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{|{{Infobox aircraft begin
|name = BMW IIIa
|image = BMW IIIa.jpg
|caption = The Smithsonian [[National Air and Space Museum]]'s preserved BMW IIIa, shown with quick-change propeller hub
}}{{Infobox Aircraft Engine
|type=[[Inline engine (aviation)|Inline engine]]
|manufacturer=[[BMW]]
|national origin=
|first run=1917
|major applications=[[Fokker D.VII]]
|number built =
|program cost =
|unit cost =
|developed from =
|developed into =
|variants with their own articles =
}}
|}
'''BMW IIIa''' was an [[Inline engine (aviation)|inline]] six-cylinder [[SOHC]] valvetrain, [[water-cooled]] aircraft engine, the first-ever engine produced by [[BMW AG|Bayerische Motoren Werke 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.VII|Fokker D.VIIF]], which outperformed any allied aircraft.
==Design and development==
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 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 {{convert|100|bhp|abbr=on}}, such as the [[Gnome Lambda]]-clone {{convert|80|hp|abbr=on}} Oberursel U.0 rotary engine, Class I was reserved for engines from 100 to {{convert|120|bhp|abbr=on}}, with Class II for engines of between 120 and {{convert|150|hp|abbr=on}}.  The BMW engine was {{convert|185|bhp|abbr=on}} 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 {{convert|200|hp}} 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&nbsp;hp.  This corresponds to British ratings of the Mercedes DIIIa engine being rated by the British as 180&nbsp;hp (German rating of 170&nbsp;hp) and the DIIIau at 200&nbsp;hp (German-180&nbsp;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#Definitions|mechanical horsepower]] was based on the older 33,000&nbsp;ft-lb/min figure, which translates to 745.7 watts instead.
[[File:Flugmotor BMW IIIa.jpg|thumb|BMW IIIa at the Luftwaffenmuseum]]
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:{{cquote|There can be no doubt that the BMW engine was the absolute highlight in power unit development towards the end of the war.  The only bad thing was that it came too late.<ref>Barker, R. (2002); ''The Royal Flying Corps in World War I''; Robinson. {{ISBN|1-84119-470-0}}</ref>}}
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.{{Citation needed|date=January 2010}}
==Applications==
* [[Aero A.18]]
* [[Aero A.26]]
* [[Aero Ae 04]]
* [[Avia BH-3]]
* [[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)==
[[File:100 4123.jpg|thumb|BMW IIIa specifications]]
{{pistonspecs
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|ref=[https://airandspace.si.edu/collection-objects/bmw-model-iiia-line-6-engine/nasm_A19710908000 Smithsonian NASM BMW IIIa specifications]
|type=6-cylinder, inline, water-cooled, piston engine
|bore={{convert|150|mm|in|abbr=on|1}}
|stroke={{convert|180|mm|in|abbr=on|1}}
|displacement={{convert|19.1|L|cuin|abbr=on|0}}
|length={{convert|1540|mm|in|abbr=on|2}}
|diameter=
|width={{convert|510|mm|in|abbr=on|0}}
|height={{convert|1040|mm|in|abbr=on|0}}
|weight={{convert|285|kg|lb|abbr=on|0}}
|valvetrain=SOHC (single overhead camshaft)
|supercharger=
|turbocharger=
|fuelsystem=Two-barrel altitude compensating (to 2 km) carburetor
|fueltype=
|oilsystem=
|coolingsystem=Water-cooled
|power={{convert|185|hp|kW|abbr=on|0|disp=flip}} at 1,400 rpm
|specpower=
|compression=6.4:1
|fuelcon=
|specfuelcon=
|oilcon=
|power/weight=
}}
==See also==
* [[BMW IV]]
* [[Liberty L-6]]
* [[Mercedes D.III]]
*[[List of aircraft engines]]
==References==
{{Commons category|BMW IIIa}}
{{reflist}}
{{BMW aeroengines}}
[[Category:BMW aircraft engines]]
[[Category:1910s aircraft piston engines]]
[[Category:Straight-six engines]]