Stirling Engine

[Burnit]

I am in the process of designing & machining an Alpha Stirling engine, as most are aware there is a HOT cylinder & a cold cylinder.
My question is in regards to the hot cylinder & what suitable materials can be used for Cylinder Bores & Piston Rings or Seals with lubrication free operation.
There is a need for good sealing & high temperature resistance as well as self lubricating features.

I thought of maybe a cast iron bore with a Graphite piston but then still i require seals or rings.

Is there a suitable type of Nylon or something that may do the job maybe even with an aluminium piston which would be helpful.

Any advise would be greatly appreciated.

Thank You

 

[patprimmer]

What temperature and load will it be exposed to.
One type of nylon has some useful properties to nearly 300 deg C

PTFE has some useful properties to about 400 Deg C from memory.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Burnit]

Hi Pat,

Temperature at the sealing surface i will try to keep as low as possible & i would think it will be around the 300 to 400 deg C hopefully less. The actual heat source temp will be higher but an attempt will be made to cool the sealing surface.

At this stage the loading factor is an unknown to me as the output of these engines to date is very low compared to everything i have worked on--IC engines & airflow development for performance engines.
Your point about loading has now got me thinking about the hot cylinder inclination, horizontal vs vertical vs inclined etc?
If a soft type of seal is used this may be an important issue as well.

What's your thought's on this?

 

[patprimmer]

I don't really know what effect seal loads on a Stirling engine.

If you want a soft polymer seal, PTFE or Nylon 4.6 wll be hard to beat.

DuPont (trade name Teflon) or Asahi (trade name Fluon) or Ticona? (trade name Hostaflon) can provide info on PTFE.

DSM (trade name Stanyl) can provide info on nylon 4.6. Carbon fibre filled might help if cost is no barrier.

I would think some softish metals might do better.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[Burnit]

Thanks for the guide to some useable products, i will look into this.

Yes, i agree that some soft types of metals may well be the way to go, it's the lubrication quality that has me stumped on this issue at the moment.

I was thinking that soft seals may not be as abrasive as some metals on the bores hence the enquiry about Nylon type materials.

 

[ivymike]

maybe look at Vespel.

 

[EdDanzer]

When designing any type of seal you need to take P/V, pressure and velocity into consideration as much or more than temperature. Most of the polymers have high expansion ratios and poor pressure capabilities at high temperatures.
You may find the seal being the limitation in design. If you lubricate between two seals it will provide many more material options.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[tbuelna]

At one time, in the not-too-distant past, there were piston rings available in Torlon (polyamide-imide) or Vespel (polyimide) resins. Torlon and Vespel both have excellent strength, high Tg limits, and can be made with a graphite fill so that they are somewhat self-lubricating. They were used as lightweight 2nd rings on drag racing engines. But obviously they were oil-lubed and only had a very short life span requirement.

http://www.solvayadvancedpolymers.com/static/wma/pdf/3/1/2/T4435_030402.pdf

Unfortunately, both Torlon and Vespel are also very expensive. And while they may be "self-lubricating", I doubt they would have adequate P-V capability to survive for long in an oil-free thermal engine piston ring application.

In fact, I don't know of any ring/liner material combination that would give adequate service life in a recip piston thermal engine without oil. The only reason that piston rings last in a conventional IC engine is because of the perfectly ultra-thin boundary layer oil film left behind by the oil control ring after every piston stroke.

Good luck.

 

[willeng]

Some time ago i was asked to look into a similar project for a customer, after testing many seal types etc i settled on Rulon.
We also settled on Water lubrication between an upper & lower seal arrangement which worked great.

The placement of the ring lands was critical in design & were kept low on the piston compared to IC engines.
This kept the seals away from the major heat source & helped a lot.

Due to the low rpm nature of the Stirling engine a lot of things work that will not in a higher rpm IC engine.

It may be a reciprocating engine but she is a SSLOW

http://WWW motion

devise & needs a good flywheel to keep her ticking over.
All said & done i was happy with the results & so was the customer & it gave a needy break from the hum drum of the oil burning much outdated inefficient IC engines.

 

[GregLocock]

So how efficient was your stirling engine willeng?




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[willeng]

Gday Greg,

FFFOORRD would love it, sorry that word is hard to say around here HaHa, especially on some Sunday's.

Like always the BS stops when the flag drops.
So, to banty around numbers without others actually being there to verify things becomes a little unrealistic, some will say WOW others will say BS without knowing the facts.

I will say however that what started out as a novel project for a customer & to relax for a while has turned into an interesting eye opening engineering experience.

The efficiency is far more than i have ever encountered before & i have pursued this with a new interest in life.
Getting this engine to "operate" rather than puff & to get it to acellerate has been a major brain strain but nice progress is being made finaly.
By the end of 2009 i will be very close to releasing some surprising "real world" statistics. Well they surprise me now!

A more compact design with a nice power transition is at hand as i speak & the work continues.

This is one interesting engine!

 

[globi5]

PTFE has the highest operating temperature of all plastics, but it has comparatively little strength and deformates over time. PEEK (Polyetheretherketone) has also a high operating temperature but much higher strength than PTFE.

PEEK PVX contains carbon fibre, graphite and PTFE and is probably one of the best options for sealing purposes, where little wear and friction is required.

http://www.ensinger.ltd.uk/docs/datasheets/TECAPEEK PVX.pdf

But, if you want to exceed 300 deg C for longer periods of time, you'll probably have difficulties to find any suitable plastic material at all.
In that case you should consider conventional piston rings.

 

[sbev]

I am familar with a New Zealand company that produces an 800 watt electrical generator that runs on diesel, natural gas, or propane. It uses a Stirling engine for its power engine. I believe it uses ceramics for its high temp materials. Check out this link for Whisper Gen at

http://www.whispergen.com/main/technology/

 

[marcusaurailius]

Burnit, with regards to your ring queery, have you thought about some rings called...CLUPERTS...
These are used in a lot of areas where there is no ..oil.. as such,ie steam engines,pistons
these are a double ring,in a ring!!
the bloke who makes them,is called
Alan Smith, and is in Maryport England,.
goes under the name of , The Clupert Piston Ring CO
look up clupert piston rings

Marcus

 

[vwhammer]

Have you considered building both the cylinder and piston out of Al then hard anodize the cylinder and coat the piston with xylan.
Then you could develope a gas bearing arrangement to locate the piston in the bore
After employing this method you could get the tolerance tight enough that you would not need rings at all.

 

[patprimmer]

The tried and proven coating for aluminium bores is Nikasil.

On anything with more than say 1" bore and less than say 6000 rpm, rings are necessary due to different expansion rates as he piston normally runs hotter.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers for professional engineers

 

[marcusaurailius]

Mr Burnit, I got you the full name of the ring manufacturerfor the Clupet,s i mentioned earlier

the Clupet piston ring and gauge company
they on the solway industrial estate
grasslot
maryport
cumbria
england

the bloke is Alan Smith

give him a ring, and he will be able to sort you out,
if he cant ,then no one will!!

just out of interest, Volvo did a lot of work on this in the early seventies, as i think did Leyland.but you may already know this

and a bit of humour, a well know respected engineer,at the turn of the 19th century said to its designer,

I can see that it works, but i cant see how it works!!
any one know who???

Regards, Marcus

 

[marcusaurailius]

Burnit, waas talking to the bloke who makes piston rings, and while in his place he told me that he was makink rings for 3 stirling engines at the moment

he aked what size you wanted, i did not no

but he has got ..hundreds of rings ,all cluperts on the shelf,

so if you no got fixed up, go and crak with him

regards Marcus

 

[thundair]

Two points that I don't think were covered;

1. PTFE has a high expansion rate at temperature. I use to use Teflon rods inside my carbon fiber wrist pins to push from the inside as I cured them.

2. If you would use all ceramic you could precision (grind) machine mating sleeve and piston and lap them into final fit. The piston would incorporate an air labyrinth to help maintain seal.

Just spinning off the top of my head, but it sounds good to me.

Cheers

I don't know anything but the people that do.

 

[TDIMeister]

The best thermal efficiencies I've seen in recent literature for commercial and research Sterling engines are in the high-30s percent. Much higher and I would be suspect. There's only so much one can do if peak cycle temperatures are in the range of 1300 K compared to IC engines that routinely go over 2500 K.