I have been doing some research on auto gas and below is some of what I have come up with.
In 1982 EAA received approval from the FAA to provide an STC that allowed certain types of aircraft engines to use auto fuel instead of more costly avgas. This was the result of years of research and testing conducted by EAA staff and volunteers.
Total money saved by EAA Auto fuel STC’d aircraft in 1999.
19,109,776.4 x $.65 gallon price difference between auto fuel and 100LL = $12,421,345.66
Aircraft owners choose automotive fuel rather than aviation fuel for a variety of reasons. The primary reason is cost. The savings in fuel prices between auto fuel and avgas are quite significant. In an airplane using 15 gallons per hour, a savings of only 50 cents per gallon will be $7.50 per hour less than the cost of using avgas. In 200 hours, this adds up to $1500.00. Savings of even half this amount would clearly justify the installation of an auto fuel STC.
Auto fuel is safe for use in approved aircraft, however not all auto gas is the same there are over 100 different blends of auto gas in the United States and depending on the time of year the blends change.
I ran a check and found 34 STC’s for use of auto fuel. If you would like a list send me your address and I can send you a hard copy of the list by manufacture and type of aircraft to use it on.
Please note that certain STC's are only for engines that are certified for the use of 80 octane fuel. Engines requiring 100 octane fuel must still use 100 octane avgas. This is because of detonation. Detonation will not be a problem when using any grade of automobile gasoline with an aircraft engine approved for use of 80 octane fuel (hopefully).
The number which is posted on the automobile service station pump is not a true octane number. It is what is called an "antiknock index" number (AKI). This number is the average of two octane numbers arrived at by two different kinds of tests. One is called ASTM Research Method and is often abbreviated R or RON. The other is the ASTM Motor Method, M or MON. The antiknock index number on the pump is then this average, or R + M divided by 2 = AKI. A rule of thumb is that the Motor Method octane number (MON) is approximately five points less than the AKI. The significance of the MON is that this is identical to the octane number for aviation gasoline.
Vapor lock is a problem to consider regardless of what kind of fuel is being used. When using automobile gasoline, vapor lock is an important consideration because automobile gasoline has been designed to facilitate engines starting in the winter time and thus has a higher volatility.
There are other important considerations such as the effect of high ambient temperatures, very high engine operating temperatures under conditions of takeoff with high volatility fuel, and the complexity of the fuel system (many bends and fittings). All of these factors and many more effect the likelihood for vapor lock. Another area of concern is high altitude vapor lock.
Automobile gasoline volatility has been generally higher than aviation gasoline volatility. If critical operating conditions, as mentioned, reach extremes, vapor lock can occur earlier with automobile gasoline than with aviation gasoline. Operation conditions that encourage the formation of vapor in aviation or automobile gasoline are those which raise the under-cowl temperatures to extremes and provide a source for the transfer of excessive heat into the fuel lines. After any prolonged period of heat soak (e.g., hot day ground idling or engine restart a short time after a long period of engine operation), perform full power check before taking off. Ensure recommended fuel pressure is indicated on aircraft so equipped. Follow this precaution also with aviation gasoline.
At the present time, in most US metropolitan regions, the EPA limits the volatility of automobile gasoline to about the same as 100LL aviation gasoline. In California as of June 1, 1996, regular automobile gasoline, with the exception of gasoline that has alcohol, is for all practical purposes identical to 80 Grade aviation gasoline. Future automobile gasoline changes to meet EPA requirements find both aviation gasoline and automobile gasoline approaching identical characteristics with the exception of meeting the 100 octane rating. Oxygenates required in these urban areas are primarily Methyl Tertiary Butyl Ether (MTBE) and ethanol alcohol. Alcohol additives, other than some de-icing fluids, are not approved by the FAA.
Oxygenates include a broad range of alcohol's and ethers. While there are several alcohol's and ethers that are being used in unleaded gasoline, two such components, Methyl Tertiary Butyl Ether (MTBE) and ethanol have seen significant level of commercial use. Another ether used is Ethyl Tertiary Butyl Ether (ETBE) and it to has been tested by EAA and approved by the FAA. All alcohol's and ethers add oxygen to the gasoline.
MTBE is manufactured by the chemical reaction of methanol and isobutylene. It has been a blending agent in gasoline to raise the octane number for over 20 years. The conversion of methanol to MTBE eliminates the unfavorable characteristics associated with alcohol's, such as materials compatibility problems, water tolerance and corrosive action. More recently, MTBE has been classified as an oxygenate by the EPA.
Here in California MTBE was found to pollute the ground water and cause cancer. I would like to thank Senator Barbara Boxer for forcing MTBE on all California’s driving up the price of unleaded gas in the San Francisco to $2.35 a gallon this morning. Thank you again Ms. Boxer for polluting my drinking water.
MTBE eats the rubber fuel hose up big time. Lowers gas mileage and clogs up my fuel systems. Thanks again Ms. Boxer it costs me extra money to fix it. As you may of noticed I LOVE Ms. Boxer for screwing all of us California’s. Did I mention she changed the laws to allow an Indian gaming club because they gave her tons of money. The best senator money can buy!!! Sorry I get carried away when talking about money grabbing politicians with a power problem.
Back to auto gas. Ethanol is often confused with methanol. These two alcohol's have distinctly different characteristics; however, all STC's prohibit the use of gasoline containing alcohol - either methanol or ethanol.
Known Problems:
Alcohol attacks some seal materials and varnishes on cork floats of fuel level indicators. This could cause leakage of seals and release particles of varnish from floats, causing blocked screens in fuel lines or blocked carburetor jets. Excessive entrained water carried by alcohol could lead to fuel lines blockage or blockage at screens or valves when operating at low ambient temperatures at ground level or at high altitude. Fuel volatility is also increased with the addition of alcohol in a manner that is not detected by the Reid Vapor Pressure test which is used to determine if a fuel meets the automotive specification. For example, a gasoline with alcohol will meet the Reid Vapor Pressure limit of 13.5 psi but it will behave as though it has a volatility of roughly 20 psi. Gasoline's with alcohol will also phase separate. Phase separation occurs as the gasoline/alcohol blend cools, such as when a plane climbs to a higher altitude. When water that is absorbed in the fuel by the alcohol comes out of solution, it takes most of the alcohol with it. The quantity that comes out of solution cannot be handled by the sediment bowl and tank sumps. Furthermore, if the alcohol is used to raise the octane of the base gasoline, the gasoline that remains will not have sufficient octane to prevent detonation. A good reference for this phase separation problem is: Paul Corp., Laboratory Investigations into the Effect of Adding Alcohol to Turbine Fuel, DOT/FAA/CT-TN88/25 July, 1988, FAA Technical Center, Atlantic City International Airport, NJ 08405.
Stache
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