Avgas - Avgas Properties and Varieties

Avgas Properties and Varieties

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. All grades of avgas that meet CAN 2-3, 25-M82 have a density of 6.01 lb/U.S. gal at 15 °C, or 0.721 kg/l, and this density is commonly used for weight and balance computation. Density increases to 6.41 lb/US gallon at -40 °C, and decreases by about 0.5% per 5 °C (9 °F) increase in temperature. Avgas has an emission coefficient (or factor) of 18.355 pounds CO₂ per U.S. gallon (2.1994 kg/l) or about 3.05 units of weight CO₂ produced per unit weight of fuel used. Avgas has a lower and more uniform vapor pressure than automotive gasoline so it remains in the liquid state despite the reduced atmospheric pressure at high altitude, thus preventing vapor lock.

The particular mixtures in use today are the same as when they were first developed in the 1940s, and were used in airline and military aero engines with high levels of boost supercharging; notably the Rolls-Royce Merlin engine used in the Spitfire and Hurricane fighters, Mosquito fighter-bomber and Lancaster bomber (the Merlin II and later versions required 100-octane fuel), as well as US-made liquid-cooled Allison V-1710 engines, and numerous radial engines from Pratt & Whitney, Wright, and other manufacturers on both sides of the Atlantic. The high octane ratings are achieved by the addition of tetraethyl lead (TEL), a highly toxic substance that was phased out of automotive use in most countries in the late 20th century.

Avgas is currently available in several grades with differing maximum lead concentrations. Since TEL is an expensive and polluting ingredient, the minimum amount needed to bring the fuel to the required octane rating is used; actual concentrations are often lower than the permissible maximum. Historically, many post-WWII developed, low-powered 4- and 6-cylinder piston aircraft engines were designed to use leaded fuels and a suitable unleaded replacement fuel has not yet been developed and certified for most of these engines. Numerous current (2010) certificated reciprocating-engine aircraft require high-octane (leaded) fuels.

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 fuel-efficient Diesel engines that 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.