Aviation fuel

Hardly marked down as a "war-winner" and rarely given an airing, improvements in aviation fuel technology played a vital part in the Battle of Britain, providing timely increments to the power of engines and the speed of such aircraft as the Spitfire and Hurricane.

In fact, even prior to the outbreak of war, the technology of fuel for the internal combustion engine was advancing. It has gone though rapid development in the 20s and 30s when motor fuel had been of variable quality, and aviation fuel little better.

The search was on for fuel which would solve the problem of detonation in the cylinder, where the air-fuel mixture does not burn in a controlled fashion when ignited by the spark plug. Instead, it explodes prematurely as a result of heat and pressure, known as "knocking".

In high performance engines, this could cause considerable damage to piston and bearings. It thus became essential to find a way of operating an engine so it would not suffer detonation, to prevent power output being limited by fuel performance.

All aircraft engines of the Battle of Britain era were mechanically-supercharged internal combustion engines. The supercharger used engine power to compress intake air to feed to the cylinders. The centrifugal compressor had a fixed compression ratio at any given rpm. Let's take the 1940 R-R Merlin III as an example. This engine had a maximum power of 1030HP at 16.250 feet, using 6.25lbs boost*.

This implies a compression ratio of around 2.7. (manifold pressure 20.95 psi, atmospheric pressure at 16250 ft, 7.86 psi.) The supercharger takes 100hp or so at 3000rpm engine speed, whether the power is being used or not.

You can only use all the boost at 16,250ft. Below that height you are compressing the air but must run the engine throttled to avoid exceeding the boost limit. 16250ft in this case is the full throttle height. This is the case when using the 87 octane fuel available to both the RAF and the Luftwaffe.

In the spring of 1940 supplies of 100 octane fuel from the USA became available to the RAF. This made it possible to run the Merlin at a higher boost pressure without detonation. Limited by the greater detonation resistance of 100 octane fuel and by the mechanical strength of the Merlin itself, a new boost limit of +12lbs was imposed, for use in emergency with a time limit of five minutes.

No change to the engine itself was required, just a new gated position on the throttle and a change to documentation. The increased boost gave an extra 300 horsepower, a 30% increase, up to a new full throttle height of around 9000ft where the compressor's ratio delivered the +12lbs.

A combat for an RAF fighter would typically involve a scramble takeoff and a formation climb directed by the ground-based controller, who would try to put the formation in a position to intercept a raid.

If successful there would be an engagement at the height of the incoming bombers, which would be above the heights where the 100 octane performance boost took place. Inevitably, fighter combat tends to be downhill, as any intensive manoeuvring uses up energy faster than the engine can provide it.

The RAF fighter pilot might find himself chasing, or being chased by, a 109 downwards into the area where he had the advantage of 100 octane. The effect of the extra 300hp was to give the Spitfire pilot a clear performance advantage over the 109 in every way, speed, climb and ability to sustain a turn.

The pilot of a Hurricane would find his position improved from being at a marked disadvantage to something approaching parity. The availability of 100 octane fuel meant a significant increase in effectiveness and survivability for RAF aircraft.

The Luftwaffe had limited quantities of 96 octane fuel for new marks of Me 109s only in the later stages of the battle. Their engines were of larger capacity, more lightly loaded and with less aggressive supercharger output. The normal Me 109E operated with a boost equivalent to +5. Only in 1944 were pressures equivalent to +12 allowed in the later 109s. The RAF ended the war using +25 boost and 150 Octane fuel in the later Spitfires, giving twice the power of a 1940 Merlin with the same rpm and capacity.

* British engines measured their manifold pressure in pounds of boost. Sea level ambient pressure is zero pounds boost, or a pressure of 14.7 psi. So the actual manifold pressure at +6.25 lbs boost is 14.7 + 6.25 = 20.95 psi.