August 31st, 2006

Let's Clear the Air

CNG – Just the Facts

by NAFTC student worker and contributing writer Matt DeLiso

Let’s face the facts: Gasoline prices across America will continue to increase and reach new records. With these prices rising, people are searching for new fuel sources to power our mobile economy. This has renewed consumer interest in compressed natural gas (CNG) vehicles.

CNG has been used in the transportation industry for more than eighty years, most commonly in city fleet vehicles. For the most part, the general public would not even notice that vehicles are CNG-powered except for the “Powered by CNG” signs on buses or other fleet vehicles.

CNG systems work by filling the tanks located on the vehicle to 3,000 or 3,600 PSI depending on the system and the manufacturer. This fuel is then fed to the engine through a pressure regulator to drop the pressure between 100-180 PSI in the fuel rail. At this point, the fuel then is injected into the engine.

As consumers, some people are concerned with the limited range of CNG-powered vehicles due to the fact that there are no CNG filling stations like there are for gasoline-fueled vehicles. However, there is a bifuel option for CNG vehicles, meaning that the vehicle is able to run on CNG or gasoline with the driver choosing what fuel type they wish to use. Both dedicated and bifuel systems are being made a factory option by the major OEMs.

Several safety concerns pertaining to these vehicles come to mind since they store CNG, a flammable gas, on board. Although this may be hard to believe, CNG vehicles are as safe as the average gasoline-powered vehicle. This is mainly due to the inspection and design of these systems. The fuel tanks are designed to resist fire, accidents, and gunfire. The fueling lines bursting pressure is approximately 20,000 PSI. Also, there are pressure relief devices in the fuel system that will release the pressure when it builds to an unsafe level.

CNG, although flammable, is safe. CNG needs at least a 5.3 percent concentration in air to ignite where gasoline needs only a 1 percent concentration in air to ignite. It is nontoxic, although in large quantities it will displace air and cause asphyxiation. CNG is lighter than air, so if there is a leak, the gas will go up in the air and not pool on the ground like gasoline does.

Comparison of gasoline, diesel, and CNG.

Comparison of gasoline, diesel, and CNG. Credit: U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy

The best way to ensure safety is to inspect the CNG systems. These inspections are completed on a rigorous scale to make sure that the systems are performing as intended. CNG systems, their storage cylinders in particular, are inspected every three years or 36,000 miles. During these inspections, certified inspectors look at the tanks checking for dents, cuts, etc., and will fail the cylinder if it is not safe.

Participants learn about CNG cylinder inspection at NAFTC headquarters.

Participants learn about CNG cylinder inspection at NAFTC headquarters. NAFTC file photo

Although nothing is 100 percent safe, CNG vehicles have an advantage to gasoline-powered engines because of fuel characteristics. With AAA reporting the current gas price as averaging $2.848 per gallon for low grade, a gallon of CNG, not including taxes (taxes differ by state), is $1.753 per gallon, making it a valuable alternative. To learn more about the NAFTC’s CNG Cylinder Inspection course, visit

Share this: