Source: an article from Scientific American, Vol. IV, n°14 - Part 1
Date: April 6, 1861
Title: Important Discussion on the Air Engine
In 1861, one month after having written the excellent article about the Wilcox Hot Air Engine, the Scientific American trackbacks the air engine history in publishing the following paper about the Stirling engine of 1840. It gives a structured summary of a lecture done by Patrick Stirling, son of Robert Stirling the co-inventor of the below described engine together with his brother James Stirling.
Fifteen years before, James Stirling already did a presentation of the 1840 engine before the same Institution of Civil Engineers. Why do again a presentation about the same engine so many years later, knowing that the engine was discontinued 3 years after it start? Moreover the time elapsed between 1841 and 1861 did not show any change in the Stirling technology, meanwhile quite a few inventors brought up improvements on air engines.
The context was feverish and Ericsson, the great competitor for inventing a working air engine, at last succeeded in building one, that was then successfully sold to the industry. Ericsson patented his new engine in 1858, a substantially different than Stirling's one, two years before this 1861 lecture.
Stirling, along with several erudite scientists like Rankine, Thomson, genuinely thought that his engine just needs some ajustment. Being a forerunner, he might have wanted his technology to received a deserved recognition. But, although being the third and last attempt of Stirling, this engine has again been failure ; Stirling being unable to solve the cracking, due to overheating, of the heaters that weighed about four tons each.
No other Stirling air engine came up afterwards.
At the third meeting of the session of the Institution of Engineers in Scotland, held in the Philosophical Society’s Hall, in Glasgow, on Wednesday, 26th December, 1860, the President in the chair, the following paper was read by Mr. Patrick Stirling.
The subject of this paper may require some apology for being introduced at this time; but at a recent meeting of this institution there was one of Mr. Ericsson’s air engines exhibited and explained, without any account of its performance as to power, consumption of fuel, etc., being given; and it has been considered that a description and statement of the performance of Stirling’s air engine might be interesting to members of the institution.
The engine forming the subject of this paper was constructed by Mr. James Stirling, at the Dundee Foundry, in 1842, for the purpose of driving the machinery there, and was erected in room of the steam engine, by removing the boiler, cylinder, air pump and condenser, and making use of as many of the parts of the steam engine as could be made available, which will account for the apparent want of arrangement of the different parts of the engine.
In this engine, which is represented in the engraving, there were two strong air-tight vessels A A, connected by passages with the opposite ends of the working cylinder B, in which last was a piston of the ordinary construction used in the steam engine. The lower ends of the air vessels were kept at a high temperature by a furnace which was common to both, and the upper ends of the vessels were kept from accumulating heat by a series of water pipes, through which there was a constant flow of water.
In each of these vessels there was an air-tight vessel or plunger filled with a non-conducting substance, such as pounded bricks, to prevent the radiation of heat. These plungers were slung to the opposite ends of a lever, and were capable of being moved up and down in the interior of the air vessels, and their use was to shift a body of air from the hot ends of the vessels to the cold ends alternately, and in such a manner that the quantity in one would be at the hot end whilst that in the other was at the cold end.
If we consider, then, that the movements of the air engine depend upon the well-known principle in pneumatics that air has its bulk or pressure increased when it is heated and decreased when it is cooled, there will not be much difficulty in understanding that the movement of the plungers up and down will cause a pressure to be exerted on the opposite sides of the piston alternately; and upon the difference of pressure obtained on the opposite sides of the piston depends the power of the engine.
It may be mentioned that the plungers were moved by an eccentric or crank on the crankshaft of the engine, in the same way as the slide valve of a steam engine, and at nearly the same angle to the crank.
This engine was made to work on the high pressure principle, as it was found that engines working at the simple atmospheric pressure gave so little power in proportion to their size as to render them unfit for practical use. It was found necessary, therefore, to apply a double-acting air-pump for the purpose of increasing the density of the air in the air vessels, and the usual minimum pressure was ten atmospheres, which, on being thrown to the hot end of the air vessels, was converted into a pressure of fifteen and a half atmospheres by the addition of heat.
The difference, then, in the pressure of the air when hot and cold constituted the disposable pressure upon the piston for the purpose of producing power.
When the pump had got up the full working pressure in the engine, the air, instead of being blown off, was allowed to pass into an air-tight magazine, where a sufficient quantity was kept overnight to fill the engine up to full pressure at starting in the morning, and this done, the suction valves of the pump were nearly closed together, the leakage of the engine being so small that scarcely any addition of air was necessary.
Having explained in a general way the principles of the air engine, and by what means power was obtained from two opposing volumes of air, it will be necessary to consider the means by which economy in fuel was effected, as it must be evident to the most casual observer that, were the whole heat that was necessary in making one stroke taken from the hot end of the air vessel and thrown away at the cold end, the power produced by its expansion and contraction would be more expensive than that which is gained by the use of steam.
To obviate this waste of heat, Dr. Robert Stirling discovered that the air could be divested of its heat to a great extent, on its passage from the hot to the cold end of the air vessels, by dividing the air into a multitude of thin films by means of strips of thin sheet iron kept apart from each other, and presenting a great metallic surface for receiving the heat.
Now, as everybody, by contact, will give out heat to one that is colder than itself, the air, when it enters the narrow passages, must give out a portion of it heat even at the hottest end of the passages, and must continue to give out more and more heat in its progress upward, as the temperatures of the passages diminish, until it ultimately escapes into the cold end of the vessel, where there is only a small portion of heat to be extracted to reduce it to the required temperature.
Thus, the temperature of the air at the hot end may he 600° F (approx 315° C), and when it arrives at the cold end it may be down to 150° F (approx. 65° C), so that the whole heat constituting the difference of these two temperatures must have been left in the sheets of iron forming the narrow passages; and this being the case, there is no room to doubt that the cold air, when again made to enter the narrow passages for the purpose of being heated, immediately comes in contact with metal that is hotter than itself, and consequently has its temperature increased by so many degrees every inch it travels downward, until, on its arrival at the hot end, it requires but a comparatively small addition to its temperature to complete the necessary pressure to move the piston.
The thin sheets radiate from the center of the air vessel, and fill up the space between it and the plunger. In this may be said to lie the grand principle of the air engine, and when it was applied to highly compressed air it produced a large amount of work for the fuel consumed.
The engine under consideration had a working cylinder of 16 inches (approx. 40 cm) diameter, with a stroke of 4 feet (approx. 1.20 m), and when tested with a friction brake, it was found capable of sustaining a weight of 1,250,000 lbs. raised 1 foot per minute ; or 37 horses’ power for a whole day, on a consumption of 1,000 lbs. (approx. 454 kg) of Scotch Chew coal, including the quantity necessary to get up the heat in the morning.
This gives a consumption of 2.7 lbs. (approx. 1.22 kg) per horse-power per hour; but when the engine was not fully burdened, the consumption was considerably under 2.5 lbs. (approx. 1.13 kg) per horse-power per hour.
This was considered a very fair result to be obtained eighteen years ago; and it is not unreasonable to suppose that, had the construction of engines of this kind been persevered in, still greater economy in fuel would have resulted. The engine drove the works at the Dundee Foundry for several years at a very small cost for maintenance.
The whole interior of the machine being entirely free from dust and moisture, there was little or no tear and wear of the different parts, and the piston, and piston and plunger-rods, did not consume a gill of oil in a week.
The principal cause of the failure of the air engine was the difficulty experienced in getting heat to pass through the lower ends of the air vessels with sufficient rapidity to supply the place of the heat that was carried away by the water pipes or refrigerator at each stroke; and in order to compensate for the slowness of the conducting power of the metal, which was necessarily pretty thick, it was necessary to keep the outside of the vessel at a very high temperature, which induced irregular expansion and contraction and incipient decay, resulting in the cracking of the metal and consequent destruction of the vessels.
Notwithstanding this hitherto unsurmounted defect, the writer is of opinion that small engines upon this principle could be constructed and used with economy, in situations where the use of steam is impracticable from want of room to erect steam boilers, or from other causes. There would be less smoke emitted from the chimney; there would be no noise as with a steam boiler blowing off, or a high pressure engine exhausting; and accidents from explosion would be entirely avoided, as, when the air vessels did give way, a very small opening made its appearance, which allowed the air to escape in a few seconds without doing the slightest injury.