Citroën engines for light aircraft were converted from horizontally-opposed automobile engines built by Citroën in the 1940s through the 1970s. André Citroën (1878-1935) had his own engineering firm from 1902 and before WWI also worked for the Mors automobile company in France. During WWI, he interested the French government in his ideas for mass production of munitions and built a large plant at quai de Javel in Paris, France to accomplish this. After WWI he converted the plant to produce automobiles. The Citroën company still exists as a part of the Peugeot firm. A brief outline of Citroën history can be found here. The only known (to the compiler) US conversion of the 2CV engine below was by Joe Horvath of Revmaster to a requirement from Moulton Taylor for Taylor's Micro-IMP ultralight.
O-37 -- {N/A / N/A / 37} / {N/A / N/A / 602}
2cyl; Citroën 2CV; 18hp@N/A rpm; 1960-1985; Wt = N/A.
Largest of the 2CV engines and known to have been converted for aircraft; earlier 2CV engines had 22.9 cu. in. / 375 cc, 25.9 / 424, and 26.5 / 435 displacements. The Taylor Micro-IMP was underpowered with the O-37 engine, so Revmaster modified it to increase the displacement to 48.8 cu. in. / 800 cc, but could not increase performance beyond 20hp. Taylor selected other non-HOAE for his aircraft.
BGP; J81-82toJ85-86; SA9/80, 10/80.
Applications: (Argentina) Arrambide/Marino Armarl Gorrion parasol monoplane. (US) Moulton Taylor (Aerocar, Inc.) Micro-IMP [N9HT].
O-75 -- {3.031 / 2.583 / 74.6} / {77 / 65.6 / 1222}
4cyl; Citroën GS 1.2; 65hp@5700rpm; 1973-1983; Wt = 220#.
Geared to 0.5 and adapted to drive two pusher propellers; this was one of the engines developed for the Citroën GS automobiles; other GS engines of the same configuration had 61.9 cu. in. / 1015 cc, 68.9 / 1129, 79.3 / 1299, and 121.4 / 1990 displacements. Starck, too, found that his aircraft was underpowered with this engine and turned to a converted Porsche engine of 100hp.
J78-79toJ82-83; SA7/75, 8/76.
Applications: (France) Starck A.S. 37A [F-WYBQ], A.S. 37B [F-WXDU].