|
|
|
|
Avgas
|
| |
|
| |
Avgas is a high-octane aviation fuel used to power many aircraft and racing cars. Avgas is a portmanteau for aviation gasoline, as distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars.
Discussion
Ask a question about 'Avgas' Start a new discussion about 'Avgas' Answer questions from other users |
Encyclopedia
Avgas is a high-octane aviation fuel used to power many aircraft and racing cars. Avgas is a portmanteau for aviation gasoline, as distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars. Some light aircraft also use automobile fuel instead of avgas.
Avgas is used in aircraft that have piston or Wankel engines. Gas turbines can operate on avgas, but typically do not. Turbine and diesel engines 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 still widely available 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 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 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 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 based 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 grams 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. As of 2008, 82UL is not being produced and no refiner has announced plans to put it into production..
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 grams 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.
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 lean mixture rating, the second (higher) number is the rich mixture rating. For mogas, the octane rating is typically expressed in the U.S. as an anti-knock index (known as "pump rating"), which is the average of the octane rating based on the research and motor test method ((R+M)/2).
Identification dyes
Fuel dyes aid both ground crew and pilots in identifying the proper fuel grade:
- 80/87 - red
- 100/130 - green
- 115/145 - purple
- 100LL - blue
- jet fuel (JET A, A1. B) - clear or straw (being undyed)
Automotive gasoline
Automotive gasoline (known as Mogas or Autogas among aviators) that does not contain oxygenates may be used in aircraft that have a Supplemental Type Certificate for automotive gasoline, and in experimental aircraft. 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 Cherokee's 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, primarily because automotive gasoline is not subject to the stringent quality-control standards necessary for an aviation fuel.
The requirement for a non-oxygenated fuel is a major problem with these STCs. In the name of reducing tailpipe emissions, most autogas contains additives designed to boost the oxygen available to the fuel, such as alcohol. The FAA doesn't allow alcohol-containing gasoline to be used in aircraft. Therefore, the STC is largely useless since allowable fuel isn't readily available.
Properties
Avgas has a density of 6.02 lb/US gallon at 15 °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.
Avgas has an emission coefficient (or factor) of 18.355 pounds CO2 per US gallon, or about 3.05 units of weight CO2 produced per unit weight of fuel used.
Consumption
The annual U.S. usage of avgas was 236 million gallons (893 million liters) in 2006.
Avgas compared to other fuels
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 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 or 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.
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.
Environmental regulation
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.
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.
The notice of petition stated:
The public comment period on this petition closed on 17 March 2008.
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 microgram/m3 from the previous standard of 1.5 microgram/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 US 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 microgram/m3.
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.
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."
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.
Future Availability
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 will develop a diesel engine in the range for certification in 2009 or 2010. This engine will be followed by lower power engines, as well.
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 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.
Future alternatives
Due to the problems of continuing to use an aviation fuel made from oil and containing tetraethyl lead there has been work on future replacement fuels. One initiative is a fuel developed by Swift Enterprises and known initially as Swift Fuel. This fuel is made from switch grass and agricultural waste cellulosic biomass and comprises acetone compounds obtained via biomass fermentation. The FAA's Hughes Technical Center in Atlantic City, New Jersey has tested the fuel and determined its octane rating as 104.4. It contains 13% more heat value than avgas, but weighs 7 lb/US gal, which is 17% more dense than avgas. It meets most requirements of the Avgas ASTM D 910 standard. The company claims the fuel can be manufactured for USD$2 per gallon.
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 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".
See also
External links
- - information on STC to use mogas in aircraft, by Petersen Aviation, Inc., which sells such STC's
- advocating for Aviation Grade Ethanol
- ethanol advocacy site telling story of "Shauck and his wife flew a single-engine airplane across the Atlantic Ocean in 1989, using 100% Ethanol"
|
| |
|
|
