Compressed Natural Gas (CNG) is a practical alternative fuel derived from natural gas, the same source used to heat homes and generate electricity. Natural gas is a colorless, odorless hydrocarbon energy source. Compression is necessary to store a usable amount of fuel in a vehicle by significantly reducing its volume for use in internal combustion engines.
Defining CNG: Composition and Pressurization
Compressed Natural Gas is composed predominantly of methane (CH₄), the primary component of natural gas. This simple hydrocarbon structure contributes to its clean-burning properties compared to more complex liquid fuels. To be viable as a vehicle fuel, natural gas is compressed to less than one percent of its volume at standard atmospheric pressure.
The gas is stored in high-pressure tanks, usually cylindrical or spherical, made from materials like steel, aluminum, or composites. Typical storage pressure for CNG in vehicles ranges from 2,900 to 3,600 pounds per square inch (psi), or about 200 to 250 bar. This high-pressure storage allows for a sufficient amount of energy to be carried for a practical driving range.
CNG must be distinguished from Liquefied Natural Gas (LNG), another natural gas vehicle fuel. While both share a similar chemical composition, CNG remains gaseous at ambient temperatures. LNG is cooled to a cryogenic temperature of approximately -162°C (-260°F) to become a liquid. CNG’s storage method is simpler because it avoids the complex, insulated cryogenic tanks and the management of “boil-off” required by LNG.
Primary Applications in Transportation
CNG’s storage requirements and lower energy density compared to liquid fuels dictate its primary use in the transportation sector. It is mainly used in centrally fueled fleets that have predictable, high-mileage routes. These applications benefit from reduced fuel costs and the convenience of returning to a central depot for refueling.
Specific examples of fleet operations include public transit buses and school buses operating within fixed urban corridors. Refuse trucks and delivery vehicles also utilize CNG, benefiting from dedicated fueling infrastructure and limited daily travel distances. While CNG can be used in light-duty passenger vehicles, the large high-pressure tanks often reduce cargo capacity, making it a less common choice for general consumers.
Operational and Environmental Comparison to Gasoline
CNG offers distinct advantages over traditional gasoline, particularly in environmental performance. As a cleaner-burning fuel, CNG results in significantly lower tailpipe emissions of harmful pollutants. Vehicles running on CNG produce reduced levels of carbon monoxide, unburned hydrocarbons, and particulate matter compared to gasoline and diesel counterparts.
The lower carbon content of methane means CNG-powered vehicles emit 10 to 15 percent less carbon dioxide (CO₂) than comparable gasoline vehicles. This reduction in greenhouse gas emissions positions natural gas as a cleaner “bridge fuel” away from petroleum. Using CNG in urban fleets also improves local air quality due to the lower output of smog-producing nitrogen oxides (NOx) and soot.
From an operational standpoint, CNG presents an economic case for large fleets. The cost per energy-equivalent gallon of CNG is often lower than that of gasoline or diesel, sometimes by 15 to 50 percent, leading to long-term savings. The clean-burning nature of the fuel reduces engine wear and deposits, extending the life of engine oil and spark plugs and lowering maintenance costs.
However, CNG’s lower energy density requires a larger volume of storage to achieve the same driving range as a gasoline vehicle, often resulting in a shorter overall range. Although CNG has a high octane rating that translates to good engine performance, the limited availability of refueling stations remains a hurdle for widespread public adoption. Furthermore, the initial investment for a CNG refueling station is higher than for a liquid fuel station, complicating infrastructure growth.