Junkers Jumo 205

Jumo 205
Jumo 205 cutaway
Type	Aircraft Diesel engine
Manufacturer	Junkers
First run	1930s
Major applications	Junkers Ju 86 Blohm & Voss BV 138 Blohm & Voss BV 222
Developed from	Junkers Jumo 204

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.

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.

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Wikipedia