Fredrik Ljungström (16 June 1875 in Stockholm – 18 February 1964 in Stockholm) was a Swedish engineer, technical designer, and industrialist.
Considered one of the foremost inventors of Sweden, Fredrik Ljungström accounted for hundreds of technical patents alone and in collaboration with his brother Birger Ljungström (1872–1948): from early bicycling free wheeling hubs techniques and mechanical automatic transmissions for vehicles, to steam turbines, air preheaters, and circular arc hulls for sailing boats. He co-founded companies such as The New Cycle Company, Ljungström Steam Turbine Co. and Swedish Turbine Manufacturing Co. (STAL), and associated with other industrialists such as Helge Palmcrantz, Alfred Nobel, Gustaf de Laval, Curt Nicolin, and Gustaf Dalén. As innovative as his ideas were in function, they also often turned out in terms of unconventional external design, such as his steam turbine locomotives and sailboats.
During the resource scarcity of World War II, Fredrik Ljungström's innovative technology for oil shale underground gasification by electrical energy, called the Ljungström method, provided a strategical impact for the Swedish Armed Forces. In addition, Ljungström's technology contributed to the first Swedish jet engine, torpedoes, and more.
With Fredrik Ljungström's air preheater implemented in a large number of modern power stations around the world until this day with total attributed worldwide fuel savings estimated to 4,960,000,000 tons of oil, "few inventions have been as successful in saving fuel as the Ljungström Air Preheater". In 1995, the Ljungström air preheater was distinguished as the 44th International Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers. His works are represented by the Swedish National Museum of Science and Technology, the Nordic Museum, and the Swedish Railway Museum, as well as internationally such as by the Science Museum in London, England and by The science museum in Milan.