Aviation From My Rocking Chair: Auto Fuel and Octanes. Is It Smoke and Mirrors?

Hello Chapter members.

Have you ever said or written something you just knew was going to cause controversy? Well, I’m about to! For the next 3 months, you and I are going to discuss Autogas vs Avgas, and I fully expect as many opinions as we have members. I originally wrote this series in 2002, at a time when the GAO listed at least 45 Autofuel blends. Now there are at least 140 different recipes. In that time, Avgas went from 3 blends (80/87, 100/130, 100LL) to just 100LL.

Let me get my feet wet with what Mr. John Szmanski and the Zeltex Corp. called, “Octane 101”. “It is vitally important for we as pilots to have clean fuel with the proper octane for the powerplant we trust our lives to. With more and more Autofuel STCs all the time, let’s take a look at how Octane is determined and some of the possible problems that might be encountered”.

Octane rating of gasoline refers to the fuel’s anti-knock quality, and Autogas is rated differently than Avgas. Two CFR(Cooperative Fuel Research) knock test engines are used to test Autogas according to ASTM D2699 and D2700. D2699 refers to the Research Octane Number (RON) and is the essential index of Acceleration Knock. D2700 refers to Motor Octane Number (MON) and is a knock index for High engine speeds. The MON method differs from the RON test by using heated fuel mixtures, higher engine speeds, and variable ignition timing, all of which mean more stringent demands on the tested fuel. The MON is typically 8-10 octane numbers lower than RON. According to ASTM D2700, the Autogas MON octane is similar to the Avgas rating up to 100 octane. Now, here is the rub; Autogas octane rating is the average of RON and MON (R+M)/2 and that formula will be posted on the pump. You will have no way of actually determining if the MON octane number meets your engine requirements unless you have documentation or a means to test on site.

Contrary to what some people think, high octane gasoline is not more powerful than the low octane stuff. High octane fuel does feature a higher ignition temperature and a slower burning rate. High ignition temperature reduces the chance of Detonation from cylinder hot-spots and helps minimize Pre-ignition. A slower burn rate allows more efficient use of the fuel’s pressure buildup to be converted to mechanical energy instead of heat. Gasoline with octane and ignition temperatures too low will cause Pre-ignition. Typical ignition temperatures for Autogas are: 300deg.c (87 octane), 400deg.c (93 octane) Avgas is blended to ignite at 500deg.c. O.K., gang, before I put this column to bed for the month, let’s go over what Pre-ignition and Detonation are and how they relate. A combination of high compression and high cylinder temperature causes a fuel charge to ignite BEFORE the sparkplug fires. In a car it causes that dreaded “pinging” heard during uphill climbs with low octane fuel. Unfortunately, in an airplane you can’t hear it due to noise of everything else. Detonation occurs after the sparkplug fires. Normally, when fuel is ignited, an even flame path develops across the combustion chamber and produces a smooth temperature and pressure increase which pushes the piston down. Detonation happens when secondary ignition sources such as red-hot carbon bits and cylinder hot-spots cause 2 or 3 flame paths to develop. The result is an instantaneous fuel charge burn with a violent increase if cylinder temperature and pressure.

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Using fuel with lower than recommended octane for your engine can cause serious damage to pistons, valves, cylinders, and bearings in very short order. Low octane fuel ignites quickly and produces a pressure/ temperature peak that can exceed the engine design limits. In addition, the more the engine is run, the worse the problem becomes. Heat buildup can not be dissipated fast enough from the cylinder, causing more pre-ignition (hot spots) and uncontrolled detonation.

One more item. If your engine is designed for low octane fuel, high octane gas will not improve it’s performance. The fact is, the retarded timing and slower burn rates of high octane fuel can and will cause burned exhaust valve and seats. This happens because the fuel charge is still burning when the exhaust valve opens. Valve and seat erosion occurs very quickly.

Well, Troopers, I’ll let you go for this month. Next time Continental and Lycoming will check in, and after that it’s Autogas STC, EAA, and your turn.

Rick Vaux TC 4130 (gotta love that number!)