FLYING DIESELS

Whether you’ve seen them in person or not, you know there are diesel in ships and boats, road vehicles of all types, off road vehicles of all types and in every conceivable stationary use. If you are a long time reader of this column, you will also know there were, and still are, diesels in the air.

In the January, 2018 issue, we covered the Guiberson aircraft diesel story. In the September 2018 issue, we covered the earlier Packard diesel (see it online at www.dieselworldmag.com/diesel-engines/first-in-flight/). Those were the two main American contributions but the Europeans had many more. Let’s round up some of the other more important aircraft diesels we found and tell you a little bit about them. Along the way, you’ll learn what was good about them and what wasn’t.

A period pic from about 1935 showing an LOF-6 being assembled. Each cylinder was individual and protruded deeply into the block. There were four valves per cylinder with a compression ratio of 16:1. You can see the four Bosch Type A injection pumps, two for each bank. The injection pressure is listed at a low 1600 psi. The LOF-6 had coolant and oil heaters to facilitate starting in cold weather or starting at cold high altitudes.

1933-46 Junkers Jumo

Dr. Hugo Junkers, 1859-1935, may be one of the most unrecognized diesel geniuses of all time. A German, he was the first to develop a diesel engine specifically for aircraft in 1913 but his genius did not end at engines. He was also an aeronautical engineer, visionary and pioneer in the development of aircraft. He proved to be a good man as well, taking a stand against Fascism and refusing to develop aircraft for the Nazi regime. As a result, was threatened with prison for treason and died under house arrest in 1935. His company and patents were all effectively nationalized.

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There were three production variations of the basic six-cylinder Jumo 205, the C, D and E. All displaced 1,014 cubic inches from 4.13 x 6.30 inch bore and strokes, two pistons per cylinder, two-stroke, direct injected with 17:1 compression ratios. Power ranged from 600-880 horsepower at up to 3000 rpm across the different variants. The most common version, the 205C, was rated at a maximum of 600 horsepower at 2200 rpm. They used a geared-up centrifugal blower that supplied a little positive boost on top of basic two-stroke airflow needs (about 5 psi). Each cylinder had two separate injection pumps (one on each side of the engine) and four injectors, two on each side of the engine. Injection pressure was 8,100 psi… high for the era. Fuel consumption at 1600 rpm cruising speed was 23 GPH. At full power it was about 31 GPH. Approximately 900 Jumo 205 engines were reportedly built into the beginning of World War II.

His first experimental aircraft engines came in 1913-15 and shared a recurring technical theme for Junkers, opposed pistons. That’s two crankshafts connected together and two pistons facing a central combustion chamber. When the cylinder fires, two pistons react. The first Junkers engines were technically oil engines and even had spark ignition to assist combustion but later engines were true compression ignition. All were two strokes. Opposed piston engines are an idea that delivered mixed results over the years but did see notable success by Fairbanks-Morse as a submarine diesel.

At over 1,300 pounds, the engine was no lightweight but it had very little frontal area, and allowed for a very streamlined nacelle or cowling. A major downside to these engines was rough running, making them hard on airframes. That was partly due to being unable to precisely time each piston in each cylinder and also because one crankshaft carried more of a parasitical load than the other. A turbocharged Jumo 207 variant was built in small numbers that made over 1,000 horsepower. Frank Schwichtenberg via Wikipedia Commons

The power and fuel consumption curves of the LOF-6 show it was extremely economical for an engine it’s size. At the 900 horsepower, 1300 rpm speed, fuel consumption was about 46 gallons per hour. At that 900 horsepower setting, the four LOF-6 engines on Hindenburg could push the 800-foot long airship at 78 mph. That made for a pretty fast trip across the Atlantic by 1930s standards. There were no non-stop Trans-Atlantic commercial airline flights in those days, the first happening only in 1938.

The early Junkers test engines culminated in the FO-3 5-cylinder in 1926 that made 830 horsepower and was a true diesel. This engine became the basis for what would become the legendary Jumo line. The final push to production started in 1928 with the FO-4 six-cylinder that made about 750 horsepower and these would be the first in the series to actually fly in 1929. The development progressed rapidly through several other test engines (that also flew) and culminated in the Jumo 205 that debuted in 1932, was approved for aviation and went into production.

One of the Jumo 205C recipients were the Blohm & Voss Ha-139, a four-engine floatplane that mount four diesels. They were mainly used by Lufthansa on the long routes carrying mail in the late 1930s. They could fly almost 3,000 miles at 150 mph and had a top speed of 180 mph. Only three were built, plus prototypes. At the beginning of the war, the Luftwaffe took them over.

Even after the Jumo 205 went into production, experimentation continued. The most spectacular prototypes were the Jumo 223 and 224, both 24 cylinder engines, the 223 displacing 1,767 cubic inches and the 224 making 4,150 cubic inches. The 223 cranked out a maximum of 2,380 horsepower at 4200 rpm. The 224 made 6400 horsepower at 3000 rpm from 4150 cubic inches. Because the Dessau, Germany, Junkers factory fell into the Soviet Zone, the Russians captured the factory at the end of World War II and conscripted some of the Junkers engineers to continue with the 224 development. That ceased in the summer of 1948 but some 205 variants were produced during that time.

**1927-34 Bristol Phoenix Radial Diesel** Bristol Aeroplane Company started in 1910 and became legendary for numerous well-regarded aircraft but also for aircraft engines. In 1927, they started development of a diesel version of their Jupiter VIII F radial and called it the Phoenix. The first one ran in 1928 but the firsts flight took place in 1933. The 430 horsepower at 2000 rpm, naturally aspirated Phoenix was tested against the naturally aspirated Jupiter in the same aircraft. The Jupiter made only 10 more horsepower so performance was comparable. It was discovered the Phoenix made more power than the Jupiter above 13,000 feet. The big test came on May 11, 1934 when the Phoenix set the diesel aircraft engine altitude record. After the first flights in 1933, Bristol added a supercharger and the Phoenix powered a Westland Wapati test aircraft to 27,453 feet, a record that would stand for a decade. Subsequent testing with the supercharged versions yielded a maximum of 635 horsepower at 2100 rpm. Against the Jupiter, the updated Phoenix had a 15 percent rate-of climb advantage, a 4 percent increase in top speed and 35 percent better fuel economy. Only test engines were built and available sources do not yield exactly why development stopped shortly before World War II. A good guess would be the start of WWII and the need to prioritize development and production.

Only a couple of LOF-6 engines survive, this one in the Zeppelin Museum at Friedrichshafen, Germany. Not many of these were built, though they were designed and produced on the premise that German airship production would increase. The tragic Hindenburg disaster put an end to that, not to mention World War II. Stahlkocher via Wikipedia Commons

Though the Jumo diesels are legendary and mythical, they didn’t see a great deal of service in aircraft. A small number of different German aircraft used them, mostly seaplanes, but some of those were later converted to conventional gasoline engines. During World War II, a shortage of diesels led certain small patrol boats to be built with marineized Jumo engines. Jumo engines were used postwar into the ‘60s by the East German Navy to power small patrol boats as well.