Join Date: Aug 2002
Location: Shoreline, WA, USA
More On Avgas Octane Ratings
This should help with understanding Avgas
Gasoline is a mix of liquid hydrocarbons -- that is, chemical molecules that contain hydrogen and carbon atoms. The simplest such molecules, methane and ethane, have just one or two carbon atoms respectively, and are gases. The two hydrocarbons of most significance for gasoline are heptane, which has seven carbon atoms, and octane, which has eight. Both are liquids at room temperature. "Straight-run" gasoline -- directly as it comes out of a petroleum distillation plant -- consists of 62-64% octane, and the rest heptane. It's said to have an Octane Rating of 62-64.
The octane rating is significant because octane can withstand much higher compression than heptane, and high compression increases power. So to get reasonable power from a lightweight engine for aircraft use, aviation gasolines have an octane rating of 80 or higher.
The octane rating can be increased beyond the simple proportion of octane to heptane by adding anti-knock agents, which delay the onset of detonation. Until recently, the most important such additive, for both automotive and aviation use, was tetra-ethyl lead (TEL). It's found in aviation fuels in the following proportions:
Fuel Grade (Octane Rating) Color TEL per Gallon
80/87 Red 0.5 mL
100LL Blue 1.2 - 2.0 mL
100/130 Green 3.0 - 4.0 mL
115/145 Purple 4.6 mL
Source: "Maintaining and Overhauling Lycoming Engines (2ed) by Joe Christy, © 1986, TAB Books, Blue Ridge Summit, Pa.
In the U.S., only the first two grades (80/87 and 100LL) are widely available, though 100/130 remains common overseas. The first grade, 80/87 octane, is basically comparable to pre-1986 "regular" leaded automotive gas, at least as far as TEL content is concerned. Of course, modern auto gas has no TEL; instead a manganese-based antiknock agent is used, along with a sophisticated reforming process to increase the base octane number of the feed stock. (Read more about aviation gasoline and the octane rating system
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