The cruising altitude of modern airliners ranges from 10 to 12km high. So it is hard to imagine that a pilot broached these heights a full 90 years ago. It was with a BMW engine which, on 17 June 1919, propelled Franz Zeno Diemer to an altitude of 9,760 meters (9.8km).
If proof were needed of the superiority of BMW's high altitude engines, it was furnished that summer in 1919 at Munich's Oberwiesenfeld airfield. No human had previously piloted an aircraft to a greater height. When Diemer set out on his record flight from Oberwiesenfeld on 17 June, his DFW F 37/III - an altitude aircraft specially developed by Deutsche Flugzeugwerke - was powered by a BMW IV aero engine. Based on the tried and tested BMW IIIa, the new BMW IV engine had its bore and stroke increased by 10mm to boost output from 185 to 230hp.
Glorious flying weather saw the engine deliver a consistent performance on that Sunday, enabling Diemer to make a steady ascent to greater heights. After 87 minutes he had reached 9,760 meters. No aeroplane had previously attained such an altitude. Later, Diemer announced the engine still had reserves in hand, but that he himself had reached the limits of his capacity. After all, in his open pilot's seat he not only had to contend with temperatures as low as -50C, but also with the low oxygen levels at this altitude, which took their toll on him physically.
The challenge Diemer faced during his world record flight was the same underlying problem with which all aircraft engines had to grapple during the First World War: loss of power at altitude. Normally, an aeroplane required maximum output on the ground in order to take off. But with aircraft now forming a separate military arm, the requirements changed. Engine output also became crucial to survival at high altitude, for rapid climbing to escape hostile combat. With increasing altitude, however, the air density steadily diminishes, and this led to power loss.
Two different concepts were pursued in an attempt to counteract the loss of output in the thinning air. Some manufacturers - such as Daimler - turned their attention to charging, whereby air was forced into the carburetor by means of a charger. But this technology was too complex to bring into production within a short space of time, and only established itself in the 1930s.
BMW's head designer, Max Friz, opted for an over-square, high-compression unit for the first BMW aircraft engine, the forerunner of the BMW IV world record engine. This design principle involved enlarging cylinder volume as well as an increased compression ratio. To avoid engine overload, it had to be throttled back at take-off and low altitudes. To meet the demands of a high-altitude engine, Friz designed a special carburetor to deliver a steady supercharged pressure and ensure a consistent air/fuel ratio. The carburetor was controlled by two levers, one for normal operation and one for high altitudes. By means of these two levers, the pilot could control the throttle valves of the BMW carburetor in such a way that the optimum mix was prepared to suit the aircraft's altitude at any time. The throttle valves were only fully opened once an altitude of 3,500 to 4,000 meters had been reached. This configuration meant the loss of power of the IIIa at altitude was less than in other engines.