Natural gas is used as a transportation fuel in many countries around the world, and its use in vehicle applications is growing. Besides displacing imported petroleum fuels, one of the primary benefits of using natural gas as a vehicle fuel is the potential to substantially reduce exhaust emissions of harmful pollutants such as particulate matter (PM) and nitrogen oxides (NOx). Studies have shown that NOx reductions of 50–80 percent and PM reductions of 80–95 percent are possible when heavy-duty vehicles are operated on natural gas instead of diesel fuel. Similar reductions can be achieved in light-duty cars and trucks. In some cases, using natural gas to power a vehicle will also lower total greenhouse gas (GHG) emissions.
Switching to natural gas does not by itself guarantee low emissions of conventional pollutants or GHGs. Environmental benefits depend on the type of engine technology used and how it is deployed (e.g., as a retrofit of an existing gasoline engine or as a purpose-built natural gas engine)—in fact, some natural gas conversions can actually increase total emissions.
To maximize the benefits of any natural gas vehicle program, several factors must be considered, including natural gas fuel quality, vehicle engine technology, retrofits versus new vehicles/engines, and safety. All of these issues will be discussed in this paper.