Thoughts on this Stirling Enginge Design Appreciated

Moderator: stan.hornbaker

Forum rules
Be nice!
haselhurst
Posts: 8
Joined: Wed Mar 18, 2009 12:18 am
First Name: Geoffrey
Last Name: Haselhurst

Thoughts on this Stirling Enginge Design Appreciated

Post by haselhurst »

Hi Everyone,

I am in the process of building a Stirling engine. I have built one
before but it did not work - that is how I learn! Since then i have
thought of hundreds of designs and now feel that I can build a
relatively low cost Stirling engine using two petrol motors (one for
the hot side, one for the cold side).
I have put up a webpage on this with the complete details - would
really appreciate any thoughts.
http://www.spaceandmotion.com/invention ... engine.htm
Thanks,
Geoff Haselhurst
stan.hornbaker
Posts: 532
Joined: Thu Jun 20, 2002 6:01 pm
First Name: William S.
Last Name: Hornbaker

Thoughts on this Stirling Enginge Design Appreciated

Post by stan.hornbaker »

Let me suggest that you review the Post "Practical Use" about 12 threads prior to yours.
Second I have looked at the referenced web site and it appears that you have not taken into consideration some of the basic design/physical impediments to approaching a working engine.
haselhurst
Posts: 8
Joined: Wed Mar 18, 2009 12:18 am
First Name: Geoffrey
Last Name: Haselhurst

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by haselhurst »

Hi William,

Thanks for your reply. I read the other post - but did not see anything there relating to my design - just that such a machine would be useful but no one has been able to build it yet.

You wrote that i seem to have overlooked some of the required physics to make a working engine. Could you please be precise and state what I have missed - why this machine will not work.

Thanks,
Geoff
stan.hornbaker
Posts: 532
Joined: Thu Jun 20, 2002 6:01 pm
First Name: William S.
Last Name: Hornbaker

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by stan.hornbaker »

Carbon steel will deteriorate at the combustion temperatures and moisture content of most combustible fuels.

One of the big impediments to heat transfer from the outside to the gas inside the hot cylinder is the heat transfer film factor on the inner surface and oil will increase this film factor thus slowing down the rate of heat flow.

Your design lacks a heat exchanger or regenerator between the cylinders.
The cold side is 1/2 full of copper. How is this done without impacting on piston movement?

What is the advantage of the chain drive over the original crankshaft?
Why make that modification. with staggered operation of 8 cylinder pairs it would make a smoother operating machine. Send your phone number, to wa4tjj@arrl.net for a personal discussion.
haselhurst
Posts: 8
Joined: Wed Mar 18, 2009 12:18 am
First Name: Geoffrey
Last Name: Haselhurst

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by haselhurst »

Hi William,

Again, my sincere thanks for your time and patience in replying.

I have pasted in your comments below and added my replies. I have also emailed you this with my phone number enclosed (and i can phone you).

"Carbon steel will deteriorate at the combustion temperatures and moisture content of most combustible fuels."

I have not seen anything on the internet that suggest seamless carbon steel will have problems at 300 degrees Celsius. Do you have a reference for this? And the future machine, should it work, would be run from solar heat which has no moisture content.
According to http://www.engineeringtoolbox.com/astm- ... d_742.html the steel I am using has a working pressure of over 100 atmospheres at 300 degrees C.

"One of the big impediments to heat transfer from the outside to the gas inside the hot cylinder is the heat transfer film factor on the inner surface and oil will increase this film factor thus slowing down the rate of heat flow."

I am using the petrol motor's metal pistons to drive oil / liquid pistons above the metal pistons (the motor is filled with oil). So my 90mm diameter by 300 mm long heating pipes are 3 times the volume of each cylinder, which allows me to 2/3 fill the liquid piston space with copper metal AND 3 by 25mm pipes that run diagonally through the 90mm heating pipe thus helping in heat transfer. Thus on the hot side the hot gasses from the fire will actually flow through these 25mm pipes to help heat the oil and thus working gas inside the heating pipes. For the cooling side the design is the same but I will run water through these 25mm pipes.
The total surface area of the heating and cooling pipes is 1m^2 for both the hot and cold sides. i.e. 6 times 1600 cm^2 for each heating pipe, the same area for each cooling pipe. This is one advantage of using liquid pistons.

"Your design lacks a heat exchanger or regenerator between the cylinders."

The hot and cold heating / cooling pipes (90mm) are connected across the top by 25 mm pipe by 400mm. This pipe will be 1/2 full of copper pipe which acts as a regenerator. I have kept this volume small to begin. While the regenerator helps with efficiency in terms of storing heat as the working gas moves from hot to cold side and back again - it causes losses by increasing the dead space where the gas is not being heated / cooled above the liquid pistons. This can be adjusted in time to get optimum results.

"The cold side is 1/2 full of copper. How is this done without impacting on piston movement?"

My diagram is in error here - it should be 2/3 full of copper and include the three diagonal 25mm pipes that run through the heating and cooling pipes. The metal piston is 100mm below the surface of the oil liquid pistons so there is no problem with having the copper and 25mm pipes in the heating / cooling pipes.

"What is the advantage of the chain drive over the original crankshaft? Why make that modification. with staggered operation of 8 cylinder pairs it would make a smoother operating machine."

Each motor still has its original crankshaft which drives the 6 pistons. The end of each crankshaft for the hot and cold motors / pistons is connected by a chain drive so the two motors are connected and are rotating 90 degrees out of phase. Thus effectively we are building 6 Stirling engines together (3 pairs of pistons each 120 out of phase as per the design of 6 cylinder petrol motors and their crankshafts).

"Send your phone number, to wa4tjj@arrl.net for a personal discussion."

I would love to discuss this with you. I am emailing this to you now with my phone number. If you email me back your number I can call you - or you can call me collect.

I hope my comments above help clarify things - and I really really appreciate your comments!

Cheers and thanks,
Geoff Haselhurst

haselhurst at yahoo.com
bptdude___2569
Posts: 177
Joined: Sat Jan 13, 2007 3:31 am
First Name: Joe
Last Name: McLean

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by bptdude___2569 »


This is fun, a realistic exchange.

Here are some thoughts from a guy who never actually tried to build one! :)

Heat exchanger:

You may want to start with too much, rather than too little. I do not like heat exchangers, but having one, even with dead space added, may be what makes the engine turn at all, at first. Then try to back down as much as possible, to increase engine output power.

Also, remember you do not want the diameter to be so small your engine is fighting to move the air through it. Pressure is the gold that the engine creates from the temperature.

And, be carefull about heat loss out the exchanger. Good model engine heat exchangers are short, wide, and stuffed with as much steel wool or whatever is used, without restricting the flow.

The design for liquid piston is good, and feeding the heat and cold in the way you do is good.

In general, most of us would be amazed this thing works at all, simply because the heavy weight of the engine as designed for internal combustion. If I had to pick an engine for somebody to play with, I would have chosen the old BMW 700cc three cylinder motorcyle engine, that had no flywheel or counterweights. But, I am sure rooting for you.

As a design concept, I personally find it odd that you are heating and cooling the oil in such a nice way, but then the working gas has to take a second exchange from the oil, which does not seem as clever. Am I missing something?

haselhurst
Posts: 8
Joined: Wed Mar 18, 2009 12:18 am
First Name: Geoffrey
Last Name: Haselhurst

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by haselhurst »

Hi Joe,

Thanks for your comments - I have pasted them below with my replies.

And yes, I love machines - they are logical - they represent physical reality and all the wonderful things we can do in this universe. So I see the Stirling engine as a lovely logical / engineering challenge! And with great importance to humanity!
#########-

"Heat exchanger: You may want to start with too much, rather than too little. I do not like heat exchangers, but having one, even with dead space added, may be what makes the engine turn at all, at first. Then try to back down as much as possible, to increase engine output power. ... Also, remember you do not want the diameter to be so small your engine is fighting to move the air through it. Pressure is the gold that the engine creates from the temperature. And, be careful about heat loss out the exchanger. Good model engine heat exchangers are short, wide, and stuffed with as much steel wool or whatever is used, without restricting the flow."

I assume here that you are talking about the heat exchanger in the regenerator. Stirling engines should work without the regenerator - it just makes them more efficient as I understand things. But I will re-think my regenerator! I agree, you need to know flow rates of the gas and ensure there is no significant pressure loss - I can calculate this reasonably closely so hopefully it won't be a problem.

For me the critical heat exchange is in the liquid pistons on the hot and cold side, to heat and cool the working gas (Nitrogen) as quickly as possible. I have a total surface area of 1m^2 on both the hot side and the cold side - I wonder if anyone knows a useful rule of thumb for this surface area vs. the volume / mass of the working gas.?

I re-did my calculations today for the heat flow, as follows (please correct me if I made any errors);

Specific heat of Nitrogen 1.04 J/gc, Density of Nitrogen = 1.25g / litre

Thus my 3.9 litre motor at 60 atmospheres = 234 litres = 292 grams

If I assume 5 rps then mass of gas = 5 * 292 = 1,462 grams = 1.462 kg nitrogen to be raised 200 degrees C each second.

Heat (Joules) = 1,462 grams times 200 C times 1.04 = 304,200 Joules = 304 KJ= 304 KW as it does this each second.

So it takes a lot of energy to heat this much gas this quickly!!

Heat Conduction

The total surface area of my heating pipes equals 1 m^2 = 10,000 cm^2

H = kA (T2 - T1)/L (joules/second) where k = thermal conductivity of carbon Steel = 50 J/s-m-C

H = 50 by 1m^2 by 200 C / 0.01m = 280,000j/s = 1,000 KW = 1MW

So it seems that I can get enough heat flow through the steel with this surface area.

** However, I need to calculate heat from the steel to the copper inside and then the oil and nitrogen!!

Thermal conductivity of Nitrogen = 0.024, oil = 0.15 and copper wire = 290 (so having lots of copper wire in contact with the steel heating / cooling pipes is important)

I may increase the length of my heating / cooling pipes to 50 cm such that 80% can be filled with copper - hopefully that will be ok. Tomorrow I will try and calculate the heat flow through the steel to the copper and then to the nitrogen gas and oil.

"The design for liquid piston is good, and feeding the heat and cold in the way you do is good."

Thank you. I see two advantages of the use of liquid oil pistons above the Aluminum alloy motor pistons;
1. You can add lots of copper to your volume - the oil just acts to displace the nitrogen from the hot side to the cold side.
2. You get the main heat source away from the metal pistons / motor.
"In general, most of us would be amazed this thing works at all, simply because the heavy weight of the engine as designed for internal combustion. If I had to pick an engine for somebody to play with, I would have chosen the old BMW 700cc three cylinder motorcycle engine, that had no flywheel or counterweights. But, I am sure rooting for you. "

Well I will be amazed if it works too! But I like experimenting, I like physics / logic, I like machines, and I hate pollution and think our world really needs help! So I have lots to motivate me. And I don't see any problems with using the petrol motor, once it is turning slowly the weight is pretty irrelevant I think (thoughts??)

"As a design concept, I personally find it odd that you are heating and cooling the oil in such a nice way, but then the working gas has to take a second exchange from the oil, which does not seem as clever. Am I missing something?"

See two point above. The oil is really just to displace the working gas around the copper. I assume most of the heat flow will be through the carbon steel / copper, but you will get a bit through the oil as well.

Thanks Joe - your enthusiasm and kindness are two very nice human traits - I really appreciate your comments and time.

All the best,

Geoff
bptdude___2569
Posts: 177
Joined: Sat Jan 13, 2007 3:31 am
First Name: Joe
Last Name: McLean

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by bptdude___2569 »


Well, I am easily bribed by mere kinds words!
*grins*

Yes, when I said heat exchanger, I did mean regenerator. The regenerator is one of the mysteries of the Stirling. So many practical people who build engines will swear you will never get one running without one. But yes, if you eventually get rid of it, after you have a working engine, you will be the one to crack the problem of Stirlings not having usable power output.

What they do, among other things, is pre-expand the gas going into the hot side, before the cylinder is ready, as in before Top Dead Center. It does the same bad thing on the cold side. But without it, your engine probably will not clear some critical hurdle to make it go round an round.

Also, the regenerator provides a wall between the hot and cold side, that allows gas to pass through, but temperatures to remain devided. You will not find this explained often.

All these things have to do with the way Stirlings are heated and cooled. My ideal Stirling has no need for a regenerator because it will flash heat and flash cool the gas. As such, it hopefully will need no such wall between the two sides.

Enough of my dream engine, let us get back to yours.

Let us assume your method of heating the oil is good enough for now, and you need to heat the gas much much faster. { everything I say about the hot side, also applies to the cold side }.

While you have the cylinder part filled with oil having nice heating pipes from the outside, you might think of changing your engine design as follows.

When the oil in the hot side is pushed up the cylinder, it should immerse a second set of coils, copper, that are just passive. As the oil comes up, it soaks the copper mesh, heating it. When the oil level goes down, to be replaced by the working gas, the gas then has much much more surface area to heat it with. Physically, it is the same materials as a regenerator.


bptdude___2569
Posts: 177
Joined: Sat Jan 13, 2007 3:31 am
First Name: Joe
Last Name: McLean

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by bptdude___2569 »


Funny thought occured to me while running your engine in my head. I do understand the use of oil to help speed the transfer of heat. I also understand this is a cool trick to not have the oil contribute to the dead space just gas would do.

But ... because the oil seems to be just loose in the cylinder, the engine would have to be on a flat level surface, and not used in a machine that moves. Even worse, not only will the piston have to overcome the inertia of the heavy fluid on each engine cycle going up, on the way down, on the cold side, might go down faster than the oil would drop by gravity.

This is "hot side focus" issue... :) On the hot side, the expansion of gas should push the cylinder down, no problem. But on the cold side, it is not merely a matter of bleeding off excess heat, like in an internal combustion, but the cold side is suppose to do work also, so the engine both pushes and pulls. On the cold side, picture a partial vacuum trying to pull the column of oil up, and hence pull the cold side cylinder up. If you could look inside the cold side while it operated, you might be quite surprised! *grins*

haselhurst
Posts: 8
Joined: Wed Mar 18, 2009 12:18 am
First Name: Geoffrey
Last Name: Haselhurst

Response to Thoughts on this Stirling Enginge Design Appreciated

Post by haselhurst »

Hi Joe,
Comments below to your comments.
Cheers, Geoff

Yes, when I said heat exchanger, I did mean regenerator. The regenerator is one of the mysteries of the Stirling. So many practical people who build engines will swear you will never get one running without one. But yes, if you eventually get rid of it, after you have a working engine, you will be the one to crack the problem of Stirlings not having usable power output.

What they do, among other things, is pre-expand the gas going into the hot side, before the cylinder is ready, as in before Top Dead Center. It does the same bad thing on the cold side. But without it, your engine probably will not clear some critical hurdle to make it go round an round.

GH - I agree with the regenerator - as you said earlier. As I see it, it needs to store a lot of heat, it needs to have a small volume to reduce dead space, and it needs to have minimal flow resistance (as gas flows at around 100m/s through it). I will use one - probably just a shorter version of my 90mm heating / cooling pipes 2/3 full of copper. It is complex, as you mention there are bad things about it - so a bit of experimenting over time is a good approach. See what works best for the particular machine.

Also, the regenerator provides a wall between the hot and cold side, that allows gas to pass through, but temperatures to remain divided. You will not find this explained often.

GH - Completely agree. You want it to store heat, but not have the heat flow from hot side to cold side. I will add insulation between the copper - in the middle of it . I have thought of this!

All these things have to do with the way Stirlings are heated and cooled. My ideal Stirling has no need for a regenerator because it will flash heat and flash cool the gas. As such, it hopefully will need no such wall between the two sides.

GH - I have read of this idea. But how do you do this?

Enough of my dream engine, let us get back to yours.
Let us assume your method of heating the oil is good enough for now, and you need to heat the gas much much faster. { everything I say about the hot side, also applies to the cold side }.

While you have the cylinder part filled with oil having nice heating pipes from the outside, you might think of changing your engine design as follows.

When the oil in the hot side is pushed up the cylinder, it should immerse a second set of coils, copper, that are just passive. As the oil comes up, it soaks the copper mesh, heating it. When the oil level goes down, to be replaced by the working gas, the gas then has much much more surface area to heat it with. Physically, it is the same materials as a regenerator.

GH - This is correct and how I plan on building the heating and cooling pipes - they are 2/3 full of copper pipe / wire with good contact to the steel pipe they are in. So I expect most heat to flow through metal rather than metal to oil to metal. But both are good.

Funny thought occured to me while running your engine in my head.

GH - One of the lovely things about being human, a creative imagination. I am a philosopher (yes we are odd creatures!) and you soon realise that imagination is a two edged sword, the source of all truth, and the source of all error! But that is how I build things - with theory and imagination to begin. But experience / experiments are the ultimate source of truth about reality. That is one thing I like about machines - they confirm or deny our theories!

I do understand the use of oil to help speed the transfer of heat. I also understand this is a cool trick to not have the oil contribute to the dead space just gas would do.

GH - Exactly. You can 2/3 fill your heating and cooling spaces with copper and use liquid pistons / oil to move working gas from hot to cold side around the copper. You can't do that with solid metal pistons alone, obviously.

But ... because the oil seems to be just loose in the cylinder, the engine would have to be on a flat level surface, and not used in a machine that moves. Even worse, not only will the piston have to overcome the inertia of the heavy fluid on each engine cycle going up, on the way down, on the cold side, might go down faster than the oil would drop by gravity.

GH - We will find out. I shall try and calculate the acceleration and see what physics says about this. And it is being pushed down on the expanding cycle. I don't see why a bit of movement would cause any real problems, so what if the oil splashes around a bit. But it is designed as a stationary machine (maybe on wheels so you can move it around a bit)! It will weigh 500 KG!

This is "hot side focus" issue... :) On the hot side, the expansion of gas should push the cylinder down, no problem. But on the cold side, it is not merely a matter of bleeding off excess heat, like in an internal combustion, but the cold side is suppose to do work also, so the engine both pushes and pulls. On the cold side, picture a partial vacuum trying to pull the column of oil up, and hence pull the cold side cylinder up. If you could look inside the cold side while it operated, you might be quite surprised! *grins*

GH - Because the engines are in a pressured cylinder you will find that the pistons are actually being pushed up from the crank side when it is cooling / contracting. This use of heat to expand and work then transfer and cool to contract and work is what I love about Stirling engines! So simple and clever and efficient.

It is nice discussing this - big thanks again.

Geoff
Post Reply