German engineer who pioneered in building internal-combustion engines.

In 1864 Langen formed a partnership with Nikolaus A. Otto (Otto, Nikolaus August), with whom he collaborated for the rest of his life. In 1867 they designed their first internal-combustion engine. Later, recognizing the theoretical advantages of a four-stroke cycle, they incorporated it in their “silent engine” (patented 1877), the first operating example of the modern automobile engine. Langen also conceived the idea of an overhead suspension monorail, as put into operation in 1901 at Wuppertal, Ger.