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"Van Arsdell's clever toy belongs to a diverse class of heat engines called air engines, or, more commonly, Stirling engines, after a Scots clergyman, Robert Stirling, who filed the first patent for the type in 1816. Actually, rudimentary Stirling engines existed before Stirling, going back to the late 1600's; but Stirling added a refinement that he called an 'economizer'. It is now called a 'regenerator', and it increases the engine's yield of work for a given input of heat increases, in other words, what we call its 'efficiency'. A regenerator is now a standard part of every Stirling engine design." Like the gasoline, diesel, and jet engines with which we are all familiar, the Stirling is a heat engine; that is, an engine that derives its power from heat. But unlike those other engines, the Stirling obtains its heat from outside, rather than inside, the working cylinders. In this respect, the Stirling is similar to that charming old workhorse of the industrial revolution, the steam engine.
This difference, between external and internal combustion, is one of the main reasons for the widespread current interest in the Stirling. An internal combustion engine, that is an engine that burns its fuel inside its working cylinders or chambers like the gasoline, diesel, or gas turbine engine, is generally rather particular about its fuel. A gasoline engine may be modified to run on hydrogen, methane, or propane; but it will not run on salad oil, straw, coal, or peat. When one thinks of a small gasoline engine, one tends to think of it as a self-contained little powerpak; perhaps it would be more appropriate to think of it as a power plant with an oil refinery attached to it!
Of the three internal combustion engines mentioned, the gas turbine is probably the most omnivorous with respect to fuel; yet even it is limited indeed when compared to the Stirling. Quite literally, any source of heat, as long as its temperature is high enough, will do to power a Stirling. This last statement is no doubt true of any other externally heated engine, like the steam engine, but interest has focused on the Stirling as the externally heated engine of choice because it holds the promise of making the most power for a given supply of heat of the practical alternatives presently known.
Thus, the Stirling can directly use concentrated solar energy, or it can burn kerosene, coal, straw, wood, sawdust, cardboard, discarded Christmas trees, or any other combustible substance imaginable.
It can also use stored heat. Certain salts give off great amounts of heat for their weight in the process of cooling from their liquid to their solid state. With a salt such a lithium fluoride, this change of state occurs at a high temperature, in the neighborhood of 1550°F, which is an excellent temperature for the hot end of a Stirling engine. When space-age techniques of thermal insulation are employed in conjunction with these salts, it has been found that a very good thermal storage battery can be made from which to run a Stirling engine. This is similar to the electric motor-storage battery system, except heat is stored instead of electricity. The thermal battery-Stirling system is superior, however, in that it can be recharged (electrically reheated) much faster and it can store over eight times more shaft power per pound than a lead-acid battery.
But even when it is burning a conventional fuel, such as gasoline or diesel fuel, a Stirling has an important advantage resulting from its basic design. That is, its combustion takes place against hot surroundings at atmospheric pressure, and not against cooled cylinder walls at elevated pressures as is the case in the gasoline engine. Combustion under the conditions in the Stirling produces practically no carbon monoxide or unburned hydrocarbons, and the level of nitrogen oxides is also relatively low. In terms of these three pollutants, the Stirling is about the cleanest heat engine imaginable.