Junker Jumo 205

The Junkers Jumo 205 aircraft engine was the most famous of a series of aircraft diesel engines that were the first, and for more than half a century the only successful aviation diesel powerplants. 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, in the 1930s and through most of World War II.

Junkers Jumo 205D illustration
Junkers Jumo 205D.
[Source: Unknown]

Design and development
These engines all used a two-stroke cycle with twelve pistons sharing six cylinders, piston crown to piston crown 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 port apertures cut into the cylinder liners instead of valves during their manufacture, 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, somewhat offset upwards on the engine's front end. 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 engine's propeller. In all, about three-quarters of the power to the engine's propeller 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.

General characteristics
• Type: 6-cylinder 12-piston liquid-cooled opposed piston inline 2-stroke diesel engine
• Bore: 105 mm (4.13 in)
• Stroke: 160 mm (6.3 in)
• Displacement: 16.63 L (1,015 in³)
• Length: 1,934 mm (76.5 in)
• Width: 547 mm (21.54 in)
• Height: 1,325 mm (52.17 in)
• Dry weight: 595 kg (1,312 lb)

• Supercharger: Spülgebläse
• Fuel system: Fuel injection
• Fuel type: Diesel
• Oil system: Forced with one pressure and two scavenge pumps
• Cooling system: Liquid-cooled

• Power output: 880 PS (868 hp, 647 kW) 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)

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