BMW III
| BMW IIIa | |
|---|---|
 |
|
| Preserved BMW IIIa, shown with quick-change propeller hub | |
| Type | Inline engine |
| Manufacturer | BMW |
| First run | 1917 |
| Major applications | Fokker D.VII |
BMW IIIa was an inline six-cylinder SOHC valvetrain, water-cooled aircraft engine, the first-ever product from BMW GmbH. Its success laid the foundation for future BMW success. It is best known as the powerplant of the Fokker D.VIIF, which outperformed any allied aircraft.
On 20 May 1917, Rapp Motorenwerke (which later that year became BMW GmbH) registered the documentation for the construction design for the new engine, dubbed BMW III. Designed by Max Friz and based on the Rapp III engine, it was an SOHC in-line six-cylinder, just as the earlier Mercedes D.III was, which guaranteed optimum balance, therefore few, small vibrations. It was designed with a high (for the era) compression ratio of 6.4:1. The first design drawings were available in May, and on 17 September the engine was on the test rig. After a successful maiden flight for the IIIa in December 1917, volume production started up at the beginning of 1918.
The military authorities were responsible for the fact that the first BMW product was designated with a III instead of a I. As early as 1915, the IdFlieg German military aviation inspectorate introduced uniform model designations for aero engines, with the Roman numeral referring to the performance class. IdFlieg's Class 0 (zero) engine power category was for engines of up to 100 bhp (75 kW), such as the Gnome Lambda-clone 80 hp (60 kW) Oberursel U.0 rotary engine, Class I was reserved for engines from 100 to 120 bhp (89 kW), with Class II for engines of between 120 and 150 hp (110 kW). The BMW engine was 185 bhp (138 kW) and was assigned to category III.
The engine was successful, but the real breakthrough came in 1917, when Friz integrated a basically simple throttle butterfly into the "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane fuel of gasoline blended with Benzole, the carburettor adjusted the richness of the fuel-air mixture according to the aircraft's altitude. It enabled the engine, now dubbed BMW IIIa, to develop a constant 200 horsepower (150 kW) up to an altitude of 2000 meters -a decisive advantage over competitors' engines.
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Wikipedia