Natural gas is used for different applications such as power generation, domestic use, transportation, and as a feedstock for the production of ammonia-based fertilizers. It is the dominant alternative road transport fuel in addition to ethanol. Since natural gas contains mainly methane, biomethane or electro-methane can be used as a substitute. In road transport vehicles, methane is mostly used in compressed form (compressed natural gas, CNG, or compressed biogas, CBG) in light-duty vehicles. However, in heavy-duty long-haul vehicles the interest is also in liquefied form (liquefied natural gas, LNG, or liquefied biogas, LBG). For long distance travel, natural gas is usually shipped as LNG, and then re-gasified in coastal terminals to be injected into the natural gas grid. Biomethane and electro-methane could be produced locally. Therefore, it is not as dependent on gas grid or shipping as natural gas. In all pathways, the composition of the natural gas has high variability.

Methane is traditionally used in the Otto engine, either under stoichiometric or lean-burn conditions. In last years, other engine technologies have been developed, e.g. high-pressure direct-injection (HPDI) compression-ignition engines with dual fuel operation for methane and diesel fuel. For HDPI concept, energy efficiency is higher than for stoichiometric gas engine. On the other hand, the stoichiometric engine can control emissions efficiently with a three way catalyst; also NO_x emissions that are problematic for lean-burn natural gas engines. DF engines need to be equipped with similar aftertreatment technology as diesel engines to meet emission legislation in many regions. All natural gas engines deliver low PM emissions. Methane emissions from current state-of-the-art natural gas vehicles are in general below the emission limits. Many of the emission species have been reported to be lower for natural gas vehicles than for gasoline or diesel vehicles.

Natural gas is the dominant alternative road transport fuel in addition to ethanol. The global fleet of natural gas vehicles is approximately 22.4 million according to the NGV Journal (http://www.ngvjournal.com/worldwide-ngv-statistics/, entered 17.7.2016). With number of light-duty and heavy-duty vehicles, their mileages and fuel consumptions, global natural gas consumption in road transport would be in maximum 60 Mtoe. Synthetic diesel fuel is produced from natural gas by liquefaction (Gas-to-Liquid, GTL). According to the IEA Headline Energy Data 2015, global use of natural gas in the transport sector in 2013 was 96.2 Mtoe, which indicates potential use of natural gas as GTL in the range of 30 Mtoe. Additionally to convering natural gas to GTL for use in road transportation, it can be converted to liquid methanol or synthetic gasoline, or to other gaseous fuels, such as DME. Hydrogen can be produced from NG via methane reforming, and electricity can be generated at a NG -powered plant for on-road vehicles. Emphasis is needed on cost, the environmental benefits, energy use, and energy security that each fuel pathway can offer to a particular nation. In the AMF Task 48, the feasibility of the different natural gas pathways used in motor vehicles were assessed to determine the advantages and disadvantages of each option. To demonstrate how differently each factor can weigh in, case studies were conducted in six different countries spanning three continents. (AMF Task 48: Sikes et al. 2015).