The Jumo 204 was an opposed-piston, inline, liquid-cooled 6-cylinder

aircraft Diesel engine The aircraft diesel engine or aero diesel is a diesel-powered aircraft engine. They were used in airships and tried in aircraft in the late 1920s and 1930s, but were never widely adopted until recently. Their main advantages are their excellent ...

produced by the German manufacturer

Junkers Junkers Flugzeug- und Motorenwerke AG (JFM, earlier JCO or JKO in World War I, English language, English: Junkers Aircraft and Motor Works) more commonly Junkers , was a major German aircraft manufacturer, aircraft and aircraft engine manufactu ...

. It entered service in 1932. Later engines in the series, the Jumo 205, Jumo 206, Jumo 207 and Jumo 208, differed in stroke, bore, and supercharging arrangements.

Design and development

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 Jumo 204 by the

Reichsluftfahrtministerium The Ministry of Aviation (, abbreviated RLM) was a government department during the period of Nazi Germany (1933–45). It is also the original name of the Detlev-Rohwedder-Haus building on the Wilhelmstrasse in central Berlin, Germany, which ...

(RLM), where the first number indicates the manufacturer; 2 – Junkers Motoren. These engines used a

two-stroke cycle A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes of the piston, one up and one down, in one revolution of the crankshaft in contrast to a four-stroke engine which re ...

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. 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 Volumetric efficiency (VE) in internal combustion engine engineering is defined as the ratio of the equivalent volume of the fresh air drawn into the cylinder during the intake stroke (if the gases were at the reference condition for density) to th ...

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 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 Junkers G 24 was a German three-engine, all-metal low-wing monoplane passenger aircraft manufactured by Junkers from 1925. Junkers F 24 was the designation for single-engine versions of the same aircraft. Development The increased German ...

. The Jumo Fo3 and 204 were licensed to

Napier & Son D. Napier & Son Limited was a British engineering company best known for its luxury motor cars in the Edwardian era and for its aero engines throughout the early to mid-20th century. Napier was founded as a precision engineering company in 1 ...

, who built a small number as the

Napier Culverin The Napier Culverin was a licensed built version of the Junkers Jumo 204 six-cylinder vertically opposed liquid-cooled diesel aircraft engine built by D. Napier & Son. The name is derived from the French word, culverin, for an early cannon ...

just prior to the war. Late in the war, they mounted three Culverins in a triangle layout to produce the

Napier Deltic The Napier Deltic engine is a British opposed-piston valveless, supercharged uniflow scavenged, two-stroke diesel engine used in marine and locomotive applications, designed and produced by D. Napier & Son. Unusually, the cylinders were di ...

, which was for some time one of the most powerful and compact diesel engines in the world.

Variants

''Data from Junkers Aircraft & Engines 1913–1945.'' ;Mo3: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 World War I with the Fo3 and later opposed piston diesels. ;Fo3: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 at 1,200 rpm. Mostly successful, the Fo3 did highlight the need for accurate dynamic balancing of the rotating components. ;Fo4: (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 The Junkers G 24 was a German three-engine, all-metal low-wing monoplane passenger aircraft manufactured by Junkers from 1925. Junkers F 24 was the designation for single-engine versions of the same aircraft. Development The increased German ...

from 30 August 1929. ;Jumo 4: After the successful bench runs and flight tests, the Fo4 was refined into the certificated Jumo 4, enjoying limited success powering re-engined Junkers F24kay airliners of

Deutsche Luft Hansa ''Deutsche Luft Hansa A.G.'' (from 1933 styled as ''Deutsche Lufthansa'' and also known as ''Luft Hansa'', ''Lufthansa'', or DLH) was a German airline. It served as flag carrier of the country during the later years of the Weimar Republic and t ...

, remaining in service until 1939. ;Jumo 204:Later production of refined Jumo 4s: Jumo 204A, 204B, and 204C, differing in airscrew drive arrangements. ;Napier Culverin:The Jumo 204 and 205 were licensed to Napier, who built a small number of Jumo 204 developments as the

just prior to the war. After the war, they mounted three Culverins in a triangle layout to produce the

, which was for some time one of the most powerful and compact diesel engines in the world. Work on this engine started in 1947 and engines were running by 1950.

Applications

* Junkers F.24kay *

Junkers Ju 52/1mdo The Junkers Ju 52/3m (nicknamed ''Tante Ju'' ("Aunt Ju") and ''Iron Annie'') is a transport aircraft that was designed and manufactured by German aviation company Junkers. First introduced during 1930 as a civilian airliner, it was adapted int ...

''D-1974'' (''D-UZYP'' from 1937) *

Junkers Ju 86 The Junkers Ju 86 is a monoplane bomber and civilian airliner designed and produced by the Germany, German aircraft manufacturer Junkers. It was designed during the mid-1930s in response to a specification for a modern twin-engined aircraft suit ...

* Junkers G.38 and Japanese licensed

Mitsubishi Ki-20 The Mitsubishi Ki-20 is a Japanese bomber variant of the Junkers G.38 airliner. Mitsubishi manufactured six aircraft under license from Junkers. These aircraft, designated Army Type 92 Heavy Bomber, served through the 1930s. During World War II, ...

Blohm & Voss BV 138 The Blohm & Voss BV 138 ''Seedrache'' (Sea Dragon) was a trimotor flying boat designed and built by the Nazi Germany, German aircraft manufacturer Blohm & Voss (aircraft), Blohm & Voss. It served as the ''Luftwaffe''s primary seaborne long-range ...

Specifications (Jumo 204A)

References

External links

Multi-crankshaft opposed piston engines (french)

Description and cutaway view (Jumo 205)

OpposedPiston Engines.com's Jumo 204 Page
{{Junkers Jumo aeroengines Two-stroke diesel engines Aircraft diesel engines Opposed piston engines Junkers aircraft engines 1920s aircraft piston engines