BMW VII VIII - successors of BMW VI

[Aurum]

What is definitely known regarding BMW VII BMW VIII engines that were successors of famous widely spread BMW VI?

What were their specs (rotation/compression/power norm./take-off)?

Were they of cast cylinders blocks design or not? If not what were differences compare to parental BMW VI engine?

As far as I understand, these engines were produced in valuable quantities only by Kawasaki, why?

What planes (full list) they were installed on?

Does anybody have photos of these models?

 

[rinkol]

Information is scarce, but it does appear that these engines were all based on BMW VI technology. The following is from the book "Flugmotoren und Strahltriebwerke".

The BMW VIIaU was a 12 cylinder V type engine with 160x190 mm cylinder dimensions, a 7.3:1 compression ratio and a 750 hp rating. The main difference from the BMW VI was the introduction of a 0.62 propeller reduction gear ratio (the earlier engine was direct drive) and the resultant increase in weight to 615 kg from 546 kg.


The BMW VIII U was a 6 cylinder engine with slightly reduced stroke (180 mm) and a 0.5 gear ratio. Power was 530 hp at 2400 rpm, a much higher value than th 1650 rpm specified for the BMW VI and VII.


There was also a BMW IXaU that differed in using a supercharger to provide 800 hp at 1780 rpm and a BMW 106A/D, a development of the BMW VI, apparently intended for the evaluation of fuel injection techniques.

I imagine that the lower weight and cost of the BMW VI favored its choice for large scale production. The relatively low rpm minimized any adverse effects on propeller efficiency that would other otherwise have resulted from the absence of a reduction gear. Reliablility considerations may have been a further factor in avoiding the versions with the reduction gear. I suspect that the BMW IX probably suffered from some reliability issues associated with the increased power output and the use of the reduction gear, which would have been immature technology at the time.

Note that the ratings given are all for take-off.

There were probably minor variations in construction, but all of these engines appear to be similar in construction to the BMW VI in using separate cylinder construction. The later BMW 116 and 117 were inverted V12s with smaller cylinders and likely employing more advanced construction techniques. However, neither of these reached service use.

 

[krieghund]

here is a picture I found of the VIIa

 

[WJPearce]

(Most of) the following is from BMW Aero Engines by Jakobs, Kroschel, and Pierer (2009). Please note some of the info below differs from what rinkol reported. I have no idea which source is correct, but I do find it suspect how an engine can produce 400 hp at 1,000 rpm and 530 hp at 1,100 rpm—an increase of 130 hp for just 100 rpm. In general, all the presented BMW VIII rpm data appears low. Perhaps it is really propeller speed and the engine speed is twice what is listed. Again, the data is from the book.

BMW VIIa
First run in 1928, the BMW VIIa was basically a geared V-12 using the BMW Va (straight-six) cylinders. The crankcase was offered in aluminum or Elektron. The engine had a bore of 160 mm (6.30 in) and a stroke of 190 mm (7.48 in). Total displacement was 45.8 L (2,797 cu in). The engine had individual cylinders with two exposed valves per cylinder. The valves were actuated by an overhead camshaft. Three different compression ratios were offered, 5.5:1, 6.0:1, and 7.3:1. The 5.5:1 engine had a normal output of 600 hp at 1,565 rpm. The 6.0:1 engine had a maximum output of 750 hp at 1,650 rpm. The engine was 2.025 m (79.7 in) long, .864 m (34.0 in) wide, 1.045 m (41.1 in) tall, and weighed 525 kg (1,160 lb). The BMW VIIa passed government tests in June 1930. The engine was installed in early, single-engine Junkers Ju 52 aircraft and also the sole Dornier Do J IIb Bos Wal (D-2053), which Wolfgang von Gronau flew around the world in 1932. Only a small number of engines were made.

BMW VIII
The BMW VII was a geared, straight-six engine developed at the same time as the VIIa. The engine had a bore of 160 mm (6.30 in) and a stroke of 190 mm (7.48 in). Total displacement was 22.9 L (1,399 cu in). The engine had individual cylinders with four valves per cylinder. The valves were enclosed and actuated by an overhead camshaft. Three different compression ratios were offered:
5.5:1 – normal output of 400 hp at 1,040 rpm and max output of 470 hp at 1,100 rpm.
6.0:1 – normal output of 400 hp at 1,025 rpm and max output of 500 hp at 1,100 rpm
7.3:1 – normal output of 400 hp at 1,000 rpm and max output of 530 hp at 1,100 rpm
The engine weighed 365 kg (805 lb). Only test examples were built, and it is doubtful the engine ever flew.

Both engines were offered during a time of great economic depression. Both engines were not fully developed. These issues combined to prevent the engines from gaining any financial success.

 

[krieghund]

Here's some pictures of the BMW VI

 

[Denniss]

Info from BMW archives - BMW Classic (note 4000 PS typo at BMW VIII)

BMW VIIa.

Bauzeit:
1929 - 1931
Leistung:
Dauerleistung 600 PS
Hubraum:
46,9 l ccm
Motorart:
12-Zylinder-V-Motor
Einheiten:
nur wenige
Eingebaut in:
Dornier "Wal"
Junkers Ju 52/ 1

Beschreibung:
Wassergekühlter Zwölfzylindermotor, der vom erfolgreichen BMW VI abgeleitet wurde. Im Unterschied zu diesem waren die Magnete vor den Zylinderreihen angeordnet, um eine einfachere Wartung zu ermöglichen. Mit dem BMW VII wurden auch erste Lader-Versuche mit einem "Gemischbläser" (die Radialgebläse waren hinter dem Vergaser angeordnet) durchgeführt, die dann zu dem Flugmotor BMW IX führten. Nur wenige Exemplare des BMW VII kamen zum Einsatz, darunter in der einmotorigen Version der Ju 52 und im Dornier Wal, mit dem Wolfgang von Gronau 1932 seinen Weltflug durchführte


BMW VIII

Bauzeit:
1928 - 1930
Leistung:
Dauerleistung 4000 PS
Hubraum:
23,0 l ccm
Motorart:
6-Zylinder-Reihenmotor
Einheiten:
Versuchsmuster

Beschreibung:
Wassergekühlter Sechszylinder-Reihenmotor, der ausschließlich zur Erprobung von verschiedener Detail-Verbesserungen diente. Unter anderem wurde bei diesem Typ auch ein Vierventil-Zylinderkopf erprobt. Gegenüber dem hubraumgleichen BMW Va konnte die Dauerleistung von 320 auf 400 PS angehoben werden.

 

[rinkol]

(Most of) the following is from BMW Aero Engines by Jakobs, Kroschel, and Pierer (2009). Please note some of the info below differs from what rinkol reported. I have no idea which source is correct, but I do find it suspect how an engine can produce 400 hp at 1,000 rpm and 530 hp at 1,100 rpm—an increase of 130 hp for just 100 rpm. In general, all the presented BMW VIII rpm data appears low. Perhaps it is really propeller speed and the engine speed is twice what is listed. Again, the data is from the book.

BMW VIIa
First run in 1928, the BMW VIIa was basically a geared V-12 using the BMW Va (straight-six) cylinders. The crankcase was offered in aluminum or Elektron. The engine had a bore of 160 mm (6.30 in) and a stroke of 190 mm (7.48 in). Total displacement was 45.8 L (2,797 cu in). The engine had individual cylinders with two exposed valves per cylinder. The valves were actuated by an overhead camshaft. Three different compression ratios were offered, 5.5:1, 6.0:1, and 7.3:1. The 5.5:1 engine had a normal output of 600 hp at 1,565 rpm. The 6.0:1 engine had a maximum output of 750 hp at 1,650 rpm. The engine was 2.025 m (79.7 in) long, .864 m (34.0 in) wide, 1.045 m (41.1 in) tall, and weighed 525 kg (1,160 lb). The BMW VIIa passed government tests in June 1930. The engine was installed in early, single-engine Junkers Ju 52 aircraft and also the sole Dornier Do J IIb Bos Wal (D-2053), which Wolfgang von Gronau flew around the world in 1932. Only a small number of engines were made.

BMW VIII
The BMW VII was a geared, straight-six engine developed at the same time as the VIIa. The engine had a bore of 160 mm (6.30 in) and a stroke of 190 mm (7.48 in). Total displacement was 22.9 L (1,399 cu in). The engine had individual cylinders with four valves per cylinder. The valves were enclosed and actuated by an overhead camshaft. Three different compression ratios were offered:
5.5:1 – normal output of 400 hp at 1,040 rpm and max output of 470 hp at 1,100 rpm.
6.0:1 – normal output of 400 hp at 1,025 rpm and max output of 500 hp at 1,100 rpm
7.3:1 – normal output of 400 hp at 1,000 rpm and max output of 530 hp at 1,100 rpm
The engine weighed 365 kg (805 lb). Only test examples were built, and it is doubtful the engine ever flew.

I think the interpretation that the BMW VIII figures are for the propeller rpm is correct. For a six cylinder engine to provide well over half the output of the V-12 engines based on the same technology, it would be necessary to increase the rpm, though using four valves per cylinder would help a bit too.

I'm not sure of the precise explanation of the difference in the maximum and normal engine output figures, though it would seem that the full throttle operation corresponds only to the maximum output rating. It may be that use of the engine at maximum output was not recommended under ordinary conditions due to shortening overhaul intervals and adversely affecting reliability. The BMW VI was light for an engine of its displacement.