Letters to the Editor

Junkers single-cylinder opposed-piston explained


| January 2009


I saw Bob Comer's letter in the October 2008 issue on the "single-cylinder opposed-piston engine diesel." Unfortunately I believe the reply may have led him astray.

The cross section of the Junkers opposed-piston engine shows the correct opposed-piston engine layout with two crankshafts in a vertical inline configuration. These were made in different bores and strokes, and were called the 204, 205, 206, 207 and 208. The schematic shows a single crankshaft layout opposed-piston engine design. To my knowledge, Junkers did not make any "production" single-crank opposed-piston engines. They also made many inverted V-type engines, single crank, but they were not of the opposed type. Mr. Wendel may own a single-crank prototype design made by Junkers.

The problem with the single-crank design is the mass of all the secondary connecting rods, tie rods and cross bar, which would limit the engine speed to low rpm use only. William Doxford & Sons Ltd., Sunderland, England, were well known for building large marine-type single-crank diesel engines for many years in the early 1900s. The low speed was a big advantage in this application as they could be direct connected to the propeller without a reduction gear. According to an old engine website, they were made into the early 1960s.

The Fairbanks-Morse opposed-piston engines were most likely derived from the Junkers Jumo 205 design as they did hire engineers from Germany during the 1930s when the first 6-cylinder 5-by-6-inch opposed-piston was developed. The Fairbanks OP has gone through several design changes over the years in different models. The Model 38 is the designation for the opposed-piston design engines that includes the current production model 8-1/8-by-10 engine with ratings to 4,800 hp at 900 or 1,000 rpm. Several models no longer in production include the 8-by-10, the 6-3/4-by-8, the 5-1/4-by-7-1/4 and, of course, the infamous 20-by-21 38A20.

A couple of things different between the Jumo 205 and the Fairbanks OP are the 205s took power from the upper crank, and had exhaust pistons on top and spur gearing to connect the cranks. The Fairbanks OPs take power from the lower crank, and had exhaust pistons on the lower crank and a vertical shaft with bevel gearing to connect the cranks.

I hope this will help explain the opposed-piston design engines.






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