With the recent dramatic rise in fuel prices nationwide, I have been looking again at using Autogas in my Long-EZ.
The main issue (which has led to more discussion threads than I care to remember on the Canard forums) is that while there is a national standard for 100LL in the USA, no such standard exists for auto fuels. Instead there is a complex patchwork quilt of Federal and State regulations, which means that everything from octane ratings to additives can be different from town to town, city to city and state to state. As a result, refineries generally pump out a limited number of grades of generic gasoline that meet basic "lowest common denominator" standards, and leave it up to local distributors and filling station brand owners to supplement the fuel with required local additives and special additives for brand differentiation. You can see this system at work down the road from where I work, in Euless. There is a major gasoline distribution depot that supplies fuel all over the D/FW metroplex. When I drive past that depot, you can see fuel trucks from all sorts of filling station chains - Mobil, Chevron, RaceTrak, Joes Garage etc. all waiting in line to be filled. Clearly they are all being filled with the same basic fuel, and then the individual operators are adding whatever extras they deem necessary for branding purposes.
The differences in auto fuel specifications start with the displayed octane number at filling stations, which is actually the average of 2 different measurements - the RON (Research Octane Number) and the MON (Motor Octane Number). The technical abbreviation for the formula used is (R+M)/2. The difference between RON and MON is not supposed to exceed 10. So a fuel with a displayed value of 87 could have a RON of up to 92 and a MON of down to 82.
However, the differences that lead to the most trouble are the special standards implemented for fuel sold in major urban areas whose pollution levels exceed certain EPA and state standards. Fuel in those areas has to meet different standards (the Reformulated Gasoline standards), which place strict limits on levels of sulphur, levels of oxygenates etc. In order to meet those standards, fuel refiners, distributors and filling station owners add all sorts of chemicals to gasoline. Some of the additives (such as detergents and fuel system cleaners, which are usually added by the filling station chains) are fairly benign. However, other permitted additives - oxygenators such as ethanol and methanol, are nothing but trouble for aircraft use.
Historically, gasoline distributors used MTBE as an oxygenating additive; however, emerging evidence about the long-term toxicity of MTBE (including its appearance in local groundwater supplies) has led many states to enact MTBE bans. So far, Texas has not yet gotten around to banning MTBE (what else could we expect from a state with such a laissez-faire approach to environmental protection generally?), but a number of fuel suppliers, conscious of the bad PR from the continued use of MTBE, are already phasing it out in favour of ethanol.
Ethanol is a double-whammy no-no for aircraft - it attacks fuel system seals, gaskets etc. and also absorbs water, which can lead to significant operational issues (like the replay of "The Sound of Silence" at altitude).
Another variation is that some states (including Texas) mandate that fuel sold in the Summer (1st May to 1st October) in a list of 90+ counties has a lower Reed Vapor Pressure (RVP) to reduce vapor lock issues. This is actually good from an aviation standpoint.
Here where I live in the D/FW metroplex, fuel additives can include ethanol, and the 3 counties (Dallas, Collin and Tarrant) that largely comprise the geographical area appear to be exempt from the lower Summer RVP requirement. this is probably because adding ethanol to gasoline increases the RVP by at least 1 psi, reducing the chance of this blended gasoline passing the wider RVP Summer pressure standard.
This is bad news for pilots on 2 counts - not only could your autogas contain ethanol, but it may have too high an RVP in the Summer for safe aircraft use.
My tentative conclusion is that if you live in D/FW, and buy auto fuel in Dallas, Collin or Tarrant Counties for use in aircraft, you would be well advised to test if before use to make sure it does not contain ethanol, and that its RVP is safe for aircraft use. It might be better to buy fuel outside of those 3 counties altogether.
One positive advantage of using autogas for aviation use is that you can reclaim the state tax on it, if you save the receipts and get a special form from the Texas State Comptroller's office. Unfortunately it appears that you cannot reclaim the Federal tax on the autogas, unless you can prove that the fuel was used for charitable activities.