Hot air and stirling engines

[Ian S CParticipant @iansc]

I,v been looking at the other of our photos and I see a bit I missed at first,the holes in the piston,two tapped yes I saw those,but I missed the other two,I’v just been letting the air come up the gap if any between the piston and the cylinder,but I think you have an answer to that.

[Mark Smith 3Participant @marksmith3]

No Ian, The other two holes are a mistake. I broke a tap in the hole, not enough oil, and had to turn the cap 90 degrees and redrill and tap. That’s why the extra holes, But you may have hit on something to push the lip against the wall. I used a taper on the top cap to allow the pressure to get at the cup and force it against the wall. The compression in an unpressurised engine is impressive. I don’t know how teflon is made but if it could be shaped like leather that would be even better.  I am so impressed with the results I will use the system I described above on future engines until someone shows me something better. In his book, Making Model Stirling Engines Volume 2 James Rizzo also describes a leather sealed piston for the Lehmann engine. 

I think that I will make the piston like a bobbin to make it lighter on Beta type engines. This allows the displacer rod to pass up the centre, a flat on one side to locate the main con rod and reduce friction from the whole piston wall.

[Ian S CParticipant @iansc]

Oh well nice try,thought I was on to something there,I might try it any way.I’v tried a little valve in the crown of the piston,I think a reed valve would have been better in that case,didn’t notice any improvement,so I took it out.I’v got one motor with a piston in the form of a bobbin with the recessed area filled with a ring of teflon,it runs on plain as supplied brass tube 5/8″ bore.I find teflon is a bit stiff for my idea of a cup seal,it can work though.Can’t remember but I think I turned the teflon cup seal out of solid.IAN S.C

[Ian S CParticipant @iansc]

I think I’v found the problem with the low temp motor,it appears that very fine particles are coming of the foam displacer,so I’ll have to find some of higher density.I’m sure that these particles adhere to the cylider and piston.Took two of my motors down to the opening of the new airstrip on Saturday but didn’t run them(would have if someone interested),great day ,lots of people,lots of flying.If after changing the displacer the LTD motor doesn’t work,I’ll take a thou of the piston and fit a cup seal.

[Mark Smith 3Participant @marksmith3]

I changed a bearing in the connecting rod at the piston end and now the engine won’t go. It has good compression no tight spots and timing is right bu it just refuses to run after several hours of reliable activity. Something is wrong but at this stage i don’t know what that is. Oh well, tomorrow…

[Ian S CParticipant @iansc]

Hi Mark,you’v just got to look a the thing the wrong way and they go on strike.Did you shorten the piston?Is the new bearing square.No leaks in the displacer.I might have a cure for my motor,in my paint supply I found some silver paint that is ok on foam(I think its for making christmas decorations),I’ll see if that seals the fluffy surface on the displacer,then I’ll have another try,it only went once,and I’m not sure what happened,but the other day I had it outside and took he piston out,turned it upside down and turned the flywheel and noticed very small particals get blown out of the power cylinder(they glistened in the sunlight).

[Mark Smith 3Participant @marksmith3]

Were the particles from the displacer do you think?

I had another look at my engine tonight, made many adjustments but no go. It has heaps of compression. It gets even harder to turn over as it heats up and bounces back to bottom dead centre when I let the flywheel go (this is an upside down engine). Is it possible to have too much compression do you think? I tried a heavier flywheel which lets it turn over more times before bouncing back but it just won’t go on.

Previous to this episode, all I had to do was heat with meths for 30 seconds and give the flywheel a push and away it went, now it won’t go with the hot cap glowing red!  The bearing is straight, no changes to the piston or anything, all I did was change a bronze bush to a roller bearing. I am going to strip the whole thing down and check every part for leaks, but given how much pressure there is I don’t think that is the problem. I will also renew the leather cup washer as I think it has a lot of oil in it. I will also set it up with its old cast iron piston and see if it goes then, but that piston has vey little compression even though the engine ran quite well; except it jumps around the bench.

Edited By Mark Smith 3 on 17/09/2009 11:10:19

[Ian S CParticipant @iansc]

Yes I think the particles are fluff of the foam,I’v sprayed it with silver paint to neutralise the problem,I also have reduced the stroke of the power piston to 3/8″ from 1/2″,and I think I’ll drop it down to 1/4″,at the moment it just about keeps going.The new bearing on your motor,is it a needle roller type?Is it sealed or shielded,how is it lubricated?Ball races may be better,nothing wrong with good plain bearings.The answer may be with the piston,is the new one aluminium?Might be a bit tight(ally expands more than cast iron and with a cup seal the extra gap won’t hurt)also ally has more friction,CI tends to be self lubricating.Could you reduce the weight of the old piston?I tend to run with fairly heavy flywheels,but one of my bigger motors will run with no flywheel or pully-just a bare crankshaft,no power plenty revs just difficult to grab anything to turn it to start it.

[Mark Smith 3Participant @marksmith3]

Thanks I’ll take your suggestions on board but the being hard to turn over is not friction, it’s air pressure as this has more bounce than anything I’ve experienced. I’m conflummoxed but learning. ,

[Mark Smith 3Participant @marksmith3]

Ian, you were right thedisplacer has a leak. I remember dropping it when I did the new bearing. I took it out and had a close look and sure enough the brazed cap had cracked about a centimetre long on the edge. I will replace the cap tomorrow and see what happens then. It’s amazing how little it takes to stop these engines working.

[Mark Smith 3Participant @marksmith3]

Have you noticed how many people are reading this thread? I wish some one else would leap in and tell us what they are up to. I hope we are staying on subject or we might hear more of circlip’s shopping expeditions, at least we are talking about our engines,LOL.

[Ian S CParticipant @iansc]

Hi Mark ,thats a monster hole,I’v had trouble in the past with a pin hole.I might have found one reason for the LTD motor not doing too well,I’v been reading throughmy old MEs,about twenty yrs,came across a similar size motor,its power cylinder is 9/16″ and the stroke is 1/4″,mine is 3/8″ stroke x 1″ bore,bit of a difference.Oh well hope all the ones that arn’t saying anything are learning something,although there must be someone that knows a bit more than we do,all I do is just create scrap metal and swafe,occasionally something works,accident perhaps!

[CirclipParticipant @circlip]

Oh you are a Bu—r Mark, reading your exploits avidly, What is amazing is that such a simple principle is so difficult to get working properly. A mate (Klank) was driven to destruction nearly with one of these things.  Small point, most who use propper bearings seem to wash ALL the lubricants out.

 

  On the shopping front, had to go buy a new microwave yesterday, its predecessor only lasted for 26 years, damned Italian junk.

 

   Regards  Ian.

[Ian S CParticipant @iansc]

Hi Ian,keep the old microwave,I use the magnets out of the magnetron in a linear alternator on my feer piston hot air engine,lots of other goodies and sheet metal,the glass door is the window for a small sand blasting cabinet.Most microwaves here come from China,so we don’t have to wait so long for them to die!IAN S.C

[CirclipParticipant @circlip]

I know I’m about to be castigated for doing it, but we were so heartbroken at the loss of such an endearing family member, Zan was given full burial at the local depot WITHOUT having the indignity of his entrails being ripped out after the cause of the non communication lines were found. Bloody printed tracks/wireing, and before any say “conductive silver inks” I used to earn a living in the electronics sector.

 

   Dead is Dead, trust me I’m a Yorkshireman.

 

  Regards  Ian.

 

  Sorry guys for the highjack, put it down to the mourning process.

[Mark Smith 3Participant @marksmith3]

No worries mate good to see another voice so to speak.

Problem solved brazed a new cap on the displacer this morning and she fired with just half a dozen flicks. I can now carry on with my next one with some belief. I generally use found objects that I modify for my engines (got that phillosophy from Rizzo); at the mo I have started a pressurised engine of 32mm bore x30mm stroke. t he crankcase is machined from an old car generator case which is quite hard steel difficult to machine. The part is in  my lathe facing one side to take the cylinder flange. I am also using the bearings, crank and seal from a junked line trimmer engine, hence the 30mm stroke, the cylinder come displacer cylinder is all in one and is from a large shock absorber. I will also feature leather cup seal and teflon stuffing box in the top of the piston.

Wish me luck it will take a while.

[CirclipParticipant @circlip]

How did you form the Leather cup seal Mark??

[Ian S CParticipant @iansc]

Hi Ian see page 4 Marks got a description there.Made new power piston and cylinder,I had one 15mm bore I’ll try that with 3/8″ stroke,might make a 1/4″ crank.Mark that old generator case is pushing things in the old Barns,or did you get a new lathe?I’ll be interested in the new engine as it grows,should have plenty torque,maybe not much revs.I’v used quite a few shock absorbers,but my supply of metal and odd bits has dried up since the council closed the local tip,our stuff goes with the city stuff,very sad.Next time I’m in town I must get to the free shop at Hornby,or the Shed at Bromley,see what I can pick up.Ian I could get you a good microwave for $10-£3 or£4.IAN S.C

[wheeltapperParticipant @wheeltapper]

Posted by Mark Smith 3 on 18/09/2009 10:27:06:

Have you noticed how many people are reading this thread? I wish some one else would leap in and tell us what they are up to. I hope we are staying on subject or we might hear more of circlip’s shopping expeditions, at least we are talking about our engines,LOL.

 

Hi

I wondered if anyone would like to see my first attemt at a Stirling engine ( my first attemt at anything really ),

 

 

 

It’s Jan Ridders coffee cup engine featured in Model Engineer.

there are a few differences, mainly the plates are only 1.5mm instead of 5mm, I couldn’t get any.

so the temperature equalises quite fast unless I put something frozen on the top, then it runs at approx 60 rpm for about 20 mins.

 

because the plates are so thin I can’t put a groove for the displacer walls to seal into so I had to put insulating tape round the edge to seal the displacer cyclinder, removed for pic.

 

the displacer piston is 4mm balsa.

 

It didn’t run first time so I made a better fitting displacer rod and changed the piston to balsa (the first one was a cut down CD.

 

I was quite amazed when it ran.

 

cheers

the chuffed side of Roy

 

[CirclipParticipant @circlip]

 Thanks Ian, saw the pictures on that page, didn’t read the text,  DOH.!

 

  And thanks for the kind offer for a successor to Zan(ussi) but the same day as his departure, Ken Wood came to our rescue

 

  Another place to pick up one of Jans engines is :-

 

    http://www.projectsinmetal.com/

 

  Regards  T’other Ian.

 

  Sorry Roy, lurve the dual use of a cup of Coffee, the LTD’s are little crackers.

Edited By Circlip on 19/09/2009 15:17:54

Edited By Circlip on 19/09/2009 15:20:41

[Mark Smith 3Participant @marksmith3]

welcome, Roy, beautiful job. If your perspex ring is thick enough you could turn a groove in it to take a thin o ring, just an idea.

Ian my lathe has a very big 4 jaw chuck about 9″ dia. and very heavy. I live just down the road from the super shed 513 pages rd. drop in if you are passing. Another source of shock absorbers is your local mechanic they are usually happy for you to take away some of his junk.

Have you heard of the Little Metals co. in Stuart st off Moorhouse Ave. They sell all non ferrous metals cut to any size you like.

T’other Ian, leather piston seal is really easy to make I use leather of 1.5mm thick. This method reduces the need for such a perfect piston fit.

Edited By Mark Smith 3 on 19/09/2009 22:11:41

[Ian S CParticipant @iansc]

No Mark hadn’t heard of Little Metals Co, must look them up,havn’t bought much metal since Mico Wakefields got rid of thier scrap bin.Got to get down that end of town,I’v got some castings(40)at Wickham Foundry in Wickham st’,got to find some one to polish them,I did the first lot,never again!The leather I use was some of my Grandfathers from his factory,I think its Kangaroo,very soft but could be too dry.Wheeltapper,thats a great start,it looks good,and it goes,better than mine at the moment,then again some of mine have taken a while to get right,I use mostly my own designs and don’t always get it right first(sometimes 2nd-3rd)time.

[Mark Smith 3Participant @marksmith3]

These guys are Mico wakefields ex empoyees. They went out on their own, quite successful from what I have seen.

[Gordon WParticipant @gordonw]

I’ve made a couple of little hot air motors, which don,t go, am now planning a larger model. This will have 4″ bore hot cap, and about 2.5″ bore power cyl., depending on what I can find. Questions:- Will a leather washer seal,on a cut away piston be suitable, and can it be made the same way as a small one? Also, would a small hole in the displacer be a good idea? I’m thinking of pressure relief after temp. rise. NB:-I’m not much of a model maker, still think of M6 as a small bolt.

[Mark Smith 3Participant @marksmith3]

Sorry Gordon, but is you see from my post  earlier a hole in the displacer is a no no, some people recommend a tiny hole in very light weight displacers where they are subject to collapse but that hole would have to be miniscule. Yes larger pistons can take leather cup washers made the same way, after all (I think i’m right here) Rider Ericsons had leather piston seals.  Have you been able to work out why your engines didn’t work? Think friction, leaks, and bounce – although as you have read one of my engines had great bounce and compression but refused to run until I found a split in the top of the displacer.

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