Compressed Natural Gas (CNG)
Natural gas (85-99% methane) is clean burning, cheap and abundant in many parts of the world. Because natural gas is mostly methane, Natural Gas Vehicles (NGVs) have much lower non-methane hydrocarbon emissions than gasoline vehicles, but higher emissions of methane. Since the fuel system is sealed, there are no evaporative emissions and refueling emissions are negligible. Cold-start emissions from NGVs are also low, since cold-start enrichment is not required. In addition, this reduces both VOC and CO emissions.
NOx emissions from uncontrolled NGVs may be higher or lower than comparable gasoline vehicles, depending on the engine technology, but are typically slightly lower. Light-duty NGVs equipped with modern electronic fuel control systems and three-way catalytic converters have achieved NOx emissions more than 75% below the stringent California Ultra Low-Emission Vehicle (ULEV) standards.
As a substitute for diesel, NGVs should have somewhat lower NOx and substantially lower PM emissions.
Given equal energy efficiency, greenhouse gas (GHG) emissions from NGVs will be approximately 15-20% lower than from gasoline vehicles, since natural gas has a lower carbon content per unit of energy than gasoline. NGVs have about the same GHGs as diesel fuel vehicles.
Obstacles to the widespread use of NGVs include the absence of transportation and storage infrastructure, cost, loss of cargo space, increased refueling time, and lower driving range.
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