My contribution to the ECE

[VincentG]

My duplicate engine should be arriving next week. I will hook the pressure guage up with the same temperatures and record the results.

Tom thats an interesting idea...two mixed gasses in the same engine. So many things to try.

I have to take a closer look at your thread for that engine. I don't quite understand the operating principles, but then again, I don't quite understand the ringbom in general. I'll do some research.

[Tom Booth]

.... I don't quite understand the operating principles, but then again, I don't quite understand the ringbom in general....

You could probably convert your little LTD to Ringbom. Basically take the displacer off the crank and let the pressure build up when the piston is at TDC drive the displacer, due to the pressure differential between the inside of the engine and atmosphere.

It helps if the displacer rod is wider, and/or the displacer is counterbalanced.

The LTD Ringbom in the following video is kind of interesting, and has the type of generator I mentioned earlier:

Note the long wire spring used to counterbalance the displacer, with the kind of pressure your engine is generating, probably not necessary.

https://youtu.be/56MRdsmicsw

[VincentG]

I'll give that a shot. I've seen them run, just seems counter intuitive.

[VincentG]

It seems I have been going backwards with my housing design. Save for the brief period of time before my water cooled housing ingests water, the original displacer cylinder seems to be more effective on a long run. But why?

Then, it hit me like a ton of bricks. Or more accurately, a ton of heat! All I have done is create a more efficient pressure cooker. While thinking about efficiency, it dawned on me that what could be a more inefficient system than using a tin of boiling water to power a sub 5w engine. It's the equivalent of playing ping pong with a sledge hammer.

We have to think about heat as we think about ELECTRICITY! Voltage is temperature, amperage is btu, and resistance is the combination of specific heat, mass and the thermal conductivity of the heat exchanger.

I have been essentially powering this engine with 2 volts at 10 amps, when the efficient thing to do is crank the voltage and reduce the current and resistance.

The ideal hot plate would have and infinitely high thermal conductivity and surface area, with an infinitely low thermal capacity, and specific heat. The plate should ideally heat instantly, and have a heat capacity to only operate one engine cycle without further input. I find it's easier to chase ideal materials than ideal concepts(like the idealized Stirling cycle).

The ideal heat source would be of high temperature and as LOW a BTU output as possible and, further, it need only be adding heat during the end of the compression stroke.

A high estimate of air volume in my engine is 6 cubic inches. The weight of air is only 0.000045009
lb/cu in. So thats about .00025 lbs. The specific heat of air is .24, not accounting for water content. So to raise 5 cu in. by 200 degrees requires....someone please check that math for me...I get .208 btu.

So to efficiently transfer heat into the engine in a real world scenario, we need a flame hotter than our target delta F of 200(easy
) and a BTU output of no more than what testing should suggest.

Lastly we need the ideal hot plate. Well I imagine graphene is the answer, but next I will try VERY thin copper or aluminum, folded into an accordion like shape for both surface area and stiffness.

I think the efficiency potential of such a system along with proper timing, compression and insulation and especially combining an intermittent heat source could be off the charts.

It is really a wonder these LTD engines run at all on the mega BTU output of boiling water without instantly heat soaking.

[matt brown]

A high estimate of air volume in my engine is 6 cubic inches. The weight of air is only 0.000045009
lb/cu in. So thats about .00025 lbs. The specific heat of air is .24, not accounting for water content. So to raise 5 cu in. by 200 degrees requires....someone please check that math for me...I get .208 btu.

It is really a wonder these LTD engines run at all on the mega BTU output of boiling water without instantly heat soaking.

I didn't check your math, but it looks about right, except for the 200F increase which would stall engine. Tom likes to say I love to rain on guys parades, but I'm only giving a reality check. The takeaway here is that LTD were never meant to be power producing engines, so I consider the Kolin/Senft LTD 'contribution' to SE a distraction. There's no substitution for basic thermo, and trying to learn it via trial & error will require multiple lifetimes.

adiabatic index.png (289.07 KiB) Viewed 5567 times

The adiabatic index for diatomic gases shows the relationships between variations in PVT values. Yes, these are ideal gas values, but a good basic reference where (dry) air can be considered diatomic. Note that, for example, when air is compressed 6x, the pressure rises ~12x, and the temperature rises ~2x. As always, volume & pressure can be in any units, but temperature will be deg K.

A table like this would require many hours to arrive at thru trial & error...

[matt brown]

My recent deep dive into air compressors started with me pondering why an air nozzle never freezes up like a propane tank. IOW assuming a 60 gal tank stores compressed air at 7 atms (100psi) & 300k (80F) then a prolonged venting to ambient would have pressure drop by 7, whereby according to previous adi index table, temperature would drop by 1.74, such that 1 atm ambient discharge should be 172k or -150F and this NEVER happens. The ice ball on a propane tank discharge is due to latent heat absorption which varies whether propane is pure or mixed with butane. However, the reason why the air compressor discharge never gets frosty is simply due to...you guessed it...moisture. Yep, the moisture in air supplies 'some' heat during expansion. OK, this is simple enough to grasp, but what's happening during the compression side of the cycle is a head banger and what my deep dive was all about.

[matt brown]

This sketch shows how the displacer rests on the bottom of the cylinder, blocking the heat from entering the system, but at the same time traps a small volume of air that has time to become super heated. The added benefit is the displacer is not in direct contact with the hot plate and stays cooler this way. When the displacer lifts, the super heated air instantly imparts its energy into the cold air above. This is the theory anyway... but the engine that lifted the 1 pound weight incorporated this design.

air gap displacer.jpg

I applaud scheming Otto, but you're running into similar stuff that I did years ago. The heater area under displacer will have greater temperature AND pressure than gas on top, thus requiring some type of 'backwork' to keep displacer down. However, it still might be possible to 'balance' the displacer by vary the higher pressure bottom area vs lower pressure top area (similar sketch).

[VincentG]

Don't worry Matt, my skull is thick enough to keep the rain off. I'll bite, why would an actual delta T of 200F stall the engine? I also fail to see how the LTD layout is not just a gamma with a better flow path to the working cylinder. More importantly for me, its just easy to make design changes to.

Beyond that, I think the very reason hot air engines can be so effective is because of that very chart, in combination with the incredibly low specific heat of air. All we have to do is keep the temperature of compression down and add heat to the cold compressed air, and reap the benefits of the return on pressure. Especially on a large scale, think like a 10ft diameter diaphragm, that air can do an incredible amount of work.

I have to add here that the point of this post is for home power generation with a biomass or solar fuel source. We are not concerned about absolute efficiency, just power production with a real world engine build. And big industries have proven it can be done.

[VincentG]

I applaud scheming Otto, but you're running into similar stuff that I did years ago. The heater area under displacer will have greater temperature AND pressure than gas on top, thus requiring some type of 'backwork' to keep displacer down. However, it still might be possible to 'balance' the displacer by vary the higher pressure bottom area vs lower pressure top area (similar sketch).

True, in a later post I had mentioned keeping the displacer down mechanically, and how this may be used to our advantage. But as I progress with design theory, I think we may be able to discard the displacer all together.

[Tom Booth]

... I will try VERY thin copper or aluminum, folded into an accordion like shape for both surface area and stiffness.
...

I can attest to the high heat conductivity of copper. I usually tend to the wood stove with a long steel poker, for shifting logs around or whatever, but somehow one day misplaced the poker.

Looking around I found an old copper pipe about 4 foot long, poked it in the fire and instantly had to drop it as it felt like it was searing my hand, almost felt like a high voltage electric shock. The instant the tip of the copper pipe touched some hot coals the heat traveled the length of the pipe and certainly would have burned my hand had I not dropped it right away.

I thought steel to be a rather good conductor of heat but compared with copper it's like night and day. I can normally hold the steel poker with bare hands. The heat dissipates before getting that far.

I'm not sure about aluminium, as aluminium tends to throw off heat as quickly as it takes it in. It is probably better at reflecting heat, but I haven't stuck any aluminium pipes in the fire. A bake potato in the oven wrapped in aluminum foil can be picked up without burning your fingers. It throws off heat very quickly.

In other words, if copper were used, I don't think you would necessarily need to worry about making it especially thin as it conducts heat incredibly fast even through several feet of pipe.

Maybe copper outside clad with aluminum inside?

[Tom Booth]

...The ice ball on a propane tank discharge...
...However, the reason why the air compressor discharge never gets frosty...

Not sure what this has to do with anything but are you talking about a compressor tank or the compressor itself?

If you aren't talking about compressed air STORED in a tank, than that is apples to oranges, as you did say propane TANK.

Compressed air STORED in a tank certainly does frost up when discharged.

https://youtu.be/2hYQtB4QkEY

[matt brown]

Don't worry Matt, my skull is thick enough to keep the rain off. I'll bite, why would an actual delta T of 200F stall the engine? I also fail to see how the LTD layout is not just a gamma with a better flow path to the working cylinder. More importantly for me, its just easy to make design changes to.

Consider a Lenoir cycle which is a 3 legged cycle with isochoric heating, adiabatic expansion, and isobaric 'cooling'. Using your delta 200F cycle values, such a cycle would (as non compression Lenoir) have isochoric heating (constant volume) from 80F>>>280F, or300k>>>411k. Then, 'this volume' could expand how far ??? Hmmm, since this isochoric heating was 1.37 (as in 300k x 1.37 = 411k) then 1 atm P at start will only be 1.37 atm P. Note that line 2 in adiabatic index shows delta T = 1.32 so let's consider this 1.32 (per index) the same as your cycle value delta T = 1.37 and we find that a 1:2 volume expansion will bring the temperature from 411k>>>300k, but to use all this heat would require the pressure to be reduced by 2.64 as in 1.32 atm>>>.5 atm with a 1:2 adiabatic expansion. No way, Jose.

Playing with my calculator, I find that when the volume ratio = 1.22, that the pressure ratio = 1.32, and the temperature ratio = 1.083 due to 1.22^1.4=1.32 (delta V to gamma equals delta P) and 1.22^.4=1.083 (delta V to gamma-1 equals delta T). Now, we find that the 1.32 atm P @ 411k>>>1 atm P @ 380k with a paltry 1.22 (diatomic) adiabatic expansion ratio.

Indeed, the adiabatic game is far from intuitive and requires some busy math, but al least no calculus. Moving from 3 legged Lenoir to 4 legged Otto is similar, no free lunch. The bigger question is how to get any meaningful volume ratio from anything like an LTD mech when the unswept volume is multiples of the swept volume ???

[matt brown]

Consider a Lenoir cycle which is a 3 legged cycle with isochoric heating, adiabatic expansion, and isobaric 'cooling'. Using your delta 200F cycle values, such a cycle would (as non compression Lenoir) have isochoric heating (constant volume) from 80F>>>280F, or300k>>>411k. Then, 'this volume' could expand how far ??? Hmmm, since this isochoric heating was 1.37 (as in 300k x 1.37 = 411k) then 1 atm P at start will only be 1.37 atm P. Note that line 2 in adiabatic index shows delta T = 1.32 so let's consider this 1.32 (per index) the same as your cycle value delta T = 1.37 and we find that a 1:2 volume expansion will bring the temperature from 411k>>>300k, but to use all this heat would require the pressure to be reduced by 2.64 as in 1.32 atm>>>.5 atm with a 1:2 adiabatic expansion. No way, Jose.

Interestingly, Tom will probably spot the way out...simply modify this Lenoir cycle from isobaric compression to 2:1 isothermal compression whereby .5 atm P>>>1 atm P and everything is hunky-dory (aka the green cycle).

[matt brown]

Compressed air STORED in a tank certainly does frost up when discharged.

Requires really dry air and really large pressure drop, both of which are rarely encountered. I was surprised that even the large compressor brands shrug off the moisture issue.

[VincentG]

but to use all this heat would require the pressure to be reduced by 2.64 as in 1.32 atm>>>.5 atm with a 1:2 adiabatic expansion. No way, Jose.

This does not seem to be taking into account that the displacer will shut the heat off for us. There is no need to use all the heat on the expansion stroke, we are just using the pressure spike to drive a piston. The temperature will drop 200 degrees at BDC(play along with me here) and return the piston under vacuum.