Junkers Jumo 205

Junkers Motoren
Industry	Aircraft engines
Founded	1932
Headquarters	Germany
Products	Diesel aircraft engines (Jumo 204, 205, 206, 207, 208)

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The Jumo 205 aircraft engine was the most widely produced member of a series of diesel engines designed by Junkers. The initial model, the Jumo 204, was introduced in 1932. Subsequent developments in this line included the experimental Jumo 206 and Jumo 208, while the Jumo 207 was manufactured in larger quantities for use in the Junkers Ju 86P and -R variants, which were high-altitude reconnaissance planes, as well as the six-engine, 46 m (151 ft) wingspan Blohm & Voss BV 222 Wiking flying boat. Each of these three versions featured variations in bore, stroke, and supercharging systems. Overall production exceeded 900 units during the 1930s and throughout much of World War II.

Design and development

File:Jumo 207 im Technikmuseum Hugo Junkers Dessau 2010-08-06 01.jpg

Junkers Jumo 207 aviation diesel engine with a similar sectional design

These engines operated on a two-stroke cycle principle, incorporating twelve pistons arranged in six cylinders with an opposed piston layout, where pistons faced each other within each cylinder. This distinctive design necessitated two crankshafts—one positioned at the base of the cylinder block and another at the top—interconnected via gearing. During operation, the pistons moved inward toward each other. Intake ports were situated at one end of the cylinder, while exhaust ports were located at the opposite end, effectively assigning one piston to control intake and the other to manage exhaust flow. Each cylinder employed two cam-driven injection pumps, each supplying two nozzles, totaling four nozzles per cylinder.

In line with typical two-stroke engine architecture, these Jumos did not utilize valves; instead, fixed intake and exhaust ports were machined into the cylinder liners. These ports were uncovered at specific points during piston travel. Usually, such port arrangements result in low volumetric efficiency because intake and exhaust openings overlap in timing and are generally opposite each other within the cylinder. This often causes inadequate scavenging of combustion gases, leading to smoky exhaust and reduced efficiency.

The Jumo engines mitigated this issue through an innovative port configuration: the intake port was located beneath the "lower" piston, whereas the exhaust port was beneath the "upper" piston. The lower crankshaft was timed to lag behind the upper crankshaft by approximately 11°, causing exhaust ports to open and close slightly before intake ports. This sequencing improved scavenging effectiveness significantly. Consequently, these two-stroke diesels achieved emissions and efficiency levels approaching those of four-stroke valve-equipped engines but maintained simpler mechanical complexity.

However, this arrangement had drawbacks. Since opposing pistons did not reach their top positions simultaneously—the lower piston lagging behind—the engine's running smoothness was compromised compared to true opposed-piston configurations. Additionally, power from the dual crankshafts required gearing together, increasing mechanical complexity and weight—a challenge also found in H-block designs.

In practice, some of these issues were alleviated by extracting primary power from only the upper crankshaft, which was positioned slightly higher at the front of the engine. Auxiliary components such as fuel pumps, injectors, and the scavenging compressor were driven by the lower shaft, consuming over half its output power. The remainder was then transmitted via gearing to drive the propeller through the upper shaft.

Although its flat engine layout theoretically allowed installation within thick wings of large aircraft like airliners or bombers, features of its oil scavenging system indicate it required vertical orientation during operation—as implemented in all aircraft that utilized this engine.

The comparatively lower exhaust gas temperatures produced by Jumo diesel engines versus carbureted counterparts facilitated turbocharger integration for enhanced high-altitude performance. This approach was demonstrated in the Jumo 207 model, which harnessed exhaust energy to boost power output at altitude. Its turbocharging system worked alongside a mechanically driven blower: initially, the turbocharger provided primary compression while the blower supplied secondary boost. During low-load or startup conditions, when exhaust energy was insufficient for turbocharging, the mechanical blower ensured adequate air supply for combustion. At higher loads, abundant exhaust gases enabled the turbocharger to deliver full supercharging independently, rendering the mechanical blower unnecessary and improving overall efficiency without a significant increase in specific fuel consumption.

Variants

Jumo 205
Jumo 206: An experimental model whose development was discontinued in favor of the Jumo 208.
Jumo 207A: Designed for high-altitude performance, equipped with two inline centrifugal superchargers and a precooler.
Jumo 207 B-3: Featured an enhanced turbocharger and incorporated GM-1 nitrous oxide injection.
Jumo 207 C: Tailored for medium-altitude operation, produced in limited quantities primarily for the Blohm & Voss BV 222.
Jumo 207 D: Also optimized for medium altitude, with cylinder bore increased from 105 mm to 110 mm. Ground-level maximum output reached 1,200 hp (880 kW). Only prototypes were made.
Jumo 207 E: Comparable to the 207 C but designed to deliver improved high-altitude performance. This variant remained a project only.
Jumo 207 F: Focused on higher altitude efficiency, featuring a two-stage turbocharger. Development ceased in 1942.
Jumo 208: Increased displacement allowed a peak power of 1,500 hp (1,100 kW) at medium altitude. It underwent bench testing but was never mass-produced.
Jumo 218: A planned 12-cylinder variant named Jumo 218 was designed but never constructed.
Jumo 223: A single unit with 24 cylinders and four crankshafts was built and tested.
Jumo 224: Larger than the Jumo 223, created by combining four Jumo 207 C engines.
CLM Lille 6As: Manufactured under license by CLM Lille, this version generated 650 hp (480 kW). (CLM later evolved into the Indenor [fr] engine manufacturer, affiliated with Peugeot).
CLM Lille 6BrS: An advanced iteration of the 6As utilized to power the Bernard 86.

Applications

The Jumo 205 was initially installed in early models of the Junkers Ju 86 bomber but proved insufficiently responsive for combat roles and prone to failure when operating at maximum power—common conditions in military aircraft. Subsequent adaptations employed the engine for extreme high-altitude missions, such as those undertaken by the Ju 86P and Ju 86R variants used for reconnaissance flights over Britain. In January 1940, the Luftwaffe trialed the Ju 86P prototype equipped with turbocharged Jumo 207A-1 diesel engines, achieving greater success in this configuration. The engine found its greatest effectiveness powering airships due to its favorable characteristics and was also well-suited for civilian uses like the Blohm & Voss Ha 139 airliner. Its fuel efficiency made it ideal for Germany's limited number of maritime patrol flying boats during World War II, including models such as the BV 138 and BV 222.

Applications list

Specifications (Jumo 205E)

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Other significant opposed-piston engines

Commer TS3, known as the "Commer Knocker," a commercial vehicle engine
Leyland L60 tank engine, utilized in the Chieftain tank, featuring a design comparable to the Junkers Jumo 205 and Napier Culverin
Rolls-Royce plc#K60 K60 engine, applied in the FV430 armored vehicle series and the Swedish Strv 103 tank
Napier Deltic
Soviet 5TDF engine powering the T-64 tank
Soviet 6TD engine employed in the T-80UD, T-84, and Al-Khalid tanks
Fairbanks Morse 38 8-1/8 diesel engine

References

External links

Template:Junkers Jumo aeroengines