BMW IIIa was an inline six-cylinder
SOHC An overhead camshaft (OHC) engine is a piston engine , internal combustion piston engine. A reciprocating engine, also often known as a piston engine, is typically a heat engine (although there are also pneumatic and hydraulic motor, hydraulic r ...
water-cooled file:KKP Auslauf.jpg, Cooling tower and water discharge of a nuclear power plant Water cooling is a method of heat removal from components and industrial equipment. Evaporative cooling using water is often more efficient than air cooling. Water is ...
aircraft engine, the first-ever engine produced by Bayerische Flugzeugwerke AG, who, at the time, were exclusively an aircraft engine manufacturer. Its success laid the foundation for future BMW engine designs. It is best known as the powerplant of the
Fokker D.VIIF
Fokker D.VIIF
, which outperformed any allied aircraft.

Design and development

On 20 May 1917,
Rapp Motorenwerke Rapp Motorenwerke GmbH was a German aircraft engine manufacturer in Munich. Founded in 1913, the company changed its name in 1917 to Bayerische Motorenwerke GmbH, which evolved into the automotive manufacturer known today as BMW. Early engines Kar ...
(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 Max Friz (October 1, 1883–June 9, 1966) was a German mechanical engineer specializing in engine design. He was the key contributor of engine design and innovation that led to the founding of Bayerische Motoren Werke AG (BMW) in 1917. Early lif ...
and based on the Rapp Rp III, Rapp III engine, it was an Overhead camshaft#Single overhead camshaft, 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 an 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 , such as the Gnome Lambda-clone Oberursel U.0 rotary engine, Class I was reserved for engines from 100 to , with Class II for engines of between 120 and . The BMW engine was 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 twin-barrel "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane rating, 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 up to an altitude of 2000 meters – a decisive advantage over competitors' engines. German and British horsepower ratings apparently differed. Postwar British tests put the rating of the BMW IIIa at 230 hp. This corresponds to British ratings of the Mercedes DIIIa engine being rated by the British as 180 hp (German rating of 170 hp) and the DIIIau at 200 hp (German-180 hp). This discrepancy may explain the significant difference in performance of the BMW IIIa equipped Fokker D.VIIF both against Mercedes powered D.VII's and their Allied opponents. The standard German ''Pferdstärke'' metric horsepower unit was expressed in the early 20th century as being a unit of almost exactly 735.5 watts, while the British unit for Horsepower#Current definitions, mechanical horsepower was based on the older 33,000 ft-lb/min figure, which translates to 745.7 watts instead. The ability to gain power at higher altitudes was why this engine had unique superiority in air combat. It was primarily used in the Fokker D VII and in the Junkers Ju A 20 and Junkers F13, Ju F 13. When equipped with the BMW IIIa engine, the Fokker D VII could outclimb any Allied opponent it encountered in combat. Highly maneuverable at all speeds and altitudes, it proved to be more than a match for any of the British or French fighter planes of 1918. The water-cooled straight-6, in-line 6-cylinder engine's reputation grew very quickly after its abilities were proven in air combat by ''Jasta 11'', the "Manfred von Richthofen, Red Baron's" squadron. Ernst Udet, squadron leader of Jasta 11 in World War I, acknowledged the outstanding performance of the BMW IIIa engine: About 700 engines were built by BMW, however, a large demand for the new BMW IIIa aircraft engine in Munich (coupled with a lack of production capacity) caused part of the production to be transferred to the Opel factory in Rüsselsheim. On September 13, 1919, Franz Zeno Diemer set a world altitude record for a passenger aircraft (eight people on board, 6750 meters) in a Junkers F13, Ju F 13 powered by a BMW IIIa aircraft engine.


* Aero A.18 * Aero A.26 * Aero Ae 04 * Fokker C.I * Fokker D.VII * Dobi-III * Junkers D.I * Junkers F.13 * Letov Š-3 * LFG Roland D.XV * LFG Roland D.XVII * TNCA Serie E

Specifications (BMW IIIa)

See also

* Liberty L-6 * Mercedes D.III *List of aircraft engines


{{BMW aeroengines BMW aircraft engines 1910s aircraft piston engines Inline-six engines