four-stroke

The term "four-stroke" is a modern technical compound, transparently built from two foundational English words. "Four" descends from the ancient Proto-Indo-European numeral kʷetwóres, which evolved into the Germanic fedwōr and eventually Old English fēower. "Stroke" originates from the Old English verb strācian, meaning to pass the hand gently over a surface, which broadened to signify any single, complete movement or action. In mechanical engineering, this concept of a "stroke" was applied to the linear motion of a piston. The compound thus logically and literally describes an engine operating cycle defined by four such distinct piston movements, a principle patented by Nikolaus Otto in 1876, which became the dominant design for gasoline engines.

Structure: four (number) + stroke (action) - four: From Old English fēower, of Proto-Germanic origin; related to Dutch vier and German vier, from an Indo-European root shared by Latin quattuor and Greek tessares. Functions as a numerical quantifier. - stroke: From Old English strācian ("to stroke"), of Germanic origin. In this compound, it refers to a single complete movement of a piston in an engine.