China's Global Times on Wednesday reported a research institution in the country had developed a new heat engine, which it described as the most powerful type of its kind in the world.

The report claimed the 711 Research Institute under the China State Shipbuilding Corporation (CSSC) had developed the new 'Stirling' engine.

What Is A Stirling Engine?

A Stirling engine is a 'closed cycle' engine; that is, the propellant in it is not ejected out, but is continuously circulated. This is different from 'open cycle' engines, such as those used in cars, in which propellant is burned and ejected out.

A Stirling engine converts heat into mechanical energy by the movement of two or more pistons inside cylinders. According to Interesting Engineering, “The gas inside a Stirling engine never leaves the engine. They are heated and cooled down again and again, so are never released in the form of an explosive exhaust. This regenerative engine has the ability to use the same gas repeatedly to generate power; therefore, a Stirling engine can prove to be much more efficient than the internal combustion engine used in modern-day vehicles.” The heat for the engine is provided by an external source, such as fossil fuels (petrol or coal), nuclear power, solar power or even heat from decaying plants.

Robert Stirling, a Scottish clergyman and engineer, built the first engine of this type in 1816 and patented it. The engine was created with the aim of competing with the steam engine. In the early 1800s, the steam engine was the most powerful form of propulsion available, but was prone to the threat of explosions. The Stirling engine minimised the risk of explosions.

However, the Stirling engine's capacity to change its power output is limited, compared with open-cycle engines. Thus adoption of Stirling engines has been less.

Use In Submarines

As they lack explosive exhaust and have less vibration, Stirling engines are inherently quieter. This makes them useful for purposes where silent operation is critical, such as powering submarines. Swedish shipbuilder Kockums (now part of Saab) developed a Stirling engine that burns pure oxygen and diesel fuel in a pressurised combustion chamber. This system has been used as an alternative power source for Swedish Navy diesel-electric submarines such as the Gotland class, which entered service in the late 1990s. In such submarines, the Stirling engine acts as an air-independent propulsion (AIP) system, which propels the submarine underwater without needing to run its diesel engines to recharge its batteries.

Kockums has supplied its Stirling engine to the navies of Japan and Singapore. Submarines using the Kockums engine have a submerged endurance of up to two weeks, according to reports.

While fuel cell-based AIP systems are considered more efficient and are quieter, they have been harder to develop and are more expensive.

Reports in the mid-2000s claimed China's People's Liberation Army Navy (PLAN) equipped its Yuan class of diesel-electric submarines with a Stirling engine of unknown power output.

According to the US Department of Defence, the PLAN is estimated to have 17 Yuan class submarines as of 2021 and could have 25 boats of the same class by 2025. Interestingly, the new class of eight diesel-electric submarines Pakistan is receiving from China is expected to have AIP, which some experts believe will be a Stirling engine of Chinese design.

Nuclear Reactors

Stirling engines have been considered as a substitute for the steam turbines that generate electricity in most nuclear power plants. Interesting Engineering notes “liquid sodium could be used as coolant and water may not be required at all, Stirling engines can also increase the output of a nuclear reactor and decrease the amount of radioactive waste generated by the same...”

NASA has used Stirling engines on space vehicles in which decay of radioisotope fuel provides heat energy. NASA has said Stirling engines connected to nuclear reactors could be a source of electricity for space colonies.

The 'Most Powerful' Stirling Engine?

Explaining the features of the new Stirling engine, Global Times claimed “The prototype ran at a rated power of 320 kilowatts with a power conversion efficiency of 40 percent...” In comparison, the Stirling engine used on the Swedish Navy's Gotland submarines is rated at 75KW.

The report noted the new Chinese Stirling engine “can spawn a series of variations to suit the need of power supply, ranging from 100 kilowatts to several megawatts, and is very suitable to be developed into a portable micro reactor power generator used in special environments such as polar regions, remote islands and Gobi Desert regions.”

Interestingly, the report noted the new Stirling engine could be used with a “sodium-cooled fast reactor” and potentially also be used on submarines.