basic dimentions needed for gama design

Discussion on Stirling or "hot air" engines (all types)
Wellington
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basic dimentions needed for gama design

Post by Wellington » Sun Sep 11, 2016 11:18 am

Ok, got lots of tubing of various sizes and am ready to begin my first engine. Can anyone give a brief explanation or link to the basic dimensions and volumes of a gamma engine. my main concern is how do you calculate how much volume of air to pump into the cold side with the power piston?

Wellington

Ian S C
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Re: basic dimentions needed for gama design

Post by Ian S C » Mon Sep 12, 2016 4:55 am

for a GAMMA motor, as with a BETA, the swept volume of the displacer is 1.5 times the swept area of the power cylinder. The next ratio is the actual size of the displacer, it should be about three times it's diameter long. the gap between the displacer and the hot cap is generally not critical, depending on the size of the motor it could be from 1 mm to 3mm wide. No leaks, minimum friction, and lots of luck. This is for a conventional high temp hot air motor.
Ian S C

Wellington
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Re: basic dimentions needed for gama design

Post by Wellington » Mon Sep 12, 2016 5:27 am

Ian S C wrote:for a GAMMA motor, as with a BETA......Ian S C
ok thanks Ian. any other dimensions that I need to know about? like how near the tip of the cold end should the displacer stop? Also how do you match up the crank turning circle diameter (say 4cm) with the total distance of displacer movement?
Wellington.

Alfista
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dimensions of gamma engine

Post by Alfista » Mon Sep 12, 2016 7:40 pm


I refer you to Robert Sier's Notes on Hot Air Engines. It is probably the best one page summary to be found.


http://www.stirlingengines.org.uk/modeng/note.html

Ian S C
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Re: basic dimentions needed for gama design

Post by Ian S C » Tue Sep 13, 2016 1:29 am

The gap at each end of the displacer should be similar to that down the side2 mm is a good guess, as the motors get bigger, the gap can be increased a bit to allow for expansion of the metal.
I agree about Robert Sier's writtings on hot air engines, another to look for is James G. Rizzo. Another author to look up on Google is the Japanese engineer Koichi Hirata, you will find a collection of his designs ready for you to build, interesting chap.
Ian S C

Wellington
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Re: dimensions of gamma engine

Post by Wellington » Tue Sep 13, 2016 3:50 am

Alfista wrote:
I refer you to Robert Sier's Notes on Hot Air Engines...


Just what I wanted. Thank you Alfista. Have you tried the delft clay kit I recommended yet?
Wellington

Wellington
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Re: basic dimentions needed for gama design

Post by Wellington » Tue Sep 13, 2016 3:52 am

Ian S C wrote:The gap at each end......
Thanks Ian. These two links should keep me busy a while.
Wellington.

Wellington
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Re: basic dimentions needed for gama design

Post by Wellington » Tue Sep 13, 2016 5:09 am

Wellington wrote:
Ian S C wrote:The gap at each end......
Thanks Ian. These two links should keep me busy a while. You once recommended putting the crank and alternator all in one housing for a pressure design. The v twin alpha design looks most suitable for making a pressurised engine. Do the following dimensions given on the Robert Sier link also apply to an alpha engine design? ie:

i. length of displacer chamber L = 3 times its diameter.
ii. length of heater chamber = 2/3L
iii. length of cooler = 1/3L
iv. swept volume of displacer = 1.5 times swept volume of piston cylinder.
v. length of displacer = 2/3L and stroke = 1/3L.

Regards. Wellington

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Re: basic dimentions needed for gama design

Post by Ian S C » Wed Sep 14, 2016 12:37 am

The ALPHA motor is a totally different machine, but it works in a similar way, no displacer, both hot and cold pistons are sealed and produce power, the volume ratio is 1:1.
Ian S C

Wellington
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Re: basic dimentions needed for gama design

Post by Wellington » Wed Sep 14, 2016 1:02 am

Ian S C wrote:The ALPHA motor is a totally different machine.....Ian S C
Thanks Ian. Can you recommend any good links to learn about the basics of the alpha design?
Wellington.

Ian S C
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Re: basic dimentions needed for gama design

Post by Ian S C » Thu Sep 15, 2016 2:13 am

It's worth downloading Andy Ross's book "Making Stirling Engines", it's a freeby, It doesn't actually have plans for motors, But lots of how.
Ian S C

Alfista
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choice of one's first engine

Post by Alfista » Thu Sep 15, 2016 9:50 am


Wellington,
I would encourage you to first try your hand at a gamma engine first before trying to build an alpha. The alpha poses the particular problem of building a compound piston & displacer, possibly two of them and possibly sealing a low friction, hot piston & cylinder combination. If you are especially good at making low friction, good sealing cast iron rings, then you may be all set.
Please ignore my concerns if you have this all figured out, I just think that in general, a gamma configuration is the best choice for a first engine.

Alfista





......

Ian S C
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Re: basic dimentions needed for gama design

Post by Ian S C » Fri Sep 16, 2016 2:21 am

It's best not to use piston rings on any hot air motor (friction), it is possible to make an ALPHA motor if you have a lathe, you want two cylinders, and two pistons, one the cold piston made to the normal fit for a Stirling Engine, the other, the hot one needs a minute fraction more clearance to allow for expansion as the motor heats up. This is my ALPHA motor, no regenerator, unpressurised, using the ROSS YOKE linkage. I Since this photo was taken I have removed the water jackets around the bottom of the cylinders, these have been replaced with aluminium fins for air cooling, and the steel cylinder head has been replaced with aluminium, it didn't actually make any difference to the performance of the motor.
Ian S C
Test 008 (640x480).jpg
Test 008 (640x480).jpg
Test 008 (640x480).jpg (185.55 KiB) Viewed 6530 times

Alfista
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more about piston rings

Post by Alfista » Sat Sep 17, 2016 12:38 pm


Here is a little more information on piston rings.
I understand that there are many ways to build an engine but it would be a shame to dismiss the topic of piston rings as "something not used" on hot air engines. On smaller engines, it is probably an unnecessary complication. In glass cylinders, it is probably best to use graphite or epoxy or something like that for the piston and no piston rings. However, in larger sizes, piston rings can be very useful.

The Heinrici engine produced by Myers engine works calls for a 1.5" cast iron piston ring. It is quite reasonable to substitute the cast iron ring with something like oil grooves otherwise known as labyrinth packing.

Rizzo speaks of using O rings and also lipped ptfe washers and even stranded ptfe. Darlington writes about making rings from leather and also from rulon. Leather is a common and traditional practice in making piston seals for ram water pumps as well as steam engines. In the case of ram water pumps and hot air engines, sometimes this takes the form of a leather piston ring, other times as a cylinder seal, such is the case with the leather seals on the large Rider-Ericsson alpha engines.

In his introduction to alpha engines :
http://www.douglas-self.com/MUSEUM/POWE ... hotair.htm
Douglas Self writes "The alpha type: there are two pistons in separate cylinders which are connected in series by a heater, regenerator and cooler. This means that the hot piston must have seals, eg piston rings, that can cope with hot gases." Hmm ... piston rings capable of coping with hot gases. Perhaps cast iron would work ...

Ross 65cc & 300 cc
Andy Ross used cast iron rings in his 65cc and 300cc engines. He notes that the rings are modified with less spring (than automotive rings), presumably for less friction.

I also (tried to) include two photos of my aluminum piston for my 3" Stirling engine. It has two cast iron rings separated by an undersized aluminum ring. It has good sealing and low friction. (If the board returns to normal, I will post the photos later.) The oil sort of sits on the upper ring and is also trapped between the rings. This sort of design should also work well in an alpha engine where the piston may be exposed to much higher temperatures than a typical gamma engine. In general, piston rings are used to lower overall friction while increasing the seal. The piston can then be made a little undersized. With less surface area on the cylinder wall, one should in theory have less friction as compared with the full length of the piston fitted closely to the cylinder wall.

Alfista


Ian S C
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Re: basic dimentions needed for gama design

Post by Ian S C » Mon Sep 19, 2016 1:50 am

Here is a plain piston with a leather cup seal. This is the power head for a stove top fan motor.
Ian S C
Test 044 (640x480).jpg

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