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Also known as aero diesel, an aircraft diesel engine is a type of aircraft engine that runs on diesel fuel. While there have been a few occasions in which it has been employed, aircraft diesel engines have never seen popular use. The engine was never widely used due to a variety of factors that favored gasoline-fueled aircraft engines, which mainly involved the low cost and high availability of gasoline. The aero diesel was most commonly used with airships, or dirigibles, where its superior fuel economy made it necessary for long-distance voyages. Due to a steady decrease in the availability of gasoline, however, diesel engines have been reconsidered for use in modern aircraft.
In the 1920s and 30s, a variety of companies manufactured diesel engines for aircraft. One of the most notable was the Packard radial diesel, which was manufactured from 1928 to 1929. This engine was used in the Simon SM-IDX "Detroiter," which made the first diesel-fueled flight on September 18th, 1928. Another fixed-wing aircraft that employed an aircraft diesel engine was Bloom & Voss Ha 139 floatplane, whose power plant consisted of four Junkers Juno 205 diesel engines. All other fixed-wing aircraft that used diesel engines, such as the Soviet bomber Petlyakov Pe-8, eventually adopted gasoline engines.
Despite its lack of popularity among fixed-wing aircraft, the aircraft diesel engine was widely used in airships. The most significant benefit for using these engines for airships was the low flammability of the fuel; despite still being flammable, its chance of catching fire was far less than that of gasoline. Since almost all airships at the time used hydrogen, which is extremely flammable, as a lifting gas, reduced fuel flammability was a great benefit. Airships such as the LZ 129 Hindenburg and the LZ 130 Graf Zeppelin II employed diesel engines. These engines could be put into reverse by changing the gears on the camshaft, allowing for an engine that not only had a low chance of catching fire, but was also very maneuverable.
After the decline of airship use, the aircraft diesel engine became even more uncommon. Modern necessity, however, has fueled its resurgence; with the increasing price of aircraft fuels, such as a avgas, aero diesels have undergone modern development. Germany has led the way in the development in modern aircraft diesel engines, offering some models which can use both diesel and aircraft fuel. Other countries, most notably France and the United States, have also been involved with the development of modern diesel aircraft engines.