Natural gas vehicle
- "NGV" redirects here. For the art gallery in Melbourne, Australia, see National Gallery of Victoria.
A natural gas vehicle (NGV) is an alternative fuel vehicle that uses compressed natural gas (CNG) or liquefied natural gas (LNG) as a cleaner alternative to other fossil fuels. Natural gas vehicles should not be confused with vehicles powered by propane (LPG), which is a fuel with a fundamentally different composition. Worldwide, there were 14.8 million natural gas vehicles by 2011, led by Iran with 2.86 million, Pakistan (2.85 million), Argentina (2.07 million), Brazil (1.70 million), and India (1.10 million). The Asia-Pacific region leads the world with 6.8 million NGVs, followed by Latin America with 4.2 million vehicles. In the Latin American region 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 to run on ordinary gasoline.
As of 2009, the U.S. had a fleet of 114,270 compressed natural gas (CNG) vehicles, mostly buses; 147,030 vehicles running on liquefied petroleum gas (LPG); and 3,176 vehicles liquefied natural gas (LNG). Other countries where natural gas-powered buses are popular include India, Australia, Argentina, and Germany. 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. An increasing number of vehicles worldwide are being manufactured to run on CNG. Until recently, the Honda Civic GX was the only NGV commercially available in the US market., however now Ford, GM and Ram have bi-fuel offerings in their vehicle lineup. Fords approach is to offer a bi-fuel prep kit as a factory option, and then have the customer choose an authorized partner to install the natural gas equipment. Choosing GM's bi-fuel option sends the HD pickups with the 6.0L gasoline engine to IMPCO in Indiana to upfitt the vehicle to run on CNG. Ram currently is the only pickup truck manufacturer with a truly factory-installed bi-fuel system available in the U.S. market. Outside the U.S. GM do Brasil introduced the MultiPower engine in August 2004 which was capable of using CNG, alcohol and gasoline (E20-E25 blend) as fuel, and it was used in the Chevrolet Astra 2.0 model 2005, aimed at the taxi market. In 2006 the Brazilian subsidiary of FIAT introduced the Fiat Siena Tetra fuel, a four-fuel car developed under Magneti Marelli of Fiat Brazil. This automobile can run on natural gas (CNG); 100% ethanol (E100); E20 to E25 gasoline blend, Brazil's mandatory gasoline; and pure gasoline, though no longer available in Brazil it is used in neighboring countries.
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.
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.
- 1 CNG/LNG as fuel for automobiles
- 2 Chemical composition and energy content
- 3 Storage and transport
- 4 Implementation
- 4.1 Overview
- 4.2 Europe
- 4.3 North America
- 4.4 South America
- 4.5 South Asia
- 4.6 Southeast Asia
- 4.7 East Asia
- 5 See also
- 6 References
- 7 External links
CNG/LNG as fuel for automobiles
Differences between LNG and CNG fuels
Though LNG and CNG are both considered NGVs, the technologies are vastly different. Refueling equipment, fuel cost, pumps, tanks, hazards, capital costs are all different.
One thing they share is that due to engines made for gasoline, computer controlled valves to control fuel mixtures are required for both of them, often being proprietary and specific to the manufacturer. The on-engine technology for fuel metering is the same for LNG and CNG.
CNG as an auto fuel
CNG, or compressed natural gas, is stored at high pressure, 3,000 to 3,600 pounds per square inch (21 to 25 MPa). The required tank is more massive and costly than a conventional fuel tank. Refueling stations are more expensive to operate than LNG stations because of the energy required for compression. Time to fill a CNG tank varies greatly depending on the station. Home refuelers typically fill at about 0.4 GGE/hr. "Fast-fill" stations may be able to refill a 10 GGE tank in 5–10 minutes. Also, because of the lower energy density, the range on CNG is limited by comparison to LNG.
LNG as an auto fuel
LNG, or liquified natural gas, is natural gas that has been cooled to a point that it is a cryogenic liquid. In its liquid state, it is still more than 2 times as dense as CNG. LNG is dispensed from bulk storage tanks at LNG fuel stations at rates exceeding 20 GGE/min. Because of its cryogenic nature, it is stored in specially designed insulated tanks. Generally speaking, these tanks operate at fairly low pressures (about 70-150 psi) when compared to CNG. A vaporizer is mounted in the fuel system that turns the LNG into a gas (which may simply be considered low pressure CNG).
Advantages over gasoline and diesel
LNG – and especially CNG – tends to corrode and wear the parts of an engine less rapidly than gasoline. Thus it's quite common to find diesel-engine NGVs with high mileages (over 500,000 miles). Emissions are cleaner, with lower emissions of carbon and lower particulate emissions per equivalent distance traveled. There is generally less wasted fuel.
Inherent Advantages/Disadvantages between autogas (LPG) power and NGV
Autogas, also known as LPG, has different chemical composition, but still a petroleum based gas, has a number of inherent advantages and disadvantages, as well as noninherent ones. The inherent advantage of autogas over CNG is that it requires far less compression (20% of CNG cost), is denser as its a liquid at room temperature, and thus far cheaper tanks (consumer) and fuel compressors (provider) than CNG. As compared to LNG, it requires no chilling (and thus less energy), or problems associated with extreme cold such as frostbite. Like NGV, it also has advantages over gasoline and diesel in cleaner emissions, along with less wear on engines over gasoline. The major drawback of LPG is its safety, the fuel is heavier than air, which causes it to collect in a low spot in the event of a leak, making it far more hazardous to use, as more care is needed.
Current advantages of LPG power over NGV
In places like the US, Thailand, and India, there are five to ten times more stations thus making the fuel more accessible than NGV stations. Other countries like Poland, South Korea, and Turkey, LPG stations and autos are widespread while NGVs are not. In addition, in some countries such as Thailand, the retail LPG fuel is considerably cheaper in cost.
Though ANG (adsorbed natural gas) has not yet been used in either providing stations nor consumer storage tanks, its low compression (500psi vs 3600 psi) has the potential to drive down costs of NGV infrastructure and vehicle tanks.
Chemical composition and energy content
The primary component of natural gas is methane (CH4), the shortest and lightest hydrocarbon molecule. It may also contain heavier gaseous hydrocarbons such as ethane (C2H6), propane (C3H8) and butane (C4H10), as well as other gases, in varying amounts. Hydrogen sulfide (H2S) is a common contaminant, which must be removed prior to most uses.
Combustion of one cubic meter yields 38 MJ (10.6 kWh). Natural gas has the highest energy/carbon ratio of any fossil fuel, and thus produces less carbon dioxide per unit of energy.
Storage and transport
The major difficulty in the use of natural gas is transportation. Natural gas pipelines are economical and common on land and across medium-length stretches of water (like Langeled, Interconnector and Trans-Mediterranean Pipeline), but are impractical across large oceans. Liquefied natural gas (LNG) tanker ships, railway tankers, and tank trucks are also used.
CNG is typically stored in steel or composite containers at high pressure (3000 to 4000 psi, or 205 to 275 bar). These containers are not typically temperature controlled, but are allowed to stay at local ambient temperature. There are many standards for CNG cylinders, the most popular one is ISO 11439. For North America the standard is ANSI NGV-2.
LNG storage pressures are typically around 50-150 psi, or 3 to 10 bar. At atmospheric pressure, LNG is at a temperature of -260°F (-162°C), however, in a vehicle tank under pressure the temperature is slightly higher (see saturated fluid). Storage temperatures may vary due to varying composition and storage pressure. LNG is far denser than even the highly compressed state of CNG. As a consequence of the low temperatures, vacuum insulated storage tanks typically made of stainless steel are used to hold LNG.
CNG can be stored at lower pressure in a form known as an ANG (Adsorbed Natural Gas) tank at 35 bar (500 psi, the pressure of gas in natural gas pipelines) in various sponge like materials, such as activated carbon and metal-organic frameworks (MOFs). The fuel is stored at similar or greater energy density than CNG. This means that vehicles can be refuelled from the natural gas network without extra gas compression, the fuel tanks can be slimmed down and made of lighter, less strong materials.
Conversion kits for gasoline or diesel to LNG/CNG are available in many countries, along with the labor to install them. However, the range of prices and quality of conversion vary enormously.
Recently, regulations involving certification of installations in USA have been loosened to include certified private companies, those same kit installations for CNG have fallen to the $6,000+ range (depending on type of vehicle).
|Top ten countries
with the largest NGV vehicle fleets - 2013
|World Total = 18.09 million NGV vehicles|
Natural gas vehicles are popular in regions or countries where natural gas is abundant and where the government chooses to price CNG lower than gasoline. The use of natural gas began in the Po River Valley of Italy in the 1930s, followed by New Zealand in the 1980s, though its use has declined there. At the peak of New Zealand's natural gas use, 10% of the nation's cars were converted, around 110,000 vehicles. In the United States CNG powered buses are the favorite choice of several public transit agencies, with a fleet of more than 114,000 vehicles, mostly buses. India, Australia, Argentina, and Germany also have widespread use of natural gas-powered buses in their public transportation fleets.
Germany hit the milestone of 900 CNG filling stations nationwide in December 2011. Gibgas, an independent consumer group, estimates that 21% of all CNG filling stations in the country offer a natural gas/biomethane mix to varying ratios, and 38 stations offer pure biomethane.
Bus Eireann Introduced the first NGV on 17 July 2012. It will operate on the 216 city centre to Mount Oval, Rochestown, route until mid-August on a trial being undertaken in partnership with Bord Gáis. The Eco-city bus is made by MAN.
Natural gas traction is quite popular in Italy, due to the existence of a capillar distribution network for industrial use since the late 50s and a traditionally high retail price for petrol. As of April 2012 there were about 1173 filling stations, mainly located in the northern regions, while the fleet reached 730,000 CNG vehicles at the end of 2010.
Ukraine’s first compressed natural gas refueling station (CNGS) was commissioned in 1937. Today, there is a well-developed CNGS network across the country. Many buses were converted to run on CNG during the 1990s, primarily for economic reasons. The retrofitted cylinders are often visible atop the vehicle's roof and/or underneath the body. Despite their age, these buses remain in service and continue to provide reliable public transport combined with the environmental benefits of CNG.
With the recent increase in natural gas production due to widespread use of fracking technology, many countries, including the United States and Canada, now can be self-sufficient. Canada is a substantial net exporter of natural gas, though the United States still has a net import of natural gas. Natural gas prices have decreased dramatically in the past few years and are likely to decrease further as additional production comes on line. However, the EIA predicts that natural gas prices will start increasing in a few years as the most profitable natural gas reserves are used up. Natural gas prices have decreased from $13 per mmbtu (USD) in 2008 to $3 per mmbtu (USD) in 2012. It is likely therefore that natural gas-powered vehicles will be increasingly cheaper to run relative to gasoline powered vehicles. The issue is how to finance the purchase and installation of conversion kits. Some support may be available through the Department of Energy. Private initiatives which essentially lease the conversion equipment in exchange for slightly higher natural gas refueling can be self-financing and offer considerable advantages to liquidity strapped consumers.
Natural Gas has been used as a motor fuel in Canada for over 20 years. With assistance from federal and provincial research programs, demonstration projects, and NGV market deployment programs during the 1980s and 1990s, the population of light-duty NGVs grew to over 35,000 by the early 1990s. This assistance resulted in a significant adoption of natural gas transit buses as well. The NGV market started to decline after 1995, eventually reaching today’s vehicle population of about 12,000.
This figure includes 150 urban transit buses, 45 school buses, 9,450 light-duty cars and trucks, and 2,400 forklifts and ice-resurfacers. The total fuel use in all NGV markets in Canada was 1.9 petajoules (PJs) in 2007 (or 54.6 million litres of gasoline litres equivalent), down from 2.6 PJs in 1997. Public CNG refuelling stations have declined in quantity from 134 in 1997 to 72 today. There are 22 in British Columbia, 12 in Alberta, 10 in Saskatchewan, 27 in Ontario, and 1 in Québec. There are only 12 private fleet stations.
As of December 2009, the U.S. had a fleet of 114,270 compressed natural gas (CNG) vehicles, 147,030 vehicles running on liquefied petroleum gas (LPG), and 3,176 vehicles running on liquefied natural gas (LNG). The NGV fleet is made up mostly of transit buses but there are also some government fleet cars and vans, as well as increasing number of corporate trucks replacing diesel versions, most notably Waste Management, Inc and UPS trucks. As of 12-Dec-2013 Waste Management has a fleet of 2000 CNG Collection trucks; as of 12-Dec-2013 UPS has 2700 alternative fuel vehicles. As of February 2011, there were 873 CNG refueling sites, 2,589 LPG sites, and 40 LNG sites, led by California with 215 CNG refueling stations in operation, 228 LPG sites and 32 LNG sites. The number of refueling stations includes both public and private sites, and not all are available to the public. As of December 2010, the U.S. ranked 6th in the world in terms of number of NGV stations.
The natural gas vehicle market is limited to fleet vehicles and other public use vehicles like minibuses in larger cities. However the state-owned bus company RTP Of Mexico City has purchased 30 Hyundai Super Aero City CNG-Propelled buses to integrate with the existing fleet as well as to introduce new routes within the city.
CNG vehicles are common in South America, with a 35% share of the worldwide NGV fleet, where these vehicles are mainly used as taxicabs in main cities of Argentina and Brazil. Normally, standard gasoline vehicles are retrofitted in specialized shops, which involve installing the gas cylinder in the trunk and the CNG injection system and electronics.
As of 2009 Argentina had 1,807,186 NGV's with 1,851 refueling stations across the nation, or 15% of all vehicles; and Brazil had 1,632,101 vehicles and 1,704 refueling stations, with a higher concentration in the cities of Rio de Janeiro and São Paulo.
Peru had 81,024 NGVs and 94 fueling stations as 2009,. In Peru several factory-built CNVs have the tanks installed under the body of the vehicle, leaving the trunk free. Among the models built with this feature are the Fiat Multipla, the new Fiat Panda, the Volkswagen Touran Ecofuel, the Volkswagen Caddy Ecofuel, and the Chevy Taxi.
GM do Brasil introduced the MultiPower engine in August 2004 which was capable of using CNG, alcohol and gasoline as fuel. The GM engine has electronic fuel injection that automatically adjusts to any acceptable fuel configuration. This motor was used in the Chevrolet Astra and was aimed at the taxi market.
In 2006 the Brazilian subsidiary of FIAT introduced the Fiat Siena Tetra fuel, a four-fuel car developed under Magneti Marelli of Fiat Brazil. This automobile can run on 100% ethanol (E100), E20 to E25 blend (Brazil's normal ethanol gasoline blend), pure gasoline (not available in Brazil), and natural gas, and switches from the gasoline-ethanol blend to CNG automatically, depending on the power required by road conditions.
Since 2003 and with the commercial success of flex cars in Brazil, another existing option is to retrofit an ethanol flexible-fuel vehicle to add a natural gas tank and the corresponding injection system. Some taxicabs in São Paulo and Rio de Janeiro, Brazil, run on this option, allowing the user to choose among three fuels (E25, E100 and CNG) according to current market prices at the pump. Vehicles with this adaptation are known in Brazil as tri-fuel cars.
Pakistan was the country with the second largest fleet of NGV with a total of 2.85 million by the end of 2011. Most of the public transportation fleet has been converted to CNG. Also, in Pakistan and India, there have been on-going (last several years now) series of CNG fuel shortages which periodically waxes and wanes, getting the fuel into a tank can be a major problem. In July 2011, petrol usage shot up 15% from the month before due to shortages. Pakistan also has reported that over 2,000 people have died in 2011 from CNG cylinder blasts, because of low quality of cylinders there. In 2012, the Pakistani government took the decision to gradually phase out CNG sector altogether beginning by banning any new conversions to CNG and banning the manufacturing of new NGV's. In addition the government plans to close down all refueling stations in the next 3 years.
In 1993, CNG had become available in Delhi, India's capital, though LPG is what really took off due to its inherently far lower capital costs. Compressed Natural Gas is a domestic energy produced in Western parts of India. In India, most CNG vehicles are dual fueled, which means they can run both on CNG and gasoline. This makes it very convenient and users can drive long distances without worrying about availability of natural gas (as long as gasoline is available). As of December 2010 India had 1,080,000 NGVs and 560 fueling stations, many of the older ones being LPG rather than CNG. In addition, it is thought that more illegally converted LPG autos than legal ones ply the streets in India, some estimates are as high as 15 million "autos" (running the gamut of everything from LPG motored pedal bicycles to CNG buses)
In 1995, a lawyer filed a case with the Supreme Court of India under the Public Interest Litigation rule, which is part of the Constitution of India and enables any citizen to address directly the Supreme Court. The lawyer’s case was about the health risks caused by air pollution emitted from road vehicles. The Supreme Court decided that cars put into circulation after 1995 would have to run on unleaded fuel. By 1998, India was converted to 100% of unleaded fuel after the government ruled that diesel cars in India were restricted to 10,000 ppm after 1995. At the beginning of 2005, 10,300 CNG busses, 55,000 CNG three-wheelers taxis, 5,000 CNG minibuses, 10,000 CNG taxis and 10,000 CNG cars run on India’s roads (1982-2008 Product-Life Institute, Geneva). The Delhi Transport Corporation currently operates the world's largest fleet of CNG buses for public transport.
By the end of 2011, Iran had the world's largest fleet of NGV at 2.86 million vehicles. The growth of NGV market in Iran has in large part been due to Iranian government intervention to decrease the society's dependence on gasoline. This governmental plan was implemented to reduce the effect of sanctions on Iran and make the nation's domestic market less dependent on imported gasoline. Iran has been manufacturing its own NGV's through local manufacturing using dedicated CNG engines which use gasoline only as a back up fuel. Also by 2012, Iranian manufacturers had the capacity to build 1.5 million CNG cylinders per year and therefore Iranian government has banned their imports to support the local manufacturers. In addition CNG in Iran costs the least compared to the rest of the world. In 2012, Iranian government announced a plan to replace the traditional CNG cylinders with Adsorbed Natural Gas (ANG) cylinders.
In view of a generous supply of natural gas but relying on imported oil, the Thailand government heavily promoted alternative fuels like LPG, natural gas and ethanol to replace gasoline beginning around 2003, yet NGV was very slow to take off due to cheaper LPG fuel, a pre-existing LPG fleet, and very low conversion cost of local LPG conversion shops as compared to factory installed CNG or conversion. A significant effort was taken when the state-controlled oil company PTT PCL built a network of natural gas refueling stations. The cost of subsidy was estimated at US$150 million in 2008.
As price of oil climbed rapidly, it was estimated more than 40,000 new cars and trucks powered by natural-gas were purchased in six months in 2008, including many buses. That year, about half of the taxi fleet in Bangkok used LPG, and were prodded to convert to CNG, with little success. Since 2008 there has been a government arm-twisting to switch from LPG to CNG, with a rollout of CNG stations near Bangkok around 2007 and then upcountry in 2010, at times replacing LPG stations. Operators of used vehicles have balked at the massive conversion cost (up to quadruple that of LPG in Thailand), especially given Thailand's strong ultra-competitive domestic LPG conversion industry, as well as retail CNG fuel cost (one and a half times). Thailand had some 700,000 LPG fueled vehicles, and 300,000 CNG fueled, with 1,000 LPG stations and 600 CNG as of 2011. Demand has increased 26% over 2011 for CNG in Thailand. As of the end of 2012, Thailand has 1,014,000 LPG fueled vehicles, and consumed 606,000 tonnes in 2012 of LPG, while 483 stations serve up some 380,000 CNG vehicles., showing that LPG conversion continues to enjoy heavy favor over NGVs despite massive government push for CNG. CNG vehicles are more likely to be bought factory installed while LPG is likely to be an aftermarket conversion. LNG vehicles in Thailand are almost non-existent except for lorries.
In Malaysia, the use of compressed natural gas was originally introduced for taxicabs and airport limousines during the late-1990s, when new taxis were launched with NGV engines while taxicab operators were encouraged to send in existing taxis for full engine conversions, reducing their costs of operation. Any vehicle converted to use CNG is labelled with white rhombus "NGV" (Natural Gas Vehicle) tags, lending to the common use of "NGV" when referring to road vehicles with CNG engine. The practice of using CNG remained largely confined to taxicabs predominantly in the Klang Valley and Penang due to a lack of interest. No incentives were offered for those besides taxicab owners to use CNG engines, while government subsidies on petrol and diesel made conventional road vehicles cheaper to use in the eyes of the consumers. Petronas, Malaysia's state-owned oil company, also monopolises the provision of CNG to road users. As of July 2008[update], Petronas only operates about 150 CNG refueling stations, most of which are concentrated in the Klang Valley. At the same time, another 50 was expected by the end of 2008.
As fuel subsidies were gradually removed in Malaysia starting June 5, 2008, the subsequent 41% price hike on petrol and diesel led to a 500% increase in the number of new CNG tanks installed. National car maker Proton considered fitting its Waja, Saga and Persona models with CNG kits from Prins Autogassystemen by the end of 2008, while a local distributor of locally assembled Hyundai cars offers new models with CNG kits. Conversion centres, which also benefited from the rush for lower running costs, also perform partial conversions to existing road vehicles, allowing them to run on both petrol or diesel and CNG with a cost varying between RM3,500 to RM5,000 for passenger cars.
There were about 400 CNG-fueled vehicles in Singapore in mid-2007, of which about 110 are taxis operated by Smart Automobile. By February 2008, the number has risen 520 CNG vehicles, of which about half are taxis. All vehicles had to refuel at the sole CNG station operated by Sembcorp Gas and located on Jurong Island until the opening of the first publicly accessible CNG station at Mandai in 2008, operated by Smart Automobile. The company plans to build another four stations by 2011, by which time the company projects to operated 3,000 to 4,000 CNG taxies, and with 10,000 CNG public and commercial vehicles of other types on Singapore's roads. Sembcorp Gas opened its second CNG station a week after the Mandai station at Jalan Buroh.
CNG is almost unheard of as a transport fuel before 2010 in the archipelago except in Jakarta, where a very relatively minor amount of vehicles, most notably Transjakarta buses, use the fuel. However, since 2010 there has been a government emphasis to push usage of CNG not only for vehicle fuel, but also for domestic consumption over wood burning (which can produce deadly methanol) and kerosene.
China had 450,000 NGV's and 870 refueling stations as of 2009. China in 2012 has 1 million NGVs on the roads, 3 million forecast for 2015, with over 2000 stations (both CNG and LPG), with plans for 12,000 by 2020.
For the purpose of improving air quality in the metropolitan area of Seoul, CNG bus was first introduced in July, 1997. In 2009, it was reported that 17,445 out of 32,280 buses was running on compressed natural gas. Especially, most of Seoul buses are now operate exclusively on CNG.
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|Wikimedia Commons has media related to Compressed natural gas vehicles.|
- Natural Gas Vehicle Availability, U.S. Department of Energy, 2012-07-30
- Greener Cities – International project dedicated to the development of an ever growing demand for environmentally friendly eco-sustainable vehicles, specifically to promote the use of cleaner fuels such as CNG and Biogas
- "Natural Gas Vehicles".
- The Italian community of Natural Gas Vehicles – Forum, technical info, maps (also in English, German, and French)
- A Student's Guide to Alternative Fuel Vehicles: Compressed natural gas - natural gas under high pressure, California Energy Commission, April 22, 2002
- Consumers' Guide to Compressed Natural Gas, U.S. Department of Energy[dead link]
- "Boost for natural gas cars: Home fueling". NBCnews.com. 9/10/2004.
- "CNG California".