Aviation fuel has been continuously improving for nearly a century, and it continues today with the development of unleaded aviation gasoline.

Jimmy Doolittle

Jimmy Doolittle set many speed records while burning early versions of Shell avgas.

As far back as the 1930s, the advantages of high-octane aviation gasoline (avgas) were recognized. Jimmy Doolittle left the active Army but was still in the Army reserves when he was hired by Shell Oil company in 1930. He was in charge of Shell’s aviation department and strongly advocated developing higher octane aviation fuels. Shell was able to produce 100/130 octane, but because of the process used, it was over one hundred times more expensive than normal automotive gasoline.

The higher octane fuel allowed higher compression ratios with the resultant greater horsepower, without increasing the engine size. Doolittle set the world’s high-speed record for land planes in 1932 using Shell avgas. That same year, he won the Bendix Trophy Race from Burbank, California, to Cleveland, Ohio, in a Laird Biplane and he took the Thompson Trophy at Cleveland in a Gee Bee racer with an average speed of 252 miles per hour.

Luckily for the Allies in World War II, a French engineer, Eugene J. Houdry, immigrated to the United States in 1931. He developed a process to refine 100/130 avgas that was far less expensive. In France, he had been researching converting coal into gasoline. Working with Sun Oil in the United States, he perfected a process using a regenerated catalyst to convert vaporized petroleum into high octane gasoline. It was known as the Houdry Process, and it came about in the nick of time for the Allies in WWII. The first large-scale plant came online in 1940, and the British were able to modify their aircraft engines to use the new higher octane aviation fuel for the Battle of Britain. The boosted (turbocharged) engine performance shocked the Germans. The speed of the Spitfire increased from 340 miles per hour in 1939 to 425 miles per hour in 1944.

The normal avgas used by Germany throughout WWII was B-4, with an octane rating of 91/100. They also had quantities of C-3, which had an octane rating of 95/120. By 1943 (war ended in 1945) they were catching up, producing a limited amount of a super C-3 with an octane rating of 150. Destruction of refining capability by the Allies prevented widespread use of the higher octane avgas. Japan’s aviation gasoline peaked out at 87/91. The first number is the octane rating at a lean mixture, and the second number is the octane rating at a rich mixture.

The development of 100/130 avgas was initially a case of Catch-22. The engine manufacturers needed a fuel that could withstand the higher compression ratios and not detonate prematurely. At the same time, the fuel refiners needed a large enough customer base to afford to set up the refining capacity for high octane avgas. The aviation demands in WWII satisfied both requirements. By the end of then war, 17 Allied refineries were producing high octane avgas.

Avgas characteristics

In the early days of avgas development, other factors besides the octane level needed to be solved for powering aircraft engines at high altitudes. The fuel had to have a low vapor point to prevent vapor lock. High altitude = low outside pressure, which requires a low vapor point. A low vapor point means you can climb to higher altitudes (low pressure) before the avgas turns to a vapor. “Sea level” gasoline turns into vapor at mid altitudes due to their high vapor points. Fuel pumps don’t work well on vapors! The viscosity also needs to be fairly constant and not gel at the low temperatures in the high altitudes (the ISO temp at 25,000 feet = -30 degrees F). The later stage WWII piston engine fighters could fly above 30,000 feet.

Lead warning

Tetraethyl lead improved avgas significantly, but it came with serious drawbacks.

The main effort, however, was on raising the octane level. Tetraethyl Lead (TEL) was the main additive used to prevent premature detonation or explosion in the cylinder. A rapid burn that causes the gas to expand on a wavefront is the desired condition. Sustained detonation, or knocking, can blow a hole in the piston. TEL was cheap, and it did the trick. On the flip side, too much TEL could result in spark plug fouling with no detonation improvement. While hotly debated today, lead was thought to provide some lubricant for the piston rings and reduced wear on the valves and valve seals.

Previously there were several grades of aviation gasoline available from the major oil companies: 80/87, 91/96, 100/130, 108/135, and 115/145. But with the reduction of avgas production and the increase of jet/turbine fuel production, the various grades were reduced over time to one primary grade of 100/130 and subsequently 100/130 LL commonly referenced just as 100LL.

Characteristics and specifications for avgas are published by ATSM International. The specification for 100/130 and 100LL is ASTM D910.

EPA actions to get the lead out

The elimination of lead additives started in the 1970s to prevent children from getting lead poisoning. This primarily happened when they chewed woodwork. Infants often chewed on their crib rails or other trim work and ingested lead from the paint. Unfortunately, the lead additive in paint had beneficial properties. Lead gave the paint durability and generally lasted longer than water-based paints of the 1970s and 1980s. It took approximately ten years before the paint manufacturers developed a way to make water-based paint as durable as the “old” lead-based paint.

The campaign moved on to eliminate lead from most products. Next, lead was removed from automotive gasoline. Again, the lead additive helped even burning rather than the detonation of the fuel in the cylinder. Lead in solder was next. Solder used to have a large percentage of lead, which helped it melt at a “low temperature” and then hold solid when cooled. Lead-free solder doesn’t work as well in electronic applications: “tin whiskers” are a more significant problem with lead-free solder. Aviation electronics manufacturers were previously able to get a waiver to use lead-based solder due to the critical nature of the avionic computers, but no longer.

Each time the regulations were tightened to eliminate more uses of lead, it was several years before “workarounds” were developed. We are now on the cusp of elimination of the waver for 100LL avgas. In January 2022 the EPA said it will issue an Unleaded Fuel Endangerment Finding, which could be the first step in making 100LL illegal. Fortunately, work has been ongoing for several years to develop a viable alternative.

Reduction in emissions

DC-6

The US airline fleet used to consume vast quantities of leaded avgas.

While the elimination of 100LL avgas is the current target of environmental groups and the EPA, there has already been a significant reduction in lead emissions. In the 1940s and 50s, we had hundreds of Douglas DC-3,-4, -6, -7 aircraft and the Lockheed Constellations, Boeing Stratocruisers, and numerous other military and civilian piston aircraft in service. Many of the air transport and military transport aircraft had four engines with a much higher displacement. They consumed vast quantities of leaded avgas. At that time, the avgas had 0.12% TEL. Today 100 LL has a maximum of 0.056% TEL. In 1945 the US produced 28.4 million tons of leaded aviation gasoline. Today it is approximately 0.56 million tons.

Even with the reduction in lead emissions, the widespread elimination of 100LL is expected soon. This is not just due to the environmentalists but due to the economics of serving a dwindling market. As shown previously, there were five grades of avgas, but today only one is in volume production. Other than some custom racing blends, 100LL is the only gasoline product that still contains TEL.

Compounding the economic situation, leaded and unleaded fuel cannot normally be stored or transported in the same tanks due to contamination from the leaded fuel into the unleaded fuel. The availability of TEL is another issue. Outside of China, there is only one manufacturer of TEL, and that is in Europe. It is inefficient for the refiners to produce leaded fuel for such a small market that continues to decrease. The refineries’ output of kerosene-based Jet/turbine fuel has far outpaced complex hydrocarbon-based avgas since the 1960s.

Evolution to unleaded 100 avgas

Several companies have produced unleaded avgas. Hjelmco Oil company introduced grade 91/96UL which was approved by the Swedish Civil Aviation Authority in 1991. In 2006 Hjelmco introduced the unleaded grade 100 UL which at the same time was tested and evaluated by another European Civil Aviation Authority. Hjelmco states that the fuel meets ASTM avgas standard D910 parameters (100 LL standard) in all respects except for energy content (gives 1-2 % potential higher fuel consumption).

In the US, Shell Oil has produced small quantities of lower octane unleaded avgas. Exxon Mobil and Philips 66 have done research and development work. According to Phillips 66 Frequently Asked Question on Unleaded Aviation Gasoline, in 2020, they said they are targeting 2025/2026 to “commercialize” UL100. Swift Fuels is producing UL94 and is working on a no-lead 100 octane avgas. But General Aviation Modifications Inc (GAMI) has a considerable advantage. They are the first to obtain an STC for use of their G100UL™ unleaded 100 octane avgas, which was announced at EAA AirVenture Oshkosh 2021. They are also actively expanding the Approved Model List (AML). As of October 2021, they had 611 aircraft/aircraft engines listed on the AML.

GAMI at OSH

GAMI appears to be in the lead for now, after making a splash at AirVenture 2021.

The significance of the STC should not be underestimated. GAMI went through the FAA Issue Paper process, which is enough to deter even experienced certification engineers. It would take the oil companies considerable time and effort in an FAA certification program. It’s also an area in which the big oil companies have little experience. Having said that, the FAA’s Piston Aviation Fuels Initiative Project (PAFI) seems to continue to sputter along.

Advantages and hurdles

Additional advantages for GAMI are:

  1. Their unleaded avgas (G100UL™) can be mixed with 100LL with no adverse interactions.
  2. It produces slightly more power (BTUs) per gallon and weighs slightly more than 100LL at 6.25 vs 6.0 pounds per gallon.
  3. Since both fuels are approved for aircraft use, transportation facilities don’t need separate equipment for 100LL and G100UL.
  4. No engine modifications are required.
  5. There is the possibility of longer TBOs due to cleaner-burning with a lack of lead deposits.
  6. There is also the possibility of the use of synthetic motor oils with longer oil change intervals.
  7. Less frequent spark plug changes.

It is unclear if the big oil companies will be willing to incorporate additives that they didn’t develop or invent. Some large engineering companies have a bias that if they didn’t invent it, it can’t be any good. It even has its own acronym, “NIH” – Not Invented Here.

The companies that blend fuels will likely be the early adaptors for G100UL production. Avfuel, a national fuel distributor, will handle all the interaction with the oil companies or blenders’ production side. Avfuel will also handle the distribution to the FBOs. GAMI will maintain overall quality control.

The evolution of avgas continues as we move into the era of mogas and unleaded 100 avgas. Finding mogas without ethanol may be the next challenge.

Bob Teter
Latest posts by Bob Teter (see all)
30 replies
    • Bob Teter
      Bob Teter says:

      Thanks. When I wrote the article, I thought GAMI had the STCs in hand, but the FAA headquarters (AIR-1) has been slow to approve G100UL. I’ve been involuted in several Avionics STC’s but never had one stopped at the last minute by Washington. Additionally, the FAA replaced PAFI with EAGLE and saga continues.

      Reply
      • Duane Mader
        Duane Mader says:

        We’re at 8 months now on a simple registration N number change for a new airplane. RVSM approval in the past was similar. It costs us every time we can’t use our CAA fuel discount for the temporary tail number.

        Reply
  1. Rich R
    Rich R says:

    If you want a deep dive on WWII engine development challenges across multiple issues (including octane rating, testing) “The Secret Horsepower Race” is an excellent read (by Calum E Douglas; Amazon carries). A friend who built Can Am engines in 60′-70’s who read this book chuckled at all the tech/concepts he worked then that had already been done in the 30’s-40’s.

    Reply
  2. Joe Henry Gutierrez
    Joe Henry Gutierrez says:

    Thank you Mr. Teter, very well said and excellently put, This sounds very good to the reality of what is going on. All the other lengths of words and alleged reasons and explanations as to why a reasonable unleaded gas for GA cannot be made is very hard to believe. It truly seems that the American public is still a product of getting screwed !! My true feelings, and still hold firm with that thought for all the past years this has been dragged on…I’m sorry but I know here in America, we do have the technology and expertise to be able to do this and do it beyond any dought !!!

    Reply
  3. Nick Paz
    Nick Paz says:

    Interesting article. I fly and aircraft that has a Lycoming 0-435-A engine and it kept fouling the plugs. We ended up changing the compression from 6/1 to 8/1 and problem solved. It burns the fuel and no more fouling of plugs.

    Reply
  4. RH Meyers
    RH Meyers says:

    Excellent background Bob. AW&ST should have included your article in their latest issue on the development and government approval status of the unleaded fuel replacement of 100LL. I cannot help but be left with the impression that the delay in the use approval is either a usual bureaucratic display of ineptitude or interference by a NIH lobby. Maybe loud accusations along this line would increase attention to what appears to be an unnecessary delay.
    – RH, former long time and heavy user of 115/145

    Reply
    • Bob Teter
      Bob Teter says:

      I appreciate the complement. I just let my AW&ST subscription lapse, but now I’ll have to renew it and read their article. As I mentioned to Joe Grimes, when I wrote this article, I thought GAMI had the STCs in hand, but the FAA headquarters (AIR-1) has been slow to approve it. I’ve been involuted STC’s before but never had one stopped at the last minute by Washington. Just recently Earl Lawrence the Executive Director, Aircraft Certification Service was replaced so maybe things will move along. However, the FAA also came out with their new and improved EAGLE program, so we’ll see what happens. It’s also interesting that nobody talks about Hjelmco Oil’s unleaded 100 octane aviation gasoline.

      Reply
  5. RH Meyers
    RH Meyers says:

    I have a one page pdf of the current AW&ST 4-17 Apr article re current fuel development Pg 13 commentary written by William Garvey if there is a way to email it to you.

    Reply
  6. Eric
    Eric says:

    Thank you, Mr. Teter, for taking the time for this well written, very interesting and insightful read.

    Having only worked with JP5 for so long, it was a treat to learn so much about compression fuel(s).

    Thanks, again!

    Reply
  7. David Reinhart
    David Reinhart says:

    The primary reason for getting the lead out of auto gas wasn’t toxicity. Lead poisons catalytic converters, and catalytic converters were seen as the solution to air pollution.

    Reply
    • Bob Teter
      Bob Teter says:

      David, you make a very good point. The excerpts from the EPA’s Nov. 28, 1973 Press Release (in the order listed) are: “According to EPA, a significant portion of the urban population, particularly children, are over-exposed to lead through a combination of sources including food, water, air, leaded paint, and dust. Although leaded paint is a primary source of exposure for poisoning in children, leaded gasoline is also a significant source of exposure which can be readily controlled. The total amount of lead used in gasoline amounts to well over 200,000 tons a year.” Later in the Press Release it said: “On January 10, 1973, EPA required the general availability of one grade of unleaded gas by July 1, 1974, in order to protect the catalytic converters which will appear on many new cars in 1975. Lead in gasoline may cause disintegration of the converters, which control auto air pollution emissions.” Thanks for pointing out there were multiple reasons for elimination of lead in automobile gasoline.

      Reply
      • Bob Teter
        Bob Teter says:

        An interesting footnote is that the same Eugene J. Houdry who developed a more cost-effective refining technique for 100/130 was also the inventor of the automobile catalytic converter.

        Reply
  8. Mike Tucker
    Mike Tucker says:

    Thanks Bob…I learned a lot from your article. My experimental aircraft will have a Rotax 912ULS engine which at present can run on mogas or 100LL. If 100LL is used there are more frequent oil changes (every 25 hours I think). By the time I am ready to purchase my 912ULS maybe better fuels will be readily available.

    Reply
  9. George Braly
    George Braly says:

    Bob,
    Well done !
    Send me an email and let’s arrange to have a phone call ?

    I will be glad to bring you up to speed on all of the most recent FAA theatrics on the road to the final “Fleet-Wide” (All spark ignition engines & all aircraft that use those engines – – found anywhere in the FAA TCDS data base) approval by the good folks at 800 Independence Avenue, Washington, D.C.
    George Braly
    Head of Engineering
    GAMI
    [email protected]

    Reply
    • Bob Teter
      Bob Teter says:

      Hi George. I’ll shoot you an email. The article was a little dated by the time I got it published but I tried to bring it up to date with my responses back to Joe Grimes and RH Meyers. I saw a portion of your recent presentation at Sun & Fun. Thanks, Bob

      Reply
  10. John Marshall
    John Marshall says:

    Bob,

    Interesting article. As an aside, at the beginning of World War II, a Pan American Airways Boeing 314 flying boat, fitted with Wright R2600 engines, was caught on the far side of the Pacific after the Japanese bombed Pearl Harbor. Their only way home was to fly the airplane back the long way, westbound, across Australia, the Dutch East Indies (now Indonesia), India, Arabia, Africa, and across the South Atlantic to Brazil, then north to Laguardia Airport, New York. The R2600s normal diet was 100/130 gas, but in many places they were forces to upload auto gas of undetermined quality instead. The flight engineers devised various ways of cooling the engines, and varying the power settings, to accommodate the lower octane fuel, and it worked. The engines didn’t care for it much, but they ran. They made the historic flight of over 31,500 miles with only a single blown cylinder.
    I have written about this flight in Smithsonian/Air&Space magazine (July 1995) and Airways magazine (several issues).

    Reply
    • Bob Teter
      Bob Teter says:

      I remember hearing about the Pan Am “trip” in 1941, but I didn’t know about using automobile gasoline. That was a remarkable trip. I also look forward to reading your account. As an offshoot of that story, Jimmy Doolittle demonstrated Curtiss aircraft in several parts of the world. Many of these countries did not have good aviation gasoline. To counter this he often carried a can of TEL in the cockpit to add to the available gasoline. On one South American trip in the 1930s, the can tipped over and spilled. He was sick for several days afterward. (Story from Gen. James H. Doolittle’s Autobiography “I Could Never Be So Lucky Again”)

      Reply
  11. Karrpilot
    Karrpilot says:

    I read an article about how the world pumps more unleaded fuel in a day than what leaded fuel is used for a whole year. And the keyword here is low lead. I am not advocating leaded fuel. I was just stating facts.

    Reply
  12. Bill Miles
    Bill Miles says:

    Several items in the hustorical background are incorrect or misleading. Reference is made to higher octane allowing higher compression ratios. This is correct, however the main effect in aircraft engines of the period was allowing higher boost pressure. Reference is also made to British turbocharged engines, which I don’t believe they ever had, the engines were surpercharged.

    Reply
    • Bob Teter
      Bob Teter says:

      Bill, thanks for the clarification. I used turbocharged as a generic term when the correct term for the British engines would be supercharged (driven by the engine vs driven by the engines’ exhaust gas). Thanks for the input.

      Reply
    • Bob Teter
      Bob Teter says:

      Bill, I went back to the books and checked on “compression ratio.” Somewhere along the line, I thought that the boost increased the compression ratio. I was wrong. I now realize that boost and compression ratio are two completely different things. The compression ratio is a ratio of volumes, not a ratio of pressures. The amount of boost determines how much fuel can be added. More boost = more fuel = more power for the same compression ratio. Both high compression and high boost require high octane fuel, but I agree with your assessment that the main effect in aircraft engines of the period [1930s & 1940s] was allowing higher boost pressure. Thanks for pointing out my error.

      Reply

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