Avgas (aviation gasoline) is an aviation fuel used to power piston-engine aircraft. Avgas can be distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars and some non-commercial light aircraft. Unlike mogas, avgas has added tetra-ethyl lead (TEL), a toxic substance used to enhance combustion stability.
Avgas is used in aircraft that have piston or Wankel engines. Gas turbines are able to operate on avgas, but typically do not. Turbine and diesel engines are designed to use kerosene-based jet fuel.
Avgas properties and varieties
The main
petroleum component used in blending avgas is
alkylate, which is essentially a mixture of various
isooctanes, and some refineries also use some
reformate. Avgas has a
density of 6.02 lb/
U.S. gal at 15
°C, or 0.721 kg/l, and this density is commonly used for
weight and balance computation. Density increases to 6.40 lb/US gallon at -40 °C, and decreases by about 0.5% per 5 °C (9 °F) increase in temperature. Avgas has an
emission coefficient (or factor) of 18.355 pounds
CO2 per
U.S. gallon (2.1994 kg/l) or about 3.05 units of weight CO
2 produced per unit weight of fuel used. Avgas has a lower and more uniform
vapor pressure than automotive gasoline, which keeps it in the liquid state at high-altitude, preventing
vapor lock.
The particular mixtures in use today are the same as when they were first developed in the 1940s, and were used in airline and military aero engines with high levels of boost supercharging; notably the Rolls-Royce Merlin engine used in the Spitfire and Hurricane fighters, Mosquito fighter-bomber and Lancaster bomber (the Merlin II and later versions required 100-octane fuel), as well as U.S. made liquid cooled Allison engines, and numerous radial engines from Pratt & Whitney, Wright, and other manufacturers on both sides of the Atlantic. The high-octane ratings are achieved by the addition of tetra-ethyl lead (TEL), a highly toxic substance that was phased out for car use in most countries in the late 20th century.
Avgas is currently available in several grades with differing maximum lead concentrations. Since TEL is a rather expensive (and polluting) additive, a minimum amount of it is typically used to bring it up to the required octane rating and actual concentrations are often lower than the maximum.
Jet fuel is not avgas. It is similar to kerosene and is used in turbine engines. Confusion can be caused by the terms Avtur and AvJet being used for jet fuel. In Europe, environmental and cost considerations have led to increasing numbers of aircraft being fitted with highly fuel-efficient diesel engines; these too run on jet fuel. Civilian aircraft use Jet-A, Jet-A1 or in severely cold climates Jet-B. There are other classification systems for military turbine and diesel fuel.
Consumption
The annual U.S. usage of avgas was 186 million gallons (704 million liters) in 2008, and was approximately 0.14% of the motor gasoline consumption. From 1983 through 2008, U.S. usage of avgas declined consistently by approximately 7.5 million gallons (28 million liters) each year.
Grades
Gasoline used for aviation fuel is generally identified by two numbers associated with its
Motor Octane Number (MON). The first number indicates the octane rating of the fuel tested to "
aviation lean" standards, which is similar to the
anti-knock index or "pump rating" given to automotive gasoline in the U.S. The second number indicates the octane rating of the fuel tested to the "
aviation rich" standard, which tries to simulate a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure.
Fuel dyes aid ground crew and pilots in identifying the proper fuel grade:
Many older grades of avgas are designated with a dual number, such as 100/130. This indicates that the fuel's octane or performance rating is 100 at lean settings and 130 at rich settings, which is why rich settings are normally used for take-off when full power is used.
100LL
The most commonly used aviation fuel is dyed blue for easy visual identification. 100LL, spoken as "100 low lead", contains a small amount of
tetra-ethyl lead (TEL), a
lead compound that reduces gasoline's tendency to spontaneously explode (detonation or "knock") under high loads, high temperatures and high pressures. Sustained detonation causes catastrophic engine failure.
TEL is known as an anti-knock compound. TEL's effect on detonation resistance is known as the "octane rating," derived from comparing performance of the gasoline under test to mixtures of iso-octane and normal heptane in a special test engine. If the product tests like 90% iso-octane and 10% normal heptane, it receives a "90 octane" rating. As higher levels of anti-detonation capability were required, the curve was extended beyond 100% iso-octane and called "performance rating." Example: a 118 octane rated gasoline is more detonation resistant than an 87 octane rated gasoline.
Historically, many post-WW2 developed, low-powered 4- and 6-cylinder piston aircraft engines were designed to use leaded fuels and a suitable unleaded replacement fuel has not yet been developed and certified for most of these engines—although some were designed for 91/96 octane AVGAS (long since discontinued) and many Continental and Lycoming light airplane engines designed for 80/87 continued in production. Engines designed for 80/87 can use AVGAS 100 or 100LL with special precautions to prevent lead buildup and lead fouling of the spark plugs.
Some of the lower powered () aviation engines that were developed in the late 1990s are designed to run on unleaded fuel, but use 100LL if unleaded fuel is not available, an example being the Rotax 912. per US gallon (0.3–0.5 g/l) and is the most commonly available and used aviation gasoline.
One gram of TEL contains 600 milligrams of lead.
82UL
82UL is the specification for an unleaded fuel similar to automobile gasoline but without additives. It could potentially be used in aircraft that have a
Supplemental Type Certificate for the use of automobile gasoline with an aviation lean MON of 82 or less or an antiknock index of 87 or less. It could not be used in engines that require 100LL. The US
Federal Aviation Administration highly recommends installing placards stating the use of 82UL is or is not approved on those airplanes that specify unleaded autogas (
mogas) as an approved fuel. As of 2008, 82UL is not being produced and no refiner has announced plans to put it into production. If produced its specifications call for it to be dyed purple.
80/87
Dyed red, avgas 80/87 had the lowest lead content prior to commencing its phase out in the late 20th century, with a maximum of 0.5
gram lead per U.S.
gallon (0.13 g/l), and was only used in low compression ratio engines. Currently commonly called AVGAS 80, its availability is now very limited.
100/130
Dyed green, avgas 100/130 had a higher octane grade aviation gasoline, containing a maximum of 4 grams of lead per US gallon (1.1 g/l). 100LL "low lead" has replaced avgas 100/130 in most places, but Avgas 100/130 is still sold in
Australia and
New Zealand as one of the two manufacturers in Australia is unable to make Avgas 100LL. Currently commonly called AVGAS 100.
91/96 & 115/145
In the past other grades were also available, particularly for military use, such as avgas 115/145 (dyed purple) and 91/96 (brown). Limited batches of 115/145 (commonly called AVGAS 115) are produced for special events such as unlimited air races.
Automotive gasoline
Automotive
gasoline (known as mogas or autogas among aviators) that does not contain ethanol may be used in
certified aircraft that have a
Supplemental Type Certificate for automotive gasoline,
experimental aircraft and
ultralights. Other oxygenates are approved, but not ethanol. Most of these applicable aircraft have
low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87
anti-knock index automotive gasoline. Examples of this include the popular
Cessna 172 or
Piper Cherokee with the variant of the
Lycoming O-320. Some aircraft engines were originally certified using a 91/96 avgas and have STC's available to run "premium" 91
anti-knock index automotive gasoline. Examples of this include some Cherokees with the
Lycoming O-320 or
O-360 or the
Cessna 152 with the
O-235. However, for most aircraft, automotive gasoline is not considered to be a viable replacement for avgas, because many airplane engines require 100 octane fuel and modifications are necessary in order to use lower octane fuel in high octane engines.
Rotax allows up to 10% ethanol fuel to be used in 912ULS engines. Light sport aircraft that are specified by the manufacturer to tolerate alcohol in the fuel system can use up to 10% ethanol.
On 16 November 2007, the environmental group Friends of the Earth formally petitioned the EPA, asking them to regulate leaded avgas. The EPA responded with a notice of petition for rulemaking.
Under a federal court order to set a new standard by 15 October 2008, the EPA cut the acceptable limits for atmospheric lead to 0.15 micrograms per cubic meter from the previous standard of 1.5 µg/m3. This was the first change to the standard since 1978 and represents an order of magnitude reduction over previous levels. The new standard requires the 16,000 remaining U.S. sources of lead, which includes lead smelting, airplane fuels, military installations, mining and metal smelting, iron and steel manufacturing, industrial boilers and process heaters, hazardous waste incineration and production of batteries, to reduce their emissions by October 2011.
The EPA's own studies have shown that to prevent a measurable decrease in IQ for children deemed most vulnerable, the standard needs to be set much lower, to 0.02 µg/m3.
In December 2008 AOPA filed formal comments to the new EPA regulations. AOPA indicated that piston-powered aircraft produce "one-tenth of 1 percent" of national lead emissions and that they are 0.55% of all transportation emissions. AOPA has asked the EPA to account for the cost and the safety issues involved with removing lead from avgas. They cited that the aviation sector employs more than 1.3 million people in the USA and has an economic "direct and indirect effect that "exceeds $150 billion annually." AOPA interprets the new regulations as not affecting general aviation as they are currently written.
Publication in the US Federal Register of an Advance Notice of Proposed Rulemaking by the US EPA occurred in April 2010. The EPA indicated: "This action will describe the lead inventory related to use of leaded avgas, air quality and exposure information, additional information the Agency is collecting related to the impact of lead emissions from piston-engine aircraft on air quality and will request comments on this information."
Despite assertions in the media that leaded avgas will be eliminated in the USA by 2017 at the latest date, the EPA confirmed in July 2010 that there is no phase-out date and that setting one would be an FAA responsibility as the EPA has no authority over avgas. The FAA administrator stated that regulating lead in avgas is an EPA responsibility, resulting in widespread criticism of both organizations for causing confusion and delaying solutions.
At Sun 'n Fun in April 2011, Pete Bunce, head of the General Aviation Manufacturers Association (GAMA) and Craig Fuller, President and CEO of the Aircraft Owners and Pilots Association indicated that they are both confident that leaded avgas would not be eliminated until a suitable replacement is in place. "There is no reason to believe 100 low-lead will become unavailable in the foreseeable future," Fuller stated.
Final Results from EPA’s Lead Modeling Study at the Santa Monica Airport shows off airport levels below current 150 ng/m3 and possible future 20 ng/m3 levels.
Phase-out of leaded aviation gasolines
The 100LL phase-out has been called "one of modern GA's most pressing problems", because 70% of 100LL aviation fuel is used by the 30% of the aircraft in the general aviation fleet that cannot use any of the existing alternatives.
In 1979 Swedish Hjelmco Oil developed and introduced unleaded AVGAS 80/87 to the Scandinavian market. This fuel met the US standard for AVGAS ASTM D910 valid at that time. This fuel was extensively used by the Swedish Air Force for about 10 years. In 1991 Hjelmco Oil introduced an unleaded AVGAS 91/96 UL meeting leaded grade 91/98 also in standard D910 with the exception of transparent colour and no lead. Engine manufacturers Teledyne Continental Motors, Textron Lycoming, Rotax and radial engine manufacturer Kalisz have cleared the Hjelmco AVGAS 91/96 UL which in practise means that the fuel can be used in more than 90% of the entire world piston aircraft fleet. AVGAS 91/96 UL has been produced in Sweden since 1991 and used in thousands of aircraft for many million flight hours.
In February 2008, Teledyne Continental's new president, Rhett Ross, announced that the company is very concerned about future availability of 100LL avgas, and as a result, they would develop a line of diesel engines. In a February 2008 interview, Ross indicated that Continental Motors believes that the aviation industry will be "forced out" of using 100LL avgas in the near future, leaving automotive fuel and jet fuel as the only alternatives. In May 2010 Continental announced that they had licenced development of the SMA SR305 diesel engine.
In November 2008 National Air Transportation Association President Jim Coyne indicated that the environmental impact of aviation is expected to be a big issue over the next few years and will result in the phasing out 100LL, due to its lead content.
Swift fuel
John and Mary-Louise Rusek founded Swift Enterprises in 2001 to develop renewable fuels and hydrogen fuel cells. By 2006, they had submitted a sample of "Swift 142" to Oracle Airmotive Research & Development of
Delphi, Indiana, which found that the fuel gave a 10% longer runtime than an equal volume of
100LL fuel in the engine of a
Bakeng Deuce aircraft. The following year, Swift Enterprises filed US and international patent applications for non-alcoholic Renewable Engine Fuels of a variety of octane ratings, to be blended from
"one or more low carbon number esters, one or more pentosan-derivable furans, one or more aromatic hydrocarbon, one or more C4-C10 straight chain alkanes derivable from polysaccharides, and one or more bio-oils...
[and, optionally,] triethanolamine", which could be derived from biomass fermentation. Swift Enterprises initiated "Carroll County Swift Development Inc.", to plan a pilot plant at the Delphi Municipal Airport in Indiana, and submitted fuel to the FAA for testing.
In 2008, an article by technology writer and aviation enthusiast Robert X Cringely, stating that the fuel was renewable, cleaner-burning, and potentially cheaper than avgas or even mogas, attracted popular attention to the fuel.
AOPA's Dave Hirschman took a cross-country flight using the fuel in fall of 2009 stating "Swift fuel has made the leap from the purely theoretical to a real product, and it appears to hold great promise for shifting GA to an unleaded, non-petroleum-based future."
The company claims the fuel can be manufactured for USD$2 per gallon, although Swift Enterprises' cost per gallon for laboratory batches was USD$60. The Swift Enterprises website claims their product has 15% more volumetric energy for a 15-25% increase in range over 100LL. The FAA found SwiftFuel 702 to have a motor octane number of 104.4, 96.3% of the energy per unit of mass and 113% of the energy per unit of volume as 100LL, and meets most of the ASTM D 910 standard for leaded aviation fuel. Following tests in two Lycoming engines, the FAA concluded it performs better than 100LL in detonation testing and will provide a fuel savings of 8% per unit of volume, though it weighs 1 pound per gallon (120 g/l) more than 100LL. GC–FID testing showed the fuel to be made primarily of two components—one about 85% by weight and the other about 14% by weight.
Soon afterwards, AVweb reported that Teledyne Continental Motors had begun the process of certifying several of its engines to use the new fuel.
Swift fuel has been criticized by aviation analysts on a number of grounds, including that the forecast price is likely unattainable, that the USD$2 per gallon is a refinery price and not a retail price and that biomass yields are critical to the project and are unproven. As a replacement for 100LL avgas In March 2009 Paul Bertorelli of Aviation Consumer termed it "one that's still a long shot", but conceded "...if Swift Fuel's real manufacturing cost is $3 a gallon and that translates to $5 or a little more at retail, they've got a player. GA in the U.S. can and has adapted to $5 avgas. If Swift can deliver, this project could have legs".
Swift fuel was approved as a test fuel by ASTM International in December 2009, allowing the company to pursue certification testing. Mary Rusek, president and co-owner of Swift Enterprises predicted at that time that "100SF will be comparably priced, environmentally friendlier and more fuel-efficient than other general aviation fuels on the market".
In February 2010 AVweb reported that Swift fuel's production costs had not proven economical and in May 2010 it was confirmed that the initial retail cost would be about US$10 per gallon. John Rusek, president of Swift Enterprises "strongly disputes" the $10 per gallon figure and insists that Swift fuel can be produced for the same price as 100LL. In July 2010 company representative David Perme indicated that Swift Fuel 100SF could be made from either biomass or natural gas as a feedstock for its base constituent of acetone. He stated that this flexibility should produce a fuel that is US$5–6 per gallon.
Swift's patents indicate that the product is a binary fuel made from acetone. The acetone is converted into a blend of isopentane and mesitylene to make the final fuel. 100SF can be made from biomass or from petrochemical bases, including natural gas.
In October 2010 Purdue University reported on Swift Fuel as part of their program of testing that will run until at least April 2012. Purdue used six aviation piston engines, including the highest octane requirement engine, a Lycoming TIO-540-J2BD and a 1933 model Ranger L-440 which requires 65-octane. David Stanley, principal investigator, stated, "SwiftFuel appears promising as a replacement for 100LL general aviation fuel".
94UL
In March 2009
Teledyne Continental Motors announced that it has done testing on a 94UL fuel. This fuel is essentially 100LL with the tetraethyl lead omitted during the production process. The company has indicated that this may be the best solution to the lead problems inherent with 100LL. The 94UL has been shown to meet the avgas specifications, including for vapor pressure, but has not been completely tested for detonation qualities in all Continental engines or under all conditions. Flight testing has been conducted on an
IO-550-B powered
Beechcraft Bonanza and also ground testing on
Continental O-200,
240,
O-470 and
O-520 engines. In May 2010 Continental indicated that, despite industry skepticism, it is proceeding with 94UL and that certification is expected in mid-2013.
In June 2010 Lycoming Engines indicated their opposition to 94UL. Company General Manager Michael Kraft stated that aircraft owners do not realize how much performance would be lost with 94UL and characterized the decision to pursue 94UL as a mistake that could cost the aviation industry billions in lost business. Lycoming believes that the industry should be pursuing 100UL instead. The Lycoming position is supported by aircraft type clubs representing owners of aircraft that would be unable to run on lower octane fuel, such as the American Bonanza Society, the Malibu Mirage Owners and Pilots Association and the Cirrus Owners and Pilots Association. In June 2010 these types clubs collectively formed the Clean 100 Octane Coalition to represent them on this issue and push for unleaded 100 octane avgas.
G100UL
In February 2010
General Aviation Modifications announced that they were in the process of developing a 100LL replacement to be called G100UL, indicating "unleaded". The new fuel is made by blending existing refinery products and produces detonation margins comparable to 100LL. The new fuel is slightly more dense than 100LL, but has a 3.5% higher thermodynamic output. G100UL is compatible with 100LL and can be mixed with it in aircraft tanks for use. The production economics of this new fuel have not been confirmed but it is anticipated that it will cost at least as much as 100LL.
In demonstrations held in July 2010, G100UL performed better than 100LL that just meets the minimum specification and equal to average production 100LL.
Other uses
In some remote communities in Australia where
petrol sniffing has been endemic, automobile gasoline has been replaced by aviation gasoline for use in all automobiles. The lower vapor pressure and slightly different composition of aviation gasoline make it less 'usable' as an inhalant. However, this has recently been replaced (with similar success) by a new, unleaded, low-aromatic mogas by the name of
Opal fuel, produced by
BP Australia.
See also
Relative CO2 emission from various fuels
References
Category:Aviation fuels
