| Avgas | |
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| An American Aviation AA-1 Yankee being refuelled with 100LL avgas. |
Avgas (aviation gasoline, also known as aviation spirit in the UK) is an aviation fuel used to power piston-engine aircraft. Avgas is distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars and some non-commercial light aircraft. Unlike mogas, avgas contains tetraethyl 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.
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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.[1] Avgas has an emission coefficient (or factor) of 18.355 pounds CO2 per U.S. gallon (2.1994 kg/l)[2][3] or about 3.05 units of weight CO2 produced per unit weight of fuel used. Avgas has a lower and more uniform vapor pressure than automotive gasoline so it remains in the liquid state despite the reduced atmospheric pressure at high altitude, thus 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 V-1710 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 tetraethyl 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 an expensive and polluting ingredient, the minimum amount needed to bring the fuel to the required octane rating is used; actual concentrations are often lower than the permissible maximum. 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. Numerous current (2010) certificated reciprocating-engine aircraft require high-octane (leaded) fuels.
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.
The annual U.S. usage of avgas was 186 million US gallons (700,000 m3) 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 US gallons (28,000 m3) each year.[4]
Many grades of avgas are identified by two numbers associated with its Motor Octane Number (MON).[5] 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. For example, 100/130 avgas has an octane rating of 100 at the lean settings usually used for cruising and 130 at the rich settings used for take-off and other full-power conditions.[6]
Fuel dyes aid ground crew and pilots in identifying and distinguishing the fuel grades.[7]
The most commonly used aviation fuel is 100LL, i.e., "low lead". It is dyed blue and contains a relatively small amount of tetraethyl lead—though the amount is greater than what was contained in many automotive grades of leaded fuel before such fuel was phased out. As of Jan 2010, 100LL has a TEL content of 1.2 to 2 grams TEL[8] 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. This grade of AVGas is dyed blue.
Many Continental and Lycoming light airplane engines designed for 80/87 remain in production. Engines designed for 80/87 can use 100LL with special precautions to prevent lead buildup and lead fouling of the spark plugs.
Some of the lower-powered (100-to-150-horsepower or 75-to-110 kW) aviation engines that were developed in the late 1990s are designed to run on unleaded fuel, but can run on 100LL if unleaded is not available, an example being the Rotax 912.[9]
82UL is the specification for an unleaded fuel similar to automobile gasoline but without automotive 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.[says who?] 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.[10] 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.[7][11]
80/87, which is dyed red, had the lowest lead content prior to commencing its phase out in the late 20th century, with a maximum of 0.5g per U.S. gallon (0.13 g/l). It was used in engines with low compression ratio. Currently commonly called Avgas 80, its availability is now very limited.
100/130 avgas, now commonly called Avgas 100, is dyed green. It contains a maximum of 4 g of lead per US gallon (1.1 g/l). 100LL has replaced 100/130 in most places, but Avgas 100/130 is still sold in Australia and New Zealand.
In the past other grades were also available particularly for military use, such as avgas 115/145 (dyed purple) and 91/96 (dyed brown). Limited batches of 115/145, commonly called Avgas 115, are produced for special events such as unlimited air races; in the past 115/145 was used as the primary fuel for the largest, boost-supercharged radial engines which needed this fuel's anti-detonation properties.[12]
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. Some oxygenates other than ethanol are approved. 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 150 hp (110 kW) 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 160 hp (120 kW) Lycoming O-320 or 180 hp (130 kW) O-360 or the Cessna 152 with the O-235. However, for most aircraft, automotive gasoline is not 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.[13][14]
Rotax allows up to 10% ethanol to be in the fuel 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.[9]
Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard for North American automobiles today. Direct conversions to run on automotive fuel are fairly common and applied via the supplemental type certificate (STC) process. However, the alloys used in aviation engine construction are chosen for their durability and synergistic relationship with the protective features of lead, and engine wear in the valves is a potential problem on automotive gasoline conversions.
Fortunately, significant history of mogas-converted engines has shown that very few engine problems are actually caused by automotive gasoline. A larger problem stems from the wider range of allowable vapor pressures found in automotive gasoline; this can pose some risk to aviation users if fuel system design considerations are not taken into account. Automotive gasoline can vaporize in fuel lines causing a vapor lock (a bubble in the line), starving the engine of fuel. This does not constitute an insurmountable obstacle, but merely requires examination of the fuel system, ensuring adequate shielding from high temperatures and maintaining sufficient pressure in the fuel lines. This is the main reason why both the specific engine model as well as the aircraft in which it is installed must be supplementally certified for the conversion. A good example of this is the Piper Cherokee with high-compression 160 or 180 hp (120 or 130 kW) engines. Only later versions of the airframe with different engine cowling and exhaust arrangements are applicable for the automotive fuel STC, and even then require fuel system modifications.
Vapor lock typically occurs in fuel systems where a mechanically driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow. If an electric boost pump is mounted in the fuel tank to push fuel toward the engine, as is common practice in fuel-injected automobiles, the fuel pressure in the lines is maintained above ambient pressure, preventing bubble formation. Likewise, if the fuel tank is mounted above the engine and fuel flows primarily due to gravity, as in a Cessna high-wing airplane, vapor lock cannot occur, using either aviation or automotive fuels.
In addition to vapor locking potential, automotive gasoline does not have the same quality tracking as aviation gasoline. To help solve this problem, the specification for an aviation fuel known as 82UL has recently been developed. This fuel would be essentially automotive gasoline that has additional quality tracking and restrictions on permissible additives. This fuel is not currently in production and no refiners have committed to producing it.[11]
The main consumers of avgas at present (mid-2000s) are in North America, Australia, Brazil, and Africa (mainly South Africa). Care must be taken by small airplane pilots to select airports with avgas on flight planning. For example, U.S. and Japanese recreational pilots ship and depot avgas before flying into Siberia. Shrinking availability of avgas drives usage of small airplane engines that can use jet fuel.
In Europe, avgas prices are so high that there have been a number of efforts to convert the industry to diesel instead, which is common, inexpensive and has a number of advantages for aviation use. However, avgas remains the most common fuel in Europe as well.
The tetra-ethyl lead found in leaded avgas and its combustion products are potent neurotoxins that have been shown in scientific research to interfere with brain development in children. The United States Environmental Protection Agency (EPA) has noted that exposure to even very low levels of lead contamination has been conclusively linked to loss of IQ in children's brain function tests, thus providing a high degree of motivation to eliminate lead and its compounds from the environment.[15][16]
| “ | While lead concentrations in the air have declined, scientific studies have demonstrated that children's neurological development is harmed by much lower levels of lead exposure than previously understood. Low level lead exposure has been clearly linked to loss of IQ in performance testing. Even an average IQ loss of 1-2 points in children has a meaningful impact for the nation as a whole, as it would result in an increase in children classified as mentally challenged, as well as a proportional decrease in the number of children considered "gifted."[16] | ” |
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.[11]
The notice of petition stated:
| “ | Friends of the Earth has filed a petition with EPA, requesting that EPA find pursuant to section 231 of the Clean Air Act that lead emissions from general aviation aircraft cause or contribute to air pollution that may reasonably be anticipated to endanger public health or welfare and that EPA propose emissions standards for lead from general aviation aircraft. Alternatively, Friends of the Earth requests that EPA commence a study and investigation of the health and environmental impacts of lead emissions from general aviation aircraft, if EPA believes that insufficient information exists to make such a finding. The petition submitted by Friends of the Earth explains their view that lead emissions from general aviation aircraft endanger the public health and welfare, creating a duty for the EPA to propose emission standards.[17] | ” |
The public comment period on this petition closed on 17 March 2008.[17]
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.[15][16][18]
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.[16]
The EPA has previously named avgas as one of the most "significant sources of lead", but it was not clear how this current change in standards will affect aircraft burning 100LL fuel.[15][16]
At an EPA public consultation held in June 2008 on the new standards, Andy Cebula, the Aircraft Owners and Pilots Association's Executive Vice President of Government Affairs stated that general aviation plays a valuable role in the US economy and any changes in lead standards that would change the current composition of avgas would have a "direct impact on the safety of flight and the very future of light aircraft in this country."[19]
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.[20]
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."[21][22]
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.[23][24][25][26][27]
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.[28]
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.[29]
The 100LL phase-out has been called "one of modern GA's most pressing problems",[30] 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.[31][32][33]
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.[9][34][35][36] 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.[37] 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.[38][39][40]
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.[41]
John and Mary-Louise Rusek founded Swift Enterprises in 2001 to develop renewable fuels and hydrogen fuel cells.[42] 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 100 hp (75 kW) engine of a Bakeng Deuce aircraft.[43] 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.[44] Swift Enterprises initiated "Carroll County Swift Development Inc.",[45] to plan a 2,500-square-foot (230 m2) pilot plant at the Delphi Municipal Airport in Indiana,[46] and submitted fuel to the FAA for testing.[47][48][49][50]
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.[51] 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."[52] The company claims the fuel can be manufactured for USD$2 per gallon, although Swift Enterprises' cost per gallon for laboratory batches was US$60.[47][53] 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.[54] [55] Soon afterwards, AVweb reported that Teledyne Continental Motors had begun the process of certifying several of its engines to use the new fuel.[56]
Swift fuel has been criticized by aviation analysts on a number of grounds, including that the forecast price is likely unattainable, that the US$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".[57]
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".[58][59]
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.[60][61][62][63]
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.[63]
In August 2010 the FAA's William J. Hughes Technical Center in Atlantic City, New Jersey issued a report on endurance testing of Swift Fuel carried out on a new Lycoming IO-540 engine involving 150 hrs of run time. Other than deterioration of the fuel pump, the tests were satisfactory.[64]
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".[65]
Unleaded 94-octane fuel, known as '94UL, is essentially 100LL without the lead. In March 2009 Teledyne Continental Motors announced they had tested a 94UL fuel, and said it may be the best replacement for 100LL. The 94UL was shown to meet the avgas specification including 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, they are proceeding with 94UL and that certification is expected in mid-2013.[66][67]
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 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; in June 2010 such clubs as the American Bonanza Society, the Malibu Mirage Owners and Pilots Association, and the Cirrus Owners and Pilots Association collectively formed the Clean 100 Octane Coalition to represent them on this issue and push for unleaded 100 octane avgas.[68][69][70][71]
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.[60][72]
In demonstrations held in July 2010, G100UL performed better than 100LL that just meets the minimum specification and equal to average production 100LL.[73]
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.[74] 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.[75]
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