SYSTEMS TECH heat from its surroundings than does a less volatile fuel and tends to allow Autogas vs 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 , 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 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 . 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 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. Several major fuel companies to. Typically refinery supplies a large fuels, the upper cylinder problems of the were contacted (Shell, Texaco etc.) that amount “generic,” single-grade, past are now mostly history. supply both auto and avgas confirmed unleaded autogas to a pipeline Secondly, the initial high wear of that the quality control is not the same system with fuel shipped to several cylinders and components at breakin once the product leaves the pipeline. 2) storage far ties in major metropolitan have been reduced significantly if avgas Alcohol mixed in autogas is not safe for areas throughout the US. was used during the break-in period and aviation use, and it is up to the user to When the fuel reaches the subsequently used occasionally to keep determine the presence of alcohol in the storage facilities it is transferred to some lead “varnish” on the cylinders fuel. tank trucks and at this time various and valves. All autogas STC limitations spe- additives, dyes, detergents, and other cifically state that alcohol mixed with chemicals are added to the fuel give Industry Disdain autogas is not approved. 3) Vapor the fuel its branded identity a grade. In discussions with many aviation pressure of the autogas can vary widely No specific tracking or tracing of the engine, airframe and fuel system as formulations are changed seasonally, autogas is done to determine what manufacturers as well as avgas and and according to local requirements. was added to the fu who transported autogas manufacturers, all have stated High vapor pressure can promote vapor it, and what ret outlet received the that they do not recommend, approve or lock in aircraft fuel systems causing fuel. suggest that autogas be used in aircraft. engine power to be reduced or the Furthermore, many go so far as to engine to completely fail. A Pinch of This... disallow warranty claims because Additionally, tests by the FAA Additives are added to satisfy myriad autogas has been used in their products. technical center have concluded that state, federal (EPA) and local A few engine shops polled and at autogas with high vapor pressure can regulations, and seasonal least one aftermarket cylinder manu- accelerate the formation of carburetor requirements, including wintertime facturer did not come out and say ice in aircraft equipped with float-type oxidizers. Alcohol is frequently warranty would not be honored, but carburetors. Some aircraft are more mixed with gasoline in varying highly discouraged its use and said that susceptible to carburetor ice than others, amounts up 10% so as to satisfy the the decision would be made on a case- so this is an issue of concern. requirement for air quality and by-case basis. sometimes as a way to boost octane There are several more reasons for Just the Facts for less cost than other additives. this industry discouragement for the use To address this issue for the use of These additives can have an of autogas in airplanes. This is in spite autogas, consider the following: Avgas adverse impact on the energy content of the acknowledgment by the FAA that usage in the US is approximately two of autogas, so the lower cost autogas autogas has been approved (after many percent of the autogas used for cars and can sometimes be offset lower hours of testing) for use in many low- trucks. With these proportions and the energy values and decrease range on octane airplanes EPA, it’s clear we’re fighting an uphill an airplane. Autogas test rig as used by Petersen battle for avgas status-quo. With the discovery of MT] Aviation for dynamic testing of The quality of avgas is maintained (autogas additive) as a contaminant engines to obtain autogas STCs for and traced from the refinery to the of ground water systems, the specific engine models. Petersen aircraft fuel tank as evidenced by recent addition of alcohol to autogas is photo. troubles of contamination of becoming more prevalent. avgas supplies and wholesale Alcohol/gasoline mixtures, replacement of aircraft engines whether or Methanol, ha at the expense of fuel never been approved for use in a companies as a result. craft with autogas STCs. Alcohol is In addition to the tracking known to entrain water, which could and purity verification during cause water in fuel to freeze at low the transportation of avgas, the temperatures when aircraft climb leaded aviation fuel can only higher altitudes. be transported in dedicated Fuel volatility is increased with tanks as leaded fuel cannot be the addition of alcohol, and the vapor transported trucks or pipelines pressure obtained with the use of the used to transport unleaded Hodges Reid vapor pressure fuel.

LIGHT PLANE MAINTENANCE • JULY 2000 7 tester sold by Petersen Aviation will autogas is changed from time to time, altitude airport operations, check to see reflect alcohol in the fuel by its read- and additives contained in autogas alter that your fuel pressure gauge is ing—but not the alcohol itself, which the apparent vapor pressure. The more indicating within limits, and make a must be checked by a different means. varied sources of autogas you use, the full-power run up check prior to takeoff Therefore, it is important that fuel be more critical your individual testing to confirm the presence of maximum tested for alcohol presence, as fuel becomes. available power. station operators frequently do not know Autogas (intended for use with autos) Failure of vapor lock testing during if the fuel they are selling contains reacts differently in aircraft fuel systems the STC process is the principal reason alcohol. than automobiles since autos rarely why some aircraft are not approved for There is an easy way to check for experience rapid temperature and autogas use although the engine is alcohol in the fuel you buy with an altitude changes (unless dropped out the approved. inexpensive, simple test kit available back of a cargo plane in a TV That’s why both the engine and from Petersen Aviation at (308) 832- commercial). airframe must be approved as a unit for 2050, www.webworksltd.com/petersen Moreover, most modern autos are the autogas STC. It’s also why just or Sporty’s pilot shop (800) 543-8633. equipped with electronic fuel injection, because a particular engine is fine with They offer improved test kits for octane in-tank fuel pumps, and higher fuel autogas in airplane “A,” it may not range and alcohol presence. operating pressures to lessen the work reliably in airplane “B.” Alternatively, another graduated chances of vapor lock. People, unfortunately, do make this cylinder method with instructions is High altitude operations, high under- erroneous assumption and don’t bother available from the EAA (920) 426- cowling temperatures with highly with obtaining the STC, if it exists, and 4843) as listed on their web site volatile fuel, and complex fuel systems operate illegally. www.EAA.org, regarding autogas all increase the likelihood of vapor lock, In part deux, next month, we delve STCs. as do the convoluted designs of more deeply into the specifics of the Generally, with a careful mix of nine carburetor air boxes designed for avgas. STCs, as well as 82-unleaded fuel. Stay parts gasoline and one part water, Poor fuel system maintenance such as tuned. You know our email address if shaken thoroughly, and allowed to stand substitution of parts (90 degree elbows you would like to add your opinion. for 10 minutes, this mixture should yield vs 45 degree as specified), replacement the same amount of water and gasoline of insulated (fire sleeved) fuel hoses Mike Berry is an airline captain and an by volume. If the amount of “water” with plain fuel hoses, (affecting heat active IA. He is a frequent contributor appears to increase, then suspect alcohol loss or gain) or removal of heat shields to LPM and lives in Spencer, Mass. mixed in the fuel and separated out with in the engine compartment, have Below, testers from Petersen Aviation. the water. contributed to vapor-lock problems. In Inexpensive, $10 tester for alcohol is some cases, avgas was the fuel being fast and reliable. Bottom, the Hodges The Vapors used. Reid Vapor Pressure tester is designed As a precaution during periods of for quick testing of autogas to make sure Vapor pressure problems have been high ambient temperature or higher it’s up to STC standards; costs addressed throughout this article and are under$50. an important factor to consider when using autogas. Vapor pressure is important as high vapor pressure increases the likelihood of vapor-lock during flight. Avgas vapor pressure remains constant at a 6.5-psi level regardless of the time of year or area of the country it is used. Autogas vapor pressure is often changed in some parts of the country from a high of 15 psi to a low of 7 psi. (California regulations define a maximum of 7 psi.) While vapor pressure of automo tive gasoline supplied in major metropolitan areas has been limited in recent years in an attempt to reduce air pollution, there are still issues to be concerned with when using autogas in many parts of the country. The chemical makeup of