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Junkers Jumo 205

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.

Technical details

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 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 run smoky 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 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 the pistons were not firing at the same time, but ran "ahead" of one another, the engine could not run as smoothly as a true opposed style engine. In addition, the power from the two opposing crankshafts has 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 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.

Usage

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.

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.

Developments

The Jumo 203 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.

It is also highly likely the Fairbanks Morse 5-1/4" (piston diameter) diesel generator was derived directly from the Jumo 205. It was the back-up source for the U.S. Navy's nuclear-powered Sturgeon-class submarines. The Fairbanks-Morse engine differed in having 10 cylinders and 20 pistons, and the cylinders were separate pieces from the upper and lower crankcases. The major benefit of the design in this application was the entire engine could be disassembled, and the pieces could be passed through a 33"-diameter hatch. The powerplant proved rugged, simple, and utterly reliable. As a diesel, it needs no sparking system, as a two-stroke, it has no intake or troublesome exhaust valves. It used an air-start system from an accumulator which could be filled by the ship's air compressors, or by hand pump.