Hot Potato Engine

[Tom Booth]

I've never previously given much attention or consideration to how a lamina or laminar (?) flow Stirling engine works.

Recently I've been playing around with it though.

Taking the information about thermodynamics in the other thread, I've come up with the basic theory that the "regenerator" in a laminar flow engine simply acts as a temporary heat accumulator.

When the air does work pushing the piston it looses energy and looses heat which makes it easier to compress.

I think that as the piston moves inward, the cool air is forced through the "choke" and similar to how when you purse your lips to blow out a candle, the air being compressed by the piston blows through the choke in a narrow stream into the hot regenerator and heats up and expands forcing the piston back outward.

If this is what is going on then it would seem to me that there is probably not much air penetration very far into the regenerator and most of the heat transfer is taking place only at the end of the regenerator closest to the piston. The rest of the regenerator constituting not much more than a dead air space, possibly also constituting a pressure buffer and heat drain - drawing heat away from where it is needed, at the point where the cool air is passing in a stream through the choke. In other words, the regenerator is probably bigger than necessary and the excess furthest away from the piston only serves to reduce the efficiency of the engine, dissipating heat away from the piston and occupying dead air space that reduces the compression ratio.

There probably isn't a very great volume of air passing through the regenerator for heat transfer. This would explain why the heat source must be positioned at the very end of the regenerator nearest to the piston for the engine to run at all.

I then thought that if the air could be forced through the regenerator in a more positive fashion, if the "dead air space" could be eliminated, the regenerator reduced in size to only what is required for temporary heat accumulation, the power and efficiency of this type of engine might be increased significantly.

I think that the idea of the necessity for a long regenerator heated at one end to produce resonant sound waves is just so much nonsense.

The way I look at it, the heated end of the regenerator only accumulates heat, similar to the way a baked potato wrapped in aluminum foil absorbs and holds heat.

Very little heat is transferred from the candle of burner to the air inside the tube. Rather the heat passes into and accumulates in the regenerator and mostly stays there until hit with a blast of compressed air as the piston travels inward pushing air through the choke.

I thought that if the regenerator itself could be moved through the air in some way as the piston is approaching TDC it would transfer much more heat to the compressed air throughout the length of the regenerator rather than just at the end where the air is hitting it directly.

So putting all these ideas together I came up with what I've dubbed a "hot potato engine".

Rather than a long wad of stainless steel turnings for a regenerator, there is just a relatively short slug.

I thought also that using an aluminum tube rather than glass would make for better heat flow from the heat source into the "potato" or slug of stainless steel or perhaps copper wire mesh constituting the regenerator.

Add a spring to the "potato" and a push rod to the end of the piston and you have a "hot potato" that gives up its heat to the air when pushed by the piston a few degrees before TDC.

I think that this should release a healthy blast of heat and result in rapid expansion of the air just at the point where it is most needed, when the piston is approaching and passing through TDC (Top dead center).

Anyway, I made up this little gif animation to illustrate the idea:

http://prc_projects.tripod.com/hot_potato_engine.html

It seems to me that if this works as well as I think it might, it may be a way to easily convert an internal combustion engine into a laminar flow-like Stirling engine.

Just take out the cam shaft and valve lifters, remove the cylinder heat, bolt on an extension cylinder with a spring loaded "potato" inside, drill a hole in the top of the piston and add a push rod. Apply heat to the "potato" tube and you have converted a four cycle IC lawn mower engine into a two cycle laminar flow "hot potato" Stirling engine.

I wonder if it would work.

[goat]

Hi Tom,

You have a load of ingenious ideas, I like it. I'm not sure that you can call your 'hot potato' a regenerator, though. The regenerator in a Stirling engine absorbs heat from the working fluid as it passes from the hot source to the cold sink and gives it back to the working gas when it passes back the other way.

Looking at your animation, the hot potato absorbs heat from the source and then passes that to the working fluid, which is a different thing to what the regenerator does. Just because it's not what might technically called a 'regenerator', it could still improve the performance of your engine by doing what you've designed it to do: Improve heat transfer from the source to the working fluid.

Have you considered having a 'cold potato' at the sink end, too?