The intriguing design of this engine, invented and patented in 1816 by Dr. Robert Stirling, is still relevant today. Initially sought as a safer alternative to steam engines (whose boilers could explode), this engine never saw wide spread use because it was generally very heavy and required high temperatures. The principle of the engine is simple: an external heat source and heat sink create a temperature difference in a cylinder and the air inside expands and contracts, moving a piston and a crank shaft in a rotary motion. The energy source can be any source of heat or cold, including solar or even an open flame. The Stirling engine can be more efficient and quieter in comparison to the more familiar internal combustion engine.