Philips generator-set with hot-air engine, c. 1950. Philips' research on Stirling engines began about1938. Initially, the work was aimed at development of small thermal-power electric generators for radios and similar equipment, for use in remote areas where storage batteries were not available but kerosene or other liquid fuel might be had. Improvements in radio valves, batteries and the invention of the transistor later removed the need for these small generating sets. 400 pre-production machines were made.