What are rules to design a Gamma from scratch?

[Sched44]

If I wanted to build a Gamma walking beam engine from scratch, what would the "rules of thumb" be that I would have to follow in order to assure a working engine when completed?

I guess what I am asking is:

How do I calculate unswept volume of displacer to power piston?

What would I want the stroke to be?

What is the best heat source to use?

Cooling: Water or air?

Can a sealed displacer cylinder be made that isolates the hot end from the cold end?

What is the lightest displacer piston material that can still stand up to the heat?

Any other things I should consider?

Thanks in advance for any advice and help.

[Longboy]

............Your model Stirling engine scale is usualy determined by the cyl. size. Sweep volumes not even an an issue unless you enjoy the math. You should build a close to square up engine ( same size bore and stroke) as you can. My engines are either 15 0r 20 mm. cylinders and stroke of 14 -18 mm. Rubbing alcohol (70%) and a meth cup and wick is the simplest and cheapest source to power your model. Rubbing alcohol at 50 % doesn't burn well and 91% will smoke. For cooling, an air cooled radiator can be spun out of aluminum round stock in one piece. Water cooled.........multiple pieces! Radiator, resorvior, water jackets,tubing, seals, connectors......basic packaging and fabrication headaches. You can isolate the hot /cold cylinders but not nessesary. If inclined to do so, mounting your hot and cold cylinders on separate blocks and connecting these blocks together with the air transfer tube stops heat migration to the cold side. Graphite for pistons is the choice here, nothing better!...................I would recomend for beginers that they buy a material or assembly kit Stirling to get there feet wet in the hobby, Keyboard to death everything on the web about Stirling and watch alot of "You Tube" vids on Stirling!

[Sched44]

Thank you for the reply and the pointers.

I have built two engines already, one from another persons’ plans and another from my head that does not deviate too much volumetrically from the first engine I built from the plans. They are fairly large for model engines standing 17 and 24 inches respectively. They both run quite nicely. Needless to say, the propane one is the stronger of the two and runs its’ own water pump to circulate its’ own cooling water.

Both engines can be seen on the post, “Something hotter than Sterno,” which is no longer an issue as my first one is propane fired. The second one runs on chafing fuel.

My biggest question is what is the ratio of the swept power piston area to the displacer piston swept area.

I will try to be clearer about this.

Say for instance that the volume my power piston displaces is 3 cubic inches, how do I figure the size of the displacer piston and the volume of the air that should surround it?

There must be some basic rules of design that go a long way to almost guaranteeing a more successful build.

Thanks again for the reply and helpful information.

[Cartech]

I have read the ratio should be 1.5 to 1 (displacer volume to power piston volume) This is the ratio for a typical power producing (some power anyway) engine. An LTD Stirling has a ratio of 10 to 1 or more. The higher the ratio, the lower the temp ratio becomes.. to a point. Friction and weight of moving materials become more and more critical. Building an engine that will run off the heat of your hand is VERY difficult without precission tools and ball bearings, not to mention skill in machining and assembly. I agree that a "square up" piston-stroke is usually best. If your making an LTD Stirling, short stroking the power piston a bit can be better.

[Longboy]

.

My biggest question is what is the ratio of the swept power piston area to the displacer piston swept area.

I will try to be clearer about this.

Say for instance that the volume my power piston displaces is 3 cubic inches, how do I figure the size of the displacer piston and the volume of the air that should surround it?

Thanks again for the reply and helpful information.

.........You're welcome. Think in terms of clearance between the outside dia. of the displacer to the inside dia. of the hot tube instead of swept volumes. The engine doesn't care about volumes but definitely cares about clearances so you need to know length and diameters. Mechanicaly, your displacer is gonna push all the air you need for a running engine with clearances of .040in between hot tube and displacer and it won't rub! The length of the displacer will be determined by two things. 1)The length of the hot tube, 2) the stroke of the displacer. Example: If your hot tube is is 3 in long and the displacer has a .5 inch stroke, its length (displacer) will be about 2 in. long. The difference in lengths between the hot tube & displacer will accomodate the stroke and a .25 in. clearance in the hot tube at the top and bottom of displacer stroke.

[Sched44]

Thanks once again for the replies.

I also belong to the Home Model Engine Machinist Forum where many of the members also build Model Stirling engines with machinery they have at home. Some of the machine tools they own are full size but many of the members seem to own the smaller but more capable machines, often 3 in 1’s, from companies like Smithy, Grizzly, Harbor Freight and others. I myself have a full machine shop at work but have a Smithy Midas 1220 XL machine in my shop at home.

There is a gentleman on the Home Model Engine Machinist Forum who is a regular poster and appears to be similarly as knowledgeable as you gents are. He placed the following on that forum in reference to basics for Stirling engine design. He credits the book The Stirling Engine Manual/ by James G. Rizzo (ISBN 0 9519367 3 5). I can tell you that this is a very expensive book, if you can get it at all. He was kind enough to email this information to me.

The following was posted:

1. The length of the displacer chamber = 3 times is diameter.

2. The length of the heated chamber = 2/3 of the length of the displacer chamber (cylinder).

3. The length of the cooling chamber = 1/3 of the length of the displacer cylinder.

4. Swept volume of the displacer = 1 1/2 times the swept volume of the power piston.

5. Length of the displacer = 2/3 of the length of the displacer cylinder.

6. Stroke of the displacer = 1/3 of the length of the displacer cylinder.

Is there anything that you can add or do you feel the above rules pretty much get one pointed in the right direction to start designing/scaling an engine from scratch?

The engines that you gentlemen display in these posts are beautiful pieces of work and being a First Class Machinist myself, I realize the effort you put forth and talent you possess in producing this end product.

I personally have way too many hobbies and attempt to spread my time amongst them all. My concern, and why I initially asked this question, is that if I am going to put the time and effort into another engine, I want to preplan to the point that I can guarantee the result within an acceptable margin for error. This engine will be the largest one I will build to date and I do not want to end up using it as a boat anchor.

I appreciate your continuing opinions.

[Sched44]

I have some questions in reference to your previous posts:

I understand piston displacement (in cubic inches) or swept volume to be one and the same. So if the volume that my power piston displaces by its” total stroke = 3 cubic inches, I would multiply that by 1.5 to get a displacer cylinder volume. In this question the displacer cylinder would have a volume of 4.5 cubic inches. Seems simple enough but having a beautiful mind, it is not.

My question: Is the displacer cylinder volume the total internal volume of the displacer cylinder disregarding the volume of the displacer piston that will be installed in it?

Now, if I install a displacer piston that displaces 2 cubic inches in the displacer cylinder, does my displacer cylinder volume now have to be 6.5 cubic inches in internal volume because I will always have to maintain the 4.5 cubic inches?

There is a thing associated with the strength and efficiency of these engines call Dead Space and besides friction appears to be enemy number one. I realize a regenerator constitutes dead space but how about excessive power piston stroke as well as excessive displacement cylinder volume?

If I wanted to install a regenerator on the displacement cylinder, I have to force most or all of the air in the displacement cylinder to go through the regenerator each time the piston in that cylinder makes a stroke. I believe that I would have to negate the clearance next to the displacement cylinder to the bare minimum somehow without creating any undue friction. How can I accomplish this or how is it generally done?

Realizing that I do not want to lose power due to the weight of engine parts like the displacer piston, can anyone recommend a displacer piston material that is both light and will stand up to temperature? A camp stove regulated propane burner will fire this engine. It will also be water cooled, possibly by a small screen cascade or closed radiator. The engine is for Antique Power Show demonstration purposes only but has to be robust enough to accomplish some sort of “work” in order to have any credentials. Looking Antique is a plus.

I have displayed/demonstrated my two existing model engines in the past at similar shows and they have drawn considerable interest. I strive to build an engine that will run for hours without the need to shut them down to cool and have been lucky thus far. My luck is due to run out and that is why I ask these questions.

Sorry for the long post.

Thanks again for the responses.

Sched44

[Longboy]

..............I'm going to stick with my original response. Clearance, not volumes. Mr. Rizzo arrives at varrious ratios in Stirling components thru mechanical posibilitys. As in his #5) Length of dispacer....... You want a displacer that travels in a tube that doesn't rub the inside of the tube nor bottoms out at either end of the tube when connected to the crank. An he is right that its 2/3rds. Don't bother with a regenerator, they have a purpose thats well explained but most likely your engine won't care. I have seen your beemer and it apperes to be a larger scale than my models, you can always add one in the air tranfer tube later and record an RPM difference for a given heat input. You have to push the expanded air so keep dead space volumes short. These model Stirling engines are pretty liberal in variances to the guidelines set forth in Mr. Rizzo's book and you have all the info needed to start your project except maby a design. Basicly if you understand a hot and cold cylinder connected by an air tube and the pistons in these cyl. driven by a flywheel separated by 90 degrees, your engine layout can be something unique. Your job........figuring out all the linkages to join the components! I understand not all of us are hip to configure their own design, nothing wrong with sticking to generic tried and true Stirling designs!

[jwegmann]

Hi buddies,
1.-I remind that 1.5 is a variable coeficient with influence in Diameter ratios,hot and cold.
2.-Sirling wrote L/3=stroke, L=3 D, 3D/3=D=stroke it means displacer zone ,I understand the drum displacer Volume swept=o,785 Dexp3,In L length of drum,1/3 cold,2/3 hot.
My above colleague confused me,please clarify,I am veteran Master,in love with SE.
I watched animation beta,in hot space you need one stroke=D(square)+3D(cylinder Length)+D when displacer drum goes behind power piston.So I think that Cylinder(shell) measures 5 D, then you have space for stroke at Hot space, and space for cold air backwards compression.
Probably col compression to power piston is a few mm approach.Please correct me as you want.I need yr Help.Thanks.Is chamber=drum displacer or to displacer cylinder?semantic.
3.-I found a tough criteria to reach Engine Vol. sarting by tha tail,PTO=Tau x omega,
you can over run and destroy the engine when no load occur.Tau is Torque and Omega is radians/s, the secret is check the max m/s between piston and liner is acceptable,for us is
8,5 m/s=2 pi xr x n/60 .2r=stroke sq,Now Beale formula, and fix the Power Out,For modells and prototypes use Scotch Yoke, and be Happy.Jwegmann

[Ian S C]

You'll do alright if you follow James G. Rizzo's books, I know of three of them, "The Stirling Engine Manual" volumes one and two, and a paper back, published under the "model engineering guide" series, called "Modelling Stirling and Hot Air Engines, And I think he has one other, "Stirling Engines - Models", the paper back contains a good bit of vol one of the "Stirling Engine Manual".
Best material for a displacer is thin walled stainless steel tube, say about .010" to .015" thick, with the hot end TIG welded on, the cold end can be made of aluminium, and loctited in place. The hot cap on the displacer cylinder can be of similar construction. A displacer made this way will isolate the hot, and cold ends quite well, if you strip the motor after a good length of running, you will find (with a bit of luck)that the hot 1/3 is a bit blue, the colour fades as it goes down the displacer.
Water cooling is the most effective, takes a bit of work, but well worth it, but air cooling can be quite good enough, have a look at the stove top fan in my album. Ian S C