What is a Stirling engine? A Stirling engine is an engine that uses the expansion and contraction of a gas when heated and cooled to carry out mechanical work.
A Stirling engine works on the principle that gases expand when heated and contract when cooled. When thermal energy is applied to a gas, the molecules receive more energy and that makes them vibrate more. When they vibrate more, they spread out and they take up more volume. When the gas is cooled, the energy is removed from the molecules and they vibrate less, move less, and take up less volume.
In a Stirling engine, there is a fixed amount of gas in a sealed chamber attached to a piston. The chamber has a heating area and a cooling area. The number of pistons varies depending on the power required. Heat is applied to the heating area of the chamber, and the gas expands, pushing the piston up, which supplies the mechanical work. As the piston moves up, a crank moves a displacer, which moves the heated gas to the cooling area in the chamber. The gas cools, contracts, and creates a small vacuum in the sealed chamber, sucking the piston back down. The displacer is removed, and the cooled gas flows back to the heating area of the chamber to repeat the process. There is a regenerator in every chamber, which is made of a porous material and temporarily stores the heat energy that is removed from the gas as it is cooled. This saved heat can then be added back with the external heat source to reheat the gas. They can work on any gas, but they are most often run with hydrogen or helium.
A Stirling engine has several advantages over the regular internal combustion engine that we use in our cars. The main advantages are that they are far more efficient and much quieter. An internal combustion engine works by igniting a small amount of air mixed with gasoline to make an explosion to push up a piston. This obviously works, but most of the energy held in the gasoline is lost to heat, sound, and light. The average internal combustion engine is only about 30% efficient. They are also driven by small explosions, which is why they are so noisy. A Stirling engine is very quiet. The only sound comes from the movement of the pistons, and that is negligible. For this reason, they are sometimes used on submarines because silence is important. A Stirling engine is also very efficient because a lot of the heat energy that is taken out is saved and put back in again. In a closed system, in the laboratory, in theory, they could reach close to 70% efficiency. Stirling engines also don’t release any exhaust gases because the gas they use is in a closed system. They also have far fewer moving parts than a combustion engine and are less likely to fail. Another advantage is that they will run off any heat source at all, be it a fire or solar power.
So, if they are that quiet and that efficient, why aren’t they used in cars? The main reason is that they are only really efficient in controlled conditions. When they are used outside, say to move a car, they become a little impractical. This is partly down to the fact that heating the gas with the heat source is not an immediate process, like the explosion in a combustion engine. When you push down on the accelerator in a car, you immediately get power. If you pushed down on the accelerator in a car with a Stirling engine, it would take a while for the heat to transfer to the gas. You would accelerate, but it would take a while. It is also very difficult to reduce heat loss on the way to heating the gas. Stirling engines are also fairly expensive to produce. They don’t have many moving parts, but the parts they do have are complex and difficult to produce. For these reasons, they are not used in cars. They are used in some submarines where silent running is more important than a rapid burst of speed.
The Stirling engine was invented by Robert Stirling, who was a Scottish engineer. He is often called the “father of hot air engines”. He spent his life trying to improve his heat engine, but he realized that it would never be able to produce enough power to rival other forms of engines. It was, however, much safer than the steam engines that existed at the time, and that was one of his primary goals. And this is what I learned today.
Sources
https://en.wikipedia.org/wiki/Robert_Stirling
https://en.wikipedia.org/wiki/Stirling_engine
https://www.sustainabilitybynumbers.com/p/inefficiency-ice
http://www.bekkoame.ne.jp/~khirata/academic/kiriki/begin/general.html
Image By Paul U. Ehmer – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=36691633