Piston Engines - Fuel — Page 77, Lesson 101

Let’s start with the fuel itself. The preferred fuel for aircraft piston engines today is derived from mineral oil, and it’s a blend of hydrogen and carbon. Jet and diesel fuels come from the same source. The different types are produced by a process called cracking, which breaks down the oil into different fractions. The gasoline we use in piston engines is called AVGAS. Now, the dispensing equipment for AVGAS is colour coded red. That’s a deliberate safety measure to prevent cross-contamination with other fuels. You never want to put the wrong fuel in an aircraft. Next, manufacturing specifications. AVGAS is made to exacting specifications issued by the Directorate of Engine Research and Development, abbreviated DERD. The specification number for gasoline is DERD 2485. Fuel grades lie within a specification, so they carry a blanket DERD number followed by a grade that is not prefixed by the DERD notification. Let’s look at the popular grades. AVGAS 100LL has a performance number of 100/130, it’s blue, specific gravity 0.72, and it’s low lead. AVGAS 100 has the same performance number 100/130, it’s green, specific gravity 0.72, but it’s high lead. AVGAS 80 has a performance number of 80/87, it’s red, specific gravity 0.72, and it’s very low lead. Note that AVGAS 100 and 100LL share the same performance number, but they’re easily distinguished by colour. Some aviation authorities allow car petrol for certain aircraft, referred to as MOGAS, motor gasoline. In the UK, aircraft authorised for MOGAS are laid down in Airworthiness Notices numbers 98 and 98a. Because MOGAS has higher volatility, carburettor icing and vapour locking are much more likely. Information on MOGAS use is also in CAA Safety Sense leaflet number 4a. Finally, calorific value. This is a measure of the amount of heat released during combustion, measured in British Thermal Units, BTU, per pound. It varies with chemical composition, and fuels with high hydrogen content are superior. Calorific value is related to specific gravity — the higher the specific gravity, the higher the calorific value.