Avgas

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Avgas is a high-octane fuel used for aircraft and, in the past, racing cars. Avgas is an abbreviation for aviation gasoline, as distinguished from mogas (motor gasoline), which is the everyday gasoline used in cars. Avgas is used in aircraft that use 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.

[edit] Avgas properties and varieties

Avgas has a lower volatility than mogas and does not evaporate as quickly, which is important for high-altitude use. 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 fairly 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 similar to kerosene and is used in turbine engines. It is also used in the few aircraft with diesel engines. 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.

[edit] Gasoline

Gasoline used for aviation fuel generally has two numbers associated with its octane rating. Examples of this include the (now almost completely extinct) 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.

100LL, spoken as "100 low lead", contains a lead based anti-knock compound but less than the "highly-leaded" 100/130 avgas it effectively replaced. Most piston aircraft engines require 100LL but it is scheduled to be phased out in the United States because of the lead toxicity. An alternative 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 is an unleaded fuel similar to automobile gasoline but without additives. It may 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 may 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 as an approved fuel[1].

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 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 anti-knock index automotive gasoline. Examples of this include some Cherokee's with the 160 hp Lycoming O-320 or 180 hp O-360 or the Cessna 152 with the O-235.

Avgas 80/87 has the lowest lead content at a maximum of 0.5 grams lead per U.S. gallon, and is only used in low compression ratio engines.

Avgas 100/130 is 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" was designed to replace avgas 100/130.

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).

To aid pilots in identifying the fuel in their aircraft, dyes are added to the fuel. 80/87 is red, 100/130 is green, and 100LL is blue, while jet fuel, JET A1, is clear or straw, being undyed. Untaxed diesel fuel for off-road use is also dyed red.

[edit] Avgas compared to other fuels

Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard for 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 mogas conversions. Fortunately, significant history of mogas-converted engines has shown that very few engine problems are actually caused by running mogas. A larger problem stems from the wider range of allowable vapor pressures found in mogas; this can pose some risk to aviation users if fuel system design considerations are not taken into account. Mogas 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 to ensure adequate shielding from high temperatures and the existence of sufficient pressure and flow in the fuel lines.

In addition to vapor-locking potential, mogas does not have the same quality-tracking as avgas. To help solve this problem, an aviation fuel known as 82UL has recently been introduced. This fuel is essentially mogas that has additional quality tracking and restrictions on the additives that can be included.

The main large consumers of avgas these days are in North America, Australia, Brazil, and Africa (mainly South Africa).

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.

[edit] 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.[2]

[edit] Other meanings

In some Scandinavian languages avgas means "exhaust gas". This usage may confuse English-speaking readers.

[edit] External links

[edit] References

  1. ^ Federal Aviation Administration (2000-04-05). Revised Special Airworthiness Information Bulletin (SAIB) Number CE-00-19R1. Retrieved on 2006-10-28. “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.”
  2. ^ MacDonald, Sandy A. F., Isabel L. Peppler [1941] (2004). “Chapter 10. Airmanship”, From The Ground Up, Millennium Edition, Ottawa, Ontario, Canada: Aviation Publishers Co. Limited, pp. 265, 261. ISBN 0-9680390-5-7.
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