Junkers Jumo 205

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Junkers Jumo 205a. Note the supercharger driven from the lower crankshaft, and the complex gearing used to connect the two shafts and still lie within the flat profile.

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 diesel aircraft 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.^{[citation needed]}

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.^{[citation needed]}

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 scavaging of the burnt charge, which is why valve-less two-strokes generally run smoky and are inefficient.^{[citation needed]}

The Jumo designs 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 scavaging. 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.^{[citation needed]}

Jumo 205 cutaway

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. Additionally the power from the two opposing crankshafts has to be geared back together, adding weight and complexity, a problem the design shared with the H block engines.^{[citation needed]}

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 supercharger, were run from the lower shaft, meaning that 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 flowing to the propellers came from the upper crankshaft.^{[citation needed]}

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 scavaging system suggest that this was not possible and that the engine had to be run "vertically", as it was on all designs that used it.^{[citation needed]}

The Jumo 205 powered the early versions of the Junkers Ju 86 bomber, but was found too unresponsive for combat and liable to failure when used at maximum power as is 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.^{[citation needed]}

A twelve cylinder version, the Jumo 218, was designed but never built, while a single example of the 24-cylinder 4-crankshaft Junkers Jumo 223 was built and tested.^{[citation needed]}

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.^{[citation needed]}

It is also highly likely that the Fairbanks Morse 5-1/4" (piston diameter) Diesel generator was derived directly from the Jumo 205.^{[citation needed]} It was the back-up energy source for the U.S. Navy's nuclear-powered Sturgeon class submarine (produced 1966-74).^{[citation needed]} The Fairbanks-Morse engine differed in that it had 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 round 33" diameter hatch, rather than cutting a hole in the hull, and then re-welding it. The powerplant proved itself to be 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 that could be filled by the ship's air-compressors, or a hand-pump.^{[citation needed]}

[edit] Specifications (Jumo 205)

General characteristics

Type: 6-cylinder 12-piston liquid-cooled opposed piston inline diesel engine
Bore: 105 mm (4.1 in)
Stroke: 160 mm (6.3 in)
Displacement: 16.6 L (1,013 in³)
Dry weight: 595 kg (1,312 lb)

Components

Valvetrain: One intake and one exhaust port per cylinder
Fuel type: Diesel
Cooling system: Liquid-cooled

Performance

Power output: 647 kW (867 hp) at 2,800 rpm
Specific power: 39.0 kW/L (0.86 hp/in³)
Power-to-weight ratio: 1.09 kW/kg (0.66 hp/lb)

[edit] Specifications (Jumo 204)

configuration: 6 cylinder, opposed-piston two-stroke diesel, 120mm bore × 2*210 mm stroke, 28.6l, 17:1 compression
power: 750 PS @ 1800 rpm, 600 PS @ 1600 rpm continuous
specific fuel consumption: 155 g/PS/h
dimensions: 1260mm length, 1510mm height, 510mm width, 750kg
reference: inter-action association, 1987