Some questions for my first engine

[DavMecha]

Hi all, I am going to build my first Stirling engine (Gamma type) which will be solar powered and which will drive an electric generator to charge mobile phones for instance.
Since I have learned about Stirling engines, I became very interested in this type of engine but never had a chance to design and build one myself till now Because I am quite new to the whole process of designing and building a practical Stirling engine prototype, I have some various related questions, so if anyone could me help me clarify those, I would be grateful :D

Here are the questions:

Q1: My solar collector will be either a Fresnel lens or a parabolic mirror, so I was wondering what minimum range of temperatures would be required on the hot side of a typical engine in order to power an electric generator (will be probably 5V)? I know that the higher the temp differential the better but the source temperature will be limited by the material being heated by the collector, so I am looking for an optimum range of source temperatures.

Q2: I am thinking of using stainless steel for the displacer cylinder and displacer piston and cast iron for the power cylinder and piston. Are there any better materials that can be used instead which will be easy to machine but also which will make the engine more efficient?

Q3: I am quite new to metalwork but I reckon that machining the power piston will be the most critical part of the engine. The lathe that I will be working on is quite old and sometimes has some "hiccups", and I fear that the piston that I will machine with the lathe might not have a good gas-tight fit. Are there any other alternatives or even some good tips to manufacture the power piston. Also, are seals critical for the power piston in a Stirling engine, because I have seen one with only grooves in it but not the rings or maybe the builder didnt show the piston with the rings assembled, so I am quite confused about this.

Q4: If I were to put a regenerator in a Gamma type engine, where will it be? Will it be in the transfer port between the power cylinder and displacer cylinder or only through the passageway across the displacer cylinder? Furthermore, I know that one form of regenerator consists of a matrix of wire which traps the heat but I have also seen another form of regenerator in a Gamma type engine where the transfer port are simply micro channels such that heat is regenerated across the larger surface area of the channels, is that more efficient?

Q5: Last question, what are the typical dimensions of the parts of a practical engine such as dimensions of the power cylinder and displacer cylinder and the displacer and power piston swept volumes that I should worked with in order to manufacture a prototype driving an electric generator?

I hope that my questions were clear enough :P

Since I am new to this whole process, I am a bit lost and confused about where to start, what materials should i used, which machining process is better, etc and etc, so additional tips and information on how to build a gamma type Stirling engine are also welcomed :)

Thanks a lot!

[Ian S C]

Hi davMecha, your materials list seams good. Bore the cylinder then make the piston to fit that(its easier to turn the piston to size) ,make it tight, then polish down to size. Its best to lap the bore with something like Brasso, or even toothpaste. The piston should have shallow narrow grooves, 4 or 5 will do, these are oil grooves, and they form a seal known as a labyrinth seal. It will work if left plain. Make your piston as light as possible, the crown does not need to be much over 1/8", and the skirt say.040" to .030". The important thing is to have patience, take your time.
If you can ,get a good reflector, big as possible. I,v thought of using a Wok (sort of frying pan), they can be quite large, but I don't know if it would foccus. I have used a fresnel lens (it one designed to sit in front of a TV to enlarge the picture), it works, but dificult to manage, and only heats a spot about 1/16" dia, The reflector will heat a ring right round the displacer cylinder.
It is possible to make the piston from graphite, I never have, and if you think of it, make the cast iron one and get things going, then maybe at a later date you could try graphite for an up grade, or build another motor, I'v only got 15 motors and still going.
Hope your in a good sunny place, I would only get about 3 months per year where I am. Ian S C

[DavMecha]

Thanks for these useful information Ian S C :) I'll keep those in mind. And yes I am in a very good sunny place, a tropical island to be specific, thats why i chose a solar powered engine.

I am going to buy a Fresnel lens soon which according to its specification will have a maximum temperature climb of about 1000°C, so i guess that it will be enough to at least get an engine speed of about 1000 rpm, if the engine is well designed and built of course, right? My objective is to at least achieve this speed in order to drive a stepper motor for generating electricity.

[Ian S C]

DavMecha, The only thing with freznel lensses,is thatthe heat is only consentrated on a very small spot, I'v tried it on the end, and side of the displacer cylinder, with no noticable difference, on a good sunny day, i think it was about 28 deg C (about 82 F), I was getting more power than I normally get using LPG. The engine used in the test was my second engine, and there is a photo in my album det up to run on LPG. That engine is based on Dyna, by James G. Rizzo in his book "The Stirling Engine Manual". Jame's motor according to the book only managed 320rpm without a load, and 150 rpm under load, mine will easily attain1500 rpm without a load, and its best working speed is 650 rpm, for the first 100 or so hours the performance was similar to the one in the book, it just took a bit of running in. It had a generator on it, and as it improved, it blew out light bulbs until I could light up 12V bulbs, and the radiator is cooled by a 12V computor fan, and it will run a radio and charge batteries, or the power hacksaw seen in another of the photos (on another motor). This motor is a BETA type. Ian S C

[DavMecha]

Ian S C, your engine based on Dyna seems to be a very interesting one indeed. I wanted to look into more details for this engine but unfortunately I have only the first volume of the Stirling Engine manual by James G. Rizzo, so i guess it is in the second one :( Anyway, thanks for these useful information.

[Ian S C]

Dyna is on page 145 of the first "Sirling Engine Manual", also page 165 in "Modelling Stirling and Hot Air Engines" by James G. Rizzo (a paper back, my first book on hot air engines), maybe you have the second Stirling engine manual. Ian S C