Tales of the Flying Tigers
[I've often wondered about those double-jointed ratings for aviation gasoline, and of course the octane of avgas played an important role in wrecking Chennault's hopes of establishing a flight of fast-climbing Curtiss-Wright CW-21 "Demon" interceptors at Kunming, when Erik Shilling's engine quit on the flight into China on December 23, 1941, and his leaderless companions crash-landed. So here's Rick Dunn to explain how 87-octane became 100/130. -- Daniel Ford]

Jimmy Doolittle, Hap Arnold, and the Battle of Britain: The 100-Octane Story

Richard Dunn

The Battle of Britain of 1940 is renowned for the skill and courage of the few RAF fighter pilots. Of course, fighting over friendly territory and radar controlled interceptions helped their cause immensely. Another British technological advantage that played an important role was the switch to 100-octane (100/130 aviation gasoline) fuel just weeks before the battle began.* This increased the speed of the Spitfire by 25-30 m.p.h. at low and medium altitude and made it equal or superior in maximum speed to the German Me 109 at all altitudes.


The invention of the internal combustion engine fueled by light distillates of petroleum (gasoline) has been one of the most profound inventions (Gottlieb Daimler, 1886) in its effects on society. Gasoline went from an unwanted and dangerous by-product of petroleum refining to a product with continually increasing demand outstripping production by the early 1900s. In 1890 there were no motor cars in the United States and only a handful elsewhere. In 1900 registered motor vehicles numbered 8,000; ten years later it was more than 450,000. In 1903 the Wright brothers used a gasoline engine in the first powered human flight.

In the first decades of the 20th Century the main focus of research and chemical engineering in the petroleum refining industry was how to obtain more gasoline from a barrel of crude oil. Thermal cracking involved breaking down complex hydrocarbons into lighter hydrocarbons. Yield per barrel was steadily increased. Later catalytic (low temperature) cracking further increased yield. In addition, the gasoline produced by these methods had superior characteristics in avoiding engine knock.

The phenomenon of knock was not well understood. Knock is a supersonic shock wave (technically detonation instead of combustion) within a cylinder initiated by temperature and pressure within the cylinder rather than by the spark. When knock (ping) becomes rapid, increasing the throttle does not increase the speed of the engine. If sustained it can reduce engine speed and ultimately damage the engine.

In the late 1920s a system for rating fuel with respect to its anti-knock characteristics was developed. The hydrocarbon n-pentane with very poor anti-knock characteristics became zero on the scale and iso-octane with excellent anti-knock characteristics became 100. A given fuel could be rated by comparing it to a mixture of the two base hydrocarbons. A fuel equal to a 50/50 mix of n-pentane and iso-octane received a rating of 50 octane.

In the early 1930s motor fuel had a rating of about 40 octane. Aviation gasoline had a rating of 70-80 octane. The market driver for gasoline was motor fuel. In the early 1930s aviation gasoline amounted to less than one-half per cent of total gasoline consumption. Of this amount about two-thirds was for military aircraft and one-third for commercial aircraft.


In 1930 James H. (Jimmy) Doolittle left active duty (retaining a reserve commission) in the US Army Air Corps and became manager of Shell Oil's aviation department. Doolittle had received the Distinguished Flying Cross for a record breaking long distance flight while on active duty. He was awarded M.I.T.'s first doctorate in aeronautical engineering. In the early 1930s he became famous as an air racer. Doolittle urged Shell executives to expand 100-octane production capacity. High octane fuel was expensive and available only in research quantities. Doolittle also used his fame to lobby legislators and other government officials on the importance of fuel developments.

By 1934 Shell was producing small quantities of high octane fuel using hydrogenation and introducing additives such as tetraethyl lead. The extremely high price ($30 per gallon) was reduced to about $2 per gallon. Motor fuel was then selling for less than $.20 per gallon. Doolittle set an aviation world speed record and won several air races. Shell invested in scaled up production of 100-octane fuel but still in small quantities. Meanwhile other refining companies began to explore increasing octane fuel for aviation purposes. The US Army created a standard of 87-octane for fuel for its combat aircraft. Beginning in July 1934 the Army began experiments with 100-octane fuel. Doolittle kept in contact with the Air Corps through successive brief periods of active duty when he tested equipment and provided advice.

The value of high octane was primarily that engines could be operated at higher manifold pressure without encountering knock. Higher manifold pressure meant that an engine could be designed to increase power without increasing the volume or weight of the engine. High octane fuel was originally associated with increased aircraft speed. As fuel became available and research was expanded, it was found high octane fuel reduced specific fuel consumption. This meant an aircraft could carry less fuel (and more payload) for the same range. This eventually caused commercial airlines to become interested in higher octane fuel.


Key events affecting the market for high octane aviation gasoline took place in 1935-1936. In 1935 when only 3% of its aircraft (about 50) could take advantage of 100-octane gasoline the Army ordered 100,000 gallons. In January 1936 Maj. Gen. H.H. Arnold became Assistant Chief of the Air Corps for Procurement. Additional purchases followed and by 1937 the Army established 100-octane as the standard for its combat aircraft. The US Navy soon followed. New aircraft such as the P-35 and B-17 were entering the inventory. Wright produced the R-1820 Cyclone engine and Pratt & Whitney produced the R-1830 Twin Wasp. In the early 1930s these engines produced about 600 h.p. As higher octane fuel became available they were modified to take increased manifold pressure. Eventually versions of both engines produced power well above 1,000 h.p. They were in service for decades in both military and civil aircraft. Experimental Allison V-1710 engines initially used 92-octane fuel soon adapted for 100-octane. The V-1710 saw service in numerous military aircraft.

These developments did not go unnoticed both by Shell's domestic competitors and internationally. The Houdry Process of catalytic cracking adapted to the production of gasoline by Sun Oil Company resulted in expanding capacity of high quality fuel which with additives produced 100-octane aviation gasoline. In addition to creating new plant capacity the Sun process was licensed to other producers. In 1937-38 7,000,000 gallons was produced by the process. Yields of up to 34 gallons of high octane fuel were obtained from a 42 gallon barrel of crude oil (compared to 18 gallons previously). By 1939 the price of high octane gasoline dropped below $.20 per gallon just pennies more than motor gas. US refineries were producing thousands of barrels per day. There were other technical developments as well. Royal Dutch Shell began refining high octane aviation fuel at its Pladjoe refinery in Borneo. British interests refined Venezuelan crude into aviation gasoline at refineries in the Caribbean and US. Later, Anglo-Iranian refineries adopted the alkylation method to produce high octane aviation fuel. During World War II the US was the primary supplier of 100-octane to the Allies including the world's second leading petroleum producer the USSR.

What about the Battle of Britain? Or, we might say the battle for 100-octane for the RAF. In January 1937 noted internal combustion engine expert F.R. Banks wrote a paper recommending 100-octane fuel become standard and British aero engines be developed to take advantage of it. His recommendation was rejected on the basis that the US was then the only source of supply and continued supply in time of war could not be guaranteed. The RAF maintained 87-octane as its standard. Personnel changes in the Air Ministry led to changed attitudes. Experiments were conducted with imported 100-octane petrol in 1938. Later an entire tanker of 100-octane reached Britain and tests were conducted at operational units. By June 1939 three months before World War II began the British started stockpiling 100-octane gasoline. To safely use the fuel at higher manifold pressure (12-lb. boost vice 6¼-lb. boost) the Merlin engines of Spitfires and Hurricanes were modified with shrouded cylinder heads. Approval was given in March 1940. When the Battle of Britain started in July the fighters had been modified and adequate fuel was available. Few Britons probably recognize that actions by Jimmy Doolittle and Hap Arnold contributed to their victory in the air at a critical moment in the island's history.

* The two scale numbers such as 100/130 relate to lean mixture versus a rich mixture of fuel in the cylinder. Numbers above 100 are technically performance numbers rather than octane ratings. A number like 130 indicates additional power is not resulting in more knock. When the USAF began phasing out reciprocating engine aircraft in the 1970s its standard avgas was 115/145.

Question? Comment? Newsletter? Send me an email. Blue skies! -- Dan Ford

Flying Tigers
revised and updated

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