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.

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.

The lower crankshaft ran eleven degrees behind the upper, effectively reducing the engine capacity but providing superior port timing and meaning that more power was transmitted to the upper crankshaft. All ancillary equipment was run from the lower crankshaft, so that three quarters of the power delivered to the output shaft came from the upper crankshaft, which was geared directly to the output shaft, and in turn to the propellers.

Some of the efficiency of the engine was robbed by the complex and heavy gearing between the two crankshafts, a problem the design shared with the H block engines. This was offset to a large degree by the high compression ratios and the superior thermodynamic efficiency of the Diesel cycle.

Intake and exhaust manifolds were duplicated on both sides of the block. There were four cam-operated injection pumps per cylinder, each driving two nozzles, for eight nozzles per cylinder in all.

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. However the engine had a definite "top" and "bottom" for oil scavaging purposes, forcing it to be installed in large nacelles instead.

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.

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.

External links

• Opposed piston engines (http://www.histomobile.com/histomob/tech/1/120.htm) (in French).
• Jumo 205 (http://www.dundee.ac.uk/museum/MECHENG/3001.htm) description and cutaway view.