Robert and his brother James submitted a new patent in 1827 (Repertory of Patent Inventions, London, 1827) for improvements in air engines for moving machinery. This patent gave no practical application but rather has been a forerunner for the 1840 patent and the Dundee Foundry air engine that entered legend!
The 1827 patent will be presented by Elijah Galloway, himself Engineer and Inventor, author of an History of the Steam Engine.
The patent of 1827 and 1840 are about the same air engine; the latter patent including several improvements. The essential difference between both lies in the economiser that was first in a separate vessel and, in 1840 was within the plunger constructed to serve as regenerator by filling it with wire-gauze and leaving holes at top and bottom for the passage of the air through it.
An important feature in them was that the air was compressed by means of a pump which formed an additional organ of the engine, so that its average pressure was kept much above that of the atmosphere.
The pressure was commonly as much as 150 lbs. per square inch with the air cold. Stirling's cycle is theoretically perfect whatever be the density of the working air, and the use of compression affects the theoretical thermodynamic efficiency only if the ratio of adiabatic expansion and compression be altered.
But it gives a higher mechanical efficiency, and also, what is of special importance, it increases the amount of power developed by an engine of given size. To see this it is sufficient to consider that with compressed air a greater amount of heat is dealt with in each stroke of the engine, and therefore a greater amount of work is done.
Practically the use of compressed air also increases the thermodynamic efficiency by reducing the ratio of the heat wasted by external conduction and radiation to the whole heat.
Compared the the 1816, the 1827 patent contained several important developments.
First, the Stirling placed the displacer and power piston in separate cylinders, as mentioned. The 1816 geometry had the displacer and piston in the same cylinder.
They took a leaf out of James Watt’s book and made the engine "double acting", meaning that it produced power on both the upstroke and downstroke.
Watt did this by valves; the Stirling’s by arranging two displacers, one to feed the foot of the power cylinder and one the top of it.
They inverted the design so that the hot ends of the displacers were underneath the machinery and they added a compressed air pump so the air within could be increased in pressure to around 20 atmospheres.
All these were very intelligent developments and if their design had been as good in practice as on paper then the Stirling engine could have been a serious competitor to steam. It was more efficient and hence more economical, and safer, an important point in an age when steam boiler explosions and engine failures were common.
Stirling's third patent of 1840 (no. 8652) included several technical improvements. The engine generated about 37 horsepower, enough to turn machines in the foundry in Dundee that he managed. It was James Stirling himself who found the big practical problem