Stirlingengineforum.com

Hi all,

I'm new to building these engines and currenty trying to build a displacer type engine from a plan on the net. Instead of using a 10mm dia power piston, I'm thinking of using a cut down 14mm glass test tube which will obviously affect the other dimensions of the engine.

Does anyone have any more info on power cylinder to displacer cylinder volume ratios? I know the suggested ratio is 1.5/1 and I've been thinking of just changing the stroke of the power piston so that the swept volume is the same even though my power piston is larger, would this work? Have read other threads on the subject and it doesn't appear to be straight forward.

Thanks for reading,

Dave. :???:

Hi Dave,

Welcome to the forum and the Stirling engine building hobby! First off, glass syringes and test tubes are an excellent material for power cylinders not made on a lathe. Be sure your piston is air tight and that you do not use any form of lubricant in the cylinder.

To answer your question, I suspect that a larger diameter piston should be fine provided certain aspects are taken into consideration.

1) All other things being equal, a shorter stroke will generate a faster motion but lower force. This means that you will have to be very careful to reduce friction any way you can.

2) If this is your first engine, I'd suggest using a higher ratio than 1.5:1 for the volumes. These are the "magic" numbers for efficiency, but they are not necessarily the best in all cases. Notice that most "tin can" style motors have big displacer cylinders in proportion to their power cylinders (or diaphrams).

3) Larger displacer cylinders mean (usually) a larger heat source. So your engine may not run on a tea candle.

Regardless of your configuration, be sure to address the following as best you can:

DEADSPACE: any volume of air within the system that is not able to be heated/cooled or compressed is going to decrease your efficiency. Keep the connection between your cylinders as short as possible.

LEAKS: the volume within the system must be sealed as well as possible without creating undue friction on moving parts. If air is able to move in and out of the system, it will decrease the force created by the power piston.

TEMPERATURE DIFFERENTIAL: be sure that you keep your hot side hot and your cool side cool. The theoretical efficiency of your engine is based on the ability to keep this difference as large as possible. Larger heat sources are one solution, but so are cooling fins, water baths, and cups of ice.

FRICTION: polish and lubricate contacting surfaces whenever possible (with the acception of a glass cylinder: adhesion between the lubricant and the moving parts may be greater than if the parts remain dry). Bearings and bushings are a great help, but may not be necessary depending upon the design. Be sure parts don't rub and that your flywheel spins freely just before sealing the system.

Ok, a bit much to digest in one setting. Don't hesitate to ask question, post pictures, etc as your project progresses. And stick to it! If you follow the general design of an already tested engine, you'll learn a great deal about how they work and how to improve your next project!

-Stefan

Hi Stefan,

Thanks for the reply, there's some great tips in there - how long have you been building Stirling Engines?

I was planning on being able to run the engine from heat sources such as the top of a TV or a cup of coffee so after reading your response, I suspect I may have to re-think the size of power piston and maybe the ratio.

I'll let you know of my progress (hopefully).

Dave.

Dave -

What you are describing is a low temperature difference (LTD) Stirling engine. There are a few designs out on the internet, but few are free. Here's one that I've found (but haven't tried personally)

http://www.pureenergysystems.com/os/Sti ... index.html

Another more conventional design that I have actually built with success is this one:

http://www.physics.sfasu.edu/astro/cour ... rling.html

It's made from very easily obtainable materials, seems to be fairly forgiving in design, and has clear instructions.

Hope they help!

-Stefan

We are truly thankful for your blog entry. You will discover a great deal of methodologies in the wake of going to your post. I was precisely scanning for. A debt of gratitude is in order for such post and please keep it up.  slotimo