Compressed natural gas (CNG) is fuel gas composed primarily of methane (CH4), compressed to less than 1% of its volume at standard atmospheric pressure. It is stored in high-pressure containers at approximately , typically in cylindrical or spherical shapes.
CNG is used in modified petrol and internal combustion engine vehicles as well as purpose-built CNG vehicles. These include dedicated CNG systems, dual-fuel vehicles that combine CNG with liquid fuel, and bi-fuel vehicles. It can serve as an alternative to petrol, diesel fuel, and liquefied petroleum gas (LPG). Compared with these fuels, CNG combustion produces lower emissions of several pollutants. Because natural gas is lighter than air, it disperses more readily when released. Biomethane derived from anaerobic digestion or landfill sources can also be used.
CNG has been used in applications including auto rickshaws, pickup trucks, transit and school buses, and trains. Municipal fleets and public transport operators have often adopted the fuel for cost and infrastructure reasons. However, fuel storage and cylinder cost remain significant factors affecting wider adoption.
Energy density CNG's energy density is the same as liquefied natural gas at 53.6 MJ/kg. Its volumetric energy density, 9 MJ/L, is 42% of that of LNG (22 MJ/L) because it is not liquefied, and is 25 percent that of diesel fuel.
History
Gases provided the original fuel for internal combustion engines. The first experiments with compressed gases took place in France in the 1850s. Natural gas first became a transport fuel during World War I. In the 1960s, Columbia Natural Gas of Ohio tested a CNG carrier. The ship was to carry compressed natural gas in vertical pressure bottles; however, this design failed because of the high cost of the pressure vessels. Since then, there have been attempts at developing a commercially viable CNG carrier. Several competing CNG ocean transport designs have evolved. Each design proposes a unique approach to optimizing gas transport, while using as much off-the-shelf technology as possible, to keep costs competitive.
In the early 2000s, Canadian company SeaNG developed the Coselle system, a novel CNG carrier design that used coiled steel pipe cartridges to store compressed natural gas more efficiently. This approach received classification approval and demonstrated a potential alternative to traditional vertical pressure vessels, which had been previously limited by high costs and weight.
Around the same time, major classification societies such as Det Norske Veritas (DNV) and the American Bureau of Shipping (ABS) introduced formal certification rules for CNG carriers. These standards helped establish safety and design criteria that allowed CNG vessel projects to progress toward commercial viability, promoting broader acceptance within the maritime industry.
Following these developments, new CNG carrier designs emerged, including EnerSea Transport's VOTRANS system, which utilized horizontal pressure vessels to reduce structural stress and improve cargo handling efficiency. Additionally, modular designs like the GASVESSEL project explored composite cylinder technologies to enhance flexibility and lower operational costs.
Uses of CNG Motor vehicles
Worldwide, there were 14.8 million natural gas vehicles (NGVs) by 2011, with the largest numbers in Iran (4.07 million), Pakistan (2.85 million), Argentina (2.07 million), Brazil (1.7 million) and India (1.1 million), with the Asia-Pacific region leading with 5.7 million NGVs, followed by Latin America with almost four million vehicles.
Several car and vehicle manufacturers, such as Fiat, Opel/General Motors, Peugeot, Volkswagen, Toyota, Honda, Maruti Suzuki, Hyundai, Tata Motors, and others, sell bi-fuel cars. In 1995, The Supreme Court of India decided that cars put into circulation after 1995 would have to run on unleaded fuel. At the beginning of 2005, 10,300 CNG busses and 20,000 CNG cars ran on Delhi's roads. In 2006 Fiat Siena Tetrafuel was introduced in the Brazilian market, equipped with a 1.4-litre FIRE engine that runs on E100, E25 (Standard Brazilian Gasoline), Ethanol, and CNG.
Any existing petrol vehicle can be converted to a dual-fuel petrol/CNG vehicle. Authorized shops can do the retrofitting, which involves installing a CNG cylinder, plumbing, a CNG injection system, and electronics. The cost of installing a CNG conversion kit can often reach $8,000 on passenger cars and light trucks, and is usually reserved for vehicles that travel many miles each year. CNG costs about 50% less than petrol, and emits up to 90% fewer emissions than petrol.
Locomotives CNG locomotives are operated by several railroads. The Napa Valley Wine Train in the US successfully retrofitted a diesel locomotive to run on compressed natural gas before 2002. This converted locomotive was upgraded to utilize a computer-controlled fuel injection system in May 2008, and is now the Napa Valley Wine Train's primary locomotive. Ferrocarril Central Andino in Peru, has run a CNG locomotive on a freight line since 2005. CNG locomotives are usually diesel–electric locomotives that have been converted to use compressed natural gas generators instead of diesel generators to generate the electricity that drives the traction motors. Some CNG locomotives are able to selectively fire their cylinders only when there is a demand for power, which, theoretically, gives them a higher fuel-efficiency than conventional diesel engines. CNG is also cheaper than petrol or diesel fuel.
Natural gas transport
CNG is used to transport natural gas by sea for intermediate distances, using CNG carrier ships, especially when the infrastructure for pipelines or LNG is not in place. At short distances, undersea pipelines are often more cost-effective, and for longer distances, LNG is often more cost-effective.
Advantages
Natural gas vehicles have lower maintenance costs than other hydrocarbon-fuel-powered vehicles. CNG fuel systems are sealed, preventing fuel losses from spills or evaporation. Increased life of lubricating oils, as CNG does not contaminate and dilute the crankcase oil. Being a gaseous fuel, CNG mixes easily and evenly in air. CNG is less likely to ignite on hot surfaces, since it has a high auto-ignition temperature (540 °C), and a narrow range (5–15 percent) of flammability. CNG-powered vehicles are considered to be safer than petrol-powered vehicles. Less pollution and more efficiency: CNG emits significantly less pollution directly than petrol or oil when combusted (e.g., unburned hydrocarbons (UHC), carbon monoxide (CO), nitrogen oxides (NOX), sulfur oxides (SOx) and PM (particulate matter)). For example, an engine running on petrol for 100 km produces 22 kilograms of , while covering the same distance on CNG emits only 16.3 kilograms of . The lifecycle greenhouse gas emissions for CNG compressed from California's pipeline natural gas is given a value of 67.70 grams of -equivalent per megajoule (gCO2e/MJ) by CARB (the California Air Resources Board), approximately 28 percent lower than the average petrol fuel in that market (95.86 gCO2e/MJ). CNG produced from landfill biogas was found by CARB to have the lowest greenhouse gas emissions of any fuel analyzed, with a value of 11.26 gCO2e/MJ (more than 88 percent lower than conventional petrol) in the low-carbon fuel standard that went into effect on January 12, 2010. Due to lower carbon dioxide emissions, switching to CNG can help mitigate greenhouse gas emissions. However, natural gas leaks (both in the direct use and in the production and delivery of the fuel) represent an increase in greenhouse gas emissions. The ability of CNG to reduce greenhouse gas emissions over the entire fuel lifecycle will depend on the source of the natural gas and the fuel it is replacing.
Drawbacks Compressed natural gas vehicles require a greater amount of space for fuel storage than conventional petrol-powered vehicles. Since it is a compressed gas, rather than a liquid like petrol, CNG takes up more space for each GGE (gasoline gallon equivalent). However, the cylinders used to store the CNG take up space in the trunk of a car or bed of a pickup truck that has been modified to additionally run on CNG. This problem is solved in factory-built CNG vehicles that install the cylinders under the body of the vehicle, leaving the trunk free, e.g., Fiat Multipla, New Fiat Panda, Volkswagen Touran Ecofuel, Volkswagen Caddy Ecofuel, Chevy Taxi, which sold in countries such as Peru. Another option is installation on roof (typical on buses), but this could require structural modifications. In 2014, a test (by the Danish Technological Institute) of Euro6 heavy vehicles on CNG and diesel showed that CNG had higher fuel consumption, the same noise and production of CO2 and particulates, but NOX emission was lower.Gravesen, Christian Hedegaard. "Måleprogram for tunge CNG-køretøjer " page 58. EUDP journal: 64011-0330. Danish Technological Institute, October 2015. Quote: "Energy consumption was generally higher on CNG"
Leakage of unburned methane as natural gas is a significant issue because methane, the primary component of natural gas, is a powerful, short-lived greenhouse gas. It is more than 100 times more potent at trapping energy than carbon dioxide (), the principal contributor to man-made climate change.
Comparison with other natural gas fuels Compressed natural gas is often confused with LNG (liquefied natural gas). Both are stored forms of natural gas. The main difference is that CNG is stored at ambient temperature and high pressure, while LNG is stored at low temperature and nearly ambient pressure. In their respective storage conditions, LNG is a liquid and CNG is a supercritical fluid. CNG has a lower cost of production and storage compared to LNG as it does not require an expensive cooling process and cryogenic tanks. However, CNG requires a much larger volume to store the energy equivalent of petrol and the use of very high pressures (3000 to 4000 psi, or 205 to 275 bar). As a consequence of this, LNG is often used for transporting natural gas over large distances, in ships, trains or pipelines, where the gas is converted into CNG before distribution to the end user.
Natural gas is being experimentally stored at lower pressure in a form known as an ANG (adsorbed natural gas) cylinder, where it is adsorbed at 35 bar (500 psi, the pressure of gas in natural gas pipelines) in various sponge-like materials, such as carbon and MOFs (metal-organic frameworks). The fuel is stored at similar or greater energy density than CNG. This means that vehicles can be refueled from the natural gas network without extra gas compression, the fuel cylinders can be slimmed down and made of lighter, weaker materials.
It is possible to mix the ANG and CNG technology to reach an increased capacity of natural gas storage. In this process known as high pressure ANG, a high pressure CNG tank is filled by absorbers such as activated carbon (which is an adsorbent with high surface area) and stores natural gas by both CNG and ANG mechanisms.
Compressed natural gas is sometimes mixed with hydrogen (HCNG), which increases the H/C ratio (hydrogen/carbon ratio) of the fuel and gives it a flame speed up to eight times higher than CNG.
Codes and standards The lack of harmonized codes and standards across international jurisdictions is an additional barrier to NGV market penetration. The International Organization for Standardization has an active technical committee working on a standard for natural gas fuelling stations for vehicles.
Despite the lack of harmonized international codes, natural gas vehicles have an excellent global safety record. Existing international standards include ISO 14469-2:2007 which applies to CNG vehicle nozzles and receptacle and ISO 15500-9:2012 specifies tests and requirements for the pressure regulator.