When World War I started the following year the
Oberursel U.I of 100 hp, a licensed copy of the
Gnome Delta 100 hp (75 kW) rotary, had the best
power-to-weight ratio of any German engine. It went on to power most of the early German fighters, such as the Fokker and Pfalz E-series monoplanes. Oberursel also built a licensed copy of Gnome's 14-cylinder
Double Lambda two-row rotary. This 160 hp (120 kW) engine, designated U.III in Germany, was difficult to build and quickly wore out in service. It was used on the Fokker E.IV and D.III designs. The 110 hp
Oberursel UR.II, the licensed copy of the
Le Rhône 9J of the same power output, was the next major success.
Fokker bought the company in 1916 in order to guarantee supplies of the UR.II. This acquisition proved advantageous because Fokker was partial to rotary powered designs, and because supplies of the
Mercedes D.III engine were limited. The UR.II was used in the
Fokker Dr.I and
Fokker D.VI. By 1917, the UR.II had been rendered obsolete by its relatively low power and poor performance at altitude. An 11-cylinder development, the UR.III, was not used operationally. Indeed, by 1918, rotary engines had largely fallen from favor with the
Idflieg and with pilots. The lack of
castor oil and the poor quality of the mineral oil substitute "Voltol" severely reduced engine life and reliability. Nevertheless, in the summer of 1918, the UR.II was installed in the
Fokker D.VIII. The light weight and aerodynamic cleanliness of the D.VIII allowed it to achieve excellent performance even with the outdated UR.II. After the war the company was purchased in 1921 by
Gasmotorenfabrik Deutz, another gasoline engine manufacturer, who moved their
two-stroke diesel manufacturing to the Oberursel factories. In 1930 they merged with Humboldt-Deutz, but with only one product line. The factory was eventually closed in 1932 during the
Great Depression, reopening in 1934 for small-scale production. ==World War II==