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Junkers Jumo 204
Junkers Fo4 / SL1
Junkers Jumo 4
Napier Culverin


Cut away view of a Jumo 205, a decreased capacity version of the Jumo 204


Development of the Junkers diesel engines started in the 1920s with the Junkers Fo3 and Junkers Fo4/Junkers SL1. The Fo4 was re-designated Junkers 4, which in turn was re-designated Junkers 204 by the Reichsluftfahrtministerium (RLM).

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 ports were duplicated at both ends of the block. There were two cam-operated injection pumps per cylinder, each feeding two nozzles, totaling four nozzles per cylinder.

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 204 solved this problem to a very large degree through a better 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.

The Jumo 204 (originally designated Jumo 4) was test flown in early 1929 installed in a Junkers G 24. The Jumo 204 first entered service in 1932.

 Later engines in the series were designated Jumo 205, Jumo 206, Jumo 207 and Jumo 208, they differed in stroke and bore and supercharging arrangements.

The Jumo Fo3 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.


First run in 1934, the Culverin name is derived from the French word, culverin, for an early cannon or musket. First flown in 1938 using a Blackburn Iris V biplane flying-boat aircraft, the engine went into production for use in the Fairey III biplane.


A research diesel engine for test bed use only, the Mo3 was a four cylinder, (eight piston), opposed-piston engine designed to run horizontally. Tested from 1913, the problems found in testing the Mo3 were progressively solved after WWI with the Fo3 and later opposed piston diesels.

From 1924, Junkers experimented with the Fo3, A vertical 5-cyl. opposed-piston diesel, which ran on a test bed in the summer of 1926, developing 620 kW (830 hp) at 1,200 rpm. Mostly successful, the Fo3 did highlight the need for accurate dynamic balancing of the rotating components.

(a.k.a. SL1) The Fo4, benefiting from the experience gained with the Fo3, was a six-cylinder opposed-piston diesel engine, tested from 1928 and flown in the nose of a Junkers G.24 from 30 August 1929.

Jumo 4
After the successful bench runs and flight tests, the Fo4 was refined into the certicated Jumo 4, enjoying limited success powering re-engined Junkers F24kay airliners of Deutsche Luft Hansa, remaining in service until 1939.

Jumo 204
Later production of refined Jumo 4s, designated Jumo 204 in three sub-variants: 204A, 204B,and 204C, differing in airscrew drive arrangements.

Napier Culverin
Licensed built version of the Junkers Jumo 204 six-cylinder vertically opposed liquid-cooled diesel aircraft engine built by Napier & Son.


Junkers F.24kay
Junkers Ju 52
Junkers Ju 86
Junkers G.38
Blohm & Voss BV 138
Blackburn Iris V
Fairey III


Jumo 204
Type: Six-cylinder 12-piston liquid-cooled opposed piston inline two-stroke diesel engine
Bore: 120 mm (4.72 in)
Stroke: 210 mm (8.27 in)
Displacement: 28.5 l (1,739 in³)
Length: 1,260 mm (49.61 in)
Width: 510 mm (20.08 in)
Height: 1,510 mm (59.45 in)
Dry weight: 750 kg (1,653 lb)
Power output: 552 kW (740 hp) at 1,800 rpm
Specific power: 19.3 kW/l (0.42 hp/in³)
Compression ratio: 17:1
Specific fuel consumption: 212 g/(kW•h) (0.35 lb/(hp•h))
Power-to-weight ratio: 0.74 kW/kg (0.45 hp/lb)


Napier Culverin
Type: 12-piston 6-cylinder vertical opposed-piston liquid-cooled diesel engine.
Bore: 4.75 in (120.65 mm)
Stroke: 8.25 in (209.55 mm)
Displacement: 1,737 cu in (28.5 lt)
Dry weight: 1,785 lb (809.6 kg)
Valvetrain: Piston ported
Fuel system: Fuel injected
Fuel type: Diesel oil
Cooling system: Liquid cooled
Reduction gear: Spur geared, reduction ratio 0.6935:1, right hand tractor
Starting system: Compressed air starter
Power output: 821 hp at 2,050 rpm
Power-to-weight ratio: 0.46 hp/lb







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