LTD Stirling Engine Development-Displacer Design

[VincentG]

Makes sense thank you. That would be a pretty good design with the plunger size being very tunable.

[VincentG]

Finally got around to testing this out. I've added aluminum foil to either side of the displacer that comes in contact with the hot and cold plates, picking up or losing temperature and doubling the effective heat transfer area. The foil has very low thermal mass so it should have a fast temperature gain/loss especially being backed up by insulation. An added benefit should be radiant shielding to keep the displacer itself at a more stable temperature.

It seems to work remarkably well, allowing the engine to run on lower temperature differentials than it would after the spring dwell system was added(the spring adds significant resistance compared to stock).

foil displacer.jpg (217.64 KiB) Viewed 4933 times

[Tom Booth]

Finally got around to testing this out. I've added aluminum foil to either side of the displacer that comes in contact with the hot and cold plates, picking up or losing temperature and doubling the effective heat transfer area. The foil has very low thermal mass so it should have a fast temperature gain/loss especially being backed up by insulation. An added benefit should be radiant shielding to keep the displacmer itself at a more stable temperature.

It seems to work remarkably well, allowing the engine to run on lower temperature differentials than it would after the spring dwell system was added(the spring adds significant resistance compared to stock).

It might be interesting to try different thicknesses of foil, or even multiple layers. Aluminum foil cools down,(releases heat) with such great rapidity there should, I imagine, be some optimal thickness that would provide a good balance between heat absorption/retention when in contact with the hot plate and heat release when raised and heating the working fluid.

I'm inclined to think that thicker should be better up to a point where weight becomes an issue because any heat not released would do no harm and just be retained by the foil so less reheating would be necessary the next cycle.

Also it might be interesting to perforate the foil to increase surface area, or possibly even use something like aluminium window screen

A quick and easy way to perforate the foil might be to press a wire brush or something similar into it. Such perforations might make multiple layers more effective without significantly increasing "dead air space" between layers.

[VincentG]

A lot of things to try, but I'm just trying to lay down a foundation for other guys to build on. Never been good at the nitty gritty single focused detail work. The more surface area the better.

any heat not released would do no harm and just be retained by the foil so less reheating would be necessary the next cycle.

In my bench testing of other displacers, I've found this is not quite true. The issue is that the displacer can build in temperature until it is overheated. With such a large surface area and the fact that its moving through the full volume of gas, it tends to prevent the gas from cooling down. I have not done the math yet, but I suspect foil is a good match to the thermal mass of the gas. In my opinion the foil should optimally give up all its heat to the gas each cycle.

This seems to be one advantage of your regenerative type displacers. They have active cooling throughout, so they maintain temperature better than other solid materials. The xps foam has so much engineering behind it that does everything right for a displacer. It just needs its own high temperature protection against the heating element if MTD or HTD.

[Tom Booth]

A lot of things to try, but I'm just trying to lay down a foundation for other guys to build on. Never been good at the nitty gritty single focused detail work. The more surface area the better.

any heat not released would do no harm and just be retained by the foil so less reheating would be necessary the next cycle.

In my bench testing of other displacers, I've found this is not quite true. The issue is that the displacer can build in temperature until it is overheated. With such a large surface area and the fact that its moving through the full volume of gas, it tends to prevent the gas from cooling down.
...

I see your point.

Inevitably heat is being transfered into the gas when the displacer is lifted. All the way up as well as all the way back down.

Less heat retention in the displacer though, can only go so far to remedy that as although the bottom of the displacer may run out of heat, the hot bottom plate will continue heating the gas through nearly the whole 360° regardless, if the motion is sinusoidal/following the crank.

I'm not sure what your plans is, but my general plans for future experiments is to take a lesson from the magnetic displacer engine that just "naturally" reduces the lift or exposure time to a very brief little "micro-hop" barely uncovering the hot plate at all and then immediately covering it back up again all very near to TDC.

That results in a lot of unnecessary "dead air space" above the displacer that is not heated, So...my plans are, though I haven't gotten around to testing it yet: reduce the throw on the displacer connecting rod (attach the displacer rod closer to the center of the crank/flywheel) and reduce the height of the displacer chamber (or make the displacer thicker) And have a very long "dwell" covering the hot plate through maybe 350° and only exposing the hot plate briefly at TDC through maybe 10° of crankshaft rotation, mimicking an IC engines brief "explosive" ignition as much as possible. Then the displacer can be "superheated"/preheated (glowing cherry red HOT) through 350° with a super hot double blast of radiant heat at TDC when the displacer is briefly lifted.

Actually my goal is, or was, to eliminate the cold side altogether by making it into a completely sealed and thermally isolated air spring.

If you are working on an open system with no piston though, that's a whole different ball game. Probably. You'd want the "lung" to "breath" as deeply as possible. I haven't really thought about it much.

Maybe with Tesla valves you could just have a continuous wind whistling through the thing. With no pistons, ... driving a turbine I presume?

[Tom Booth]

Ultimately the displacer lift volume/time should be variable so heat input can be increased or decreased as needed under a variable load. How to engineer that mechanically though, I've been drawing a blank.

Maybe a pump that could transfer gas from displacer hot space to air spring buffer space and back, more going to hot displacer space under load.

The advantage of an air spring I think, or hope, will be that it will return the pressure and "heat of compression" immediately, during the expansion stroke of the power piston, right after the heat blast at TDC.

All planned future experiments that may never happen, but maybe you or someone else could try it if I never get around to it.

[VincentG]

I actually tried the small blip of heat input when I had the ring cam engine going. It does not work with a cold side, as the pressure fell too quickly when the displacer drops back down. I do think it would work better with no cold side and just insulation. So I now better understand what you've been driving at with just cooling from expansion after a short heating phase.

Previously I was only able to think of standard operating conditions where heat is applied the whole way to BDC of the power piston.

I think about it like this...

Imagine a potato cannon of extremely low thermal mass and a fresh charge of fuel and air. Now fire the cannon. At the moment all of the fuel and air is fired out and all that remains inside is high temperature residual gas, shut a valve. Cool the cannon. Now if you open the valve the higher pressure atmosphere rushes in.

What do you call the force of the returning air. Is this energy that other engines throw away as a loss?

Maybe with Tesla valves you could just have a continuous wind whistling through the thing. With no pistons, ... driving a turbine I presume?

Thats the basic idea of my ambient engine scheme. But for the LTD project, I'm trying to stick more conventional solutions that would be better accepted by people all over the world looking to turn a crankshaft to power their existing needs.