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DIESELS ARE boring, some say. Here are two, however, that are anything but. The German Junkers Jumo was (almost) unique in providing diesel power for airplanes. The English Napier Deltic is an unorthodox offspring of the Jumo with applications on land and sea.
First, some basics of internal combustion. Rudolf Diesel promoted compression ignition in 1893, 17 years after Nicolas Otto perfected a design which employed a primitive version of spark ignition. (Actually, his 1876 Otto Silent Engine used a flame, rather than an electric spark to ignite its fuel/air mixture.)
For no particular reason I could determine, the Otto (four-stroke) cycle retains its capital O; the diesel (compression-ignition) engine typically loses its upper case.
Note, diesels and spark-ignited designs can be either two-stroke or Otto-Cycle (four-stroke) in operation. Respectively, it’s a matter of how many piston reciprocations are required to complete a cycle. Briefly, is it Bang/Blow? Or Suck/Squeeze/Pop/Phooey?
Hugo Junkers began manufacturing boilers and radiators in 1895. By the Twenties, his company was producing innovative aircraft of all-metal design and engines to power them. The Jumo 204 entered service in 1932 and others in the series followed.
All Jumos were two-stroke supercharged inline-6 diesels of opposed-piston layout. Like other two-stroke designs, the Jumo needed neither intake nor exhaust valves. Instead, intake and exhaust occurred when the pistons encountered apertures in the cylinders.
The Jumo’s opposed-piston configuration was as complex as two-stroke operation is simple: two inline-6s, one inverted atop the other and sharing a central cylinder head. An upper crankshaft drove the propeller; a lower crankshaft contributed its torque through a gear linkage to the upper one.
The Jumo’s 12 pistons moved toward and away from each other in opposing pairs, with the lower ones lagging behind the uppers by 11 degrees of crankshaft rotation (which promoted intake and exhaust efficiency of the configuration).
All of the accessories, such as fuel pumps, injectors and supercharger drive, were run by the lower crankshaft. It’s estimated that three-quarters of the propeller’s spin came from the upper pistons’ operation.
The choice of a diesel has its tradeoffs. Diesel fuel, being less volatile than gasoline, is less of a fire hazard. Its density, greater than gasoline’s, yields higher thermal efficiency (and, hence, lower fuel consumption). On the other hand, compressive pressures of diesel combustion require heavy components, to the detriment of aviation applications.
Diesels are inherently fuel-injected, a more complicated process than gasoline carburetion. However, fuel injection eliminated shortcomings of carburetion in aircraft applications, such as icing up and problems with inverted flight.
Junkers’ development of diesel direct injection led to similar hardware in gasoline engines. German aircraft in World War II had direct injection of their gasoline powerplants whereas many allied planes were still carbureted.
This technical lead translated into post-war automotive advantage. The 1950s’ Mercedes-Benz 300SLR sports racing car and W196 Grand Prix car featured direct gasoline injection; their competitors, Weber and SU carbs.
Diesels were deemed appropriate for dirigibles (the Hindenburg, for one), but not for heavier-than-air craft. Early versions of the Junkers Ju 86 bomber had Jumo 205s. However, the diesels were found unresponsive in World War II combat. Preferred applications were powering Blohm & Voss flying boats used for long-range maritime reconnaissance.
The Napier Deltic diesel evolved from this British company’s Culverin, a pre-war licensing of the Jumo 204. Whereas the Jumo/Culverin was a head-to-head pairing of inline-6s, the Deltic (as in the Greek delta) was a trio of Culverins arranged in an equilateral triangle.
To describe operation of a Napier Deltic, an animation [at History and design] is likely worth more than a thousand words. In particular, the Deltic has a quirk: One of its three crankshafts rotates opposite the direction of other two.
The British Admiralty envisioned the Napier Deltic as perfect for patrol boat power. Its diesel fuel is less flammable than gasoline. Diesel efficiency promotes long-range patrolling. The unit is compact and weighs one-fifth that of a conventional engine of equivalent power.
Fabricated largely of aluminum, the Deltic has a small magnetic signature, a crucial attribute in mine-searching vessels. Its characteristic buzzing sound in operation is quieter than the conventional thumps of naval power.
Deltic diesels remain in Royal Navy service in 12 vessels of the Hunt mine-countermeasure class. Other ex-British Deltic vessels continue in the naval forces of Greece and Lithuania.
The New York Fire Department had a Napier Deltic powering its Mack Super Pumper (now residing in the Antique Toy and Firehouse Museum in Bay City, Michigan. Others power locomotives on British railways. There’s even a British Deltic Preservation Society. ds
© Dennis Simanaitis, SimanaitisSays.com, 2015