The Air Engine

Source : The Engineer, Dec. 14, 1917, p. 516 - Part 2 - The Air Engine
Date of patent : 1816
Patent #: 4081

In his patent Stirling describes the use of the regenerator (economiser) for an air engine but also for other applications like furnaces to the saving of fuel in breweries, distilleries, dye works and other manufactures.
Here we reproduce only the application to the hot air engine.

Having thus described and ascertained the nature of my Invention I shall now describe [...] my engine for moving machinery, and the following is a general description of the manner in which this is performed.

I employ the expansion and contraction (or either) of atmospheric air or any of the permanent gases by heat and cold to communicate motion to a piston or other similar contrivance.

In order to produce this expansion and contraction I cause the air to pass from a cold to a hot part of the engine and the contrary alternately either in the same passage in the manner described at the explanation of Fig. 1, or in different passages as described at fig. 2.

I apply fire to the warmest part of the engine, in order to supply the waste of heat occasioned by its transmission from the hot to the cold parts by the radiating and conducting power of the materials of which they are formed, by the change of capacity for heat which the air suffers from condensation and rarefaction, and by the impossibility of transferring the whole of the heat from the air to the passages and the contrary and I apply a stream of cold air or water to the coldest part of the engine to carry off said waste heat.

The passages are of course hot at the one extremity and cold at the other, and in passing through them the air is alternately heated and cooled or expanded and contracted.

The following is a particular description of that form of my engine which I consider as the best.

Stirling's Economiser and Hot Air Engine - Fig. 7

AADD Fig. 7 is a cylinder composed of three parts accurately joined together by rivets or screws and rendered airtight by hammering or soldering the joinings.

The part AB is formed of cast iron and accurately bored, the part BC is made of sheet or cast iron as thin as possible (as one tenth of an inch), and the part CD is of sheet or cast iron.

To this cylinder is fitted a piston EE which is made airtight in the usual way, and provided with rods II for communicating its motion.

FFGG is a hollow cylinder also as thin as possible made of sheet iron covered with thin plates of polished brass or silver to prevent the waste from radiation and divided into compartments by plates bb for the same purpose. It is shut on all sides and air tight, kept at a small distance from the outer cylinder by wheels aa, or any similar contrivance and furnished with a rod H working through a stuffing in the center of the piston, by means of which it is moved up and down.
This inner cylinder I call the plunger.

The part BD of the outer cylinder is kept hot by the flame or heated airs of a fire C applied at DD made to descend on all sides of the cylinder to C and allowed to escape at e.

The part AB is kept cool by a stream of air or of water directed upon it, and the part BC increases in temperature from B to C. The temperature of the plunger increases form F to G and the interval between the cylinder and plunger is partially filled with wires wound round the later and kept at a small distance from it and from one another by wires laid along it at right angels to the former, in order to heat and cool the air more completely.

This intervals is on each side about one fiftieth of the whole diameter of the cylinder. Figures 7 and 8 are drawn to a scale of one half inch to a foot except the thickness of the metal which is to no scale. The space contained by the cylinder and piston is filled with atmospheric air.

Figure 8 is an elevation of the engine.

Stirling's Economiser and Hot Air Engine - Fig. 8

ABC the cylinder DD pillars supporting it, EE a beam centered at G to which the rods II of the piston are connected by a parallel joint NN: ff an arm which connects said beam to the crank g.

I o I a bent lever joined to said arm to o and connected to the fixed point A by the rod l, and to the other extremity of the beam FF by the rod k. This beam is also centered at G and moves between two plates or similar beams of which the beam EE is composed. P a rod by which FF is connected to the rod of the plunger M a slider upon the rods of the piston fixed to the rod of the plunger to render its motion steady and parallel.
hhh the fly upon the same axle with the crank.
cde the furnace and flues.

The Operation of the engine is explained as follows

The part of the cylinder surrounded by the flues is heated to a temperature of 480° F (249° C) higher than the part AB.

In the position represented at fig. 8 the plunger is in contact with the piston, by which means the included air is brought to the warm part of the cylinder has its elasticity increased and presses upon the piston with a force greater than that of the atmosphere.

The piston is thus forced downwards and the rod ff and the crank g upwards till the pressure of the included air and that of the atmosphere become equal.

The impulse communicated to the fly carries the end of the crank towards q, and the arm ff and bent lever ii are brought to such a position as to depress the rod k and thus to raise the plunger from the piston.

The included air is thus made to descend between the plunger and cylinder and brought to the cold part; it is cooled in its descent, has its elasticity diminished, and its pressure becomes less than that of the atmosphere, the piston is forced upwards, and the crank downwards.

The revolution of the fly and crank again bring the plunger towards the piston, the air ascends through the same passage by which it descended, is heated in its ascent and forces the piston downwards and the crank upwards, and so on alternately. In this manner a rotary motion is produced which may be applied to the moving of machinery.

The force of the engine is regulated by allowing a portion of air to escape outwards and inwards by a small cock which is opened and shut by a governor as in steam engines, and placed in the cold part of the cylinder immediately above the highest ascent of the piston.

The distance which the rod of the plunger H fig. 7 moves through the piston I call the stroke of the plunger and I make it equal to that of the piston when the difference of temperature in the hot and cold parts of the cylinder is 480° F (249° C). When the difference is less than 480° F (249° C) I make the stroke of the plunger proportionally greater than that of the piston, and the contrary when the difference is greater.

The length of the arms of the bent lever I o I and of the rod k which is necessary to make the plunger just touch the piston on the one hand and the upper end of the cylinder on the other determine by experiment as being the most convenient method known to me. I do not answer for the absolute correctness of those in the plan.

The cylinder is inverted to prevent the oil used to make the piston airtight from getting to the hot part and wasting the heat.

In the foregoing description, wherever I have specified more than one material or more than one method of performing the same thing, I have placed that first to which I give the preference; but I reserve to myself the power of using any materials and applying my new contrivance for heating and cooling bodies, to the purpose to which it is applicable in any form or manner which further experience may prove to be advantageous and which is not inconsistent with the terms of his Majesty’s Letter Patent.
In witness whereof I the said Robert Stirling have hereunto set my hand and seal this Twentieth day of January in the year of our Lord one thousand eight hundred and seventeen.

Signed sealed and delivered

In the presence of:
Rob. Cameron - Accountant in Edinburg
Francis F. Cameron - Preacher of the Gospel in Edinburg


Stirling's Economiser and Hot Air Engine - Added
The engraving above shows the first Stirling engine, as specified in the 1816 patent, and built in 1818 for pumping water at an Ayrshire quarry.

The top of a long vertical cylinder A is heated by flue gases from furnace B; the bottom is cooled either with water or by atmospheric convection. The cylinder contains the dpluner (or displacer) C and the power piston D.
The plunger is of rather smaller diameter than the cylinder A and centered by small rollers.

As the plunger moves up and down, air is pushed from the hot space F (above the plunger) via an economiser (or regenaretor) into cold space E located between the bottom of the plunger and the top of the piston. Then the air goes back again.

The economiser (not shown in the engraving) is in the anular space between the displacer and the cylinder, and probably consisted of thin wire wound around the plunger.
Thus the air is in the cold and hot space alternatively, and undergoes temperature and pressure variations which cause it to perform work on piston D.