The present criteria of air pollutant emission reductions from NGVs cannot be the same due to differences in fuel composition and engine configurations in each place. According to DOE and EPA 2010, the Natural gas LDVs reduce pollutants by 60–90%, Compared with conventional gasoline LDVs. Through ETP lecture, we know that EPA has ranked the Honda Civic GX on the cleanest internal combustion engine vehicle in the world. According to the EPA 2010a, the Honda Civic GX has scored 9 or higher on not only EPA’s Air Pollution Score but also EPA’s Greenhouse Gas Score as the non-hybrid. I realize that California is famous for its tough emission vehicle standard through ETP lecture. This vehicle is able to satisfy their regulation. Moreover, according to the Stern 2009, the Civic GX was awarded as the greenest vehicle by the American Council for an Energy Efficient Economy for 2009. There were clean diesel light-duty vehicles and the Toyota Prius behind GX.
Criteria of pollutant emissions of NGVs made in U.S. was not always lower than comparable petroleum-fueled models of the same engine size. When running on gasoline, in the case of the Cavalier bi-fuel CNG/gasoline vehicle, it would be worse than the comparable gasoline in terms of fuel economy due to its added storage system weight. We can compare past CNG vehicles to those of gasoline vehicles through EPA ratings. We can find that fact that the 2010 Civic GX’s criteria of air pollutant score is 9.5 in low emissions states and 9 elsewhere on the Fueleconomy.gov. On the other hand, Civic has ratings of 6 and 7.The degree of superior performance of the GX against gasoline does not appear to be inherent in CNG. American manufacturers did not reproduce this degree of superiority in past models. As to some vehicles such as bi-fuel vehicles, it is too difficult to optimize the vehicle to run on natural gas, which could be the issue. Therefore, we have to admit that the emissions of NGVs would not meet what we expect.
According to the recent EPA/CARB emission standards, heavy-duty engines are required to have extremely low emissions, especially for PM and NOx.
Engine manufacturers of NGVs have difficulty to satisfy these regulations, while diesel engine manufacturers have needed to use expensive after-treatment options, such as particulate filters.
Therefore, the gap between new heavy-duty NGVs and new diesel vehicles would be narrowed, which means the benefit of new heavy-duty NGVs are likely to decrease.
We would be able to compare the emission reduction of NGVs with the emission reduction of diesel-fueled vehicles through table released by the National Renewable Energy Laboratory and the West Virginia University Heavy-Duty Vehicle Emission Testing Laboratory and based on a sampling of tests.
 DOE and EPA: see U.S. Department of Energy, Office of Energy Efficiency and Renewable
Energy, and U.S. Environmental Protection Agency.
 EPA: U.S. Environmental Protection Agency.
 Stern, P., 2009, America’s Greenest Cars of 2009, February 11, 2009, available at: