Repsol YPF

Internal combustion engines

As the first engines used to power flight were based on the automotive engines of the time, they were fuelled with automotive petrol. During the following decades, there were great improvements made to engines and aviation fuel. Much of the development was driven by the military interest in aviation, both in the USA and in Europe. The primary objectives were always to acheive greater reliability and power, but without a proportional increase in the size and weight of the engine.

Engines and fuels are interdependent components of the same system. When new developments took place with regard to engines, better fuels were required, and when the improved fuel was used, greater engine development immediately became possible, to make the best use of the fuel available.

As the engines were brought up to date with the fuels that were available, it became evident that even better fuels would provide an even better performance. Consequently, engine manufacturers and users have, for years, constantly been demanding better fuels.

With regard to fuels, the basic challenge was to improve their antiknock properties, insofar as to prevent the engine power from being limited by knocking. The greatest developments were the use of a lead-based anti-knock additive (tetraethyl lead), the identification of crude oil with the best lead response and the identification and production of specific hydrocarbons with good anti-knock properties.

In 1930, the U.S. Army Air Corps specified a Fighting Grade fuel with a minimum octane rating of 87. This is believed to be the first time that the antiknock properties of an aviation fuel were defined in terms of octane number. By the start of World War II, 100 octane fuels were being used, which were similar to those that are used today.

Avgas reached its development peak during World War II. In 1944, the U.S. military issued a specification for Grade 115/145. This fuel, which had the highest antiknock rating of any avgas in large scale production, was used to obtain maximum output from high-performance engines.

Grades of fuel

Grades of avgas are identified by their nominal minimum lean-mixture antiknock rating. 

Previously, both the lean- and rich-mixture ratings were used; now, only the lean-mixture rating is used.

In the decade following the war, six grades were in military and commercial use.

In the same decade, turbine engines became the engine of choice for the military. As turbine engines also began to dominate commercial fleets, both ends of the avgas grade lineup were eliminated. Demand for the fuels with the higher antiknock ratings disappeared. And, at many commercial airports, an avgas tank was converted to jet fuel storage. But when some of the older 80-octane-rated engines were fueled with Grade 100/130, they encountered spark plug fouling and exhaust valve deterioration because of its higher lead content. A lower lead content version of Grade 100/130 (Grade 100LL) was developed in an effort to create a single universal grade of avgas.

Today avgas is used mainly by small airplanes and light helicopters, but there is also a significant number of military and civilian transports powered by large piston engines that use avgas. The D 910 avgas specification recognizes three grades, all of which contain lead.

As planned, Grade 100LL is the most popular, by far. Grade 100 and Grade 80 are used sparingly.

Fuel consumption

The Air Force was critical to the outcome of World War II, and toward its end, the Allies' production of avgas peaked at more than 25 million gallons per day.Two years after the war, it had decreased sharply to about 5 million gallons per day. The growth of commercial aviation together with military use resulted in a gradual increase to about 14 million gallons per day in 1957.

Later, production began to decrease again as turbine engines replaced piston engines, first in military and later in commercial applications, returning to about 5 million gallons per day

In 1999, total avgas production in the United States was 0.8 million gallons per day, a very small amount compared to the production of turbine fuel (70 million gallons per day) or automotive fuel (346 million gallons per day). Worldwide use of avgas in 1996, the most recent data available, is estimated to be about 2.2 million gallons per day.