What is Avgas (Aviation Gasoline)?
Aviation Gasoline (Avgas) comes from the petroleum fraction (mainly alkylates) and is designed to fuel the aircraft that uses the reciprocating piston engine with spark ignition. Grades of aviation gasoline are identified with names based on their antiknock quality (as measured by Octane Number):
o Grade 80 / Grade 91 – No longer available in Australia; and
o Grade 100/130; and
o Grade 100LL (Low Lead).
Different colours, obtained by the use of specific dyes, are used to differentiate the fuel grades.
Grades 100/130 and 100LL are identical in antiknock quality but differ in maximum lead content.
The most common specification for Avgas is ASTM D910 and determines the various properties of aviation gasoline such as performance (Knock Value / Octane rating), concentration of lead (addition of tetraethyl lead or TEL), appearance, colour (blue, yellow and red), volatility, vapour pressure, residue/ precipitate, density, sulphur content, freezing point, corrosion, oxidation stability and conductivity.
For avgas, the maximum vapour pressure (measured by Reid Vapour Pressure or RVP) is 7 psi and the minimum is 5.5 psi. The maximum reduces vapour lock problems. The minimum is to ensure the vapour in the tank is always above the rich explosive limit.
These properties ensure a satisfactory performance of aircraft piston engines over a wide range of operating conditions.
Requirements common for all grades either prescribe a balance of properties to ensure satisfactory engine performance, or limit the concentration of components that could have an adverse effect on engine performance.