What Is a Natural Gas Vehicle and What Does CNG Mean?

Dear Vanessa,

I feel like I’m seeing more natural gas vehicles on the road lately. Usually, they’re buses or company fleets (like UPS), but now I’m noticing passenger cars that say “CNG.” That stands for Clean Natural Gas, right? I’ve got to say, I know nothing about it. What is CNG? Is it really ‘clean’? And what are the issues if I want to get a CNG car?

— Jane, Pecos, N.M.

Hi Mom,

CNG, it turns out, does not stand for “clean” natural gas, though that's what I thought, too. No doubt the industry considers our common error a positive indication of their “clean” campaign. The ‘C’ is for compressed, as the gas must be highly compressed to power an internal combustion engine. There’s also “LNG,” which is the liquefied version of natural gas.

That’s not to say that natural gas isn’t “clean.” It certainly seems to be the cleanest of the fossil fuels (yes, CNG is, after all, a non-renewable fossil fuel). Natural gas vehicles are said to emit a good 20 percent less carbon dioxide (CO2) than standard gas vehicles. According to the U.S. Environmental Protection Agency, natural gas vehicles:

• reduce carbon monoxide emissions by 90 percent.

• reduce nitrogen oxide 35 to 60 percent.

• potentially reduce non-methane hydrocarbon emissions 50 to 75 percent.

• emit fewer toxic and carcinogenic pollutants, including at least 60 percent less particulate matter.

Also helping CNG’s “clean” profile are its methods of processing and distribution. While gasoline and diesel must be processed from crude oil in (yuck) refineries, natural gas requires relatively less processing before use. And because natural gas is distributed by underground pipeline, no rail cars, trucks or tankers are required. Gasoline and diesel on the other hand, are always delivered to fueling stations by tanker trucks. And in the event of a pipeline or compressor station leak, there is no risk of soil and water contamination (gas goes up, not down).

Then again...

Natural gas might be cleaner-burning than oil, but it’s still a hydrocarbon that has to be taken out of the ground and is in limited supply. The fuel is often sourced with or near oil reserves, and involves similarly invasive drilling methods. Though natural gas does not spill like oil to cause soil and water ecosystems, it does rise into the atmosphere, contributing directly to global warming. Natural gas is also highly combustible, and toxic.

The cleanliness of natural gas is based on extraction from ‘conventional’ sources. But much of the U.S.’s natural gas is contained in harder to access, ‘unconventional’ sources — like coal bed methane, tight sandstone and shale — entailing the same air and water contamination that accompanies the extraction of other fossil fuels. Extracting natural gas from these sources is extremely expensive, damaging to the environment, and leads to health problems — which raise health care costs and compromise the lives of already over-stressed Americans.

In the last 10 years, unconventional production of natural gas has increased from 28 to 47 percent of total output. Growing reliance on shale in particular is raising concerns about water consumption and contamination. Extracting gas from this source involves hydraulic fracturing, a process that injects water, sand and chemicals into the shale layer at extremely high pressure. The process uses millions of gallons of water and leaks chemicals into waterways.

Natural gas currently supplies 22 percent of U.S. energy demands; it's the third-largest energy source after petroleum and coal. If we embrace natural gas as a vehicle fuel, we may simply replace U.S. dependence on foreign oil with a dependence on foreign natural gas, yet another fossil fuel.

Since the late 1980s, U.S. imports of natural gas have tripled, and we already consume nearly a quarter of the world’s natural gas. The United States has about 3 percent of the world’s proved natural gas reserves (enough to meet current demand for another nine years), but there seems to be little agreement on the status of reserves generally. Nonetheless, no one denies that increased demand caused by growing fleets of natural gas cars would have a tremendous impact on supply and distribution.

On the optimistic side, the California Energy Commission says that with current rising demand, more than 15 percent of our natural gas will be imported from countries other than Canada and Mexico by 2025. The U.S. Department of Energy, however, projects that by 2016 the majority of U.S. natural gas imports will come from outside North America. Russia and Iran top the list of countries with the largest proved reserves.

But then again...

Although natural gas is a nonrenewable resource, its primary component, methane, can be derived from renewable sources. Biogas — also called digester gas, swamp gas or marsh gas — is methane produced by the fermentation of organic matter, including manure, wastewater sludge, solid waste in landfills, or any other biodegradable matter. Methane is already captured at some landfills in the U.S., and because it is the most potent greenhouse gas (over 20 times more effective in trapping heat in the atmosphere than CO2), new technologies to capture and use it are developing rapidly. Natural Gas Vehicles for America (NGVA) says that waste biomass could supply enough natural gas for about 11 million natural gas vehicles, approximately 5 percent of the nation’s automotive fleet.

Now, with all of that said, consider the Earth Policy Institute’s assertion that using natural gas in the electric grid (burning it in combined cycle power plants) is three times as efficient as burning it in a car. Because internal combustion engines are so wasteful, running a car on electricity from a natural gas power plant is at least twice as efficient as running a car on CNG. So keeping natural gas in the utility sector to power a fleet of plug-in electric vehicles is a smarter use of the resource than burning it in an internal combustion engine.

CNG is a clean fossil fuel ... but still a fossil fuel. It's a limited resource ... but it can be mimicked by capturing and burning methane. It has cleaner and simpler extraction, refining and transportation processes ... but only in relation to the dirtiest fossil fuels. It causes no direct soil and water contamination ... but causes real air and climate impacts, and soil and water contamination from extraction processes. It offers a limited infrastructure to consumers ... but is more easily adaptable to our current infrastructure systems than other alternative fuel sources. There are few public refueling stations ... but you can fuel at home. The vehicles are more expensive ... but substantial incentives are available to offset higher costs. Fuel prices are currently low ... but are bound to rise. It can be used to power internal combustion engines ... but is more efficiently used in the electric power grid.

In short, natural gas seems to be a better way to do the wrong thing.

Keep it Green,


Considering buying a CNG car? Here are some issues to consider:

• The lack of infrastructure for fueling CNG cars. While there are over 1,100 CNG filling stations in the U.S., only half are open to the public (compare that to over 200,000 gasoline stations). Home fueling systems are now available, but cost several thousand dollars. These allow you to plug into your home's existing natural gas system, but require long refueling times.

• The limited number of CNG stations is all the more relevant because natural gas vehicles have a shorter driving range than regular gas-powered vehicles (natural gas has a lower energy content than gasoline, so the cars only get about 170–220 miles on a full tank).

• Not many CNG vehicles are available, and those that are tend to be more expensive than their more established counterparts. As alternative fuel vehicles, however, they are eligible for a combination of federal, state and local incentives that could help reduce the price tag by several thousand dollars.

• The price of natural gas is considerably less than gasoline, also offsetting the higher initial cost. That’s for now; as demand increases and supplies decrease, how quickly and steeply prices will rise is a crap shoot.

• The American Council for an Energy-Efficient Economy (ACEEE) put the natural gas-fueled Honda Civic GX at the top of its 2007 environmentally friendly car list, above Toyota’s hybrid Prius. The Civic scored slightly better than the Prius on fuel economy, and scored better in terms of the pollution generated in the manufacturing processes.

• The Earth Policy Institute places plug-in hybrid electric vehicles (PHEVs) above their CNG counterparts and would urge you to use an electric vehicle and reserve natural gas for the power grid where it can be burned three times more efficiently than in a vehicle.