| On the long road towards a more environmentally friendly vehicle, there are several stopping points. Today, we have the SUV. Somewhere, far in the future, is a little car that runs on sea water, produces Snickers as waste products, and gets 300 channels of commercial-free radio. Somewhere in between is the natural gas-powered vehicle.
While still a fossil fuel, natural gas is a lighter, richer fuel than gasoline, containing more hydrogen and less carbon. This is good, because when burning a hydrocarbon the hydrogen part produces energy and the carbon part produces pollution in the form of CO2. The CO2 emissions from natural gas are between 22% and 25% less than that from a comparable gasoline vehicle. The world resources of natural gas are only supposed to last 100 years given current projections, so natural gas is not the fuel of the future. However, the infastructure used to store and deliver natural gas to consumers is very similar to that needed for hydrogen, which has potential to be the fuel of the future.
That infastructure is also the largest reason that natural gas vehicles are not more prevalent. There isn't a nationwide infastructure currently in place to provide natural gas fueling to consumers. In some locations, such as San Diego, CA, there are natural gas pumps located throughout the city, but in most locations the use of natural gas is limited to those people for whom it is financially feasable to provide their own fueling station--bus lines, delivery fleets, taxi services.
The vehicles themselves are not a problem. Natural gas, being a hydrocarbon fuel like gasoline or diesel, can be used in an engine similar to the one in a gasoline car. Differences exist; for instance, natural gas doesn't have the lubricating properties of gasoline, so the cylinder heads must be specially hardened to prevent erosion. Also, natural gas is, strangely enough, gaseous under normal conditions, and must be compressed or liquefied for easy storage. Even with this compression it takes up significantly more space than the equivalent amount of gasoline. For buses or trucks this in not an issue, but for a normal passenger car the entire trunk must be used to store the equivalent of 15 gallons of gasoline. Even with this, the range of a natural gas vehicle is about half that of a comparable gasoline or diesel car.
Safety is also an issue: the tanks must be made as leak-proof as possible as it would be very dangeous to have natural gas escape in an accident. Common policy is to separate the fuel into several tanks and install automatic shutoff valves that, in the event of an acccident, make it impossible for natural gas to escape the tank through the fuel intake system.
Though any hydrocarbon will produce toxins when burned, natural gas does have some environmental benefits over other petroleum based fuels:
Lower emissions of volatile organic compounds: 98% lower
Carbon Monoxide (CO) emissions: 90% lower
Nitrogen Oxides (NOx) emissions: 50% lower
Particulate Matter emissions : 60% lower
Sulfur Dioxide (SO2): 97% lower
Lower NMHC emissions
Zero Benzene emissions
Emissions are also reduced because natural gas is distributed via underground pipeline, which means no rail cars, trucks or tankers are required to deliver the product and transport spills are eliminated. In the event of a leak from pipelines or compressor stations, there is no risk of soil or water contamination.
Cost can also be an issue--natural gas cars are more expensive than gasoline cars, costing $3,000 to $6,000 more than a gasoline powered equivalent, and to convert an existing car to run on natural gas costs around $2000.
Full size transit buses are typically $30,000 to $60,000 more than thier diesel equivalents. However, the fuel itself is significantly less expensive. The current prices reflect about a 15% to 25% savings when compared to the cost of operating a gasoline vehicle. The New York DOT found, in a limited survey of fuel use in DOT vehicles between 1999 and 2001, that operating their gasoline-powered automobile fleet cost the department $0.07 per mile, while the average cost of operating their natural gas-powered fleet, which includes automobiles, pickup trucks and vans, was just $0.05 per mile. Government subsidies also exist to reduce the cost difference.
Natural gas vehicles are not going to save the world. They're not even going to significantly change the transportation industry. However, they are a step in the right direction, and are practical--an important consideration. | 
