(Read more about aviation gasoline and the octane rating system here.) Your Engine MattersĪ large proportion of low-compression aircraft engines from both Lycoming and Continental were originally certificated for operation on 80/87 octane aviation gas. 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. 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. In the U.S., only the first two grades (80/87 and 100LL) are widely available, though 100/130 remains common overseas. Source: “Maintaining and Overhauling Lycoming Engines (2ed) by Joe Christy, 1986, TAB Books, Blue Ridge Summit, Pa. It’s found in aviation fuels in the following proportions: Until recently, the most important such additive, for both automotive and aviation use, was tetra-ethyl lead (TEL). 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. 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. “Straight-run” gasoline - directly as it comes out of a petroleum distillation plant - consists of 62-64% octane, and the rest heptane. The two hydrocarbons of most significance for gasoline are heptane, which has seven carbon atoms, and octane, which has eight. The simplest such molecules, methane and ethane, have just one or two carbon atoms respectively, and are gases. Gasoline is a mix of liquid hydrocarbons - that is, chemical molecules that contain hydrogen and carbon atoms. To fully understand the issues involved, we’ll need to discuss a bit of petrochemistry.
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(Read the rules about using autogas in an Adobe PDF document on the FAA’s Web site.) The FAA also wrote a letter a few years ago contradicting some of the claims of the engine manufacturers.What’s the real story? In short, for normally-aspirated (non-turbocharged), low-compression engines originally designed for 80 octane aviation gas, autogas is a perfectly viable option that can save considerable money and - despite what the engine manufacturers say - won’t hurt your engine. The FAA - which approves the STCs - mostly seems to agree, though it continues to prohibit the use of autogas for Part 135 operators while carrying passengers for hire. On the other hand, the Experimental Aircraft Association ( EAA) and Petersen Aviation - who issue the necessary STCs - say that it’s perfectly safe. Indeed, Teledyne Continental Motors (TCM) explicitly states that using autogas voids their warranty on new engines and parts.
The engine manufacturers are unanimous that auto fuel should not be used in engines originally certificated for avgas. Others - usually pilots who haven’t run autogas - say, “I’ll never run that crap in my engine,” listing a range of potential problems from vapor lock to deteriorating gaskets.Look to the experts, and you’ll find a similar range of opinions. Some - particularly owners who have an autogas supplemental type certificate (STC) - will tell you that it’s perfectly safe, and even better for an aircraft engine than running 100LL. Of all the hangar talk one encounters at airports, the subject of using automotive gasoline in airplane engines is among the most contentious.
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