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performance calculations for the BMW IIIA WWI aero engine
BMW IIIA liquid-cooled 6 -cylinder inline engine 200 [hp](147.1 KW)
introduction : May 1918 country : Germany importance : *****
applications : Fokker D.VIIF
normal rating : 200 [hp](147.1 KW) at 1350 [rpm] at 2350 [m] above sea level
design : Max Friz
general information :
By mid-1918, some D.VIIs received the "over compressed" 138 kW (185 hp) BMW IIIa, the first product of the BMW firm. The BMW IIIa followed the SOHC, straight-six configuration of the Mercedes D.III but incorporated several improvements. Increased displacement, higher compression and an altitude-adjusting carburettor produced a marked increase in speed and climb rate at high altitude. Because the BMW IIIa was over compressed, using full throttle at altitudes below 2,000 m (6,600 ft) risked premature detonation in the cylinders and damage to the engine. At low altitudes, full throttle could produce up to 179 kW (240 hp) for a short time. Fokker-built aircraft with the new BMW engine were called D.VII(F), the suffix "F" standing for Max Friz, the engine designer.
It was designed with a high (for the era) compression ratio of 6.4:1.
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. Post-war 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 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 Ju F 13. When equipped with the BMW IIIa engine, the Fokker
BMW IIIa at the Luftwaffe museum
D VII could outclimb any Allied opponent it encountered in combat. Highly manoeuvrable 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.”
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 Ju F 13 powered by a BMW IIIa aircraft engine
high compression engine : cannot run at full power at sea level,
constant power to height : 2350 [m]
no reduction, direct drive, valvetrain : SOHC
weight engine(s) dry : 293.0 [kg] = 1.99 [kg/KW]
bore : 150.0 [mm] stroke : 180.0 [mm]
Mechanical efficiency/mean pressure correction factor : 0.709 [ ]
invloed hoogte > cm : 0.21 [ ]
compression ratio: 6.37 :1
calculated compression ratio : 6.37 : 1
high compression engine cannot run with full power/throttle at sea-level (only for short
period)
published volume (displacement): 19.060 [litre]
calculated stroke volume (Vs) : 19.085 [litre]
compression volume (Vc): 3.554 [litre]
total volume (Vt): 22.639 [litre]
power / stroke volume (litervermogen Nl): 7.7 [kW/litre]
torque : 1041 [Nm]
engine weight/volume : 15.4 : [kg/litre]
average piston speed (Cm): 8.1 [m/s]
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intake pressure at 2350 [m] altitude Pi : 0.71 [kg/cm2]
mean engine pressure (M.E.P.) at 2350 [m] altitude Pm : 6.34 [kg/cm2]
compression pressure at 2350 [m] altitude Pc: 7.68 [kg/cm2]
estimated combustion pressure at 2350 [m] Pe : 30.71 [kg/cm2]
exhaust pressure at 2350 [m] Pu : 2.86 [kg/cm^2 ]
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compression-end temperature at 2350 [m] Tc: 603 [°K] (330 [°C])
caloric combustion temperature at 2350 [m] Tec: 2326 [°K] (2052 [°C])
polytroph combustion temperature at 2350 [m] Te (T4): 2406 [°K] (2133 [°C])
exhaust temperature at 2350 [m] Tu: 1246 [°K] (972 [°C])
*********************************************************************************
compression pressure at sea level Pc: 10.00 [kg/cm2]
estimated combustion pressure at sea level Pe : 40.00 [kg/cm^2 ]
compression-end temperature at sea level Tc:623 [°K] (350 [°C])
mean engine pressure at sea level Pm : 8.26 [kg/cm2]
emergency/TO rating can be increased with higher [rpm}
(emergency/take off) rating at 1478 [rpm] at sea level : 285 [pk]
Thermal efficiency Nth : 0.371 [ ]
Mechanical efficiency Nm : 0.684 [ ]
Thermo-dynamic efficiency Ntd : 0.253 [ ]
design hours : 771 time between overhaul : 64
fuel consumption optimum mixture at 1350.00 [rpm] at 2350 [m]: 47.60 [kg/hr]
specific fuel consumption thermo-dynamic : 249 [gr/epk] = 339 [gr/kwh]
specific fuel consumption (cruise power) at 2350 [m] optimum mixture : 345 [gr/kwh]
estimated sfc (cruise power) at 3490 [m] rich mixture : 347 [gr/kwh]
specific fuel consumption at 2350 [m] at 1350 [rpm] optimum mixture : 324 [gr/kwh]
estimated specific oil consumption (cruise power) : 25 [gr/kwh]
Literature :
https://en.wikipedia.org/wiki/BMW_IIIa
https://en.wikipedia.org/wiki/Fokker_D.VII
DISCLAIMER Above calculations are based on published data, they must be
regarded as indication not as facts.
Calculated performance and weight may not correspond with actual weights
and performances and are assumptions for which no responsibility can be taken.
Calculations are as accurate as possible, they can be fine-tuned when more data
is available, you are welcome to give suggestions and additional information
so we can improve our program. For copyright on drawings/photographs/
content please mail to below mail address
(c) B van der Zalm 23 January 2020 contact : info.aircraftinvestigation@gmail.com python 3.7.4