Douglass DC-6 has piston engines. SEE diagram.

Piston engines, also known as reciprocating engines, are descendents of steam engines which first appeared in the early 17th century.  In simple terms, steam engines use heat to produce steam pressure, which in turn is used to generate rotating motion.
Steam engines were eventually used to drive everything from mechanical equipment to the first railway trains.  In today’s aircraft piston engines, gas propulsion replaces steam in creating pressure within the engine.