Source: an article from Scientific American, Vol. IV, n°14 - Part 2
Date: April 6, 1861
Title: Important Discussion on the Air Engine
Author: Unknown

After his presentation, Patrick Stirling answered the participants many questions.

Questions & Answers

In answer to various questions from different members, Mr. Stirling said that as long as the plunger was moving up the pressure kept up well, but of course it did not continue as great as at the commencement of the stroke. The plunger was over the center before the engine piston. When the plungers were placed at half stroke the whole was in equilibrio, and the engine was set in motion by moving one plunger up and the other down.

The heating vessels were four feet internally in diameter, and on every side there were minute air passages formed by metal plates, arranged not quite 1/32 of an inch (approx. 8 mm) apart. The plungers fitted as closely as they could make them, but there was no packing except about the piston and plunger rods. The packing of the plunger rods was peculiar. There was a copper tube filled with a solution of pitch and oil, fixed to the top of the plunger, and into this there dipped a pipe attached to the stuffing box, whilst a leather collar above encircles the rod, so that by no amount of pressure could any air get through. He had not heard of any air engine since this one was made which had been so successful as it.

This engine could be made to work at 10, 15, or even 20 horses power, with every satisfaction.

For such powers the air vessels were not so large, but that they could make their bottoms comparatively thin. If these vessels were efficiently constructed, and with their bottoms thin—for example, not thicker than the upper part of the vessel’s sides—the success of the engine would be complete.

There was no practical difficulty, except in getting air vessels to withstand the heat.

So far as the piston and cylinder were concerned, he had never seen better working machinery. The piston has worked for years without alteration, and it was observed that the sides of the cylinder were polished like mirrors. The piston packing was a pair of common cast iron rings, such as in ordinary steam engines, and made self-springing. The piston rod was packed with a leather like that of the heating vessel, and exactly like the plunger of a Bramah press. These leathers would work for three or four months.

The temperature of the cylinder varied between 120° F (approx. 50° C) and 150° F (approx 65° C). He could not say exactly what was the highest temperature of the air vessels, but the bottoms were kept red hot. The temperature in the cylinder was almost constant, and also in the tops of the air vessels, where it never rose above 150° F (approx 65° C), but it was not so easily measured at the bottom. It had been assumed, however, that it was 600° F (approx. 315° C).

In the practical working of the engine the plates in the side passages of the air vessel took up heat from anybody hotter than itself, passing over it, which heat it gave out again in the reverse process.
The air entered at 150° F (approx. 65° C), got heated during its descent by coming in contact with gradually hotter portions of the plate, and so, by the time it got near the bottom of the vessel, it had become heated to nearly 600° F (approx. 315° C).

The great difference between this engine and Mr. Ericsson’s was this:
The engine of Mr. Ericsson on board the steamer which attracted so much attention, was a low pressure one, and it took in fresh air at every stroke, and as quickly threw it away. The blowing of the air through a wire gauze was the first thing tried by his father to obtain economy, and for which a patent was taken out in 1816.
He might state that, in 1827, when his father was taking out his second patent (1827), he met Mr. Ericsson, who asked him if he confined the air before using it; to which he answered that he did. Then Mr. Ericsson said their plans were quite different, and he would not require to oppose my father’s patent.

The air vessel no doubt might be made of copper, but it would not be so strong; and there was another objection, if it became red hot it might stretch or get out of shape. No doubt platinum would be the best metal to make it of. He could not arrive at the first cost of an air engine as compared with that of a steam engine; but of course there were no boilers nor slide valves required in the air engine. Diagrams of the engine had been taken, but they could not be depended upon as absolutely perfect, from the fact that there was a great deal of friction with the indicator piston, which required to be very tight on account of the great pressure. They never got a very truthful figure on account of the friction, but the diagram was a good one so far as it went. He had not one of the diagrams now in his possession.

Mr. Mime said he had seen this air engine working, and had never seen any description of engine work more smoothly.

Mr. Stirling, in answer to an inquiry, said that he was not aware of any engine of this kind being now in operation. The engine described had worked for four years, and in that time they had to renew the air vessels once. It took very little water to keep the top part of the engine cool. They allowed it to run down into a cistern, where it cooled, and was then used over again. The temperature of the water rose to 150° F (approx. 65° C) or 160° F (approx. 71° C) on passing through the refrigerating coils.

Mr. Brownlee thought that, in some cases, one difficulty in connection with this engine would be, that it required more water than a high pressure steam engine. He considered that it would not require a very high temperature to get a pressure of five atmospheres in this engine; for, the lowest temperature of the air being 150° F (approx. 65° C), with a pressure of ten atmospheres, it would only require a temperature of 455° F (approx. 235° C) to get an additional pressure of five atmospheres.

Mr. Stirling did not admit that more water was required in the air engine than in high pressure steam engines, as they always got back the water, and so could use it again and again. With regard to the pressure obtained in this engine, he remarked that there was always a pressure of about six atmospheres at the starting, but after working a little it generally went back half an atmosphere, and at that it worked steadily. One great matter to be attended to in the construction of air engines was to have as little vacant space as possible, anywhere about it, into which the air could be compressed. Of course, great attention was paid to have all the passages in the air vessels as small and all the parts as close fitting as possible, so that the air was pumped out very completely every time the plunger came down.

The President remarked that still there would be a large quantity of air that would never leave the lower parts of the air vessels. The thin plates referred to as inserted in the sides of the vessel presented great surfaces for communicating heat. They did not, he supposed, assist in the economy of heating the air directly, but they were a means by which the heat applied through the bottom of the vessel was more rapidly distributed to the air. They took up the heat and gave it back again to the air when returning to the lower parts of the air vessels.

Mr. Stirling said that economy was undoubtedly the reason for the use of the p1ate, as they offered a large surface for picking up heat from the air when it was wanted to cool it, and which heat was given back again to the air when it was wanted to heat it, so that very little extra heat was required to raise the pressure to its maximum. These plates received their heat from the air, and not directly from the fire. They received heat in the same way as Dr. Jeffery’s respirator did. There were only about eight or nine cubic feet (approx. 255 l) of air in the vessels altogether. If this process of abstracting and giving up heat by the plates were absolutely perfect they would throw away no heat. They had only to make up for loss of heat by radiation.

Mr. Brownlee did not quite agree with that; for they knew that when air was compressed it gave out heat, so that, when the piston returned and the plunger partly returned, the consequent compression of the air must raise its temperature. If they could utilize all the heat of the fuel it would require only about a quarter of a pound (approx. 455 g) of coal per horse-power per hour. He believed that this engine might be made to work with one pound (approx. 455 g) of coal per horse-power per hour.

Mr. Lawrie asked what was the cause of the total failure of Ericsson’s engine. He thought it was very extraordinary, seeing the high success of Mr. Stirling’s engine.

Mr. Stirling replied that he could not say, as no data had been published. All that they could get were newspaper notices.

Mr. D. Rowan said if the economy of this engine was so great why did they not continue to work it?

Mr. Stirling answered, because they could not get the vessels to stand any length of time. The thickness of the vessels was about four inches (approx. 10 cm). Possibly thinner metal would have stood, and they would have lost less heat from the outside.
The vessel was the one difficulty of the engine.

The President drew attention to the principle of a new furnace, whereby fire-brick was used to save the wrought iron vessel from being burnt. He thought an air vessel might be got to stand, made on that principle.

Mr. Downie asked if, in Stirling’s engine, any means of protecting the bottoms of the air vessels by fireclay or other refractory material had been tried.

Mr. Stirling said the fire did not act directly on the vessels. The furnace was in a central space, from which the fire gases entered the two heating chambers containing the heating vessels, which chambers, with their fire-brick lining, were converted into a red hot bath. There were slips of fire-bricks between the furnace and the chambers, so that no part of the vessels were directly exposed to the fire; all the heat was got at second hand.

Mr. Downie said it occurred to him that if the bottom of the air vessel had been concave, and with fire-bricks built close up to it, it would have given better results.

Mr. Stirling said they had tried a number of bottoms, and amongst them one having a bottle shape, which gave good results, but the hemispherical one was found to stand best.