SYSTEMS TECH heat from its surroundings than does a less volatile fuel and tends to allow Autogas vs Avgas vapor (bubbles) to form in the fuel lines. We extended our questions and Bubbles in the fuel delivery system, investigation to virtually everyone involved prior to reaching the point of atomization in the carburetor, cause an in the process. Consider it all before interruption or reduction in fuel flow deciding if it’s for you. (vapor lock) and complete or partial by Mike Berry testing the fuel for alcohol or other engine failure due to improper fuel-air problem agents or contaminants. mixture. Reid vapor pressure is a here are many questions on the safe The promised benefits from using measure of fuel volatility and is closely use of autogas as a substitute for autogas are increased economy in terms monitored and controlled within an Tavgas. Our goal is to present the of the price of the fuel and a reduction acceptable range for use in aircraft. facts from a broad array of industry of wear of certain engine and airframe people, not just the engine manu- parts such as spark plugs and exhaust Achtung, Octane system components. facturers. Octane ratings for aviation gasoline This includes fuel system engineers, have been rated differently than au- autogas manufacturers, the EAA, engine Chemistry tomotive gasoline in that actual fuels makers, major autogas STC holders, and The formulation for aviation gasoline are tested in aviation engines. They are the like. We thank all the companies and continues to be relatively unchanged for rated according to an antiknock value individuals for their cooperation and the past 50 years. or “performance number” based on assis tance. It must satisfy these basic require- octane values lower than 100-octane or Petersen Aviation was especially ments: 1) Vaporization must occur performance-rated for fuel rated at 100- helpful to us. This company represents easily at low temperatures but yet not so octane or higher. the lion’s share of the autogas STCs, easy that it will cause vapor lock; 2) It The term 80/87 describes the ob- with over 30,000 in the field sold by must have a high energy content per served actual performances (resis tance them. unit weight (BTU), and permit high to detonation) when the engine is If you decide to use autogas, LPM compression engine operation without operated with a lean condition (80- can help you understand the require- detonation; 3) It must be relatively free octane) and then rich condition (87- ments in accordance with the STC. With of gum-forming compounds; 4) It must octane). more than 50,000 total STCs issued for have a low sulfur content to reduce Automotive fuels are not rated in the use of autogas in low-compression corrosive action; 5) It must be stored this way but use an “antiknock index” piston engine aircraft, it would hardly and delivered free of contaminants. and an average of the research octane seem that there is a Volatility considerations are im- method, and motor method. problem with the use of autogas in portant because of the effect on The antiknock index posted on aircraft. carburetor icing and “vapor lock.” Va- autogas pumps is approximately five That statement is based on users porization of fuel in the carburetor points higher than the actual octane closely following the STC, including venturi cannot take place without heat rating of aviation fuel—thus an an- Airplanes that have been approved for being extracted (latent heat of tiknock index of 87 posted on an use with autogas run the gamut. Being vaporization). If too much heat is taken autogas pump would equate to about an able to burn autogas is a real boon to out during the vaporization process “average” 82-octane aviation fuel. such high fuel usage engines. ft also there is danger of carburetor ice forming enables operation where avgas may Autogas STCs for 80-octane aviation not be readily available. Petersen with float-type carburetors. engines require a minimum of 87- photo. Highly volatile fuel extracts more antiknock index. Leaded aviation fuels use tetraethyl lead in small quantities, primarily to improve antiknock qualities, and is a necessary additive to aviation fuel to produce 100-octane or greater fuel. Lead is also used to provide lubrication of upper cylinder components such as valves, pistons, cylinder walls and valve guides. Currently, 100-octane 100LL) av- Of the several gas contains about 2 grams of lead per gallon lead substitutes and 80/87-octane contains 0.5 grams per sold in gallon. automotive supply stores that Additives have been tested, One problem with lead mixed with avgas is none have proven that lead requires the addition of a bromine to be beneficial for aviation compound to scavenge lead residue from the engines. engine. Automotive The burning of the lead with the bromine additives that compound produces lead bromide that is claim to remove carried out with the exhaust gasses; however, a water from fuel small amount remains within the engine. This (“dry gas”) lead bromide, when mixed with water, metals, should not be used, as often these are and lubrication oil, produces a corrosive liquid Autofuel testing and certification is not just alcohol and are not to be used in limited to the U.S. Here aJU-52is very damaging to engine components. aviation fuel systems. It cannot be being prepped for fuel testing in Significant water is produced as fuel is stressed enough to use only additives Switzerland under a Petersen license. burned. The presence of water and the proven and tested for use in aircraft. Petersen photo. corrosive properties of the lead bromide are The most unwanted impurity in autogas, this seems to be founded always lurking to damage an engine. gasoline (auto or aviation) is water. The partially on facts from problems ex- This lead bromide by-product is just one of transfer of fuel from cans, external fuel perienced several years ago. the reasons why aviation lubricating oil is tanks, or other non-aviation transfer Power plant and airframe manu- different from automotive lubricating oil. (Note systems introduces the possibility of facturers continue to not be in favor of aviation lubricating oil, according to engine water mixed into the fuel. the use of autogas in their products as makers’ specs, should always be used in Care must be taken to ensure that a well. aircraft engines regardless of the type of fuel filter system or other water removal With continued testing and used.) device is used. One such device is a monitoring of operations using autogas, Engines that are operated infrequently are fuel-water separator funnel available a wider experience level is being prime candidates for damaging corrosion from from Aircraft Spruce under part number established and autogas problems this corrosive mixture o f water and lead bro- F3C or F8C that allows the user to see generally have not been as significant mide. The corrosive effects of lead bromide water or other contaminants that are in now as in the past. can be reduced by regular flying, frequent oil the fuel filter/funnel. They’re important The major complaints of years past changes according to manufacturers’ specs, and when fueling with cans directly into an from mechanics was that upper preservation of engines not operated frequently aircraft fuel tank. (at least once a month). Aircraft fuel filters are limited in Additional additives to avgas are capability, and can remove only a small It’s imperative to check carburetor compounds such as dye, for identification amount of water or contamination in the filter screens regularly for any signs of purposes and aromatics such as benzol to resist fuel before allowing water to pass contamination of any sort to keep the knocking. Toluene is added to make for low directly into the fuel delivery system. dean side of the airplane up. freezing point, good volatility, and rubber Operators with Marvel Schebler float-type carburetors should at least solvent properties less damaging than benzol. annually drain the float bowl and check In addition to these compounds there are other for contaminants. Check the finger compounds blended in small amounts with screens for contamination whether avgas for reducing gum residue. autogas or avgas has been used. Water introduced into aircraft fuel injection Fuel Purity systems is extremely damaging and The requirement for purity in aviation fuel is a expensive, as damaging rust can given; gasoline must be free from all impurities develop in a matter of days. such as water, dirt, or other fuels and additives. Additives not certified for aircraft use should Old Issues not be added to fuels as their performance numbers may be adversely affected, and these While many mechanics and repair possible reductions of antiknock properties shops do not recommend the use of have not been tested or approved for aircraft. cylinder wear problems were the Fe-suit by the STC process. No matter where you buy a~ it is of the use of unleaded autogas. This Primarily it’s the liability issue, as the same quality, and consistency no really depends on the status of the autogas was not tested during the matter who sells ii “makes it.” That cylinders. original FAA certification and approval cannot be said autogas. With high Older engines that had not seen an process for engines and airframes. autogas volume. the distribution and overhaul in years had old parts, which Additional problems cited by every supply system is very different and were more vulnerable to unleaded fuel. manufacturer are: 1) Quality control could not n the same quality With new, hardened, valve system verification of autogas is not the same as verification that avgas is subjected components designed for 100-octane avgas.