Avgas UL91 is an aviation fuel with high anti-knock internals. Intended for use in aircraft piston engines, including the Cessna 172.
As for the composition, Avgas UL91 is a mixture of hydrocarbons, derived from primary and secondary refining processes. Antioxidant and antistatic additives are also added into the gasoline composition. Antioxidant additives prevent the formation of gums and other oxidation products. On the other hand, antistatic additives prevent the build-up of static electricity in the fuel during its pumping and filling.
The key feature of Avgas UL91 is the lack of tetraethyl lead in its composition. This element is excluded from the gasoline composition in order to increase the engine service life and minimize harmful emissions into the atmosphere.