The group of Luftwaffe Bf-109 fighters passed high above the English channel at 38,000 feet. At that altitude, they were almost invisible to the naked eye and had little to fear from ground fire. Their job was to get over London, before the HE-111 bombers trailing behind them, and clear out any British fighters. At their current speed and altitude, they were faster and flying higher than any Spitfire or Hurricane could reach. And being faster and higher than your enemy made all the difference in winning a dogfight.

The Germans had fought the Spitfire and Hurricane in the skies over France the year before and were not impressed. The Bf-109 was at least 40 mph faster and could climb several thousand feet higher.

The flight leader scanned the skies below looking for RAF fighters when his wingman began to furiously wag his wings, trying to get his attention. The flight leader dipped his wing to acknowledge and saw his wingman gesture over his right shoulder with an extended thumb. “They must be behind us and below,” he thought as he craned his neck to see behind his aircraft. He then picked up the glint of metal above and behind him. A squadron of Spitfires was at least 2,000 above them and diving fast.

The flight leader hesitated for a moment. The Spitfires couldn’t fly higher or faster than them. Yet, there they were. A whole squadron, like Furies, descending on the Germans from above. The flight leader signaled to his men to break. His flight of 109s split left and right to try and spoil the RAF fighters’ diving attack.

In the dogfight that followed the German pilots would be astonished to find that the Spitfires were now just as fast and could climb just as high as them. “These can’t be the same planes we fought against in France,” the flight leader thought as he twisted and turned trying to shake off the Spitfire on his tail. “They can’t be.”

Doolittle with a plane prior to WWII
Jimmy Doolittle and Gee Bee Super-Sportster (PW Wasp Motor Collection: Charles M. Daniels Collection, National Air Races. San Diego Air and Space Museum Archive)

In 1935 aviation legend Jimmy Doolittle had left the Army and was working for the Shell Oil Company as an Aviation Fuels Manager. He was trying to solve a problem that nobody even knew existed. Doolittle may have been the first American to be both an Aeronautical Engineer and a pilot. He held a Ph.D. in Aeronautical Engineering from MIT, one of the first-ever such degrees.

Young Major Doolittle was a speed demon as well and an air racer. In 1931 he won the Bendix Trophy, set a world speed record of 296 miles per hour, and won the Thompson Air Race trophy flying the dangerously unstable but fast Gee Bee R-1. Along with Charles Lindberg, Amelia Earhart, and Eddie Rickenbacker, Doolittle was among the most famous aviators in the country. And this was why Shell Oil had lured him out of the Army. His task was to develop aviation fuels for military and civilian applications and be the public face of Shell Oil to the military.

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What Doolittle grasped intuitively through his engineering training was that aircraft engine development was being limited by a “which came first, the chick or the egg?” paradox.

Engine makers, like Allison and Pratt and Whitney, wouldn’t spend millions designing and making higher performing engines when nobody made the fuel to make them run. Oil companies like Shell had no interest in investing millions in high-performance aviation gas refineries when there yet were no aircraft engines built that could run on that gas. So Doolittle went to work on getting the Army to adopt a new fuel standard for its aircraft. With the country in the midst of the Great Depression that would not be easy.

Up until that time, aircraft and automobile fuel were of the same grade. Fuel was already cheap to make and readily available everywhere. So engine designers made engines to run on this lower-rated fuel that had an octane rating of 87. For the military using the same grade of fuel on its aircraft and other vehicles made sense from a logistics and supply chain standpoint. But this decision, driven by a desire to keep the supply chain simple, resulted in aircraft that underperformed in speed, climb rate, max take-off weight, and service ceiling. And as war clouds gathered in Europe once again, Doolittle wanted the United States to have the fastest, highest-flying planes in the world. He had been to Germany and seen the Luftwaffe’s planes fly with fuel-injected engines by Daimler and BMW. He had seen the factories the planes were being made in. And he was impressed.

So why did the octane rating of aviation gasoline matter?

An Internal Combustion engine is comprised of an empty cylinder into which a piston is inserted. At the top of the piston, there are valves that inject a mixture of air and fuel into the cylinder which the piston compresses very tightly as it moves to fill the cylinder’s void space. When the volume of the air-fuel mixture in the cylinder is compressed down to a certain ratio, typically about 6:1 or 7:1, a spark is introduced thus explosively igniting the compressed air-fuel mixture. This drives the piston back down again. Horsepower is created by a crankshaft attached to the piston. So the fuel is matched to the compression ratio in the engine.

At low compression ratios, like 6:1, the fuel’s volatility increases making it more explosive. If you increased that ratio to 8:1, the fuel would spontaneously detonate just from the friction between its molecules being compressed by the piston. This is called pre-detonation or “knock.” It robs power from the engine and increases wear because the fuel is igniting before the piston reaches its full stroke.

Rolls Royce Merlin Engine
Roland Turner from Birmingham, Great Britain – Rolls Royce Merlin engine, Avro York, Imperial War Museum, Duxford.

Now, if you could increase compression to 10:1 or 12:1, the explosive detonation would greatly increase the power of the engine for each cycle of the piston. But 87 octane gas would explode on its own, at about 8:1. What was needed was a fuel with lower volatility that would remain stable when compressed to 1/1oth or 1/12th of its volume in the cylinder and not go off prematurely.  That was what Doolittle wanted: Aviation fuel with an octane rating of 100 so that engine designers could make higher performance engines with higher compression ratios. The aircraft that carried these advanced engines would have stunning performance, especially at higher altitudes.

So Doolittle set about getting Shell Oil to make an engine fuel no one had yet manufactured. Yet, his farsightedness was not widely shared. Within Shell Oil, the project was being mocked as Doolittle’s “Million Dollar Blunder.”

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For years Doolittle spent untold hours lobbying Congress and the Army to adopt a 100 octane fuel standard. The Army finally relented and adopted the 100 octane standard in 1938. Now Doolittle had a new problem: how to make 100 octane gas at a cost the government could afford. Initial experimental formulations of 100 octane fuel required a very expensive refining process that resulted in a prohibitively high fuel price of $25/gallon when automotive fuels were less than 20 cents a gallon. The thermal method of “cracking” high octane gas out of crude oil was wasteful and produced byproducts like olefins that gummed up engines. There was a solution to this problem, but it came from an unlikely place.

Eugene Houdry as a young Armored Officer in the French Army.

Eugene Houdry had served as a young officer in the French Tanks Corps in the First World War. He had been wounded in action, decorated for valor, awarded the Croix de Guerre, and made a Chevalier of the French Legion of Honour. After the war, Houdry took an interest in auto racing and visited the United States. He toured a Ford auto plant and attended the Indianapolis 500 race. What he saw in the U.S. was a country on the move. And the automobile was doing the moving.

America was producing its own oil and gas. On the other hand, France had to import virtually every drop of oil that fueled its military and civilian vehicles. Houdry saw this as a dangerous situation for France. Oil was becoming the lifeblood of modern economies and he thought he could help his country produce the gas and other fuels it needed.

Having earned a degree in mechanical engineering from the Ecole des Arts et Métiers in Chalons-sur-Marne Houdry set to work on converting brown coal (which France had in abundance) into fuel. He opened a small lab and by 1930 he had small samples of gasoline made from coal. This was considered pretty miraculous and Houdry became the pioneer of synthetic fuels. Soon Houdry was producing 60 tons of gasoline a day from coal using his revolutionary method. But the French government decided that importing oil was cheaper than making gas from coal and withdrew their funding of the Houdry process. France would pay dearly for relying on imported oil when WWII started.

But in America, Houdry’s work found a welcome reception from the Vacuum Oil Company (which later became Standard Oil). Houdry relocated to the U.S. and licensed his method in a joint venture to very cheaply produce high octane gas out of oil.

The venture started with a single Houdry Process plant in Marcus Hook, Pennsylvania. Soon there were 17 other plants in operation. Standard Oil had licensed the process to other oil companies like Sun Oil (Sunoco). It also licensed it to Shell Oil. There, Jimmy Doolittle immediately saw the promise of the Houdry Catalyst method of making the 100 octane gas he believed America would need if it ever fought a war again in Europe. And by 1937 this seemed increasingly likely.

After WWII began and France fell the Vichy government added insult to injury. It stripped Houdry of his citizenship for being a founding member of France Forever which sought to eject the German invader from the country. Houdry became a U.S. citizen and continued his work.

When the war in Europe began, the U.S. was manufacturing a modest 40,000 gallons a month of 100 octane gas. But by 1944, it was making 400,000 gallons of AvGas a month from 77 Houdry Process plants. The first consumer was Great Britain which did not make 100 octane gas but nevertheless made aircraft engines, like the Rolls Merlin, that needed it. The British were ordered all that the U.S. could produce.

And the Luftwaffe found that the underpowered Hurricanes and Spitfires it had slapped aside so easily in the skies over France early in the war had been reborn by the time the Battle of Brittain had started where they met or exceeded the performance of the Focke Wulfs and Messerschmitts they met in combat over London. In contrast, Germany’s planes mostly flew on 89 octane fuel and Japan never got higher than 87.

high octane fuel for WWII
Sunoco officials pour the billionth gallon of aviation fuel produced during World War II. (Courtesy Sunoco Logistics/Sunoco, Inc)

Fighter planes like the Spitfire, Mustang, Thunderbolt, Hurricane, SeaFury, Lightening, Hellcat, and Corsair all ran on 100 octane gas. This allowed the engines that powered them to attain stunning performance. The famous Rolls Merlin began the war producing just over 1,000 horsepower. By the war’s end, it was producing 1,600 horsepower. Spitfires could even approach the speed of sound in a shallow dive.

You may have read The Plane That Won the War or, The Best Aircraft Engine of WWII. Underlying it all was the contribution of 100 octane gas by a farsighted Doolittle, who understood the engineering science of airplane engines, and a maverick Frenchman who feared that disaster could befall his country if it relied solely on oil imports.

Their contributions made all the difference in making those planes and engines the legends they became and in ultimately achieving victory in WWII.