Avgas
|
|
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 only used in aircraft that use piston engines; jet aircraft and turboprops use kerosene jet fuel.
Avgas has a lower volatility than mogas and doesn't 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.
- Avgas 80/87 has the lowest lead content at a maximum of 0.5 gram lead per US gallon, and is only used in very low compression ratio engines.
- Avgas 100/130 is a higher octane grade aviation gasoline, containing a maximum of 4 gram of lead per US gallon, maximum 1.12 gram/litre. 100LL "low lead" was designed to replace avgas 100/130.
- Avgas 100LL contains a maximum of 2 gram of lead per US gallon, or maximum 0.56 gram/litre, and is the most commonly available and used aviation gasoline.
In the past other grades were also available, particularly for military use, such as avgas 115/145. 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 rating. For mogas, the "octane" rating is typically expressed as an anti-knock index, 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.
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 have 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 and 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 only large consumers of avgas these days are in North America, Australia, Brazil, and Africa (mainly South Africa). There are very few remaining supply sources outside the United States.
In Europe, avgas prices are so high that the entire general aviation industry is being wiped out. There are a number of efforts to convert the industry to diesel instead, which is common, inexpensive and has a number of advantages for aviation use. It remains to be seen whether there is enough of a market left for this to occur.