The GAfuels Blog is written by two private pilots: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.
When temperatures climb in the summer, reports of vapor lock increase, especially for those flying at hot/high altitudes. Vapor lock occurs when fuel, normally in liquid phase, transitions to vapor through increased temperatures, lowered pressures, higher RVP (Reid Vapor Pressure) of the fuel, or a combination of any of these factors.
The RVP is a physical property of the fuel related to its volatility and is modified by fuel producers depending, for instance, on the season or air quality regulations. It is simply expressed as the pressure (in PSI) where the fuel begins to transition from liquid to vapor when the fuel is held at 100° Farentheit. An RVP of 14.7 would mean that the fuel would “volatize” at sea level pressure or lower on a 100° day.
The lower the RVP, the less likely that a fuel will change to its vapor state, unless the temperature exceeds 100° or the fuel pressure drops as it does as an airplane climbs. Since the temperature under the cowl of an engine tend to be fairly high, it is the local temperature near fuel lines, gascolators, fuel pumps, carburetors, injectors, etc., that matter, not just the ambient conditions outside the aircraft. For this reason, aircraft manufacturers often shield fuel from hot components like exhaust manifolds and cylinder heads, or provide directed cooling air through baffles and blast tubes to keep the fuel in its liquid state. This is one of the reasons that it is important to maintain the baffles in an engine and assure that blast tubes point at their intended targets.
Remember too that, according to Bernoulli’s Principle, the pressure of any fluid drops with the square of its velocity. Any restrictions in fuel lines, pumps, injectors, etc., will lead to an acceleration of fuel velocity and a lowering of its pressure. That’s why engines with fuel pumps and injection systems are more susceptible to vapor lock, however the problem can occur even on naturally aspirated engines if things get too hot or the RVP is too high for the conditions around fuel systems. Simple things like a kink or pinch in a fuel line between the gascolator and a carburetor are sufficient to cause localized phase change of a fluid, bubbles of fuel vapor, and a rough-running engine.
What about variations in fuel RVP though? When and where temperatures are low, such as in northern climates in the winter, the RVP is allowed by the EPA to be increased by fuel producers so that engines start easier. In the spring and summer, RVPs are lowered again to avoid fuel’s evaporation and the air pollution this is believed to cause. This is why one hears of “summer blends” or “winter blends” of fuels.
According to How Stuff Works: “Depending on the part of the country, the EPA’s standards mandate an RVP below 9.0 PSI or 7.8 PSI [depending on location] for summer-grade fuel. Some local regulations call for stricter standards. Because of these varying RVP standards, up to 20 different types of boutique fuel blends are sold throughout the U.S. during the summer. Because RVP standards are higher during the winter, winter-grade fuel uses more butane, with its high RVP of 52 PSI, as an additive. Butane is inexpensive and plentiful, contributing to lower prices. Summer-grade fuel might still use butane, but in lower quantities — around 2% of a blend.” As one can imagine, the use of a winter-grade fuel with its higher RVP in an airplane flying in hot/high conditions would be more prone to vapor lock than one using a summer blend. This is a real problem for sport aviators, since our aircraft tend to be flown less in the winter, resulting in the use of winter-blend fuels remaining in our aircraft fuel tanks as well as the fuel tanks at airports.
Ethanol too affects a fuel’s volatility and thus the problem of vapor lock in planes burning ethanol blends. E10, the current fuel found at most gas stations, may not contain more than 10% by volume ethanol. (It should be mentioned that no state authority checks the actual level of ethanol used in our fuels; there have been cases of unscrupulous retailers selling up to 30% ethanol in their fuels, another reason to avoid ethanol blends altogether.) A 10% ethanol blend raises the RVP of gasoline by about one PSI. In principle, fuel companies should lower the volatility of the base, or “blendstock” fuel to which ethanol is added. Since this was deemed too expensive, the EPA instead granted a one-pound waiver for ethanol blends in all but the highest ozone areas of the country, allowing a higher RVP by one PSI. In regions excluded from the waiver, fuel companies are required to use a blendstock that is one pound lower in RVP before adding ethanol.
The need to modify blendstocks to comply with the EPA’s RVP regulations has an impact on autogas as a fuel for aviation. Since ethanol may not be pumped through gasoline pipelines, all 1,000+ fuel terminals in the US have, in principle, ethanol-free fuel. In many cases, this gasoline is a legal, “finished” fuel that is suitable for our aircraft. Increasingly however, fuel producers pump a “Blendstock for Oxygenate Blending”, or BOB, through the pipeline instead. BOB, as described above, may have a lower RVP than the final ethanol blend. Since ethanol also bumps the octane rating upwards by three points or more, BOB is also sub-octane. For these reasons, BOB is not a legal, or safe, fuel for aircraft. Fortunately, many fuel terminals maintain supplies of 91-93 AKI octane “clear” ethanol-free non-BOB fuel as one component of blending with lower octane BOB and ethanol to produce finished fuels. Due to the ethanol production mandates in the EISA 2007 Act from Congress, fuel producers are under significant pressure to convert to producing BOB in order to remain competitive. (As a side note, the elimination of subsidies for ethanol production, a hot political topic at the moment, would have no effect whatsoever on the production mandates and would not reverse the trend in our country towards the increasing adulteration of our fuels through ethanol. Few in Congress appear to know much about these mandates, which are the primary driver behind ethanol’s expanding use, not the subsidies.)
Back to the topic of vapor lock. One of the myths surrounding the use of autogas is that it is more susceptible to vapor lock. While, as described above, the use of a winter blend — with or without ethanol — in our engines might lead to vapor lock under hot/high conditions, it occurs seldom. One of the reasons for this is the very stringent testing procedures required by the FAA for type certification or for autogas STCs. For a description of the methods used by Petersen Aviation to determine whether an engine will experience vapor lock, see Part I of the series “Autogas for Everyone”.
Nevertheless, the problem does on occasion occur, as related to us by the owner of a Piper PA-28-140 Cherokee based in south Texas: “I’m mainly concerned about ethanol, vapor lock problems with auto fuel (that has happened to me several times – yes, I have an auto fuel STC on my Cherokee 140), and availability at airports along my route of flight, wherever I may go.”
Todd Petersen, a recognized authority on autogas, responded with the following: “Vapor lock is not impossible just because there’s an STC, even though the Cherokee 140 passed the flight tests. We include a lot of info on vapor lock and how to avoid it when we send an STC. The vapor pressure of autogas has been gradually reduced, hence vapor lock is not as likely today as it was 20 years ago. Still he can do much to reduce the chance of vapor lock, blast tube to the pumps and especially the gascolator, doing away with 90 degree fittings and replacing them with gradual bends to tubing, making sure the vents are clear and correct, that sort of thing.”
Just as an autogas STC is no 100% guarantee that vapor lock will not occur even when using high quality, ethanol-free autogas, engines burning avgas can also experience the problem. As I write this article, a friend of mine is returning from Wyoming in his meticulously-maintained, Rotax-powered S-LSA. He believes that engine roughness at altitude was caused by vapor lock, and has been burning avgas exclusively since he can not find autogas at airports in the western part of the US. The Rotax 912ULS in his airplane uses twin altitude-compensating Bing carburetors that eliminate the need for mixture control. A few suggestions by the airplane’s manufacturer have resolved the problems, he reports.
Generally, problems caused by ethanol in four-stroke engines are amplified when the fuel is used in two-stroke engines found in ultralights. Terri Sipantzi, the owner/operator of Precision Windsports Inc., in Lynchburg, Virginia, comments on the topic in this article (“Flying with Ethanol”) that recently appeared in EAA Light Plane World.
As long as one uses the optimum fuel for a given engine/airframe, and pays attention to the various factors that can cause vapor lock, this potentially serious problem can be avoided. The onus, though, is on all pilots to understand the issue and not assume that a TC or STC immunizes them from vapor lock. “It never did that before” will not result in a smooth-running engine on a hot summer day at altitude.