A natural gas vehicle (NGV) is an alternative fuel vehicle that uses compressed natural gas (CNG) or liquefied natural gas (LNG). Natural gas vehicles should not be confused with vehicles powered by LPG (mainly propane), 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 liquefaction, but requires a trade-off in terms of size/complexity/weight of the storage container, range of the vehicle between refueling 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 more than 77% (2014), 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 biomethane, itself derived from biogas from landfills or anaerobic digestion. 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.
CNG-powered vehicles are considered to be safer than gasoline-powered vehicles.
CNG/LNG as fuel for automobiles
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. Until recently, the now-discontinued 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. 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.
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, 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.
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.
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. 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 [missing citation – the initial liquefaction of natural gas by cooling requires more energy than gas 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. 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 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 usually dispensed from bulk storage tanks at LNG fuel stations at rates exceeding 20 DGE/min. Sometimes LNG is made locally from utility pipe. 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). When comparing building a commercial LNG station with a CNG station, utility infrastructure, capital cost, and electricity heavily favor LNG over CNG. There are existing LCNG stations (both 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.
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 500,000 miles). CNG also emits 20-29% less CO2 than diesel and gasoline. Emissions are cleaner, with lower emissions of carbon and lower particulate emissions per equivalent distance traveled. There is generally less wasted fuel. However, cost (monetary, environmental, pre-existing infrastructure) of distribution, compression, cooling must be taken into account.
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 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 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 volatile 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 and more care is needed in handling. Besides this, LPG (40% from Crude Oil refining) is more expensive than Natural Gas.
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.
LNG fueled vehicles
Use of LNG to fuel large over-the-road trucks
LNG is being evaluated and tested for over-the-road trucking, off-road, marine, and railroad applications. There are known problems with the fuel tanks and delivery of gas to the engine.
China has been a leader in the use of LNG vehicles with over 100,000 LNG powered vehicles on the road as of 2014.
In the United States, there were 69 public truck LNG fuel centres as of February 2015. The 2013 National Trucker’s Directory lists approximately 7,000 truckstops, thus approximately 1% of US truckstops have LNG available.
In 2013, Dillon Transport announced they were putting 25 LNG large trucks into service in Dallas Texas. They are refueling at a public LNG fuel center. The same year Raven Transportation announced they were buying 36 LNG large trucks to be fueled by Clean Energy Fuels locations and Lowe’s finished converting one of its dedicated fleets to LNG fueled trucks.
UPS had over 1200 LNG fueled trucks on the roads in February 2015. UPS has 16,000 tractor trucks in its fleet, and 60 of the new for 2014 large trucks will be placed in service in the Houston, Texas area, where UPS is building its own private LNG fuel center to avoid the lines at retail fuel centers. In Amarillo, Texas and Oklahoma City, Oklahoma, UPS is using public fuel centers.
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, California to Houston, Texas by refueling exclusively at Clean Energy Fuels public facilities. In 2014 Shell and Travel Centers of America opened the first of a planned network of U.S. truck stop LNG stations in Ontario, California. 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, and Clean Energy had 39 operational public LNG facilities.
As can be seen at the alternative fuel fueling center tracking site, as of early 2015 there is void of LNG fuel centers, public and private, from Illinois to the Rockies. A Noble Energy LNG production plant in northern Colorado was planned to go online in 1st quarter 2015 and to have a capacity of 100,000 gallons of LNG per day for on-road, off-road, and drilling operations.
As of 2014, LNG fuel and NGV’s had not achieved much usage in Europe.
American Gas & Technology pioneered 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 300-5,000 gallons 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 an 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 for high-horsepower/high-torque engines a fuel that can inherently be used to create a more energy dense air-fuel mixture is preferred because a smaller and simpler engine can be used to 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 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, 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 580C/1076F, whereas gasoline and diesel autoignite at approximately 250C and 210C 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, but at typical CNG compression pressures the fuel itself is less energy dense than gasoline or diesel thus the end 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 on European CNG cars. Despite that limitation, the 12 liter Cummins Westport ISX12G engine is an example of a CNG capable engine designed to pull tractor/trailer loads up to 80,000 lbs 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.
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 -160C, using a simple heat exchanger a small amount of LNG can be converted to its gaseous form at extremely high pressure with the use of little or no mechanical energy. A properly designed high-horsepower engine can leverage this extremely 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. The end result when compared to CNG engines is more overall efficiency in high-horsepower engine applications when high-pressure direct injection technology is used. The Westport HDMI2 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 is another example of a LNG engine leveraging advanced high pressure technology.
Westport recommends CNG for engines 7 liters or smaller and LNG with direct injection for engines between 20 and 150 liters. For engines between 7 and 20 liters either option is recommended. See slide 13 from their NGV BRUXELLES – INDUSTRY INNOVATION SESSION presentation
High horsepower engines in the oil drilling, mining, locomotive, and marine fields have been or are being developed. Paul Blomerous has written a paper concluding as much as 40 million tonnes per annum of LNG (approximately 26.1 billion gallons/year or 71 million gallons/day) could be required just to meet the global needs of the high-horsepower engines by 2025 to 2030.
As of the end of 1st quarter 2015 Prometheus Energy Group Inc claims to have delivered over 100 million gallons of LNG within the previous 4 years into the industrial market, and is continuing to add new customers.
The MV Isla Bella is the world’s first LNG powered container ship. 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.
Some airplanes use LNG to power their turbofans. Aircraft are particularly sensitive to weight and much of the weight of an aircraft goes into fuel carriage to allow the range. The low energy density of natural gas even in liquid form compared to conventional fuels give it a distinct disadvantage for flight applications.
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).
Source from Wikipedia