Simanaitis Says

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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.


An animated look at two-stroke operation.

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.


Jumo 205. Image from The Lore of Flight, Crescent Books, 1978.

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.


Blohm & Voss BV 138 Seedrache (Sea Dragon), powered by a trio of Jumo 205D diesels.

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.


Complex though it appears, the Napier Deltic provided effective power for naval vessels and, in time, railway and other duties. Image from

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.


HMS Ledbury M30, Royal Navy mine-countermeasures vessel with Napier Deltic power. Image by Brian Burnell from Wikimedia OTRS.

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,, 2015


  1. toby tuttle
    March 26, 2015

    Great story,Dennis. Have you ever looked at the Rootes Deisel? It powered some fantastic large vehicles, including the Ecurie Ecosse race transporter.

  2. mike b
    March 28, 2015

    Another opposed-piston diesel was the Fairbanks-Morse that was developed for submarine use in WW1 and later was used for many other things including locomotives. The F-M Trainmaster was the first “high horsepower” locomotive, producing 2400 hp from a single engine, without turbos, at a time when the main manufacturers (GM/EMD and Alco) needed two engines to get there. Unfortunately, they had some reliability problems (eventually fixed) and were a small fish in the pool; once GM, Alco, and by then GE came up with 2K+ hp using turbos FM was finished as a locomotive builder.

    You might want to look, also, at how GM/EMD did turbos on their 2-cycle locomotive diesels. They needed a supercharger just to work (2-cycle y’know), but IIRC instead of adding a turbo to the existing super they geared the turbo to the engine with an overrunning clutch. So at low speeds it acted as a normal supercharger, but when enough exhaust flow was available the turbo added power. One result was that a properly functioning GM diesel didn’t produce a lot of smoke, unlike the (4-cycle) Alco and GE turbo engines that were “honorary steam engines” until the turbo spooled up.

  3. Bill Pitcher
    August 17, 2017

    Interesting article Dennis. I’m researching the Leyland L60 engine and I’ve compared cutaway diagrams of the L60 and Junkers Jumo 205 and found them to be very similar. Interestingly Napier had a license to manufacture the Jumo 205 but as far as I can tell never manufactured any.
    I know Leyland and Napier had links so I’m trying to establish if Napier passed on any information to Leyland in the way of specifications or drawings that might have influenced the design of the L60.

    • simanaitissays
      August 17, 2017

      Sounds like a neat project. Good luck in your research. I wonder if the people at the Brooklands Museum (where the Napier-Railton resides) might have something in their archives?

      • Bill Pitcher
        August 17, 2017

        Hi, I’ve recently contacted the Napier Power Heritage Trust and I’m hopeful for more information, if I find anything out I’ll let you know.
        I’m a member of The Leyland Society and the research is for an article in the Society journal.

  4. John Dabrowski
    January 28, 2019

    The JUMO 205 crankshaft does not drive the propeller shaft,
    there is a larger gear wheel below it,acting as a reduction gear
    to reduce propeller RPM, as in most large aircraft engines.

    John Dabrowski

  5. Nicholas Baker
    March 15, 2023

    My father worked at Napiers on the design of a small diesel aircraft engine and one of his colleagues had worked on the Napier Deltic. Many years later, I worked for a company in Austria, where I was asked to develop the concept of a 2-stroke diesel aircraft engine. I was given the drawings of the Junkers Jumo cylinder to use as a basis, and through research, I realised that the Deltic cylinder was an exact copy, and now I was using it in the design of an axial engine that employed five cylinders arranged around a central crankshaft, with 10 pistons driving a swash plate at either end to drive the crankshaft.
    Unfortunately, the engine I was working on never came to anything. Partly because the recession of 2008/9 caught up with the company, but there were some major hurdles to overcome such as lubrication and cooling. We had to revise the cylinders to accept much bigger cooling channels as the engine casing was cast Aluminium, and revised inlet and exhaust ports.
    Quite a coincidence with my dad’s work though.

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