Natural-gas engines are more fuel-efficient than gasoline engines. The main benefit of CNG in the past was its low price (about 80 percent that of gasoline on the gallons of gasoline equivalent basis). Transport and distribution of natural gas are relatively inexpensive because the infrastructure for industrial and household use already exists (Yborra, 2006).
Despite a possibly advantageous fuel-supply situation, NGVs still have a lot of hurdles to overcome. The two main challenges faced by NGVs are insufficient refueling stations and inconvenient on-board CNG tanks that take up most of the trunk space. An NGV market can be analyzed by using the vehicle-to-refueling-station index, defined as the ratio of the number of NGVs (in thousands) to the number of natural-gas refueling stations. According to Yeh (2007), “using techniques including consumer preference surveys and travel time/distance simulations, it has been found out that the sustainable growth of alternative fuel vehicles … during the transition from initial market development to a mature market requires the number of alternative-fuel refueling stations be a minimum of 10 to 20 percent of the number available for conventional gasoline stations.” A thriving NGV market tends to have an index of 1; this gives rise to a problem: new stations are not being opened because of the lack of users, but few people use NGVs because of the lack of refueling stations.
A key disadvantage of NGVs is their low range. The average range of a gasoline or diesel vehicle is 400 miles, and the range of an NGV is only 100–150 miles, depending on the natural-gas compression. Because of the dearth of natural-gas refueling stations, the current prevalent choice is to use a bifuel NGV that can run on both natural gas and gasoline. The problems associated with bifuel engines include slightly less acceleration and about 10 percent less power than a dedicated NGV because bifuel engines are not optimized to work on natural gas. Furthermore, warranties on new gasoline vehicles are severely reduced if they are converted to bifuel NGVs. The most important barriers for NGVs might be a public perception that CNG is a dangerous explosive to have on one’s vehicle and a perception that self-service refueling with a high-pressure gas is too risky to offer to the general public.
About 22 percent of all new public-transit bus orders are for NGVs. Buses and corporate-fleet cars that stay in town have been the main market for NGVs, and both uses are mainly in response to the Clean-Fuel Fleet Program set up by the Environmental Protection Agency to reduce air pollution.