Avgas - Avgas Compared To Other Fuels

Avgas Compared To Other Fuels

Many general aviation aircraft engines were designed to run on 80/87 octane, roughly the standard for North American automobiles today. Direct conversions to run on automotive fuel are fairly common and applied via the supplemental type certificate (STC) process. However, the alloys used in aviation engine construction are chosen for their durability and synergistic relationship with the protective features of lead, and engine wear in the valves is a potential problem on automotive gasoline conversions.

Fortunately, significant history of mogas-converted engines has shown that very few engine problems are actually caused by automotive gasoline. A larger problem stems from the wider range of allowable vapor pressures found in automotive gasoline; this can pose some risk to aviation users if fuel system design considerations are not taken into account. Automotive gasoline can vaporize in fuel lines causing a vapor lock (a bubble in the line), starving the engine of fuel. This does not constitute an insurmountable obstacle, but merely requires examination of the fuel system, ensuring adequate shielding from high temperatures and maintaining sufficient pressure in the fuel lines. This is the main reason why both the specific engine model as well as the aircraft in which it is installed must be supplementally certified for the conversion. A good example of this is the Piper Cherokee with high-compression 160 or 180 hp (120 or 130 kW) engines. Only later versions of the airframe with different engine cowling and exhaust arrangements are applicable for the automotive fuel STC, and even then require fuel system modifications.

Vapor lock typically occurs in fuel systems where a mechanically-driven fuel pump mounted on the engine draws fuel from a tank mounted lower than the pump. The reduced pressure in the line can cause the more volatile components in automotive gasoline to flash into vapor, forming bubbles in the fuel line and interrupting fuel flow. If an electric boost pump is mounted in the fuel tank to push fuel toward the engine, as is common practice in fuel-injected automobiles, the fuel pressure in the lines is maintained above ambient pressure, preventing bubble formation. Likewise, if the fuel tank is mounted above the engine and fuel flows primarily due to gravity, as in a high-wing airplane, vapor lock cannot occur, using either aviation or automotive fuels.

In addition to vapor locking potential, automotive gasoline does not have the same quality tracking as aviation gasoline. To help solve this problem, the specification for an aviation fuel known as 82UL has recently been developed. This fuel would be essentially automotive gasoline that has additional quality tracking and restrictions on permissible additives. This fuel is not currently in production and no refiners have committed to producing it.

The main consumers of avgas at present (mid-2000s) are in North America, Australia, Brazil, and Africa (mainly South Africa). Care must be taken by small airplane pilots to select airports with avgas on flight planning. For example, US and Japanese recreational pilots ship and depot avgas before flying into Siberia. Shrinking availability of avgas drives usage of small airplane engines that can use jet fuel.

In Europe, avgas prices are so high that there have been a number of efforts to convert the industry to Diesel fuel instead, which is common, inexpensive and has a number of advantages for aviation use. However, avgas remains the most common fuel in Europe as well.