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Among the most successful early engines marketed in the United States were those designed and built by aviation pioneer and inventor Glenn Curtiss. Early Curtiss engines were designed to power motorcycles. The first Curtiss aircraft engines were air cooled but, to achieve higher power, Curtiss began to develop liquid-cooled engines. With the advent of World War I, Curtiss concentrated its production on the famous 67 kW (90 hp) OX-5 engine which powered the JN-4D “Jenny” trainer aircraft. The OXX-6 engine was an improved version of the OX-5. Differences from the OX-5 included two magnetos (instead of one) for greater dependability and a slightly larger cylinder bore giving an additional 7.5 kW (10 horsepower). During World War I, the OXX-6 was used on such Curtiss aircraft as the Model N9 hydroplane. Together, the OX-5 and OXX-6 were the most extensively used American-built engines during World War I and in the following decade.