Stirling engine and its working

March 04, 2021

Stirling engine: Practical demonstration

Today , I present to you a very interesting demonstration of a device. I received this one cute machine very recently. Did you know what is so intriguing about this machine? Here you can see I just placed an ice block below to that machine, it is starting to rotate. And even more intriguing is that if I replace this ice block with a cup of hot water, it is again starting to rotate but in the opposite direction. This interesting device is known as a Stirling engine, as shown in the fig below(Fig:1). And it was invented by Mr. Robert Stirling long back.

Fig:1 Demonstration of Stirling engine with hot water and ice block

Now we will understand why it happened in the above demonstration, or how does this device work? I want to enhance your knowledge by discussing this with you. So let's start.

How does the Stirling engine work?


Stirling engine is an energy conversion device, used to convert thermal energy into mechanical energy. It is an environmentally friendly engine, because there is a use of natural resources to generate heat, such as solar energy, wood, coal, biogas, hot water etc. You can not believe, this engine works on constant temperature gradients, means constant hot and low temperature will result in pulsating motion, i have illustrated in the Fig:2 below. And keep in mind it will not work when only at hot temperature or only at low temperature.

Fig:2 Hot and low temperature will result in pulsating motion of stirling engine

Did you know there are some types of stirling engine, it is generally divided into three groups known as Alpha, Beta and gamma. Alpha-type has two power pistons. The beta-type has a displacer and a power piston with the same cylinder. The gamma-type has a displacer and a power piston with independent cylinders. This article is based on the gamma type engine.

There are two separate hot and cold plates with an insulating material, it placed between these two plates as shown in the Fig:3A below. You can see, here a thick insulating cylinder called a ‘displacer’ is used. Suppose the bottom plate is hot and displacer is at the bottom, it will obviously prevent the heat flow from the bottom plate(Fig:3B left side image). This means the majority of the air undergoes a heat transfer with the upper cold plate. And the air volume approaches the surrounding air temperature. If the displacer is moved up, the exact reverse will happen and the air will heat up(Fig:3B Right side image). Because the stirling engine is a closed loop, that's why there is no way for air to go out. And their air temperature inside this volume will increase, its pressure will also increase.

Fig:3A Hot & Cold plate with an Displacer
Fig:3B Air undergoes a heat transfer with upper cold plate and &
the displacer is moved up and their air will heat up

Design and construction of a Stirling engine

Now i will explain the next brilliant idea by Robert stirling was to use this high pressure to run a power piston. The same power piston can move the displacer up or down. Let’s see how he achieved it, practically. You can see in the Fig:4 below Robert Stirling had connected one more crank on the same rotating shaft of the power piston. It fits tightly into the cylinder so that gas can be compressed. And that crank is at a 90-degree offset to the first crank, which is connected to the displacer. He also added a flywheel in the shaft. Flywheel, is a heavy wheel attached to a rotating shaft which is used to conserve angular momentum and reciprocating engines.

Fig:4 Structure of Stirling engine:- Displacer is connected to the crank, and flywheel is also added in shaft, and Power piston

Now I am moving to the main part of this article, how these arrangements achieved Robert Sterling's objectives when I put a cup of hot water below to stirling engine and their bottom plate has heated. Now the power piston is at its lowest position and the displacer is almost in the middle. When the air gets heated, so its pressure increases, which moves the piston up(Fig:5A). During the piston’s upward motion, the displacer moves up and supports the additional heat from the hot plate as shown in the Fig:5B. And prevents the cold plate heat transfer during this motion. Which means that, during the piston’s upward motion, the air’s temperature and pressure keeps increasing and the piston extracts the energy.

Fig:5A Pressure has increased and piston moves up
Fig:5B Displacer support additional heat from the hot plate

However, after reaching the top, you can see that the displacer is again in the middle, can you tell me the reason why it happened? because the heated air has lost heat to the cold plate(Fig:6). And obviously, this process reduces the pressure of the air volume.

Fig:6 Heated air has lost heat to the cold plate, so displacer is again in the middle

You have to know due to the momentum of the big flywheel, the piston wil start its downward journey, during which the air loses progressively more heat and pressure. Therefore, the piston’s downward motion happens without much difficulty. After the piston reaches the bottom position (Fig:7), we are back at the starting point and the cycle repeats. It’s a beautiful way of producing reciprocating motion.

Fig:7 Due to the momentum of the big flywheel,
the piston started its downward motion

Now, I replace the cup of hot water with an ice block and see what happens? Anybody can predict now, how this machine will behave? Here, the inside air temperature and pressure become lesser than the atmospheric temperature and pressure. The low pressure inside the chamber means the force acting on the pistons is a downward direction, so the piston will move down as shown in the Fig:8A. This way, the engine rotates in the opposite direction(Fig:8B).

Fig:8A This force acting on the pistons is a downward direction
Fig:8B Because of Low pressure piston started downward direction

In short, this engine will keep on working unless and until there is a temperature difference between the two plates. When there is a High temperature difference, then there will be high engine’s speed and power output as shown in the Fig:9 below.

Fig:9 Temperature difference between different temperature

Here is the question for you: we have seen stirling engines have amazing energy-conversion efficiency, so why don’t we use these types of engines in our cars?

1.The main reason is that Stirling engines are slow to respond to varying power demands to vary power output from a Stirling engine.

2.You have to control the upper or bottom plate temperatures, and temperature controls always take time.

3.Moreover, Stirling engines are heavier and costlier compared to their counterparts.

However, we believe that the Stirling engine can be an ideal candidate for solar power extraction. The Maricopa solar power plant, commissioned in 2010 was based on Stirling engine technology and was capable of producing 1.5 megawatt of electricity. This project was later abandoned due to environmental concerns.

Let’s hope that modern engineers will find a suitable application for Robert Stirling’s genius invention.

That’s all about the Striling engine. I hope you understood and enjoyed this explanation of Stirling engine.

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Sabin Mathew, IIT Delhi postgraduate in mechanical engineering. Founder of Lesics Engineers Pvt Ltd & 'LESICS' YouTube channel. He provide quality engineering education on his YouTube channel. And 'LESICS' covers a huge variety of engineering topics. Sabin is a very passionate about understanding the physics behind complex technologies and explaining them in simple words. To know more about the author check this link