How students built a Stirling engine

Input

I want to tell you a story about how students built a Stirling engine.. I won't tell you the result). Let it be an intrigue, as for, throughout the entire project.

Background

It all started with biology, which was unusual. Given two students and a lot of free time, and an unbearable desire to create fun. Studying did not take much time and the reticular formation was not completely occupied.

In short and simple, the reticular formation is a structure that produces psychic energy and this energy needs to be lost somewhere. For an experiment, try doing nothing for 10/15/30/60 minutes, I got bored after 5 minutes. In short, there is a book by Andrey Kurpatov “Red Pill-2. The Whole Truth About Success”.

Slightly deviated from the topic. After the discussion about what is fun for us – rockets and engines. Requirements for the project topic were put forward

1. everything that burns and explodes

2. simple, repeatable

Skills – using a grinder, knowledge – physics according to the school program (4 in Russian, don't hit too hard)). Having assessed our knowledge and skills, we decided to make a Stirling engine. It meets both requirements – it burns and, it seems, is easy to make. How naive we were, fortunately there was a curator who reduced all the chaos that was happening.

What is a Stirling engine?

We've decided on the project's theme, all that's left is to figure out what it is, assemble it and launch it. Sounds simple.

A little history, the Stirling engine is a godly thing. Robert Stirlinga Scottish priest, inherited an interest in engineering from his parents and received a higher education.

The purpose of the engine was to reduce the injuries of workers who worked with steam engines. Steam engines of that time often exploded due to poor quality metal, and better materials were not available at that time. String's proposed engine solved this problem: the engine could not explode because it worked at lower pressure than steam and was not as demanding of the material.

A Stirling engine is by definition an engine that operates on the Stirling cycle (and later). The engine is undemanding to a heat source and can operate without one. The main thing is that there was a temperature difference. For example, on a solar energyon iceon a laptop (theoretically).

Universal engine, perfectly revealed in the post apocalypse. Let all the oil run out, then gasoline engines will bend, and Stirling will live on doesn't matter.

Now they are used by enthusiasts as outboard motor.

Structure and principle of operation

The historical background is over, let's return to our time.

We started from the beginning, that is, by studying the literature on the topic. And here we were in for a bummer – there are no ready-made instructions on how to assemble a Stirling, but there are instructions for a rocket engine, by the way, but we'll talk about them sometime later.

The books turned out to be difficult for us – scary integrals for describing heat transfer and so on. But the principle of the engine and its cycle became clear.

The Stirling cycle consists of:

1-2 isothermal expansion of the working fluid with the supply of heat from the heater;

2-3 isochoric heat removal from the working fluid to the regenerator;

3-4 isothermal compression of the working fluid with heat removal to the refrigerator;

4—1 isochoric heating of the working fluid with heat supply from the regenerator.

(Stirling cycle)

Main engine parts:

1. working fluid – air/helium/hydrogen

2. hot cylinder – we heat it up. The working fluid expands, doing useful work by moving the piston.

3. cold cylinder – cool. We used water/ice

4. regenerator – retains heat when the working fluid passes from a hot cylinder to a cold one, releases heat when passing from cold to hot.

5. flywheel – a massive disk that accumulates and transfers the moment of inertia to another cylinder, spins.

There are three main engine configurations – alpha, beta, gamma. We chose the alpha type because it is beautiful…

Here The hot and cold cylinders are the same. In other types they are different, which increases the complexity of manufacturing.

Alpha Stirling Duty Cycle

1. An external heat source heats the gas in a hot cylinder. The pressure created pushes the hot piston upwards.

2. The flywheel pushes the cold piston down, thereby moving heated air into the cold cylinder.

3. Part of the heat, when air passes from one cylinder to another, is stored in the regenerator

4. The air cools and contracts, and the hot piston moves down.

5. The cold piston rises, thereby moving cooled air into the hot cylinder.

6. The air passes through the regenerator again, the accumulated heat is released. And the cycle repeats.

First iteration

Okay, we've figured out the device and the operating principle, let's move on to design. Oh no, we need to calculate the dimensions.

First I did it for myself, so that no one except me would understand what is bad when working in a team, and solo too.

Ndaaaa, 0.4 W, powerful. There is no knowledge of how to calculate the energy needed for work, maybe this was the main mistake. No time to be sad, Chinese Stirlings work even with lower parameters.

The numbers are there, we can move on to design.

Designed, beautiful. The lower container is needed for water, to remove heat from the cylinder. Snow turned out to be more effective for cooling.

All that's left is to assemble it. But there's a nuance… We didn't have the skills to work with CNC, and there's no other way to make a cylinder, piston, etc. We had to learn what it is and what to eat it with. Only while we were learning, our senior comrades kindly turned out the parts for us.

And this is what happened.

Aaaand it didn't start… It even turned with difficulty. Well, the first pancake is always lumpy.

Second iteration

This, of course, did not stop us and we began to sort out the problems:

1. The piston did not fit tightly to the cylinder walls – the pistons were reground. We tried piston rings, but it did not help.

2. The flywheel is too small, there is not enough energy to turn the pistons – the flywheel mass was increased.

3. Air in a cold piston. Um, normal, room temperature? Nonsense, heat is supplied, which means heat is dissipated through the tube – a regenerator was added.

We also switched to a different layout, just the crank on both sides is out of phase, which made it more convenient for assembly and adjustment.

The green thing (a nerf bullet) in the middle of the tube is a regenerator.

The moment of truth, the launch. Aaaand the engine almost started. It almost maintained the revs, but still stopped.

The reason for this is the regenerator. It was cold… What went wrong, physics can't break down, can it?

Versions – the burner is too weak, the cylinder itself heats up, but the air inside does not. Or it was heated so weakly that while the air was moving through the tube, it was already cooling down. This cannot be verified now, I will tell you why a little later.

I understand all this now, as I write. But then I was concerned with the parameters of the system – the phase difference between the pistons, the stroke of the hot/cold piston. These parameters can be changed and see what happens.

Interesting fact, if you spray WD-40, it will evaporate and there will be an internal combustion engine. So the universal instruction did not work, all that remains is to drink vodka water with lemon and continue to continue.

Arduino and sensors

Here we come to the final. We decided to add a revolution counter to understand how the revolutions change when parameters change – phase difference, free travel of the hot/cold piston, etc.

We could have used a bike computer, but it was boring. So we decided to develop our own.

We took an Arduino Uno, a Hall sensor. The idea is that a magnet passes near the sensor, a revolution is added, the sum is divided by the entire time that has passed since the switch was turned on. Very inaccurate, but you can get the general picture.

Then the data is transferred from Arduino to Python and a graph is built in real time. Link to gitif suddenly someone wants to repeat it.

The final

Experimentally, we obtained the optimal phase difference of 90 degrees.

The final launch. And the engine maintains the revs, even increases them a little. But there is a nuance…

We overheated it – the flywheel caught fire, the stand a little, the pistons jammed. We were tired of redoing it, and the session started to take time.

Total

And so ended the Stirling engine project, which lasted 9 months. What did we learn from it?

1. Physics, mathematics are interesting and complex things. We applied some knowledge in practice.

2. Developing a project is fun. The engine burned out, just like us in the end)

3. We gained valuable experience working on a project as a team – distributing responsibilities, setting and meeting deadlines.

4. You need to write documentation so that you can remember what happened at least after some time.

Here is the first (quality) longread from me. I thought it would be a short note, but this is what it turned out to be. Thanks for your attention.

Good night:3