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