The Stirling Engine
Robert Stirling invented the Stirling engine in 1816. At the time he was a Scottish minister. Stirling engines were the safest engines made during that time period that would not explode like a steam engine could. The Stirling engine would not explode because the pressures could not be elevated to that to such a high level. The machine simply stopped when the heater section failed from thermal stress or imperfections in the material or manufacturing process. So from that day on there was a better, safer way to produce power, far superior than that of a steam engine.
A Stirling Engine is a mechanical device, which operates on a closed looping thermodynamic cycle. Different temperature levels cause compression and expansion of the air or steam, which causes the piston arms to move back and forth keeping up with the changes in the internal pressure. The flow of the steam is controlled by changes in the volume of the hot and cold spaces, without the use of valves.
The Stirling cycle can be is still applied to day in common appliances such as a refrigerator. The heat cycle when applied to a heat cycle can produce cool air. This will occur when work is done on the Stirling device, and the heat energy is discarded into the room. The heat energy that came from the Stirling cycle would be take from the inside of the refrigerator therefore making it cooler on the inside.
In 1876 Rev. Stirling wrote in a letter about his brother James, who had just died, “…These imperfections have been in great measure removed by time and especially by the genius of the distinguished Bessemer. If Bessemer iron or steel had been known thirty five or forty years ago there is scarce a doubt that the air engine would have been a great success…It remains for some skilled and ambitious mechanist in a future age to repeat it under favorable circumstances and with complete success….” Rev. Dr. Robert Stirling (1790-1878) from “Stirling Engines” by G. Walker
Robert Stirling applied for his first patents for this engine and the economizer in 1816, only after a few months of getting nominated as a minister at the Church of Scotland. Sir George Caley had devised air engines previous to this time and other devices called air engines were known as early as 1699. Steam engines began to carry a bad name along with it because they were so dangerous. Air engines were so safe and they operated on completely different principles. The economizer or regenerator has come to be recognized as a most important part of the patent that Robert Stirling received. This patent was so outstanding because of the fact that it predated much of the study of thermodynamics. Some historians believe that the reason for Robert Stirling s efforts at such a device were driven by his concern for the working people of his parishes as steam engines were being used extensively in that area and time period. Because of the lack of strength in the materials available to construct, they would frequently explode sending shrapnel, boiling water, and steam at the people working nearby.
After the years the gasoline internal combustion engine has taken over. The reason this occurred was because of the time it takes for a Stirling engine to heat up enough to get moving. Lately with all of the problems with the environment a need for automobile engines with low emission of toxic gases has revived interest in the Stirling engine. Some Stirling engines have been built with up to 500 horsepower and with efficiencies of 30 to 45 percent. The common internal-combustion engine would have efficiency in the range of 20 to 25 percent.
One of the fastest moving technologies is that of composites. These materials have a type of plastic make up along with other properties. The strength of this type of material is superior in nature and for the purposes of a Sterling engine, I feel that it would be perfectly suited. They have already begun to experimentally replace certain motor parts in the internal combustion engine, such as the pistons. The advantages of this would be that the weight of internal parts would be significantly lighter and therefore able to run at higher speeds. Another advantage would be that the heat developed inside the engine would have less affect on the components because composites react, (shrink and expand) less that a metal would.
Lubrication also plays a huge part in engine performance. If all parts slide together easy than the total force put into overcoming friction would be reduced. Bearings and other parts would also play a large part in reducing friction.
The design technology of sterling engines over the years has changed very slowly. Over the years the materials have slowly began to change, what was once a wood wheel is now machined out of steal. The main theory of the Sterling cycle has remained the same, but because of its incapability to be convenient when placed into a car, the Sterling engine has never become a huge success. This probably explains why the manufacturing process has never moved into a mass production or and assembly line operation of larger sized engines. The model industry that produces them, as toy is probable the only type of company that does mass-produce them. Because the parts for the Stirling engine are fairly easy to machine, the materials are common, and fairly inexpensive, an assembly line type of production would be fairly easy and inexpensive to set up.
These are the 2 basic design types; the two-piston type Stirling engine is shown to the left. A space above a hot piston is always heated and the space above a cold piston is always cooled. The displacer type Stirling engine (the one to the right) has a space above a displacer piston, and it is always heated by a heat source. A space below the displacer piston is cooled always. The displacer piston displaces hot air and cold air.
The 1900 s brought on a time of industrialization and few things were still made by hand. In this age Stirling engines could be built bye assembly lines but Henry Ford had already began production of his model T Ford, which used an internal combustion engine. This pretty much killed the need for the Stirling engine. During this these old times prototypes were almost non-existent except for that of a seldom made model. The reason this occurred is if a prototype was made it was probably put into use not just set aside whit the actual models were getting manufactured. Before this time if somebody wanted something they had to build it for him or her self, or they would have to find someone to build it for them. This explains why there were very few Sterling engines that were similar back then. Most parts were built by hand which explains the wooden flywheel. Because the cost of the part presently used to make it are so cheep and the parts are adequate to the demands of this engine.
If I was the designer of the Stirling engine and I had the technology of today to help me in my work I would have to make it out of composite products or at least a good portion of it. The Sterling engine in itself if very safe and the simple facts that a cleaner fuel that is being burned will also reduce emissions making it a better engine for the environment. To increase power and make it run faster I would add piston rings to the piston to give it a tighter seal. To reduce friction in the flywheel I would make sure bearings were placer on the axle and make sure they were oiled properly. Another was to make the engine move faster would be to add some wings or flattened metal sheets to the cool piston case in order to increase surface area allowing it to cool even faster.