It is thought that the first true hot air engines were devised by Sir George Caley around 1807, but the first successful engine which could perform useful work was the Stirling Cycle engine devised by the Reverend Robert Stirling in 1816.
The Reverend was concerned that at the time many people in his parish used steam engines for pumping and other work. In the early 1800's the steam engine was considerably more dangerous than later times when materials and engineering had improved and working rules were strictly enforced. In 1816 he registered a patent for the an engine and the "Economiser". This was the first of many. The "Economiser" was probably the most important part of the application as it was what made his designs different from his predecessors.
Robert's principle introduced a degree of safety in the workplace which was not possible with the steam engine of the time. When a stirling engine fails it may destroy some part of itself, but because there is no extreme working pressure involved it is very unlikely that it would hurt anyone surrounding the machine.
The Reverend's brother James Stirling implemented many of his ideas at the Dundee Foundry Company. James most successful implementation was a two cylinder dual heat regenerator engine which was used to power the foundry for some years until material failure damaged it and it was replaced by a steam engine. The achievements of the Robert and James Stirling were particularly impressive considering that at the time neither steel had been invented meaning they had to work with soft iron. Of course, this was the major cause of steam engine and boiler explosions which led the brothers to their work. The engines developed by James typically had a working life of 20 years or so before major repair or replacement was necessary.
The Stirling engines were best known for their ability to burn almost anything as fuel, their quiet operation, low maintenance, low operating costs and safety. Watching an engine pumping water when the only sound you can here is the water flowing in the pipes is very impressive. The most common failure of engines of this style were due to the breakdown of iron in the heater surface. This same problem also affects steam engines where the fire box and fire tubes have to be renewed from time to time due to ablation.
In the 1850s M. Sadi Carnot developed the principles of thermodynamics and devised the Carnot Cycle Calorific Engine. Carnot's principle was that the energy to do work was directly available from the input of heat (or cold) into a closed system without using/consuming the working medium. This same principle is used today in the recent developments which have provided engines capable of producing sufficient horsepower to drive a generator or operate a car.
The Stirling engines were in common use from the times of the Stirling brothers up until the 1920s where internal combustion engines and electric motors made them redundant. At that time these power sources could develop much more working power than the stirlings of the same time period. The Stirling engine required close attention to manufacturing as the tolerances are finer than those required for internal combustion engines. The combination of cheaper manufacturing and higher power of internal combustion caused the Stirling engine to disappear from the commercial marketplace.
The basic principle of the heat engine has not really changed since its time of inception, todays engines use much more advanced materials and engineering methods. For example it is now possible to build a device with almost zero friction between the working surfaces, had this been possible when the principle was first devised, it is quite possible the steam engine would not have been the driving force of the industrial revolution.
After World War II the invention of high grade stainless steels, and an increased understanding of the higher mathematics which explain the operation of the Stirling cycle caused development of a cheaper more efficient engine. When the engine technology improved, its simplicity of using any combustible fuel available started to generate interest in the principles again.
Todays Stirling engine reaches efficiencies of up to 38% and are available in configurations suitable for modern power requirements. One of the most impressive I know of is a waste oil burning car engine which develops 240hp from a four cylinder configuration. Experimental engines have been built in the model engineering world with efficiencies approaching 50%. Considering that the most efficient internal combustion engines are only about 20% efficient these are major strides forward.
Although there have been many forward developments in recent times, it is only now that "real" money is being pumped into the concept as the worlds energy reserves dwindle and it becomes more politically fashionable to support "the cause".
Because the Stirling engine is external combustion and requires no replenishment it is suitable for low resource areas of the world where work needs to be done. In the 1980's for example a United States organisation USAID sponsored the development of an engine design which could be manufactured and operated using the resources of the third world. This engine was successfully completed and implemented into Bangladesh where copies of the engine were locally made and used.
A very simple engine using air as the working medium can be built from simple tin cans found around the home, while this version cannot do much in the way of real work, the principle is exactly the same in its larger cousins. The Stirling engine offers real possibilities for developing areas of the world formerly ignored or under utilised. In the Australian outback the Stirling cycle engine is being used to generate electrical power or used directly for useful work.