Stirlingengineforum.com

(ceramic) "Coatings can increase the amount of heat kept in the combustion chamber, and therefore make more power, so there is a balance there between detonation and power gain".

https://www.ferrarichat.com/forum/threa ... se.244856/

The issue of detonation wouldn't be a problem in an external combustion engine.

I have been working with engines for so long, opened my own lawnmower repair shop while still in high school, I vaguely remember hearing, seeing or reading about ceramic engines and their incredible potential a long long time ago, but information seems a bit scarce on the subject.

In my search for information the past few days I came across another article:

Awwwhhh the wonderful "thing" that is Ceramics, I do ask why aren't there any ceramic combustion engines made yet ...
...
Prototypes of ceramic engines have been made, and the results are incredible. Because of their tolerance of heat, they can burn 3 times hotter than a metal engine...
...
In addition, they need no coolant system (no radiator), and because they can be cut to very tight tolerances (parts can be made to fit within several microns of distance) no engine oil is needed. They're so efficient that a ceramic engine, when compared to a regular 4-stroke engine of identical size, will produce 250% of the horsepower using approximately 25% of the fuel. They can easily weigh 50% to 80% less.

A batch of 10 of these engines, with 1 liter displacement, producing 500hp each, has been created by the Germans and have been extensively tested. The longest one has been run the equivalent of 1,200,000 miles and still shows no signs of wear and tear.

So what's the catch? First of all, nobody's set up to build these things in mass, so for now they'd be extremely expensive. ...

https://f30.bimmerpost.com/forums/showt ... p?t=905814

.

There are some political, economic and environmental issues discussed in the article that are not relevant to improving our model Stirling engines, like emissions, as an external combustion engine doesn't have an exhaust anyway.
I fired that little cylindrical piece at about 2000°F (cone 04 judging by the resulting color) and could have gone higher. (At higher temperatures the clay would have become a darker brown, hardened more into "stoneware" and would have shrunk a bit more)

That is hot enough to soften or melt many metals, and this is just clay I made from my own clay/dirt/soil from the field not any real high tech ceramic, but should still have many of the beneficial properties.

My personal reasoning, opinion or "theory" is that there would be a benefit to having a non-heat conducting, non-heat absorbing piston and cylinder because;

When the gas molecules is approaching the piston and hits a cylinder wall that is cold, it loses energy before that energy can impact the piston to move it. Instead, the hot gas just ends up heating the cylinder walls.

Likewise when the molecules do ultimately bump into the piston with more or less energy, if the piston is heat conductive, instead of moving from the impact, the energy will just heat up the metal to be conducted away and lost.

If the gas is only heated on the hot side, being blocked by a regenerator, the hot expanding gas on the hot side is going to have to compress the gas on the cold side, in effect, or literally heating it up indirectly by "heat of compression". So, logically, IMO, some percentage of the heat that heats up gas on the hot side of the regenerator is "diluted" as it then has to transfer energy to this volume of cold gas on the other side of the regenerator to impact the piston indirectly.

I can't help but think having a cooling system in between as well could only exacerbate the situation causing additional loses

This is somewhat strange.

For comparison I fired the clay I had added the "play sand" to. About 20%

The results were rather awful.
It was supposed to be a cactus shaped toothpick holder.

The clay itself, it appears, mostly held its shape but the play sand (I guess) melted and ran out, though it had been quite thoroughly mixed together with the clay.

Since the sand has a melting temperature of around 3000° F I'm not sure how this could happen as I didn't think the kiln had gotten much above 2000° F

The melted sand also adhered to the kiln floor.

Either the mixture somehow lowered the melting point of the glass, or I somehow got the kiln much hotter than I thought.

In contrast, the pure natural clay sample with no sand or "grog" added came out perfect.

At this point, I think the most probable explanation is:

The microwave kiln has two parts, the outer refractory material and the inner heating element made of crystalline silicon carbide.

The microwaves pass right through the porous refractory so the outside of the kiln does not get hot, but the microwaves are absorbed by the dense silicon carbide crystals which do heat up.

The heating element consists of a ring that is open at the top and bottom, so at least some of the microwaves can reach the ceramic piece directly, or by reflection off the top and bottom walls of the microwave oven.

The fine clay, like the kiln is mostly refractory. I had strained out all the larger crystalline sand particles. So the microwaves would tend to pass right through the dry clay, so that the pure clay is only heated indirectly by radiation from the silicon carbide.

Adding the sand to the clay, however, would allow the microwaves to be absorbed by the sand particles dispersed throughout the clay.

The sand particles within the clay, then, would be like miniature microwave kilns of a sort. The sand absorbing the microwaves and heating up, each sand particles surrounded by the insulating refractory.

Such direct internal heating would tend to raise the effective firing temperature, but would not necessarily increase the temperature of the air inside the kiln, so would not be detected by the thermocouple.

The ceramic acted as a crucible with the individual sand particles boiling inside, the sand being heated up directly by the microwaves that would have otherwise passed straight through the ceramic piece.

I made a small clay bell, intended as part of a wind chime.

I did not add any sand or "grog" at all I burnished the bell thoroughly inside and out and put a small hole in the top for a string to hang it from.

Apparently, I was over anxious and tried to fire it too soon before it was fully dry inside and it exploded in the kiln almost immediately, and it was only on defrost. I heard an explosion within less than ten seconds of turning on the microwave.

I decided to fire the pieces that remained anyway to see how they would turn out.

Explosions continued.

I over fired the remaining fragments, though much of the piece had been reduced to powder

The burnishing apparently formed a very good air tight seal so the fragments expanded like little balloons.

The surface of the fragments came out nice and glossy, as if glazed and glass-like, and do not seem to absorb water.

The fragment I'm holding in the following photos was originally very thin. Maybe 1/8 inch. The more porous edges that were exposed to the heat in the kiln enough to dry out seem to have formed a seal.

Anyway, I've decided I need to go about this a little more methodically and scientifically and have started making some test "cones" that I will be firing at something like 2 minute intervals.

That the first pice came out so perfect seems to have been some kind of dumb luck.

One factor may have been that the burnished surface I had put on the cylinder was removed when turned on the metal lathe, so the surface was likely much more able to breath and release steam. The edges of the fragments that were not burnished did not expand.

I do however like the results of the burnishing as it makes a nice glossy surface when fired, but since it creates such a great seal, the burnished piece likely requires much more time to become completely dry before firing to avoid explosions

I guess it may take a few hours, or days for these to dry thoroughly, though I might just try firing some early, before they dry to see what happens

Mostly I want to get an idea of firing time in the microwave. A minute of two, more or less seems to make a difference, much more so than in a regular kiln, or even a pit fire.


I think the moisture may not matter so much as long as the steam can escape easily. I'm guessing that if the clay is still soft and pliable (evenly damp throughout) or extremely dry it is OK, but what causes explosions is being at all damp inside with a hard dry shell on the outside. Like a popcorn kernel.

I guess my theory was at least partially right

The clay cone, made from regular moist, unfired clay exploded in the microwave, but only at the base where the clay is thick enough to prevent the steam from escaping.

Or, is there some impurity in the clay?

My best guess is still that the clay only explodes where the steam cannot easily escape like at the base of the cone where the clay is thick, or even if the clay is very thin but sealed by being burnished.

It is notable however, that where the cone was thinner and unburnished, nothing happened except that the clay dried out very quickly.


https://youtu.be/WwE8k6-HyyM?si=p5Q0uRMF1ss0wM3I

Good and dry = No explosions!


https://youtu.be/APlWY0FITkc?si=FnsoTAQ6cMUDwQR0

I had put the rest of these cones in the conventional oven in the lowest possible setting to help them dry a little faster, but then forgot they were in there until about six hours later, so they were pretty certainly already good and dry before going in the microwave.

I'm just using a regular microwave rather than a microwave kiln because the explosions all occured within the first few seconds before the kiln had time to heat up anyway.

These cones were all made from the same batch of clay at the same time in the same way.

The one wet cone exploded, but all 25 remaining cones once fully dry did not explode, so I think it can be safely assumed the explosion was not due to any contaminant or property within the clay other than trapped water/steam.

This is curious:

I tried starting out at 5 minutes.

The first cone exploded almost immediately

Then the second cone I tried pre-heating using the defrost setting, and that also exploded within a few seconds:

Adding to the rubble:
Since these cones had all been previously thoroughly dried out in the electric oven and also heated in the microwave yesterday, I concluded the clay cones had picked up moisture overnight, as it had been raining all night long last night.

With the third cone, I tried zapping it in the microwave kiln for just 2 seconds, then let it sit for a few seconds, then zap it again for three seconds, then letting it sit for about double the time or six seconds, then heat it for four seconds and let it sit eight etc. etc. Gradually increasing the time.

After about five minutes of this treatment ( on the microwave timer, actually probably 15 minutes in real time ) without an explosion, I used "defrost" set for another five minutes. When it seemed I was out of the danger zone I gave it full power for five minutes:

Looking down through the vent hole:

After that cools, I'll go through the same procedure with each cone, increasing the firing time by 1 minute.

Lesson learned; It doesn't matter how long you let some greenware sit and dry out, it can still explode just from moisture picked up from the air during high humidity.

Quite unexpectedly, just five minutes in the microwave kiln (not counting preheating, and ramping up for 5 minutes on defrost) resulted in significant firing/color change:

On the left is an unfired cone, just raw greenware.

On the right is a five minute firing.

So now, before going to six minutes, I've gone down to four. (Again, not counting preheating and ramping up)

It was a surprise to see the terra cotta colored cone come out of the kiln after such a brief firing time.

Previously I had gone at least ten or fifteen minutes minimum, on the assumption it would take much longer to fire ceramics.

Odd results continue.

I went all the way down to just 1 minute for completeness. It did cause a slight color change even at just one minute. I may do some additional testing on these pieces at a later date for hardness, water resistance etc.
The numbers represent minutes in the microwave kiln at continuous full power.

#7 (seven minutes) ballooned out a little at the base.

#9 got cooked partially melted, turned a dark brown and stuck to the floor of the kiln. I thought there was no point in going further, but continued anyway. Then #10 other than being slightly darker perhaps, went back to "normal".

So with #10 still turning out OK, I'm now scratching my head wondering what went on with #7 and #9 number nine especially.

Did I accidently push the wrong button on the microwave and set it for 99 minutes instead of just nine?

I don't think so because I didn't go far, but I didn't babysit the microwave either. Just set it and went upstairs to do other things, but I don't recall the microwave taking an unusually long time to "beep".

Perhaps #7 and #9 still had a little moisture? They only swelled up at the base, but why did #9 turn such a dark brown?

This is just dirt from the ground so, it is certainly not impossible there is some odd particles of limestone or who knows what that might cause a "hot spot"

After seeing #9 blow up, I did notice the fragments of clay on the kiln floor were getting darker on the right hand side of the kiln, and the bloated cones were swollen on that same side nearest the magnetron. I did not bother with any rotation.

With #10 I did move the kiln slightly off center towards the "cold side" away from the magnetron, so the cone was not dead center in the microwave.

Anyway, it looks like #9 was a fluke, so I am continuing on with 11, 12 etc. until results are more consistent or conclusive.

#7, #9, #11, #12

All these deformed cones swelled or ballooned out in the same way and in the same location, which leads me to the conclusion that this is not a matter of some random particle in the clay just happening to get, by some coincidence, trapped in the center base of each of these effected cones.

I suspect, therefore, the problem is likely still a bit of moisture retention.

Also, although I used the same amount of clay for each cone, there was some variation. Cones #7 and #9 were skipped. Though fired for 10 minutes, #10 did not balloon. It just so happens that #10 is also a bit taller and skinnier.


So, the base was not as chunky, therefore probably dried out better during the pre-heating.

So, I will continue with a new batch, doubling the preheating time to ten minutes instead of 5.

My theory is that the clay, at the thicker base does not heat through all the way. The radiant heat from the kiln heats the surface very rapidly sealing in a bit of moisture which builds up pressure.

As the clay heats up more and becomes plastic, the pressure from inside causes the base to balloon out. If the cone base is thin enough it can dry out more easily and rapidly so no moisture gets trapped, so no ballooning of the narrower cones.

Increasing the preheating time should, theoretically, resolve the problem.

I'm also interested in a type of metal alloy called Invar.

Invar does not expand when heated. This is actually due to the composition, when heated Invar both expands AND contracts on a molecular level, which perfectly balances out.

I was thinking, by perhaps adding small amounts of powdered sand or calcium or something to the clay, as moisture leaves during the firing process, so that the clay tends to shrink, the added powder could release gas that would fill the space left by the escaping moisture, or create enough internal pressure to counteract the shrinkage due to the rapid drying, ...or something like that.

This particular clay seems to not shrink much to begin with anyway, but I was just thinking.

A zero shrinkage ceramic similar to Invar would probably be useful in some way. Though shrinkage during firing of clay is not exactly the same thing as thermal expansion when heated.

Well, I think I've reached the limit of practical microwave ceramic firing:

At 18 minutes, the cone looks like a meteorite or something, partly melted, fell over and stuck to the kiln floor but basically held its shape and did not balloon out of explode

Maybe you can confine the clay to some sort of mold while firing it?