Junkers Jumo 222
The Jumo 222 was a high-power multiple bank in-line piston aircraft engine design from Junkers, designed under the management of Ferdinand Brandner of the Junkers Motorenwerke. Such was the performance of the engine compared to contemporary designs that many developments of wartime Luftwaffe piston-engined aircraft designs were based around it, at least as an option. The design failed to mature even after years of intensive development, dooming the entire Bomber B program along with it. Only a small number were built, never leaving the prototype phase, but the design nevertheless continued appearing, endlessly, on proposals for new Luftwaffe multi-engined designs long after most had given up hope it would ever work.
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 each. The engine looked like a radial due to the arrangement, but the internal workings were more like a V engine with crossflow heads, 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.
Each cylinder had two intake valves and a single sodium-cooled exhaust valve, a high-pressure fuel injector between the intakes, and two spark plugs. With a bore and stroke of 135 mm (5.3 in), 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 1939/40 origin. The 222A/B model was forced to run at a fairly low 6.5:1 compression ratio, the best possible given the low-octane fuels available in Germany, but by increasing the speed of the engine to 3,200 rpm, the 222 delivered 1,850 kW (2,500 hp) at takeoff. The only disappointing feature was the simple, single-stage two-speed supercharger, but even with this limitation, the engine still generated 1,641 kW (2,200 hp) at 5,000 m (16,400 ft). The dry weight was 1,088 kg (2,399 lb), only some 17 kg (39 lb) heavier than the air-cooled Double Wasp.
Compared to the contemporary BMW 801 and Daimler-Benz DB 605, the 222 was a huge leap in performance. It had only a slightly larger displacement figure than the 801's 41.8 L (2,550.8 in³), and about ¼ more than the 605's 35.7 L (2,178.5 in³), but delivered considerably more power, 1,850 kW (2,500 hp) compared to 1,193 kW (1,600 hp) in the 801 and 1,119 kW (1,500 hp) 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. Even more impressively the power-to-weight ratio was 1.7 kW/kg (1.04 hp/lb) for the 222, whereas the 605 delivered an otherwise excellent 1.4 kW/kg (0.88 hp/lb), and the 801 a fairly poor 1 kW/kg (0.60 hp/lb). The 222 also had similar exterior dimensions as these smaller engines, 1.16 m (3.81 ft) across compared to 1.27 m (4.17 ft) for the 801, and 2.4 m (7.9 ft) long compared to 2.3 m (7.5 ft) 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 by the German aviation engine industry starting in February 1937, consisted of two DB 601's mounted to a single reduction gear case on their front ends, that delivered 1,790 kW (2,400 hp) from a whopping 1,515 kg (3,340 lb), and was 2.1 x 1.6 x 1.1 m (6.9 x 5.2 x 3.6 ft) in size. Their troubled use and deficient installation design in the He 177, 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 around 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 April, 24th 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 and right-hand 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 140 mm/5.5 in 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. Both 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 145x140 mm (5.7x5.5 in), 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 2,200 kW (3,000 hp) 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 1,439 kW (1,930 hp) at 9,000 m (29,530 ft). 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.