Aircraft of The Battle of Britain - Fighter Aircraft - Main Types: Hurricane, Spitfire and Bf 109 - 100 Octane Aviation Fuel

100 Octane Aviation Fuel

As early as 1938 Roy Fedden, who designed most of the Bristol Engine Company's most successful aero engines, pressed for the introduction of 100 octane aviation spirit from the USA, and later that year the British aero engine manufacturers Bristol and Rolls-Royce demonstrated variants of their 'Mercury' and 'Merlin' engines rated for 100 octane fuel. A memorandum by the "Department of Defence Co-Ordination", 'Proposals for securing adequate supplies of 100 octane fuel to meet war requirements', 23 December 1938, noted that there was a need to increase supplies of 100 octane fuel and discussed ways in which this could be achieved.

A meeting was held on 16 March 1939 to consider the question of when the 100 octane fuel should be introduced to general use for all RAF aircraft, and what squadrons, number and type, were to be supplied. The decision taken was that there would be an initial delivery to 16 fighter and two twin-engined bomber squadrons by September 1940. However, this was based on a pre-war assumption that US supplies would be denied to Britain in wartime, which would limit the numbers of front-line units able to use the fuel. On the outbreak of war this problem disappeared; production of the new fuel in the US, and in other parts of the world, increased more quickly than expected with the adoption of new refining techniques. As a result 100 octane fuel was able to be issued to all front-line Fighter Command aircraft starting in the spring 1940.

Although U-boats and surface raiders had begun to take a heavy toll of tankers, in the summer of 1940 there was a surplus of these ships because of the incorporation into the British merchant marine of tanker fleets from countries overrun by Germany. The combination of CS propellers and 100 octane fuel put the British fighters on par with the Luftwaffe. Throughout 1940 the supply situation and distribution of the fuel to the front line services was discussed by the "Co-ordination of Oil Policy Committee".

With 100 octane fuel the supercharger of the Merlin III engine could be "boosted" to +12 lbs/sq.in., producing 1,310 hp (977 kW) at 3,000 rpm at 9,000 feet (2,743 m) with a time limit of five minutes. This increased power substantially improved the rate of climb, especially at low to medium altitudes, and increased the top speed by 25-34 mph up to 10,000 feet. During the Battle of France and over Dunkirk RAF Hurricanes and Spitfires were able to use the emergency boost.

In the opinion of a pre-war paper by the British Air Ministry, Germany, as a large producer of synthetic fuel, was thought to be in a favourable position to produce 100 octane fuel in large quantities. The German supply of aviation fuels was largely based on the hydrogenation of coal, due to their limited supplies of natural crude oil. At the outbreak of the war, Germany already had seven destructive hydrogenation plants in operation, with a total installed capacity of 1,400,000 te/year of oil.

At the start of the war the Luftwaffe standardized on 87 octane aviation gasoline, called "B-4", made from leaded hydro-petrol extracted from brown coal. In 1940 an improved fuel, designated "C-2" was introduced having a higher aromatic content of 35-38% and giving performance equivalent to Allied 100 octane grade of that time. C-2 was used in small quantities by aircraft such as the Messerschmitt Bf 109E-4/N and E-7/N and the Messerschmitt Bf 110C when equipped with the DB 601N engine, that entered series production in October 1939. The power was increased by 20% over that of the DB 601A, to 1,260 hp at 6,900 feet (2,100 m) at 1.35 ata boost pressure and 2,400 rpm. By July, nine Bf 110 and three Bf 109 fighter Staffeln (squadrons) were equipped with the new engines, By the end of October around 1,200 DB 601N engines had been delivered. and the number of aircraft equipped with the improved engine gradually increased through the second half of the year. However, due to leaking valves there was relatively high wear on the 601N-engines, which had a life of about 40 hours.