Model LTD max power effort

[Tom Booth]

... Still have to get a power cylinder and piston. ...

Is that thermal image of the engine running, with piston installed, or just sitting on the heat, not running for 10 minutes ?

[VincentG]

Not running, just sitting on boiling water with ice on top.

[matt brown]

Hmmm, Vincent, I think your basic scheme is the following...

Vincent LTD.png (19.92 KiB) Viewed 7702 times

where S is common Stirling gamma and V is your mod.

[matt brown]

Note several things here...(1) volumes do not include typical dead volumes, so clearance volumes are merely for clarity (2) graphics include regen (3) sequence 4 shows both have the same charge pressure, but this changes sequence 1 values between S and V when heat is applied (4) DP has total volume 2x PP volume

OK, the most important thing this reveals is that scheming from input (high energy state) has major shortcomings vs scheming from low energy state. However, since we live in an ICE world, scheming from high energy state is common. This simple graphic shows how even the most ideal phasing (double dwell here) effects gas flow m which effects output. The hidden nugget is that with regen shown here, this 300-600k cycle has regen 2x PP gas flow when S, but 4x PP gas flow when V. However, this drastic difference is negated when nixing regen in both.

A key take away is that increasing efficiency is not merely increasing power per BTU, but more increasing specific power (output per engine size). I get the idea...using a hot jab of input to increase overall pressure without heating entire gas mass, but this a loose idea that requires careful consideration.

[matt brown]

Hmmm, Vincent, I think your basic scheme...

Vincent LTD.png

where S is common Stirling gamma and V is your mod.

[VincentG]

Yes, except I'm now aiming at making the power piston really driven off the hot side.

This "hot pipe" will hopefully keep the gas at max temperature on its way up to the piston. When the displacer covers the cold plate, the only path for the gas to the PP will be through and around the hot pipe and within the port cut into the displacer. When the hot plate is covered, it's impact should be minimal on cooling.

hot pipe.jpg (124.92 KiB) Viewed 7673 times

hot pipe with displacer.jpg (134.23 KiB) Viewed 7673 times

I have since added another aluminum stand 180 degrees opposed to the hot pipe to help keep the displacer from rocking in the chamber and binding on the walls. It extends almost all the way through the displacer and adds more heat exchanger area while adding little extra volume.

[Tom Booth]

...I get the idea...using a hot jab of input to increase overall pressure without heating entire gas mass,...

Not sure what VincentG is up to entirely, but I don't think "without heating entire gas mass,..." is actually anybody's goal. Not mine anyway, but maybe I'm not with the program.

My general modus operandi is to heat as much of the gas as possible (that is ALL of it) then while hot and expanding to have that hot expanding gas impact the piston, as directly and completely as possible. Then to cut off heat input while pressure is still high allowing continued expansion to continue impacting and driving the piston so as to "use up" as much as possible or practicable, of the remaining heat to drive the piston for additional power output. (Then start another cycle of heating etc./

IMO the working fluid inside the engine does not really know or care about what is outside the engine. Atmospheric pressure and/or temperature. Consider for example a hermetically sealed engine heated by plutonium "insulated" by the vacuum of outer space.

"Heat" is only more or less jiggling (mostly stationary) molecules with greater or lesser vigor. Nothing actually "flowing" anywhere

Anyway why would it ever be desirable to NOT heat ALL of the working fluid?

[VincentG]

Sorry Matt I missed that one, my idea IS most certainly to heat all off the gas at one time, and then cool it all back down again. And yes, with zero regard for efficiency lol. The goal is simply maximum watt/cc for an atmospheric engine.

[matt brown]

Anyway why would it ever be desirable to NOT heat ALL of the working fluid?

Tom, in another thread, you recently commented that hot air engines run off pressure...and I always keep this in mind. I don't get hung up on heat except as it relates to pressure. So, in Elon buzz, pressure is 1st order and heat is 2nd order.

If you can increase the PP pressure without heating the entire gas mass, why heat the entire gas mass ??? Heating a large DP volume for a small PP volume is a waste of heat and time. The more heat you add per cycle (rpm) the more heat you must convert to work since work is a PV function. The common LTD has a very high DP/PP ratio, so there's little work output potential without extreme charge pressure or thermal ratio. However, the real challenge is completing the cycle, not just the expansion phase...

[matt brown]

Hey Vincent, I like the heat pipe, but like Tom, I was lost on your configuration. Now, I think I'm getting it, something like this (mooched from my why SE suck post).

hot PP_1.png (23.11 KiB) Viewed 7636 times

In graphic, S is typical Stirling gamma and A is Vincent mod but we can nix the regen for Vincent. Note that this diagram is for clarity of comparison and real mech would be substantially different.

Hmmm, how about a pseudo Otto...

(1) displacer with 1st reed valve from cold space to hot space
(2) displacer with 2nd reed valve from hot space to cold space
(3) piston with ambient port near BDC
(4) cold plate with 3rd reed valve from ambient to cold space
(5) hot space cavity under displacer
(6) displacer with surface area in hot space vs cold space proportional PVT issues
(7) phasing to be determined by testing

[Tom Booth]

Anyway why would it ever be desirable to NOT heat ALL of the working fluid?

Tom, in another thread, you recently commented that hot air engines run off pressure...and I always keep this in mind. I don't get hung up on heat except as it relates to pressure. So, in Elon buzz, pressure is 1st order and heat is 2nd order.

If you can increase the PP pressure without heating the entire gas mass, why heat the entire gas mass ??? Heating a large DP volume for a small PP volume is a waste of heat and time. The more heat you add per cycle (rpm) the more heat you must convert to work since work is a PV function. The common LTD has a very high DP/PP ratio, so there's little work output potential without extreme charge pressure or thermal ratio. However, the real challenge is completing the cycle, not just the expansion phase...

I guess it's all theoretical to a degree, but if going with kinetic theory then each gas atom or molecule is moving independently with a lot of space in between hardly interacting with each other at all. If that is true than the idea that some of the hot gas can exert "pressure" to the colder gas is mythology. The hot gas molecule has to collide with a cold molecule which then collides with the next etc. until finally the last cold molecule next to the piston receives a final weak, diluted bump which knocks it into the piston.

The energy, in other words is scattered like pool balls at the break better that the first white "hot pool ball" goes straight to the piston to impact it directly with as few cold molecules intervening as possible .

Is the kinetic theory actually true and valid?

I kind of doubt it.

Probably the gas molecules are more like the bubbles in a foam. The foam receives heat more or less as a whole through radiation, maybe. The foam expands like a sponge swelling up with water perhaps?

Or maybe the working fluid is like the stationary balls in a "Newton's Cradle" so "heat" hits the mass of air at the hot plate and that transfers through the medium to the piston without the gas molecules actually moving much at all.

https://youtu.be/0LnbyjOyEQ8?si=LiXptnpo6ptzY-VK

Do I know?

No.

So, I rely mainly on experiment to sort it out and see what actually works in practice

[VincentG]

Matt, I'll give open cycle another try with this one for sure. I had thought about doing one way reeds through displacer and still might give it a go, but it sure adds alot of extra stuff.

I've noticed when the new aluminum top plate is cooled, the power piston and cylinder need to be isolated, or they get too cold themselves.

[matt brown]

I still don't get how the heat tube works, but suspect it's some type of slide valve. When you first started working on this dwell scheme, barring gaming actual volumes, my thinking was that if the displacer bottom had a conductive material (thin sheet of copper ?) and contacted hot plate during heater dwell, then this would kinda double hot plate surface. This assumes that the two surfaces will never mate tight enough that there's a hot space 'vacuum' but rather that pressure on both sides of displacer remains equal. Then, when displacer lifts, the momentary suction will draw in the cold air from above displacer via annular space between displacer and case.

OK, this pitch is common LTD except for dwell, but here's the problem, and what caused the original thermal lag (Tailer patent) to be a no-go. There's conduction, convection and radiation heating, and as you recently pointed out, conduction trumps convection. With hot air engines, it's best to think of conduction as direct heating from source to gas molecules and convection as indirect heating between gas molecules (like a bucket brigade). In the above pitch, when the displacer lifts, the cold gas will only 'rush in' until pressures equalize, and the old axiom that "cold gas pools" will rear its ugly head. Sure, turbulence will help negate this pooling (breakup the boundary layer) but poorly. The point here is that counterflow heating thru a single port is ripe for pooling (even if the 'single port' is the entire annular region around displacer at hot plate). The obvious alternate is uniflow scheme with distinct in port/s and distinct out port/s. Reed valves offer a simple way to draw gas over hot plate, but various PVT issues remain.

[matt brown]

Matt, I'll give open cycle another try with this one for sure. I had thought about doing one way reeds through displacer and still might give it a go, but it sure adds alot of extra stuff.

I've noticed when the new aluminum top plate is cooled, the power piston and cylinder need to be isolated, or they get too cold themselves.

Rather than common reed valve/s, how about a central/circular in-valve to hot plate thru displacer and let out-valve be annular space around displacer. The challenge is what material ??? since a diaphragm valve of this type won't take the heat (tho would be good for 'static' smoke test).

[VincentG]

Matt, the hot pipe has small holes at the bottom near the hot plate and a central hole within. When the displacer is covering the cold plate, the hot gas can only reach the power piston after having traveled through or around the hot pipe which is contained in a close fitting port through the displacer.

When the displacer is covering the hot plate, the hot pipe is largely out of the equation and the gas can reach the cold plate(and cold foil on displacer) unrestricted.