This is how the Stirling engine got new life in Swedish submarines

The early steam engines were often life-threatening structures, built of substandard metal that literally did not withstand the pressure. Explosive steam boilers killed, maimed and burned countless machine operators and others who had the misfortune to get in the way.

This worried Robert Stirling, who in the 1810s studied to be a priest at the University of Edinburgh. And young Robert was not only fulfilled by the Christian calling, but also by a burning interest in technology that was possibly inherited: his grandfather Michael Stirling had invented one of the first working threshing machines in the 18th century.

Took the patent as a 25-year-old

In September 1816, a 25-year-old Robert Stirling took over as a priest in the small town of Kilmarnock, and the same year he patented his “Stirling Air Engine”. The principle was gas and pistons in a closed cylinder which is alternately heated by a heat source outside the cylinder. This creates changing pressure differences in the cylinder, which causes the piston to be pressed up and down. And because Stirling’s engine was running at a lower pressure than the steam engine, it could not explode.

It should be said that various “hot air machines” have already been constructed before. As early as 1699, the French scientist Guillaume Amontons had invented a “fire mill” (moulin à feu), which drove a rotating wheel instead of a piston. And just a few years before Stirling’s patent, the English nobleman Sir George Cayley had developed a machine that also propelled a piston through air expansion when heated.

But Stirling’s engine used a closed system, unlike the Cayleys where the air was heated by direct contact with the fire and replaced in each work cycle. And as far as is known, Stirling’s technology became the first to be used in practical operation. One of his first engines, with an estimated two horsepower, was used for several years to pump groundwater from a quarry outside Kilmarnock.

An efficient variant from 1818 stored heat in the heated part of the cylinder while the air passed to the cold side, and transferred heat to the cooled air when it returned to the hot part. And in 1827, efficiency was further increased, when Robert Stirling and his brother James installed a pump that increased the pressure in the cylinder.

Later, a kind of cooling plate was installed in the air passage between the hot and the cold end of the cylinder, which increased the power so much that a Stirling engine could be used at a foundry in Dundee.

A Swedish challenger

Development work was also carried out in Sweden according to the same engine principle. John Ericsson is perhaps best known as the inventor of the first functional propeller, but already as a young ensign at Jämtland’s field hunting corps, he designed a wood-fired hot air machine – or caloric machine, as it also came to be called.

Ericsson requested a year’s leave from the army and traveled to the England of industrialism, where he thought he could easily find uses for his invention. But the caloric machine proved to work worse with coal, which was the common fuel in the UK, than with birch wood that Ericsson used in Sweden.

The failed company led to him exceeding his granted leave, which could have had serious consequences if Crown Prince Oscar (later Oscar I) had not personally said a word to him. Thanks to the prince’s intervention, Ericsson was able to stay in England, and continue its innovator work in steam boilers and other technical tracks.

Compared with the steam engines, the “air engines” had a higher efficiency, and were significantly safer. But despite Robert Stirling’s diligent improvement work, the machines remained difficult to regulate, and expensive to manufacture. They also worked between smaller pressure differences than the steam engines did. Therefore, the power density (power development per volume) was lower, which made the machine unreasonably large in relation to the amount of power it delivered.

And in the 1850s, the bessemer process was launched, the first inexpensive method for large-scale production of high-quality steel. With this, much more stable steam engines could be built, and the boiler explosions that Robert Stirling wanted to eliminate became much more unusual.

To a certain extent, Stirling motors were used to drive pumps and industrial cooling fans until the years around the turn of the century in 1900. Then cheap electric motors broke through on a broad front, and Stirling’s invention became practically superfluous.

Revived during the 20th century

A couple of times, the half-forgotten engine was revived. Only in the 1930s, when Philips in the Netherlands tried to operate generators with Stirling technology. The idea was that the silent machines would be well suited to power radio equipment for the army. But with the World War II giant investments in military technology came better methods for the purpose, and the Stirling engines fell into oblivion again.

In the 60s, the technology reappeared, this time in Sweden where the mechanization of mining suddenly made the Stirling engine interesting. It was thought that the exhaust-free technology would fit well in underground mines, and the state raised SEK 1 billion through the Malmfonden Foundation for research and development work. But again, other techniques proved to work better.

With Kockums as co-owner, the company United Stirling was formed in 1968, which took over the patents from the Ore Fund. One idea was that the machines, with their high heat losses, would provide electricity and heat in motorhomes – but the motorhome market plummeted when the oil crisis of the 1970s caused petrol prices to skyrocket.

When petrol engines became more expensive to operate, the company saw a new business idea: drive passenger cars with Stirling engines. But again, the technology was found to be more difficult to regulate and more expensive to manufacture than the conventional alternative. Robert Stirling’s invention finally seemed to be dismissed as a historical parenthesis.

But in 1987, Kockum’s civilian ship production ceased, and the shipyard company focused entirely on military manufacturing. And suddenly they began to investigate the patents that the subsidiary United Stirling put on: Could the technology be used as a power source in submarines?

Stirling engines can in practice be powered by any heat source, and unlike diesel engines, they emit hardly any sound or vibration. Excellent properties for those who do not want to be detected by the enemy’s sonar system.

Got new life underwater

United Stirling was admitted as its own department within Kockums, completely focused on the development of submarine engines. Soon, a system called Air Independent Propulsion (AIP) was developed, which would be a third submarine class alongside nuclear submarines and diesel-electric submarines.

The AIP system uses helium, whose small gas molecules make it easy to heat up and cool down. The heat source consists of oxygen and diesel, which burns with a hot, stable flame, in a self-developed combustion system that can handle the absence of air nitrogen. When the heated helium gas has pushed up the piston, it goes on to a heat exchanger where it is cooled with seawater.

The first submarine to be equipped with the AIP system was the HMS Näcken, which was extended by eight meters and fitted with two Stirling engines during its mid-term modification in 1988. And she not only proved to go quieter than conventional submarines: thanks to the combustion system she could stay below the surface up to four weeks at a time.

Such subsea underwater conditions had previously been reserved for nuclear-powered submarines. Diesel-electric submarines must at fairly short intervals up to the surface to stick up an air mast, and then start roaring diesel engines. Which of course means risk of being detected by the enemy.

HMS Gotland was launched in 1995, and was the first submarine to be equipped with AIP operation from the start. She was given the name of the Swedish Gotland class of submarines, today consisting of three Stirling-equipped ships: HMS Gotland, HMS Halland and HMS Uppland. Swedish AIP engines have also been exported to Japan’s submarine fleet, which is considered a very high technology rating.