A Natural Gas Vehicle (NGV) utilizes compressed natural gas (CNG) or liquefied natural gas (LNG) as an alternative fuel source. Distinguished from autogas vehicles fueled by liquefied petroleum gas (LPG), NGV's rely on methane combustion, resulting in cleaner emissions due to the removal of contaminants from the natural gas source.

Conversion of existing gasoline or diesel vehicles to NGVs is feasible, offering both dedicated and bi-fuel options. Heavy-duty vehicles such as trucks and buses can also undergo conversion, utilizing spark ignition systems or hybrid electric motor configurations.

Challenges in NGV adoption include the storage and refueling of natural gas, given its pressurized or liquefied state. While advancements in compression and liquefaction mitigate energy density differences, trade-offs regarding storage container size, complexity, and weight impact vehicle range. Despite these challenges, the safety and cost advantages of methane over hydrogen fuel contribute to its viability.

Obstacles to widespread NGV adoption for private vehicles include concerns over additional weight, technological unfamiliarity, and limited refueling infrastructure in some regions. Nevertheless, global NGV numbers reached nearly 28 million by 2019,^[1] with significant market presence in countries such as China, Iran, India, Pakistan, Argentina, Brazil, and Italy.

Advantages

CNG may be generated and used for bulk storage and pipeline transport of renewable energy and also be mixed with biomethane, itself derived from biogas from landfills or anaerobic digestion. This allows for the use of CNG in mobility with significantly lower carbon emissions compared to regular diesel-powered vehicles. It would also allow the continued use of CNG vehicles currently powered by non-renewable fossil fuels so that those vehicles do not become obsolete if stricter CO
₂ emissions regulations are mandated to combat climate change.

A key advantage of using natural gas is the existence, in principle, of most of the infrastructure and 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 of. 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.

Challenges

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 (21,000 to 25,000 kPa (3,000 to 3,600 psi)), and LNG must be stored in cryogenic cylinders (−162 to −129 °C (−260 to −200 °F)). 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 sometimes located in the vehicle's trunk or pickup bed, reducing the space for other cargo. This problem has been solved by installing the tanks under the vehicle's body or on the roof (typical for buses), leaving cargo areas free. As with alternative fuels, other barriers to the widespread use of NGVs are natural gas distribution and the lower number of CNG and LNG stations then liquid fuels.^[2]

Other challenges include relatively expensive 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 that refiners are currently set up to produce a certain fuel mix from crude oil. Aviation fuel is likely to remain the fuel of choice for aircraft due to its weight sensitivity for the foreseeable future

Infrastructure

NGV filling stations can be located anywhere natural gas lines exist. Compressors (CNG) or liquefaction plants (LNG) are usually built on a large scale, but small home CNG refueling stations are possible. A company called Fuel Maker pioneered a system called Phil Home Refueling Appliance (known as "Phil"), which they developed in partnership with Honda for the American GX model.^[3][4] Phil is now manufactured and sold by BRC Fuel Maker, a division of Fuel Systems Solutions, Inc.^[5]

Deployment

The Asia-Pacific region leads the world with 6.8 million vehicles, followed by Latin America with 4.2 million.^[1][6] In Latin America, almost 90% of NGVs have bi-fuel engines, allowing these vehicles to run on either gasoline or CNG.^[7] 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.^[8] In OECD countries, there are around 500,000 CNG vehicles.^[2] Pakistan's market share of NGVs was 61.1% in 2010, followed by Armenia with more than 77% (2014), and Bolivia with 20%.^[6] The number of NGV refueling stations has also increased to 18,202 worldwide as of 2010, up 10.2% from the previous year.^[6]

CNG/LNG as fuel for automobiles

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).^[9]

CNG-powered vehicles are considered safer than gasoline-powered ones.^[10][11][12]

Available production cars

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). However, an increasing number of vehicles worldwide are being manufactured to run on CNG.^[13] Until recently,^[when?] the now-discontinued Honda Civic GX was the only NGV commercially available in the US market.^[14][15] More recently, Ford, General Motors and Ram Trucks have bi-fuel offerings in their vehicle lineup.^{[citation needed]} Ford's 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 upfit the vehicle to run on CNG. Ram currently is the only pickup truck manufacturer with a truly CNG factory-installed bi-fuel system available in the U.S. market.^{[citation needed]}

Outside the U.S. GM do Brasil introduced the Multipower engine in 2004, which was capable of using CNG, alcohol, and gasoline (E20-E25 blend) as fuel. It was used in the Chevrolet Astra 2.0 model 2005, aimed at the taxi market.^[16][17] 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. However, it is no longer available in Brazil but is used in other neighboring countries.^[9][18]

In 2015, Honda announced its decision to phase out the commercialization of natural-gas-powered vehicles to focus on the development of a new generation of electrified vehicles such as hybrids, plug-in electric cars and hydrogen-powered fuel cell vehicles. Since 2008, Honda sold about 16,000 natural gas vehicles, mainly to taxi and commercial fleets.^[19]

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, and capital costs are 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, 210 to 250 bar (3,000 to 3,600 psi). The required tank is more massive and costly than a conventional fuel tank. Commercial on-demand refueling stations are more expensive to operate than LNG stations because of the energy required for compression; the compressor requires 100 times more electrical power. However, slow-fill (many hours) can be cost-effective with LNG stations . The time to fill a CNG tank varies greatly depending on the station. Home refuels typically fill at about 1 kg/h (0.4 GGE/h). At gas stations, where the pressure is greatly higher, it is possible to refill a 25 kg (10 GGE) tank in 5–10 minutes. Also, because of the lower energy density, the CNG range is lower compared to LNG. Gas composition and throughput allowing, it should be feasible to connect commercial CNG fueling stations to city gas networks or enable home fueling of CNG vehicles directly using a gas compressor. Similar to a car battery, the CNG tank of a car could double as a home energy storage device, and the compressor could be powered at times when there is excess / free renewable electrical energy.

LNG as an auto fuel

LNG, or liquified natural gas, is natural gas that has been cooled to the point that it is a cryogenic liquid. Its liquid state is still more than twice as dense as CNG. LNG is either dispensed from bulk storage tanks or made locally from utility pipes at LNG fuel stations. Because of its cryogenic nature, it is stored in specially designed insulated tanks. Generally speaking, these tanks operate at fairly low pressures (about 5 to 10 bar (72.5 to 145.0 psi)) when compared to CNG. A vaporizer is mounted in the fuel system, turning the LNG into a gas (which may be considered low-pressure CNG). When comparing a commercial LNG station with a CNG station, utility infrastructure, capital cost, and electricity heavily favor LNG over CNG, but the availability of piped gas is more common than LNG delivery by tanker. There are existing LCNG stations (CNG and LNG), where fuel is stored as LNG, then vaporized to CNG on demand. LCNG stations require less capital cost than fast-fill CNG stations alone but more than LNG stations.

Aside from different fuel tanks, fuel metering, and computer modules, the engines in NG vehicles could be run by CNG or LNG without requiring any modification because both are forms of natural gas.

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 is quite common to find diesel-engine NGVs with high mileages (over 800,000 km (500,000 mi)). CNG also emits 20-29% less CO₂ than diesel and gasoline.^[20] Emissions are cleaner, with lower emissions of carbon and lower particulate emissions per equivalent distance traveled. There is generally less wasted fuel. However, the cost (monetary, environmental, pre-existing infrastructure) of distribution, compression, and cooling must be taken into account. In addition, natural gas has a higher Octane rating than gasoline, with an octane rating of 112 RON/105 AKI for propane and 120 RON/120 AKI for methane, making it less likely to cause engine knocking.

Inherent advantages/disadvantages between autogas (LPG) power and NGV

Autogas, also known as LPG or propane, has a different chemical composition. However, it is still a petroleum-based gas produced from refining petroleum oil and has several 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),^[21] is denser and easier to store as at this lower pressure it is a liquid at room temperature, and thus requires 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 the extreme cold such as frostbite. Like NGVs, it also has advantages over gasoline and diesel in terms of cleaner emissions and less wear on engines than gasoline. A major drawback of LPG is its safety. The fuel is volatile and flammable, and the fumes are heavier than air, which causes them to collect in a low spot in the event of a leak, making it far more hazardous to use. More care is needed in handling this. Besides this, LPG (40% from Crude Oil refining) has historically been more expensive than Natural Gas, even though the energy density per weight is higher.

Current advantages of LPG power over NGV

In places like the US, Thailand, and India, there are five to ten times more LPG stations, thus making the fuel more accessible than NGV stations. In some countries like Poland, South Korea, and Turkey, LPG stations and autos are widespread, while NGVs are not. In addition, retail LPG fuel is considerably cheaper in some countries, such as Thailand.

Future possibilities

Though ANG (adsorbed natural gas) has not yet been used in either providing stations nor consumer storage tanks, its low compression (34 bar (500 psi) vs 250 bar (3,600 psi))^[22] has the potential to drive down costs of NGV infrastructure and vehicle tanks.

LNG fueled vehicles

Use of LNG to fuel large over-the-road trucks

LNG is being evaluated and tested for over-the-road trucking,^[23] off-road,^[24] marine, and railroad applications.^[25] There are known problems with the fuel tanks and the delivery of gas to the engine.^[26]

China has been a leader in the use of LNG vehicles^[27] with over 100,000 LNG powered vehicles on the road as of 2014.^[28]

In the United States, there were 69 public truck LNG fuel centers as of February 2015.^[29] The 2013 National Trucker's Directory lists approximately 7,000 truck stops,^[30] thus approximately 1% of US truck stops have LNG available.

In 2013, Dillon Transport announced they were putting 25 large LNG trucks into service in Dallas, USA. They are refueling at a public LNG fuel center.^[31] The same year Raven Transportation announced they were buying 36 LNG large trucks to be fueled by Clean Energy Fuels locations^[32] and Lowe's finished converting one of its dedicated fleets to LNG fueled trucks.^[33]

UPS had over 1200 LNG fueled trucks on the roads in February 2015.^[34] UPS has 16,000 tractor trucks in its fleet, and 60 of the new 2014 large trucks will be placed in service in the Houston, USA area, where UPS is building its own private LNG fuel center to avoid the lines at retail fuel centers.^[35] In Amarillo, USA and Oklahoma City, USA, UPS is using public fuel centers.^[36]

Clean Energy Fuels has opened several public LNG Fuel Lanes along I-10 and claims that as of June 2014, LNG fueled trucks can use the route from Los Angeles to Houston, USA by refueling exclusively at Clean Energy Fuels public facilities.^[37] In 2014 Shell and Travel Centers of America opened the first of a planned network of U.S. truck stop LNG stations in California, USA.^[38] Per the alternative fuel fuelling centre tracking site there are 10 LNG capable public fuel stations in the greater Los Angeles area, making it the single most penetrated metro market. As of February 2015, Blu LNG has at least 23 operational LNG capable fuel centers across 8 states,^[39] and Clean Energy had 39 operational public LNG facilities.^[40]

As can be seen at the alternative fuel fueling center tracking site, as of early 2015 there is a void of LNG fuel centers, public and private, from Illinois to the Rockies.^[41] A Noble Energy LNG production plant in northern Colorado was planned to go online in the first quarter of 2015^[42] and to have a capacity of 100,000 gallons of LNG per day for on-road, off-road, and drilling operations.^[43]

As of 2014, LNG fuel and NGV's had not achieved much usage in Europe.^[44]

American Gas & Technology pioneered the use of onsite liquefaction using van sized station to access Natural Gas from utility pipe and clean, liquefy, store, and dispense it. Their stations make 1,100–18,900 litres (300–5,000 US gal) of LNG per day.

Use of LNG to fuel high-horsepower/high-torque engines

In internal combustion engines, the volume of the cylinders is a common measure of the power of an engine. Thus, a 2000cc engine would typically be more powerful than a 1800cc engine, but that assumes a similar air-fuel mixture is used.

If via a turbocharger as an example, the 1800cc engine were using an air-fuel mixture that was significantly more energy dense, then it might be able to produce more power than a 2000cc engine burning a less energy-dense air-fuel mixture. However, turbochargers are both complex and expensive. Thus, it becomes clear that for high-horsepower/high-torque engines, a fuel that can inherently be used to create an energy-dense dense air-fuel mixture is preferred because a smaller and simpler engine can produce the same power.

With traditional gasoline and diesel engines, the energy density of the air-fuel mixture is limited because the liquid fuels do not mix well in the cylinder. Further, gasoline and diesel auto-ignite^[45] at temperatures and pressures relevant to engine design. An important part of traditional engine design is designing the cylinders, compression ratios, and fuel injectors such that pre-ignition is avoided,^[46] but at the same time as much fuel as possible can be injected, become well mixed, and still have time to complete the combustion process during the power stroke.

Natural gas does not auto-ignite at pressures and temperatures relevant to traditional gasoline and diesel engine design, thus providing more flexibility in the design of a natural gas engine. Methane, the main component of natural gas, has an autoignition temperature of 580 °C,^[47] whereas gasoline and diesel autoignite at approximately 250 °C and 210 °C respectively.

With a compressed natural gas (CNG) engine, the mixing of the fuel and the air is more effective since gases typically mix well in a short period of time. Still, at typical CNG compression pressures, the fuel itself is less energy dense than gasoline or diesel thus the result is a lower energy dense air-fuel mixture. Thus for the same cylinder displacement engine, a non-turbocharged CNG-powered engine is typically less powerful than a similarly sized gasoline or diesel engine. For that reason, turbochargers are popular in European CNG cars.^[48] Despite that limitation, the 12 litre Cummins Westport ISX12G engine^[49] is an example of a CNG capable engine designed to pull tractor/trailer loads up to 36 tonnes (80,000 lb) showing CNG can be used in most if not all on-road truck applications. The original ISX G engines incorporated a turbocharger to enhance the air-fuel energy density.^[50]

LNG offers a unique advantage over CNG for more demanding high-horsepower applications by eliminating the need for a turbocharger. Because LNG boils at approximately -160 °C, using a simple heat exchanger, a small amount of LNG can be converted to its gaseous form at extremely high pressure with little or no mechanical energy. A properly designed high-horsepower engine can leverage this high-pressure energy-dense gaseous fuel source to create a higher energy-density air-fuel mixture than can be efficiently created with a CNG-powered engine. Compared to CNG engines, the result is more overall efficiency in high-horsepower engine applications when high-pressure direct injection technology is used. The Westport HDMI2^[51] fuel system is an example of a high-pressure direct injection technology that does not require a turbocharger if teamed with appropriate LNG heat exchanger technology. The Volvo Trucks 13-liter LNG engine^[52] is another example of an LNG engine leveraging advanced high-pressure technology.

Westport recommends CNG for engines 7 litres or smaller and LNG with direct injection for engines between 20 and 150 litres. For engines between 7 and 20 litres, either option is recommended. See slide 13 from their NGV BRUXELLES – INDUSTRY INNOVATION SESSION presentation^[53]

High horsepower engines in the oil drilling, mining, locomotive, and marine fields have been or are being developed. Paul Blomerus has written a paper^[54] concluding as much as 40 million tonnes per annum of LNG could be required to meet the global needs of the high-horsepower engines by 2025 to 2030.

As of the end of the first quarter of 2015, Prometheus Energy Group Inc claims to have delivered over 100 million gallons (380 million litres) of LNG within the previous four years into the industrial market,^[55] and is continuing to add new customers.

Motorcycles

Scooters can also use CNG as a fuel source.

Ships

The MV Isla Bella was the world's first LNG powered container ship.^[56] LNG carriers are sometimes powered by the boil-off of LNG from their storage tanks, although Diesel powered LNG carriers are also common to minimize loss of cargo and enable more versatile refueling.

Aircraft

Some airplanes use LNG to power their turbofans. Aircraft are susceptible to weight; much of an aircraft's weight goes into fuel carriage. Liquified natural gas has a high specific energy (MJ/kg), a useful optimization for flight applications.

Chemical composition and energy content

Chemical composition

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.

Energy content

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

Transportedit

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.

Storageedit

CNG is typically stored in steel or composite containers at high pressure (205 to 275 bars (2,970 to 3,990 psi)). 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.^[57][58] For North America the standard is ANSI NGV-2.

LNG storage pressures are typically around 3 to 10 bar. At atmospheric pressure, LNG is at a temperature of -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^[59] and metal-organic frameworks (MOFs).^[60] 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, and the fuel tanks can be slimmed down and made of lighter, less strong materials.

Conversion kitsedit

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 the USA have been loosened to include certified private companies, those same kit installations for CNG have fallen to the $6,000+ range (depending on the type of vehicle).^{[citation needed]}

Implementationedit

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