Gasoline and diesel are the two fuels that dominate U.S. roadways. However, neither is particularly clean and both are relatively expensive. That's why companies like Clean Energy (NASDAQ:CLNE), Royal Dutch Shell (NYSE:RDS.B), and General Electric (NYSE:GE) are looking to make natural gas, which can reduce green house gas emissions by up to 30%, a bigger player.

Already doing it
Clean Energy is at the forefront of pushing natural gas as a transportation fuel. The company has over 400 fueling stations and provides natural gas for over 26,000 vehicles a day. While those numbers sound impressive, they are tiny compared to the entire U.S. transportation market.

For example, although Shell doesn't break out the United States in its branded station count, it's fair to say that of the 22,000 stations in the "Americas," most are U.S. locations. So when it comes to scale in fuel distribution, Clean Energy is a relative small fry.

However, Clean Energy has an in with fleet vehicles, providing natural gas to the likes of taxis, garbage trucks, buses, and airport vehicles. It sells natural gas at nearly 40 airports, works with giant trash haulers like Waste Management (NYSE:WM), and has a national network of more than 120 fuel stations across the interstate highway system.

In fact, Clean Energy claims to fuel 90% of natural gas trucks! Of course, less than half of one percent of trucks sold in 2013 are projected to be natural gas, which makes that 90% figure less impressive. However, Clean Energy sees big potential, expecting up to 35% of new truck sales to be natural gas powered in the next five years or so.

Partners in virtue
That is, perhaps, why General Electric has been working so closely with Clean Energy. General Electric's finance unit, for example, recently inked a deal to provide truck financing to companies that sign fuel supply contracts with Clean Energy. That's on top of an earlier agreement between the two companies and Ferus Natural Gas Fuels to develop liquified natural gas projects serving industries "...such as long-haul trucking, rail, mining, marine, and oil and gas services."

And those aren't the only natural gas vehicle fuel endeavors General Electric is working on. Fleet vehicles are a big market, but the consumer market is bigger. That's where a GE effort to build home fueling stations for natural gas powered autos comes in. Although the gasoline infrastructure will be hard to displace, letting people fuel from home could be a game changer.

Of course you don't buy General Electric as a natural gas play, since it is an industrial and financial conglomerate. But GE is clearly trying to get itself into the natural gas market, and partnerships with Clean Energy are a good start. But Clean Energy is hardly the only industry participant. Royal Dutch Shell, and its 20,000+ American gas stations (44,0000+ globally), is also a big player.

Shell has a large domestic drilling program, seeing natural gas as a way to fuel long-term growth. With regard to vehicles, Shell opened a liquified natural gas station in Canada early in the year with plans to supply its own natural gas once it builds a nearby liquefaction plant. It has plans to build two more North American liquefaction plants in the near future, too.

In fact, Clean Energy lists Shell as one of its major competitive threats because the oil and natural gas giant has plans to build natural gas stations at TravelCenters of America (NASDAQ:TA) locations—a major long-haul truck stop operator. Those locations would already have a loyal clientele that would be hard for Clean Energy to duplicate.

On the horizon
Natural gas isn't a big player in the vehicle fuel market, but it likely will be in the future. If you are looking for a direct play, the best bet is Clean Energy. Shell and General Electric, however, are much more diversified businesses that, unlike Clean Energy, are making money today and still provide exposure to the space.  

Natural gas use isn't the only emerging automotive trend