaging over $4 a gallon nationally, Kolodziej predicts there will be more and more demand for CNG-powered vehicles like the Civic GX.

"I went to a conference a couple of months ago, and every manufacturer we talked to said that their phones were ringing off the hook, from people who are saying they'd be interested in buying a CNG-powered car," Kolodziej said. "And there are more and more people and companies who want to get into the conversion business." AOL Autos: Hybrid SUVs

The cost of conversions varies by the type of vehicle, Kolodziej noted. "But most cost in the $10,000 range. Most conversion systems are certified for Ford and GM models, among others," he said.

On the financial side, natural gas is about 30 percent less expensive than gasoline when it is purchased at a refueling station. It's about 50 percent less expensive when you fill 'er up at home, via home refueling appliance that tap into your natural gas line, Kolodziej said. Owners of the Honda Civic GX in New York and California who have natural gas piped into their homes can purchase the "Phill" re-fueling system for about $3,500.

This Phill system can be used whether your CNG vehicle was manufactured that way or converted, he said. AOL Autos: Luxury cars with best gas mileage

"The Phill is about the size of a pay phone, and you hang it on the garage wall, and vent it like a dryer and plug it into a gas line
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question: who is trying to determine which fuel is best?, answer: Scientists | question: What is scientists trying to determine for our future?, answer: which alternative fuel will best strike a balance between being environmentally friendly and commercial viability. | question: What number of CNG powered models are sold today?, answer: one | question: In total, how many CNG-powered model had been sold?, answer: 130,000 | question: how many CNG stations in U.S.?, answer: 1,600 | question: What is the answer for alternative fuel?, answer: CNG | question: What is the number of CNG fueling stations in the US?, answer: refueling