Natural gas is an odorless, gaseous mixture of hydrocarbons, predominately methane. Compressed natural gas (CNG) and liquid natural gas (LNG) are considered alternative fuels under the Energy Policy Act of 1992.
CNG is natural gas that has been compressed to less than 1% of its volume at standard atmospheric pressure. The gas is stored onboard vehicles in a compressed gaseous tank at a pressure of 3600 psi.
LNG is natural gas that has been cooled to -260˚F, or -160˚C, to convert it from gas to liquid state. LNG has a higher production cost and requires special storage tanks that keep it from evaporating, which makes it less popular in alternative transportation than CNG.
Natural gas vehicles (NGVs) can come equipped to run solely on natural gas or contain two separate fueling systems to run on either natural gas or gasoline. Many light- and medium-duty vehicles designed for conventional gasoline can economically and reliably be converted to run on natural gas as well.
Natural gas powers over 160,000 vehicles in the US and roughly 15.2 million worldwide. Despite the widespread use of natural gas, only about 0.2% of the natural gas used in the country goes towards transportation fuel. The majority of natural gas is used for electric power production.
LNG uses are usually limited to medium- and heavy-duty vehicles, while CNG can be used in these applications along with light-duty vehicles. The ideal usage for CNG is a high mileage, centrally located fleet, such as buses or taxis. For vehicles that need to travel further distances, LNG is more useful because the higher density allows more energy to be stored by volume.
The horsepower, acceleration, and cruise speed of cars using natural gas have proven to be comparable to those of conventional vehicles. The driving range of NGVs is generally less because of the lower energy density of natural gas. Extra storage tanks can increase range, but the additional weight may displace cargo capacity.
Around 90% of natural gas is domestically produced, increasing the country’s energy security. The fuel has a low cost compared to conventional gasoline and diesel fuels, as shown in the figure above. A light-duty vehicle running on natural gas can reduce lifetime GHG emissions by 15%, emits little to no particle matter, and presents no threat to soil, surface water, or groundwater.
Although the United States has an extensive natural gas distribution system in place, vehicle fueling infrastructure is limited. For this reason, many fleets choose to install their own natural gas fueling infrastructure, which can be costly, or partner with other fleets to establish shared infrastructure. The driving range of NGVs is generally less than that of comparable gasoline and diesel vehicles because, with natural gas, less overall energy content can be stored in the same size tank. Extra natural gas storage tanks or the use of LNG can help increase range for larger vehicles.