A natural gas vehicle (NGV) is an alternative fuel vehicle for autonomous mobility that uses compressed natural gas (CNG) or liquefied natural gas (LNG). Natural gas vehicles should not be confused with vehicles powered by propane (LPG), which is a fuel with a fundamentally different composition.
In a natural gas powered vehicle, energy is released by combustion of essentially Methane gas (CH4) fuel with Oxygen (O2) from the air to CO2 and water vapor (H2O) in an internal combustion engine. Methane is the cleanest burning hydrocarbon and many contaminants present in natural gas are removed at source.
Safe, convenient and cost effective gas storage and fuelling is more of a challenge compared to petrol and diesel vehicles since the natural gas is pressurized and/or – in the case of LNG – the tank needs to be kept cold. This makes LNG unsuited for vehicles that are not in frequent use. The lower energy density of gases compared to liquid fuels is mitigated to a great extent by high compression or gas liquification, but requires a trade-off in terms of size/ complexity/ weight of the storage container, range of the vehicle between refuelling stops, and time to refuel.
Although similar storage technologies may be used for, and similar compromises would apply to, a Hydrogen vehicle as part of a proposed new Hydrogen economy, Methane as a gaseous fuel is safer than Hydrogen due to its lower flammability, low corrosivity and better leak tightness due to larger molecular weight/ size, resulting in lower price hardware solutions based on proven technology and conversions. A key advantage of using natural gas is the existence, in principle, of most of the infrastructure and the supply chain, which is non-interchangeable with Hydrogen. Methane today mostly comes from non-renewable sources but can be supplied or produced from renewable sources, offering net carbon neutral mobility. In many markets, especially the Americas, natural gas may trade at a discount to other fossil fuel products such as petrol, diesel or coal, or indeed be a less valuable by-product associated with their production that has to be disposed. Many countries also provide tax incentives for natural gas powered vehicles due to the environmental benefits to society. Lower operating costs and government incentives to reduce pollution from heavy vehicles in urban areas have driven the adoption of NGV for commercial and public uses, i.e. trucks and buses.
Many factors hold back NGV popularization for individual mobility applications, i.e. private vehicles, including: relatively price and environmentally insensitive but convenience seeking private individuals; good profits and taxes extractable from small batch sales of value-added, branded petrol and diesel fuels via established trade channels and oil refiners; resistance and safety concerns to increasing gas inventories in urban areas; dual-use of utility distribution networks originally built for home gas supply and allocation of network expansion costs; reluctance, effort and costs associated with switching; prestige and nostalgia associated with petroleum vehicles; fear of redundancy and disruption. A particular challenge may be the fact that refiners are currently set up to produce a certain fuels mix from crude oil. Aviation fuel is likely to remain the fuel of choice for aircraft due to their weight sensitivity for the foreseeable future.
Worldwide, there were 24.452 million NGVs by 2016, led by China (5.0 million), Iran (4.00 million), India (3.045 million), Pakistan (3.0 million), Argentina (2.295 million), Brazil (1.781 million), and Italy (1.001 million). The Asia-Pacific region leads the world with 6.8 million vehicles, followed by Latin America with 4.2 million. In Latin America, almost 90% of NGVs have bi-fuel engines, allowing these vehicles to run on either gasoline or CNG. In Pakistan, almost every vehicle converted to (or manufactured for) alternative fuel use typically retains the capability of running on gasoline.
As of 2016, the U.S. had a fleet of 160,000 NG vehicles, including 3,176 LNG vehicles. Other countries where natural gas-powered buses are popular include India, Australia, Argentina, Germany, and Greece. In OECD countries, there are around 500,000 CNG vehicles. Pakistan’s market share of NGVs was 61.1% in 2010, follow by Armenia with 32%, and Bolivia with 20%. The number of NGV refueling stations has also increased, to 18,202 worldwide as of 2010, up 10.2% from the previous year.
Existing gasoline-powered vehicles may be converted to run on CNG or LNG, and can be dedicated (running only on natural gas) or bi-fuel (running on either gasoline or natural gas). Diesel engines for heavy trucks and busses can also be converted and can be dedicated with the addition of new heads containing spark ignition systems, or can be run on a blend of diesel and natural gas, with the primary fuel being natural gas and a small amount of diesel fuel being used as an ignition source. It is also possible to generate energy in a small gas turbine and couple the gas engine or turbine with a small electric battery to create a hybrid electric motor driven vehicle. An increasing number of vehicles worldwide are being manufactured to run on CNG by major carmakers. Until recently, the Honda Civic GX was the only NGV commercially available in the US market. More recently, Ford, General Motors and Ram Trucks have bi-fuel offerings in their vehicle lineup. In 2006, the Brazilian subsidiary of FIAT introduced the Fiat Siena Tetra fuel, a four-fuel car that can run on natural gas (CNG).
NGV filling stations can be located anywhere that natural gas lines exist. Compressors (CNG) or liquifaction plants (LNG) are usually built on large scale but with CNG small home refueling stations are possible. A company called FuelMaker pioneered such a system called Phill Home Refueling Appliance (known as “Phill”), which they developed in partnership with Honda for the American GX model. Phill is now manufactured and sold by BRC FuelMaker, a division of Fuel Systems Solutions, Inc.
CNG may be generated and used for bulk storage and pipeline transport of renewable energy and also be mixed with biogas, produced from landfills or wastewater. This would allow the use of CNG for mobility without increasing the concentration of carbon in the atmosphere. It would also allow continued use of CNG vehicles currently powered by non-renewable fossil fuels that do not become obsolete when stricter CO2 emissions regulations are mandated to combat global warming.
Despite its advantages, the use of natural gas vehicles faces several limitations, including fuel storage and infrastructure available for delivery and distribution at fueling stations. CNG must be stored in high pressure cylinders (3000psi to 3600psi operation pressure), and LNG must be stored in cryogenic cylinders (-260F to -200F). These cylinders take up more space than gasoline or diesel tanks that can be molded in intricate shapes to store more fuel and use less on-vehicle space. CNG tanks are usually located in the vehicle’s trunk or pickup bed, reducing the space available for other cargo. This problem can be solved by installing the tanks under the body of the vehicle, or on the roof (typical for busses), leaving cargo areas free. As with other alternative fuels, other barriers for widespread use of NGVs are natural gas distribution to and at fueling stations as well as the low number of CNG and LNG stations.