Heat engines have been known since antiquity but were only made into useful devices at the time of the industrial revolution in the eighteenth century. They continue to be developed today.
In engineering and thermodynamics, a heat engine performs the conversion of heat energy to mechanical work by exploiting the temperature gradient between a hot "source" and a cold "sink". Heat is transferred to the sink from the source, and in this process some of the heat is converted into work.
A heat pump is a heat engine run in reverse. Work is used to create a heat differential.
1769 - James Watt patents his first improved steam engine
1787 - Jacques Charles formulates Charles's law which describes the relationship between as gas's volume and temperature. He does not publish this however and it is not recognised until Joseph Louis Gay-Lussac develops and references it in 1802.
1799 - Richard Trevithick builds the first high pressure steam engine.
1802 - Joseph Louis Gay-Lussac develops his law which describes the relationship between a gas's pressure and temperature.
1834 - Jacob Perkins, obtained the first patent for a vapor-compression refrigeration system.
1850s - Rudolf Clausius sets out the concept of the thermodynamic system and positioned entropy as being that in any irreversible process a small amount of heat energy δQ is incrementally dissipated across the system boundary
1859 - Etienne Lenoir developed the first internal combustion engine, a single-cylinder, two-stroke engine with electric ignition of illumination gas (not gasoline).
1877 - Theorist Ludwig Boltzmann visualized a probabilistic way to measure the entropy of an ensemble of ideal gas particles, in which he defined entropy to be proportional to the logarithm of the number of microstates such a gas could occupy.
1877 - Nikolaus Otto patents a four-stroke internal combustion engine (U.S. Patent 194,047)
1884 - Charles A. Parsons builds the first modern Steam turbine.
1886 - Herbert Akroyd Stuart builds the prototype Hot bulb engine, an oil fueled Homogeneous Charge Compression Ignition engine similar to the later diesel but with a lower compression ratio and running on a fuel air mixture.
1892 - Rudolf Diesel patents the Diesel engine (U.S. Patent 608,845) where a high compression ratio generates hot gas which then ignites an injected fuel.
1909, the Dutch physicist Heike Kamerlingh Onnes develops the concept of enthalpy for the measure of the "useful" work that can be obtained from a closed thermodynamic system at a constant pressure.
1929 - Felix Wankel patents the Wankel rotary engine (U.S. Patent 2,988,008)
1937 - Hans von Ohain builds a gas turbine
1950s - The Philips company develop the Stirling-cycle Stirling cryocooler which converts mechanical energy to a temperature difference.
Timeline of rocket and missile technology - Rockets can be considered to be heat engines. The heat of their exhaust gases is converted into mechanical energy.