The Junkers L1 was the first engine manufactured by Junkers to fly. It was an air-cooled, upright six-cylinder inline four-stroke petrol engine only produced in small numbers and largely used for research, but led to the successful L5 and its V-12 development, the L55.

Hugo Junkers' early engineering experience was with stationary opposed-piston two-stroke diesel engines for industrial applications and this arrangement was eventually adapted for aircraft use. Nonetheless, his company's first aero engine was a petrol-fuelled four-stroke, the six-cylinder inline air-cooled L1. L was Junkers' notation for petrol engines from the L1 to the L10, which became the Jumo 210 in 1931.^[1] It first ran in 1921^[2] and was the subject of much static testing, but the intention was always to produce a flight engine. The first aircraft to test fly the L1 was the Junkers T 19; this aircraft first flew in 1922, but the date of its first flight with the L1 is uncertain.^[1]

Notable features were the four large valves per cylinder, two inlet and two exhaust; the ball race main bearings; and the double ignition system, with twin magnetos and two sparking plugs per cylinder.^[1]

The L1 was largely an experimental engine, but a small production line was set up in 1925. Reliability was not high, however and only a few aircraft, themselves built only in small numbers, used the L1 and its variants.^[1] The large diameter, circular blower fitted to the L1a resulted in a flat fronted, circular cross section cowling, particularly noticeable on the Junkers T 19 and 26.^[3]

• L1 Original version.
• L1a Larger bore diameter and a large circular blower, gear driven, mounted at the front of the engine to enhance high altitude power.
• L1b

• Focke-Wulf A 16b
• Junkers T 19
• Junkers T 26
• Junkers J 29

Data from Kay (2004) p. 263

• Type: 6-cylinder air-cooled upright 4-stroke
• Bore: 100 mm (3.9 in)
• Stroke: 120 mm (4.7 in)
• Displacement: 5.65 L (345 cu in)
• Length: 1.115 m (3 ft 7.9 in)
• Width: 700 mm (2 ft 4 in)
• Height: 790 mm (2 ft 7 in)
• Dry weight: 128 kg (282 lb)

• Valvetrain: two large inlet and two large exhaust valves per cylinder, operated by overhead camshaft
• Fuel type: petrol
• Cooling system: air-cooled; finned cylinder heads
• Reduction gear: none:direct drive
• Ignition system: twin magnetos and two sparking plugs per cylinder

• Power output: cruise 75 hp (56 kW); take-off 85 hp (63 kW)
• Fuel consumption: 26 kg/h (44 lb/hr)

• Gunston, Bill (2006). World Encyclopedia of Aero Engines: From the Pioneers to the Present Day (5th ed.). Stroud, UK: Sutton. ISBN 0-7509-4479-X.
• Kay, Antony (2004). Junkers Aircraft & Engines 1913–1945. London: Putnam Aeronautical Books. ISBN 0-85177-985-9.

• Zuerl, Walter (1941). Deutsche Flugzeug Konstrukteure. München, Germany: Curt Pechstein Verlag.