Explained: Properties of Jet and Aviation Fuel

‍DALLAS — In this article, we'll examine all the fuel types used in aviation, their properties, the different types of kerosene available, and much more.

So, what hydrocarbons are used in aviation, what are their properties, and how is jet fuel made?

Aviation Fuel, AVGAS

Aviation fuel comprises long chains of hydrocarbons produced during the refining process. These hydrocarbon chains are derived from the raw material harvested to make fuel: petroleum. The oil comprises molecules containing mainly carbon and hydrogen atoms and molecules containing other atoms, mostly sulfur, nitrogen, and oxygen.

This mixture has to be separated to make fuel for aircraft and the hydrocarbon chains of interest to us recovered, so it is first heated to around 350-370°C. The different fractions are then separated in the distillation tower of a refinery according to their boiling point.

The oil is then purified, the various components obtained by distillation are mixed, and finally, it is converted (by modifying the chemical structure of the hydrocarbon molecules) into a finished product ready to be marketed.

Aviation fuel is often used interchangeably with kerosene, but it can also refer to gasoline used for general aviation, piston propeller engines, microlights, and sports aircraft, for example.

Known as AVGAS, this spark-based fuel has a much lower flash point than kerosene and a much higher octane rating than the petrol used in road vehicles.

Jet Fuel (Kerosene)

Jet fuel or kerosene—from the German Kerosin—refers mainly to aviation fuel used to power aircraft jet engines. Also known as aviation turbine fuel or Avtur, it is a highly refined paraffin (a type of diesel) that ignites under pressure and heat in the combustion chambers of jet engines.

There are many types of specialized jet fuel, including those designed for passenger aircraft, such as JET A-1, and military-grade fuels, such as JET F-34/JP-8. Each has different characteristics, including freezing points, octane ratings, and additives.

Properties of Different Aviation Fuels

The main differences between aviation fuels and petrol are that aviation fuel is much purer, and paraffin comprises different hydrocarbons.

For example, temperatures can drop to around -40 degrees Celsius when aircraft are in flight. Automotive petrol would freeze at this height, but because kerosene has a lower freezing point, it remains liquid, allowing aircraft to operate at high altitudes.

In addition, additives designed to improve fuel performance are present in AVGAS and standard petrol, but some, such as metal deactivators, gum inhibitors, and static dissipaters, are only available in aviation fuel.

Regarding the operating temperatures of aviation fuels, the freezing point of Jet A is -40˚C, while that of Jet A-1 is -47 ̊C. The freezing point of the AVGAS, meanwhile, can be close to -58˚C. As far as high temperatures are concerned, it does not ignite at room temperature and does not form dangerous mixtures between fuel and air because it releases few vapors.

JET-A1 boils at around 150°C - 250°C and has a flashpoint above 38˚C, which is crucial as the fuel could ignite dangerously.

Water is the enemy of kerosene. It is essential to keep water from jet fuel. When flying at altitude, extremely low temperatures can cause the water in the fuel to freeze, blocking the aircraft's fuel inlet pipes. To combat this, fuel heaters are often used to prevent the water in the fuel from freezing.

AVGAS, a petroleum-based fuel, has a typical octane rating of 91 or 100 for a lean mixture and 96 or 130 for a rich mixture. Kerozene, on the other hand, has a much lower octane rating of around 15, which makes it similar to automotive diesel and much more resistant to detonation caused by sparks or compression.

SAF: The Future of Aviation Fuels?

According to US government sources, international aviation uses more than 1 billion liters of kerosene every day, or more than 11,500 liters every second.

Several possible solutions exist for reducing the greenhouse gas emissions released into the atmosphere by the aviation sector, one of which is closely linked to kerosene: Sustainable Aviation Fuel (SAF).

This is an alternative fuel made from sustainable raw materials. They can be made from vegetable oils, cooking oils, animal fats, sugars and starches, certain algae or lignocellulose from wood residues, and certain inedible plants. Once produced, up to 50% of this biofuel is blended with conventional kerosene.

There's still the issue of the actual cost of producing SAF, but proponents continue to study the feasibility of the novel energy source.

The ECLIF3 study

In a groundbreaking in-flight study, an Airbus A350 using 100% SAF demonstrated significant reductions in soot particle emissions and contrail ice crystal formation compared to conventional aviation fuel.

The ECLIF3 study, published in June of this year via a collaboration between Airbus, Rolls-Royce, the German Aerospace Center (DLR), and SAF producer Neste, measured emissions from both engines of the A350, powered by Rolls-Royce Trent XWB engines. The results showed a 56% reduction in contrail ice crystals, suggesting a notable decrease in climate-warming effects.

According to the Airbus release, global model simulations by DLR estimate a 26% reduction in contrails' climate impact with 100% SAF. This study, published in the journal Atmospheric Chemistry & Physics, provides the first in-flight evidence of the potential climate benefits of using 100% SAF in commercial aviation.