Junkers Jumo 222
The Jumo 222 was a German high-power multiple-bank in-line piston aircraft engine  from Junkers, designed under the management of Ferdinand Brandner of the Junkers Motorenwerke.

Such was the projected performance of the engine compared to contemporary designs that many developments of wartime Luftwaffe piston-engined aircraft designs were based on it, at least as an option. These included the entire Bomber B program, which looked to replace all existing Luftwaffe bombers with a new twin-engine design that was larger, faster and more heavily armed than any aircraft in their inventory.

The design failed to mature even after years of intensive development and several major design changes. The Bomber B program failed along with it, leaving the Luftwaffe with hopelessly outdated designs during the second half of the war. Only a small number of 222s were built, less than 300. They never left the prototype phase, but the design nevertheless continued appearing on proposals for new Luftwaffe multi-engined designs long after most had given up hope it would ever work.

Design and development
Design work on the Jumo 222 started in 1937. The engine was configured with six inline cylinder banks spaced at equal angles around the crankcase, each bank having four cylinders. The engine looked like a radial due to the arrangement — this was evident from a cross-sectional drawing of the original version, using a master connecting rod with five additional connecting rods pivoted from the master rod's crankpin end casting, as with a single-row radial. But the internal workings were designed to operate more like a V engine with each adjacent pair of cylinder banks, each with a crossflow head, and it was liquid-cooled like most inlines.

Looking at a complete Jumo 222 from a "nose-on" view, the half-dozen cylinder banks were arranged at 60° equal angles from each other, such that neighbouring banks had their exhaust ports (at the "60°, 180° & 300°" spaces) and intake ports (at the "0°, 120° & 240°" spaces) facing each other, resulting in simpler "plumbing" from the rear-mounted supercharger and resulting in only three sets of exhaust headers. The trio of exhaust header sets would have been most likely present at the bottom of an engine nacelle, and on the upper quarters to either side (appearing like the exhausts for many Allied "upright" V-style aviation engines) for the shortest possible exhaust outlet routing.

The four-cylinder-long multibank design resulted in a shorter engine than the Jumo 211, by roughly , but larger cross-section nacelle design. Like the Ju 88, it could use an annular radiator to cool the 222's cylinders and motor oil. The Junkers Ju 288 intended to hide the radiators behind hollow ducted spinners with each of its four-blade propellers.

Each cylinder had two intake valves and a single sodium-cooled exhaust valve — a triple-valve configuration inherited from the first Jumo-series inverted V12 aviation engine, the Junkers Jumo 210 — a high-pressure fuel injector between the intakes, and two spark plugs. With a bore and stroke of , the original Jumo 222A/B engine design had a displacement of 46.4 L (2,831.5 in³), in the same general displacement class as the Double Wasp American eighteen cylinder air-cooled radial of 1937–1940 origin. The 222A/B model was forced to run at a fairly low 6.5:1 compression ratio, the same ratio as used by the volume-produced Jumo 211C inverted-V12 from their firm. Such comparatively low compression ratios were the best possible ones given the low-octane fuels available in Germany, but by increasing the speed of the engine to 3,200 rpm, the 222 delivered  at takeoff. The only disappointing feature was the simple, single-stage two-speed supercharger, but even with this limitation, the engine still generated  at . The dry weight was , only some  heavier than the air-cooled Double Wasp.

Compared with the contemporary BMW 801 and Daimler-Benz DB 605, the 222 was a huge leap in performance. It had only slightly larger displacement than the 801's 41.8 L (2,550.8 in³), and about 25% more than the 605's 35.7 L (2,178.5 in³), but delivered considerably more power,  compared to  in the 801 and  in the 605. That represents 40 kW/L for the 222, while only 29 kW/L for the 801, and 31 kW/L for the 605. The power-to-weight ratio was 1.7 kW/kg (1.04 hp/lb) for the 222, whereas the 605 delivered 1.4 kW/kg (0.88 hp/lb), and the 801 1 kW/kg (0.60 hp/lb). The 222 also had similar exterior dimensions as these smaller engines,  across compared to  for the 801, and  long compared to  for the 605.

The comparison was even more favourable against other high-power engines under development. The cumbersome DB 606, the first-ever "high-output" powerplant developed in Germany, consisted of two DB 601's mounted to a single reduction gear case on their front ends, that delivered  with a weight of , and was ft) in size. Their troubled use and deficient installation design in the He 177A, Germany's only heavy bomber aircraft to see production and front-line service, prompted Reichsmarschall Hermann Göring to derisively label them in the late summer of 1942 as "welded-together engines". Conversely, the RLM was excited by the possibilities of the much more compact Jumo 222's design features, and the X engine configuration, 24 cylinder DB 604, of similar weight and displacement to the 222A but with somewhat lower specific power output. The RLM based their entire Bomber B program on pairs of these engines, which would deliver a bomber with the warload of the He 177 and even better speed than the Ju 88, a truly universal design.

The first prototype engine ran on 24 April 1939, and was later air-tested on the nose-engine mount of a Ju 52. Production called for two primary models, the 222A and 222B, which differed only in the direction that they spun, intended to be used for left-hand (portside wing) and right-hand (starboard wing) engines on twin-engine designs. However, continued testing went poorly, and Junkers eventually decided it was best to stop development of these "Series I" engines and move onto a modified "Series II". The new 222A-2 and B-2 ran at a slightly slower rpm but had slightly larger cylinders of  bore (49.88 litres, 3043.86 in³) for the same net performance, while the A-3 and B-3 used a different supercharger for better performance at higher altitudes. One A-3 and B-3 powerplant each were allegedly fitted to the ninth Junkers Ju 288 prototype airframe  for flight tests. Both "uprated" models of the Jumo 222A/B versions continued to prove unreliable, and were fitted only experimentally.

By late 1941, Junkers decided the best course of action was to make more radical changes to the design, and introduced the 222C and 222D models. With a new bore and stroke of , the engine displacement increased a second time, to 55.5 L (3,386.8 in³), just very slightly larger than the contemporary Wright Duplex Cyclone American 18-cylinder air-cooled radial engine, which at the time was having its own significant problems ironed out, partially from the use of combustible  magnesium-alloy metal for its crankcase. Back at the original 3,200 rpm, the Jumo 222 C/D models could deliver just under  when they started running in the summer of 1942. However, the problems were not cured, and only a handful were built. The RLM had been waiting for three years at this point, and eventually gave up and had all designs based on it look for alternate engines. Later that year, they gave up on that as well, and cancelled the entire Bomber B program outright.

Junkers still did not give up. Using the original 46.4 litre displacement A/B design, they added a new two-stage supercharger including a trio of aftercoolers, one per pair of neighboring cylinder banks for high-altitude use, producing the 222E and F-series. Although sea-level performance was unchanged, the engine was able to produce  at . By this point it appeared that the problems were finally being worked out, but bombing of the Junkers Motorenwerke's headquarters factories in Dessau made production almost impossible. A final attempt for even higher altitude performance resulted in the turbocharged 222G and H, built only to the extent of a few testbed prototypes.

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.

Variants
None of the JuMo 222 variants achieved operational service.
Jumo 222 A / B-1: first version bore x stroke (135 × 135) = 46,380 cm³,  at 3200 min -1, single-stage two-speed centrifugal supercharger, "service-test" A-0/B-0 series, flight tested
Jumo 222 A / B-2: enlarged variant larger valve cross sections bore x stroke(140 × 135) = 49880 cm³, 2500 hp at 2900 min -1, single-stage two-speed centrifugal supercharger, zero series, flight tested
Jumo 222 A / B-3: as A / B-2, but improved centrifugal supercharger, full pressure altitude: 6000 m, pilot series, flight-tested
Jumo 222C / D: further enlarged variant bore x stroke (145 × 140) = 55,480 cm³, 3000 hp at 3200 min -1, full pressure altitude: 10.000 m, by designed V-series experimental models in the assembly
Jumo 222 E / F:, like A / B-1, with two-stage two-speed centrifugal supercharger and trio of aftercoolers, full pressure altitude: 9400 m, pilot series, flight-tested
Jumo 222 turbo:, like A / B-3, with turbocharger and intercooling, full pressure altitude: 12,300 m, 2400 hp at 3200 min -1, only test bench
Jumo 222 G / 225:, projected, lengthened version with 6 × 6 = 36 cylinder bore x stroke (135 × 135) = 69570 cm³, 3500 hp at 3000 min -1, and speed charging increasable
The direction of rotation of the propeller shaft was indicated by the letter - A, C and E turned to the left while B, D and F turned to the right. The crankshaft, however, always ran uniformly to the right; the propeller running direction was varied only by different gears.

Since the Junkers Jumo 222 has six cylinder banks, it is one of the so-called Hexagon engines. Other examples of hexagon engines are rare - for example, the 24-cylinder water-cooled Dobrynin VD-4K and the 12-cylinder air-cooled Curtiss H-1640 Chieftain. Analogously, there were also octagon engines such as the Bristol Hydra with eight cylinder banks.

Applications
Aircraft designs intended to be powered by the Jumo 222:

Dornier Do 435 (when no jet engine replaced the rear piston engine)
Focke-Wulf Fw 300
Heinkel He 219B & -C
Heinkel He 277 (as a later Amerikabomber-related alternative to fitting six BMW 801 radials, late July 1943 development proposal only)
Hütter Hü 211
Junkers Ju 288

Prototype proposals designed to use Jumo 222 engine power:

 Arado E.340 (twin 222s, a Bomber B competitor)
 Focke-Wulf Fw P.195 (six/eight Jumo 222s, very large transport aircraft)

Specifications (Jumo 222A/B)

See also

References

Bibliography

External links

 Junkers Engines - Jumo 222
 EngineHistory.org's British wartime report on the Jumo 222
 enginehistory.org CAD-based animation of Jumo 222's valvetrain

Junkers aircraft engines
1930s aircraft piston engines
Inline radial engines