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 145×140 mm (5.7×5.5 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.

Variants:
None of the JuMo 222 variants achieved operational service.
JuMo 222A-1
JuMo 222A-2
JuMo 222A-3
JuMo 222B-1
JuMo 222B-2
JuMo 222B-3
JuMo 222C
JuMo 222D
JuMo 222E
JuMo 222F
JuMo 222G
JuMo 222H

Specifications:
Jumo 222A
Type: 6-bank, 24-cylinder supercharged liquid-cooled in-line aircraft engine
Bore: 135 mm (5.3 in)
Stroke: 135 mm (5.3 in)
Displacement: 46.5 L (2,830 in³)
Length: 2,400 mm (94.5 in)
Diameter: 1,160 mm (45.7 in)
Dry weight: 1,088 kg (2,399 lb)
Valvetrain: Two intake valves and one sodium-cooled exhaust valve per cylinder
Supercharger: Single-stage two-speed centrifugal type supercharger
Fuel system: Fuel injection
Cooling system: Liquid-cooled
Power output:
1,838 kW (2,465 hp) at 3,200 rpm for takeoff
1,397 kW (1,870 hp) cruise
Specific power: 39.5 kW/L (0.87 hp/in³)
Compression ratio: 6.5:1
Specific fuel consumption: 0.29 kg/(kW·h) (0.477 lb/(hp·h))
Power-to-weight ratio: 1.69 kW/kg (1.03 hp/lb)

The Junkers Jumo 205 aircraft engine was the most famous of a series of Diesel engines that were the first, and for more than half a century the only, successful aircraft Diesel engines. The Jumo 204 first entered service in 1932. Later engines in the series were styled Jumo 206, Jumo 207 and Jumo 208, and differed in stroke and bore and supercharging arrangements. In all more than 900 of these engines were produced.

These engines all used a two-stroke cycle with twelve pistons sharing six cylinders, piston-head to piston-head in an opposed piston configuration. This unusual configuration required two crankshafts, one at the bottom of the cylinder block and the other at the top, geared together. The pistons moved towards each other during the operating cycle. Intake and exhaust manifolds were duplicated on both sides of the block. There were two cam-operated injection pumps per cylinder, each feeding two nozzles, for 4 nozzles per cylinder in all.

As is typical of two-stroke designs, the Jumos used fixed intake and exhaust ports instead of valves, which were uncovered when the pistons reached a certain point in their stroke. Normally such designs have poor volumetric efficiency because both ports open and close at the same time and are generally located across from each other in the cylinder. This leads to poor scavenging of the burnt charge, which is why valve-less two-strokes generally produce smoke and are inefficient.

The Jumo solved this problem to a very large degree through clever arrangement of the ports. The intake port was located under the “lower” piston, while the exhaust port was under the “upper”. The lower crankshaft ran eleven degrees behind the upper, meaning that the exhaust ports opened and, even more importantly, closed first, allowing proper scavenging. This system made the two-stroke Jumos run as cleanly and almost as efficiently as four-stroke engines using valves, but with considerably less complexity.

There is some downside to this system as well. For one, since matching pistons were not closing at quite the same time, but one ran “ahead” of the other, the engine could not run as smoothly as a true opposed style engine. In addition, the power from the two opposing crankshafts had to be geared together, adding weight and complexity, a problem the design shared with H block engines.

In the Jumo, these problems were avoided to some degree by taking power primarily from the “upper” shaft. All of the accessories, such as fuel pumps, injectors and the scavenging compressor, were run from the lower shaft, meaning over half of its power was already used up. What was left over was then geared to the upper shaft, which ran the propellers. In all, about three-quarters of the power to the propellers came from the upper crankshaft.

In theory, the flat layout of the engine could have allowed it to be installed inside the thick wings of larger aircraft, such as airliners and bombers. Details of the oil scavenging system suggest this was not possible and the engine had to be run “vertically”, as it was on all designs using it.

A twelve cylinder version, the Jumo 218, was designed but never built, while a single 24-cylinder 4-crankshaft Junkers Jumo 223 was built and tested.

The Jumo 204 and 205 were licensed to Napier & Son, who built a small number as the Napier Culverin just prior to the war. After the war, they mounted three Culverins in a triangle layout to produce the Napier Deltic, which was for some time one of the most powerful and compact diesel engines in the world. Work on this engine started in 1947 and engines were running by 1950.

The Jumo 205 powered early versions of the Junkers Ju 86 bomber, but was found too unresponsive for combat and liable to failure at maximum power, common for combat aircraft. Later versions of the design also used the engine for extreme high-altitude use. It was far more successful as a power unit for airships, for which its characteristics were ideal, and for non-combat applications such as the Blohm & Voss Ha 139 airliner.

Applications:
Blohm & Voss BV 138
Blohm & Voss Ha 139
Blohm & Voss BV 222
Dornier Do 18
Dornier Do 26
Junkers Ju 86

Specifications:
Jumo 205 A
Type: Six-cylinder 12-piston liquid-cooled opposed piston inline two-stroke diesel engine
Bore: 105 mm (4.13 in)
Stroke: 160 mm (6.3 in)
Displacement: 16.63 L (1,015 in³)
Length: 76.5 in (1,934 mm)
Width: 21.54 in (547 mm)
Height: 52.17 in (1,325 mm)
Dry weight: 595 kg (1,312 lb)
Fuel system: Fuel injection
Fuel type: Diesel
Oil system: Forced with one pressure and two scavenge pumps
Cooling system: Liquid-cooled
Power output: 647 kW (867 hp) at 2,800 rpm
Specific power: 39.0 kW/L (0.86 hp/in³)
Compression ratio: 17:1
Power-to-weight ratio: 1.09 kW/kg (0.66 hp/lb)

Development of the Junkers diesel engines started in the 1920s with the Junkers Fo3 and Junkers Fo4/Junkers SL1. The Fo4 was re-designated Junkers 4, which in turn was re-designated Junkers 204 by the Reichsluftfahrtministerium (RLM).

These engines all used a two-stroke cycle with six cylinders and twelve pistons, in an opposed piston configuration with two crankshafts, one at the bottom of the cylinder block and the other at the top, geared together. The pistons moved towards each other during the operating cycle. Intake and exhaust ports were duplicated at both ends of the block. There were two cam-operated injection pumps per cylinder, each feeding two nozzles, totaling four nozzles per cylinder.

As is typical of two-stroke designs, the Jumos used fixed intake and exhaust ports instead of valves, which were uncovered when the pistons reached a certain point in their stroke. Normally such designs have poor volumetric efficiency because both ports open and close at the same time and are generally located across from each other in the cylinder. This leads to poor scavenging of the burnt charge, which is why valve-less two-strokes generally run smoky and are inefficient.

The Jumo 204 solved this problem to a very large degree through a better arrangement of the ports. The intake port was located under the “lower” piston, while the exhaust port was under the “upper”. The lower crankshaft ran eleven degrees behind the upper, meaning that the exhaust ports opened first, allowing proper scavenging. This system made the two-stroke Jumos run as cleanly and almost as efficiently as four-stroke engines using valves, but with considerably less complexity.

The Jumo 204 (originally designated Jumo 4) was test flown in early 1929 installed in a Junkers G 24. The Jumo 204 first entered service in 1932.

Later engines in the series were designated Jumo 205, Jumo 206, Jumo 207 and Jumo 208, they differed in stroke and bore and supercharging arrangements.

The Jumo Fo3 and 204 were licensed to Napier & Son, who built a small number as the Napier Culverin just prior to the war. Late in the war, they mounted three Culverins in a triangle layout to produce the Napier Deltic, which was for some time one of the most powerful and compact diesel engines in the world.

First run in 1934, the Culverin name is derived from the French word, culverin, for an early cannon or musket. First flown in 1938 using a Blackburn Iris V biplane flying-boat aircraft, the engine went into production for use in the Fairey III biplane.

Variants:
Mo3
A research diesel engine for test bed use only, the Mo3 was a four cylinder, (eight piston), opposed-piston engine designed to run horizontally. Tested from 1913, the problems found in testing the Mo3 were progressively solved after WWI with the Fo3 and later opposed piston diesels.

Fo3
From 1924, Junkers experimented with the Fo3, A vertical 5-cyl. opposed-piston diesel, which ran on a test bed in the summer of 1926, developing 620 kW (830 hp) at 1,200 rpm. Mostly successful, the Fo3 did highlight the need for accurate dynamic balancing of the rotating components.

Fo4
(a.k.a. SL1) The Fo4, benefiting from the experience gained with the Fo3, was a six-cylinder opposed-piston diesel engine, tested from 1928 and flown in the nose of a Junkers G.24 from 30 August 1929.

Jumo 4
After the successful bench runs and flight tests, the Fo4 was refined into the certicated Jumo 4, enjoying limited success powering re-engined Junkers F24kay airliners of Deutsche Luft Hansa, remaining in service until 1939.

Jumo 204
Later production of refined Jumo 4s, designated Jumo 204 in three sub-variants: 204A, 204B,and 204C, differing in airscrew drive arrangements.

Napier Culverin
Licensed built version of the Junkers Jumo 204 six-cylinder vertically opposed liquid-cooled diesel aircraft engine built by Napier & Son.

Applications:
Junkers F.24kay
Junkers Ju 52
Junkers Ju 86
Junkers G.38
Blohm & Voss BV 138
Blackburn Iris V
Fairey III

Specifications:
Jumo 204
Type: Six-cylinder 12-piston liquid-cooled opposed piston inline two-stroke diesel engine
Bore: 120 mm (4.72 in)
Stroke: 210 mm (8.27 in)
Displacement: 28.5 l (1,739 in³)
Length: 1,260 mm (49.61 in)
Width: 510 mm (20.08 in)
Height: 1,510 mm (59.45 in)
Dry weight: 750 kg (1,653 lb)
Power output: 552 kW (740 hp) at 1,800 rpm
Specific power: 19.3 kW/l (0.42 hp/in³)
Compression ratio: 17:1
Specific fuel consumption: 212 g/(kW•h) (0.35 lb/(hp•h))
Power-to-weight ratio: 0.74 kW/kg (0.45 hp/lb)

Napier Culverin
Type: 12-piston 6-cylinder vertical opposed-piston liquid-cooled diesel engine.
Bore: 4.75 in (120.65 mm)
Stroke: 8.25 in (209.55 mm)
Displacement: 1,737 cu in (28.5 lt)
Dry weight: 1,785 lb (809.6 kg)
Valvetrain: Piston ported
Fuel system: Fuel injected
Fuel type: Diesel oil
Cooling system: Liquid cooled
Reduction gear: Spur geared, reduction ratio 0.6935:1, right hand tractor
Starting system: Compressed air starter
Power output: 821 hp at 2,050 rpm
Power-to-weight ratio: 0.46 hp/lb