Confusing justification for new engines for Bomber B

Question

https://en.wikipedia.org/wiki/Bomber_B

With engines in the 2,000 to 2,500 hp range, twin-engined aircraft would have considerably more surplus power, allowing for much greater payloads. In theory, the more powerful engine would take no longer to produce than a 1,000 hp design, it would simply be larger. By the late 1930s, engines of this sort of power began to be seriously considered and the British and Germans drew up bomber designs based on them.

https://en.wikipedia.org/wiki/Junkers_Jumo_222

The Jumo 222 was a massive and very costly failure. 289 examples of the Jumo 222's were built in total, none of which saw active service. It also served to seriously hamper Luftwaffe piston-engined designs from 1940 to 1942, while many personnel within the Luftwaffe's government-operated technology development offices (like Oberst Edgar Petersen's chain of several Erprobungsstelle installations) and German military aviation corporate engineering departments waited for the Jumo 222 to finally start working. Meanwhile, all calls for four-engine adaptations in place of twin-engine Jumo 222 powered designs were rejected because it was felt it would place too much strain on the German engine industry. In the end there was nothing to show for it, and late in the war the Luftwaffe was flying barely updated versions of their original pre-war designs.

A bigger engine means bigger or more numerous components. Wouldnt either cause significant increase in time?

Answer 1

As a general take on the subject: the ratio of manufacturing time vs. size mostly depends on what is the bottleneck in production.

For a piston engine, or most mechanical devices of comparable complexity, we can assume it will be the part that is hardest to manufacture due to size, complexity and number of work phases. I would assume in piston engines this would be the engine block, which is the largest single component, has to be cast and then machined in several phases with many different machines. Sleeves, plugs etc. may require installation before the engine block is ready. Another such part would be the cylinder heads, while smaller, there are more than one in large engines.

Increasing bore and/or stroke will have vanishingly small effect on casting or machining time. Adding cylinders will also be non issue for casting, and percentually small increase in machinining time. If a "bigger engine" means a totally different design with more cylinder banks for example, the case is of course different.

Production of smaller components such as pistons, rods, valves and such is relatively easy to scale as long as the raw materials are availlable, which in the case of prolonged war is not guaranteed.

Having said that, if I understand the wikipedia quote correctly it claims that producing 2000 to 2500 hp engine would theoretically require same production time as a 1000 hp engine, I must call bs on the claim. Doubling (or more) the power output of an engine is no longer a case of just adding a couple of cylinders or increasing cylinder volume. The engines of that era were already forced induction, so there's not much to gain there without losing reliability.

As the case of the Jumo 222 shows, squeezing a lot of power from a compact engine easily leads to very complex solutions, such as six cylinder banks. That's no longer as fast to produce as a V configuration engine with half the output.

Answer 2

I wouldn't call the Jumo 213 E a "barely updated" version of original pre-war designs (that would be the Jumo 210), but never mind.

There are conflicting opinions why the Jumo 222 failed. It was certainly a very ambitious design, with less frontal area than a BMW 801 but 20% more power, so development was not easy and needed its time. The first models broke their connecting rods in tests and much besides, but eventually it was the RLM bureaucracy which was the main obstacle to the engine being put to use. When a 2000 HP version passed reliability testing, the goalposts were shifted in Berlin and only a 2500 HP version was deemed fit for mass production. Then a 3000 HP version. So production was never allowed to start.

Much more hinged on the Jumo 222. The Junkers 288 was practically designed around it and, being similarly ambitious as Boeing's B-29, also swallowed lots of resources. On paper it was formidable: Easy to produce and maintain, flying at 690 km/h (430 MPH) in 11 km (36,000 ft) of altitude, it would had given Junkers an unassailable technological advantage over all other German airplane companies. Although 35% larger, it required fewer man-hours to build than the Junkers 88 but (on paper) could carry twice the bomb load over twice the distance. Similarly, a Jumo 222-engined FW-190 showed spectacular performance on paper. However, this would had led to a monopoly of Junkers in airplanes and engines, which was resisted by the RLM. One line of argument goes that Junkers was never allowed to live up to its potential to keep competition alive.

With 24 cylinders, the Jumo 222 certainly was more costly to build than a 12 cylinder in-line engine and its compactness also lets me assume extra efforts in production. However, if you consider the DB 610 as the eventual replacement for the Jumo 222, it also had 24 cylinders, so would had been equally costly to produce. On the other hand, the Jumo 211 was a scaled-up version of the Jumo 210 and had twice the power, but similar man-hours in production (which I assume to be the reasoning behind your first quote). Going for more performance without reducing engine speed will, however, force you to add more cylinders, so production cost will eventually go up with performance when starting from a mature, state-of-the-art design.

What the RLM bureaucracy insisted upon was the restriction of even large bombers to only two engines because it was argued that 4-engine bombers would be too costly to produce and would not have a performance advantage. That is probably what the second of your citations tries to hint at.