BMW IIIa
was an
inline
six-cylinder
SOHC
valvetrain,
water-cooled
aircraft engine, the first-ever engine produced by
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.VIIF
, which outperformed any allied aircraft.
Design and development
[
edit
]
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 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 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 twin-barrel "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.
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
Pferdstarke
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
mechanical horsepower
was based on the older 33,000 ft-lb/min figure, which translates to 745.7 watts instead.
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
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
in-line 6-cylinder
engine's reputation grew very quickly after its abilities were proven in air combat by
Jasta 11
, the "
Red Baron's
" squadron.
Ernst Udet
, squadron leader of Jasta 11 in World War I, acknowledged the outstanding performance of the BMW IIIa engine:
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.
[1]
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
Russelsheim
.
On September 13, 1919,
Franz-Zeno Diemer
set a world altitude record for a passenger aircraft (eight people on board, 6750 meters) in a
Ju F 13
powered by a BMW IIIa aircraft engine.
[
citation needed
]
Applications
[
edit
]
Specifications (BMW IIIa)
[
edit
]
Data from
Smithsonian NASM BMW IIIa specifications
General characteristics
- Type:
6-cylinder, inline, water-cooled, piston engine
- Bore
:
150 mm (5.9 in)
- Stroke
:
180 mm (7.1 in)
- Displacement
:
19.085 L (1,165 cu in)
- Length:
170.2 cm (67.01 in)
- Width:
50.8 cm (20 in)
- Height:
105.4 cm (41 in)
- Dry weight
:
293 kg (646 lb)
Components
- Valvetrain
:
SOHC (single overhead camshaft)
- Fuel system:
Two-barrel altitude compensating (to 2 km) carburetor
- Cooling system:
Water-cooled
Performance
See also
[
edit
]
References
[
edit
]
Wikimedia Commons has media related to
BMW IIIa
.