Avgas
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'''Avgas''' is a [[octane rating|high-octane]] [[aviation fuel]] used for [[aircraft]] and [[racing car]]s. ''Avgas'' is a [[portmanteau]] for '''aviation gasoline''', as distinguished from ''[[mogas]]'' (motor gasoline), which is the everyday [[gasoline]] used in [[automobile|cars]].
Avgas is used in aircraft that use [[piston engine|piston]] or [[Wankel engine]]s. [[Gas turbine]]s can operate on avgas, but typically do not. Turbine and [[diesel engine]]s are designed to use [[kerosene]]-based [[jet fuel]].
== Avgas properties and varieties ==
[[Gasoline]] used for aviation fuel generally has two numbers associated with its octane rating. Examples of this include the (now almost completely unavailable) 80/87 avgas, and the 100/130 avgas. The first number indicates the octane rating of the fuel tested to "aviation lean" standards, which is similar to the [[Motor Octane Number]] (MON) rating given to automotive gasoline. 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.
Avgas has a lower and more uniform [[vapor pressure]] than mogas, 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 1950s and 1960s, and therefore 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 1980s. The main [[petroleum]] component used in blending avgas is [[alkylate]], which is essentially a mixture of various [[2,2,4-Trimethylpentane|isooctanes]], and some refineries also use some [[reformate]].
Avgas is currently available in several grades with differing maximum lead concentrations. Since TEL is a rather expensive additive, a minimum amount of it is typically added to the fuel to bring it up to the required octane rating so 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. See [[Jet fuel]].
===Grades===
====100LL====
100LL, spoken as "100 low lead", contains [[tetra-ethyl lead]] (TEL), a [[lead (element)|lead]] based [[engine knocking|anti-knock]] compound, but less than the "highly-leaded" 100/130 avgas it effectively replaced. Most piston aircraft engines require 100LL and a suitable replacement fuel has not yet been developed for these engines. While there are similar engines that burn non-leaded fuels, aircraft are often purchased with engines that use 100LL because many airports only have 100LL. 100LL contains a maximum of 2 [[gram]]s of lead per US gallon, or maximum 0.56 grams/litre and is the most commonly available and used aviation gasoline.
====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 octane rating (MON) of 82 or less or an antiknock index of 87 or less. It could not be used in engines that require 100LL. See [[Octane Rating]]. The FAA 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.<ref name="FAA">{{cite web
| url = http://www.faa.gov/aircraft/safety/alerts/saib/media/CE-00-19R1.htm
| title = Revised Special Airworthiness Information Bulletin (SAIB) Number CE-00-19R1
| accessdate = 2006-10-28
| author = Federal Aviation Administration
| authorlink = Federal Aviation Administration
| date = [[2000-04-05]]
| quote = The FAA highly recommends installing placards stating the use of 82UL is or is not approved on those airplanes that specify unleaded autogas as an approved fuel.
}}</ref> As of 2008, 82UL is not being produced and no refiner has announced plans to put it into production.<ref name="AvWeb17Nov07"> {{cite web|url = http://www.avweb.com/avwebflash/news/GroupAsksEPAToGetTheLeadOutOfAvgas_196596-1.html|title = Avgas: Group Asks EPA To Get The Lead Out|accessdate = 2008-02-18|last = Pew |first = Glenn |authorlink = |year = 2007|month = November}}</ref>.
====80/87====
Prior to its phase out in the early 1990s, avgas 80/87 had the lowest lead content with a maximum of 0.5 [[gram]]s lead per U.S. [[gallon]], and was only used in low compression ratio engines.
====100/130====
Avgas 100/130 had a higher octane grade aviation gasoline, containing a maximum of 4 grams of lead per US gallon, maximum 1.12 grams/[[litre]]. 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.{{Fact|date=March 2008}}
====91/96 & 115/145====
In the past other grades were also available, particularly for military use, such as avgas 115/145 and 91/96. Note that the octanes of avgas cannot be directly compared to those of mogas, as a different test engine and method is used to determine the octane. The first (lower) number is the [[Carburetor#Carburetor adjustment|lean mixture]] rating, the second (higher) number is the [[Carburetor#Carburetor adjustment|rich mixture]] rating. For mogas, the [[octane rating]] is typically expressed in the U.S. as an [[Octane rating#Measurement methods|anti-knock index]] (known as "pump rating"), which is the average of the octane rating based on the [[research]] and [[internal combustion engine|motor]] test method ((R+M)/2).
====Identification dyes====
[[Fuel dyes]] aid both ground crew and pilots in identifying the proper fuel grade:<ref>Shell Aviation (2008-03-24). AVGAS Grades and Specifications. Retrieved on 2008-06-14 from http://www.shell.com/home/content/aviation-en/productservice/aviationfuels/detail/avgasgradesspecs_10081006.html.</ref>
<li style="color:red">80/87 - [[red]]</li>
<li style="color:green">100/130 - [[green]]</li>
<li style="color:purple">115/145 - [[purple]]</li>
<li style="color:blue">100LL - [[blue]]</li>
<li>jet fuel (JET A1) - clear or straw (being undyed)</li>
Note that untaxed diesel fuel for off-road use is also [[dye]]d [[red]], the same color as 80/87 avgas.{{Fact|date=June 2008}}
====Mogas====
[[Gasoline]] (MOGAS) may be used in aircraft that have a [[Supplemental Type Certificate]] for automotive gasoline. Most of these applicable aircraft have low-compression engines which were originally certified to run on 80/87 avgas and require only "regular" 87 [[Octane rating#Measurement methods|anti-knock index]] automotive gasoline. Examples of this include the popular [[Cessna 172]] or [[Piper Cherokee]] with the 150 hp 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 [[Octane rating#Measurement methods|anti-knock index]] automotive gasoline. Examples of this include some Cherokee's with the 160 hp [[Lycoming O-320]] or 180 hp [[Lycoming O-360|O-360]] or the [[Cessna 152]] with the [[Lycoming O-235|O-235]].
===Properties===
Avgas has a [[density]] of 6.02 lb/[[US gallon]] at 15 [[Degree Celsius|°C]], or 0.72 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 increase in temperature.<ref name="From The Ground Up">{{cite book
| last = MacDonald
| first = Sandy A. F.
| coauthors = Isabel L. Peppler
| title = From The Ground Up
| origyear = 1941
| edition = Millennium Edition
| year = 2004
| publisher = Aviation Publishers Co. Limited
| location = Ottawa, Ontario, Canada
| id = ISBN 0-9680390-5-7
| pages = pp. 265, 261
| chapter = Chapter 10. Airmanship
}}</ref>
Avgas has an [[Emission factor|emission coefficient]] (or factor) of 18.355 pounds [[carbon dioxide|CO<sub>2</sub>]] per [[US gallon]],<ref name="US DOE EIA 2007">{{Cite web
|author=US Energy Information Administration |author-link=Energy Information Administration
|title=Voluntary Reporting of Greenhouse Gases Program - Fuel and Energy Source Codes and Emission Coefficients
|year = 2007
|work=US Energy Information Administration website
|place=Washington, DC
|url=http://www.eia.doe.gov/oiaf/1605/factors.html |accessdate= 2007-12-03
}}</ref><ref name="US DOE EIA 2005">{{Citation
|author=US Energy Information Administration |author-link=Energy Information Administration
|title=Form EIA-1605EZ Short Form for Voluntary Reporting of Greenhouse Gases
|chapter=Appendix F. Fuel and Energy Source Codes and Emission Coefficients
|chapter-url=ftp://ftp.eia.doe.gov/pub/oiaf/1605/cdrom/pdf/FormEIA-1605EZ_2005.pdf
|page=22
|year = 2005
|place=Washington, DC
|url=ftp://ftp.eia.doe.gov/pub/oiaf/1605/cdrom/pdf/FormEIA-1605EZ_2005.pdf |accessdate= 2007-12-03
}}.</ref> or about 3.05 units of weight CO<sub>2</sub> produced per unit weight of fuel used.
===Consumption===
The annual U.S. usage of avgas was 236 million gallons (893 million liters) in 2006.<ref>{{cite web|url=http://tonto.eia.doe.gov/dnav/pet/pet_cons_prim_dcu_nus_a.htm|title=U.S. Prime Supplier Sales Volumes of Petroleum Products|author=US Energy Information Administration |authorlink=Energy Information Administration}}</ref>
==Avgas compared to other fuels==
Many [[general aviation]] [[aircraft engine]]s were designed to run on 80/87 octane,{{Fact|date=April 2008}} 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 rather outdated, 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 hp or 180 hp 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.<ref name="AvWeb17Nov07"/>
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.
==Future Availability==
On [[16 November]] [[2007]], the [[Environmentalism|environmental group]] [[Friends of the Earth]] formally petitioned the [[United States Environmental Protection Agency]], asking them to regulate leaded avgas. The EPA responded with a notice of petition for rulemaking.<ref name="AvWeb17Nov07"/>
The notice of petition states:
<blockquote>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.<ref name="FederalRegister16Nov07"> {{cite web|url = http://a257.g.akamaitech.net/7/257/2422/01jan20071800/edocket.access.gpo.gov/2007/E7-22456.htm|title = Federal Register: November 16, 2007 (Volume 72, Number 221)|accessdate = 2008-02-24|last = Environmental Protection Agency|authorlink = |year = 2007|month = November}}</ref></blockquote>
The public comment period on this petition closed on [[17 March]] [[2008]].<ref name="FederalRegister16Nov07"/>
In February 2008, [[Continental Motors|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 will develop a diesel engine in the 300 [[horsepower]] range for certification in 2009 or 2010. This engine will be followed by lower horsepower engines, as well.<ref name="AvWeb18Feb08"> {{cite web|url = http://www.avweb.com/eletter/archives/avflash/1054-full.html|title = Teledyne Continental Plans Certified Diesel Within Two Years|accessdate = 2008-02-18|last = AvWeb Staff|authorlink = |year = 2008|month = February}}</ref>
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.<ref name="Bertorelli"> {{cite web|url = http://www.avweb.com/podcast/podcast/197170-1.html?kw=RelatedStory|title = Make Room in the Aerodiesel Market, Thielert — TCM Tells Aviation Consumer About Some Big Engine Plans|accessdate = 2008-02-18|last = Bertorelli |first - Paul |authorlink = |year = 2008|month = February}}</ref>
== References ==
{{reflist}}
==External links==
* http://www.autofuelstc.com/autofuelstc/pa/PetersenAviation.html - information on STC to use [[mogas]] in aircraft, by Petersen Aviation, Inc., which sells such STC's
* http://www.age85.org, advocating for Aviation Grade [[Ethanol]]
* http://www.ethanolmt.org/php/novdec05.php, [[ethanol]] advocacy site telling story of "Shauck and his wife flew a single-engine airplane across the Atlantic Ocean in 1989, using 100% Ethanol"
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