Louis Heinrici, Germany, circa 1900 a small Stirling type hot air engine in which a body of air is worked constantly, being alternately heated and cooled during each revolution of the crankshaft. Heinrici hot air engines are of the valveless, closed cycle type, generally called Stirling cycle engines, after Robert Stirling, the Scottish Presbyterian minister who pioneered their development in the early 1800's. They operate by alternately heating and cooling a quantity of air, called the working fluid, contained in the engine's internal spaces. Heat is applied externally and passes through the cylinder wall, heating the working fluid, which is then expanded against a piston to do mechanical work. After heating and expanding, the working fluid is moved to a cool space where it cools and contracts before being returned to the hot space for the cycle to repeat. It has a displacer (just a loose piston), below and in the same cylinder as the power piston to which it is connected via cranks and linkages so as to lead by 90degrees of crankshaft angle. The displacer space and the piston space are connected by the annular gap around the loose fitting displacer so that the working fluid moves between these spaces and changes volume by the appropriate ratio as the engine rotates. Because they have no valves and experience no sudden pressure changes, Stirling engines are noted for quietness and reliability. Heinricis use air at atmospheric pressure for their working fluid, but for higher specific output (power for size) and better efficiency, modern Stirling cycle engines use pressurised gas- air, nitrogen, helium or hydrogen.Historic - Hot Air Engine - MotorHot Air Motor made of Steel with two drive wheels. a small Stirling type hot air engine in which a body of air is worked constantly, being alternately heated and cooled during each revolution of the crankshaft. Heinrici Motorheinrici hot air motor, puffing billy