Robinson hot air engine that will not run

[isambard]

My friend and I are trying to get a robinson type hot air engine to run but after 4 attempts and 18months making new parts etc. we have run out of ideas. Has anyone a set of plans we could have a copy of doesn't what size or type as my friend is good at scaling parts. any help will be welcome :wall:

 

[deverett]

What design are you using? If made from Polly Models (UK), then you will probably require an external supplementary cooling tank. The displacer piston made of brass is not a good idea, but if you want to still use it, make sure the wall thickness is down to about 15 thou.

Robinson type engines in full size had the regenerator incorporated in the displacer piston. The idea can be copied by making the piston from very thin walled stainless steel, perforated top and bottom (the more holes the better) and the cavity loosely filled with steel wool.

Dave
The Emerald Isle

 

[isambard]

Hi Dave I think it was a Polly engine when my friend bought it but it would never try to run as it always got to hot, so Roy it air cooled and it still won't run. When it is spun over by hand it has slight backpressure but we don't know if that's right. We have no drawings but the displacer and power cylinder seem to be the right ratio to each other. I will forward your reply as I know the displacer has no holes its an aluminium tube closed both ends. Thanks replying
Regards Tom

 

[purpleknif]

Timing ?
:wall:

 

[isambard]

It hasn't got any timing adjustment that we know of if we could get a drawing of any Robinson type it help

 

[deverett]

That is not a casting kit that I recognise - if a casting kit at all. The biggest problem I can see is that you have the fire below the cold end (the air cooled - finned part) so you are not getting adequate cooling.

On an original Robinson, the platform (where the power cylinder and bearing block are mounted) is a cooling jacket with holes on each side of the centre line near the bearing block. The water would evaporate as it got hot and was topped up from time to time.

The Polly model version did not have an adequate volume of cooling water due to poor design, which was done for simple construction. As said earlier, this needs external cooling for extended running. Have a look at http://users.moscow.com/oiseming/lc_ant_p/pic_Prj4.htm for a description of a rebuild of a Polly engine.

Dave
The Emerald Isle

 

[isambard]

It had a plate with holes for water when my friend got it but it got hot very quickly so he tried it with the cooling fins. Do you know of any drawings or plans then we can put it back to water cooled if need be thanks for your help it is appreciated

Tom

 

[isambard]

Hi Dave could you explain the regenerator please we can't find anything about it a pencil sketch would help if possible, the displacer is aluminium so we are not sure if that's OK. We would be very grateful of your help as we don't understand this engine other types we have made worked very well Stirlings etc. Tom

 

[fcheslop]

It looks to be along the lines of the ET Westbury design
I have seen this and another design on U tube.
Stilldrilling rebuilt one on madmodder called Brasso I think.
I have some drawings by E T Westbury and did start a build but there's a crate load of errors so have put it to one side for now.
I dont visit this forum often but log into the other mem often
cheers

 

[Hopper]

Have you sorted out the usual bugbears of newly built Stirling engines: friction and sealing?

A common trick is to hook the Stirling up to an electric motor of some sort and let it run gently for a few hours to wear in the mating surfaces. Avoid oiling any surfaces -- it gums up and creates friction. Ball bearings on the crankshaft help too.

What material are the displacer cylinder and piston made of? Stainless steel works best as it is a poor conductor of heat -- helps keep the cold end cold(ish).

 

[isambard]

Hi yes we have sorted the friction and sealing we have made stirling engines before but this one was built when we got it. The displacer was brass but we made one with aluminium as it had worked on other types and we are going to make a smaller power cylinder to put the ratio right.
we tried it today without the shroud round the flame and it did try to start.
We keep trying different things so hopefully we will succeed.
Tom

 

[deverett]

If you have made a new displacer piston, then try modifying that first, but Aluminium is not a good displacer material. Get the top (or bottom) off. Ensure the side wall thickness is as thin as you can make it - typically 10 - 15 thou. Then drill a series of holes in the top and bottom plates so that they both resemble perforated metal. Loosely fill the cavity with steel wool and seal up the piston again. Not guaranteed to make this one work, but will help. If it doesn't do the trick, try modifying the original brass displacer in the same way. If still no go, then you might consider changing to Stainless steel.

As others have said make sure you minimise friction and have good seals, which you say you have done.

Dave
The Emerald Isle

 

[fcheslop]

Why not simply sleeve the power cylinder down to try it
I cannot see a heat barrier gasket fitted
What is the ratio on his engine
cheers

 

[littlelocos]

Hello,
I've spent a good deal of time studying Robinsons (and other) hot air engines lately. Hopefully the following can be of help.

What is the material and thickness of the hot cap? Preference would be stainless steel with the walls as thin as possible. If made in several pieces, the cap can be copper and walls stainless steel. (silver-brazed construction)

The hot cap needs to be isolated as well as possible from the cold end. Gaskets and/or o-rings may be needed to provide a thermal break.

Judging from the fins, it looks like your engine has a tall cold end. A fan blowing on this can be a help.

The Robinson engines typically REQUIRE a regenerator and will likely not run without one. McMaster-Carr sells fine stainless steel wool. It makes a great packing for the regenerator. Picture a tuna can with holes in each end, packed with stainless steel wool and you've got it.

The key to a successful regenerator is that it holds a temperature DIFFERENTIAL from cold end to hot and serves to heat AND cool the air blowing through it.

Below is a photo of a regenerator I designed for my current project - a 1/2-scale Essex Caloric Engine. The displacer it replaces is on the left.

Let me know if this is any help or if you have any other questions,
Thanks,
Todd.

Todd & Joy Snouffer
Littlelocos Model Engineering
www.LITTLELOCOS.com
www.facebook.com/littlelocos

 

[isambard]

Hi Todd & Joy thanks for your helpful suggestions we have spent today making a new smaller cylinder as suggested by fcheslop thanks to him too.
we have ordered some gasket paper (heat proof) to make a thermal barrier and we have started a new displacer piston to include a regenerator so we will let you know if it works in due course.
many thanks from Tom & Roy

 

[littlelocos]

Tom & Roy,
We're looking forward to hearing how you make out.
Enjoy!,
Todd.

 

[isambard]

Hi Todd & Joy and everyone who helped good news yesterday July 4th we had it running at last so thanks to all who gave suggestions. We made a smaller power cylinder and used thickish heat resistant gasket material for a thermal break. Then Roy made a regenerator like Todd's and put the burner under it and it tried to run so the flame was increased and it worked we just have some finishing touches to do and it's done.

Thanks again to all who helped it took 18 months on and off but we are well pleased Best regards from Tom & Roy

 

[littlelocos]

Yahoooooooo!!!
That's GREAT news!!
Congratulations guys!


Todd & Joy.

 

[deverett]

Well done. Perseverance paid off handsomely.

Dave
The Emerald Isle