Larry,
The question you asked is what is called washing gasoline. At fly-ins about 20 or 25 years ago, there were guys selling gizmos for washing the alcohol out of gasoline. The problem is that what is left is not a finished gasoline fuel you would want to feed even to your low compression O-200.
So, the answer to your question is... Yes, you can wash the alcohol out of fuel. It's a pain in the butt as it takes some time, but you can add water to the fuel, shake it up, then drain off the water alcohol mix from the bottom after it precipitates out.
The chemical issue is as follows. The refineries don't produce a finished gasoline. They produce what is called feed stock. Then they add chemicals to boost the octane of the feed stock to something usable and that becomes the finished product you get at the gas station. One of the octane boosters used is alcohol. So, if you have an 88 AKI [Anti Knock Index (R+M/2)] that is 10% alcohol, once you wash all the alcohol out of the fuel, you now have the original feed stock which (if memory serves me correctly) is about 82 or 83 AKI. When converted to a Motor Octane Nating for aircraft, the octane number is something well down into the mid to low 70s.
Note that I use the term AKI (Anti-Knock Index) rather than Octane. In the US, go the the gas station and read the labels on the pump. What we commonly call octane rating is actually an Anti-Knock Index rating and is not the same as the Octane rating used for Avgas. There are two different methods for determining octane numbers. One is called the Research Method (RON or Research Octane Number). The other is called the Motor Method (MON or Motor Octane Number). You will also notice the octane ratings for gasoline in Europe are also significantly higher as compared to the AKI ratings at the American pumps. It isn't that Europe has better gas. They just calculate the Octane differently. Europe uses the Research Method (RON) for determining the Octane of a fuel and that is what is posted at the pumps. Aviation uses the Motor Method to determine Motor Octane Number (MON) of a fuel. The pump gas in the US uses a combination of them, thus the R+M/2 marked on the pumps. That is Research Octane Number of the gasoline, plus Motor Octane Number of the gasoline added together, then divided by two to determine the Anti-Knock Index (AKI) of the fuel. Just to add confusion to the fuel fire, the AKI number is also often times referenced as the Pump Octane Number (PON). So that gives us RON, MON and PON, which is a combination of RON and MON.
Once upon a time, I wrote a simple spread sheet used to calculate the actual Motor Octane Numberof a fuel to be used in an aircraft when mixing fuels with AKI Index ratings and MON ratings as I was mixing fuels in my aircraft and had friends with high compression aircraft engines that wanted to be able to mix fuels. So, I calculated a lookup spreadsheet that made a comparison of the RON, MON and AKI of any given fuel if you had one of the numbers. I won't propagate the spreadsheet as it was a non-scientific estimate on my part that I put together just using a bit of information gleened from various sources. But the end result was that I could plug in a quantity of one type of fuel and add in a second quantity of a differently type of fuel, and get out a usable MON Octane number for the mixed fuel for aircraft engines that was usable for my purposes.
Hope this clears up the Chemistry question and reduces (or maybe adds to) the confusion of mixing octane numbers between cars and aircraft. It's best to just remember that they aren't the same.
-Jeff Scott
Arkansas Ozarks
Sent: Friday, September 30, 2022 at 5:56 PM
From: "Flesner via KRnet" <krnet@list.krnet.org>
To: krnet@list.krnet.org
Cc: "Flesner" <fles...@frontier.com>
Subject: KRnet> question for chemist
On 9/30/2022 5:22 PM, MS wrote:
If only I could be king I would straighten these unfortunate situations out . . . ![Emoji]( "Emoji")
.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Train wreck !!!! Train wreck !!! Looks like my original post has gone off the rails a bit. My PRIMARY CONCERN is what alcohol does / will do to my fiberglass tanks and any other untested component in my fuel system. My fuel system, as well as most, is a series system of components and the failure of any one component is a 100% failure of the system. Aircraft approved for auto fuel use are approved for "alcohol free" fuel. I have no way of knowing how the dozens of seals, gaskets, fuel lines, pumps, shutoff valves, etc. designed for use with my 80 year old engine technology will react with alcohol and I don't intend to take it to 5000 feet to test it. I was simply looking for an option, if it is do-able and convenient, to the price of 100LL with it being a well proven fuel. I don't want to duplicate what a friend did years ago with his big Stinson Gullwing when he used the wrong brake fluid and it caused some "O" rings to swell and locked up his brakes. Fortunately the issue developed on the ground.
My other concern with auto fuel is when left in the carb for extended periods the gas evaporates and leaves a highly contaminating gummy brown crap. Ever wonder why you mower runs like crap the first time in the spring. I assume it flushes out with fresh fuel but there are times when the carb needs to be removed and cleaned to work again.
I'll stick with 100LL if I can't find a reliable / cheaper alternative.
Larry Flesner
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