LTD model "Stirling" uses Lenoir Cycle

[matt brown]

Lenoir 800k air.png (96.44 KiB) Viewed 8605 times

A little PV plot can go a long ways towards exposing mysteries...

Tom's thread on high eff probably has any of the lurking engineers thinking Tom is full of **it. Well, Tom may be lacking in engineer speak, but a bigger problem is the engineer mindset that's limited by edu dogma (as Tom has complained). To be fair, engineers don't go to school to learn everything about thermo, but rather only what is reasonably expected that they'll need to know. Over the years, I've discovered that what one knows is colored by how one came to know it. So, there's a subjective qualitative issue with edu that's usually hidden by the objective quantitative issues. Hence, one of my favorite quotes: "I never let my schoolin' interfere with my education" (Twain).

Anyways, notice how the Lenoir Cycle PV above matches typical 'Stirling" LTD model wherein a 'displacer' merely shuttles a gas back & forth at constant volume, except for a little adiabatic expansion followed by a little isobaric compression. A simple 3 legged cycle, but both low power and low eff (more on that later). I drew this with GeoGebra, but it's a crude app that boggles stuff, and here I couldn't get the 1-2-3 pts to coincide with a power cycle, so ignore the reverse 'refeer' direction of cycle arrows and neg output. However, the basic cycle is correct with 300k-800k isochor, 1:2 adiabatic expansion, and 2:1 isobaric compression. Note that the red lines are adiabats, the green lines are isotherms, and this shows a diatomic (air) cycle vs a monatomic (argon) cycle which would be similar except that the adiabat would drop 'faster', whereby argon engine would have lower volume ratio, less input, and less output within same thermal ratio (of isochor).

Now, feast your vulcan squinties on the (green) 600k isotherm at pt 3 and follow it to the isochor. With ideal regen, 60% of isochoric input could come from isobaric 'compression', shrinking total heat input to 600k-800k region of isochor. I didn't check eff of orig cycle or regen flavor, and merely wanted to throw this out there to demonstrate Tom's reasoning is not daft. Once you get your head around LTD=Lenoir, consider how a Manson might make a regenerated Lenoir. The major issues of 3 legged cycles are that they're extremely limited, tend towards low specific power (tho pressurization would solve this), and inventing a mech that follows a theoritical cycle (per PV) can be daunting (gadz, just think of SE).

[Tom Booth]

..., and 2:1 isobaric compression.

Do you mean isochoric?

At or during "full compression" the piston is basically motionless for probably about 90° rotation of the flywheel. Simultaneously the displacer lifts driving air into the hot plate.

So on top of the piston holding maximum compression, temperature and pressure increase as the displacer wafts air across the hot plate.

2:1 is anything but isobaric.

[matt brown]

Yes, isobaric compression from pt 3 @ 600k/2v to pt 2 @ 300k/1v, thus 2:1. I'm trying to make several points with this cycle: (1) it's a well known (to some) cycle that doesn't require a 'conventional' compression process (2) the cycle may be open or closed (3) a corresponding mech (engine) which matches the cycle was the 'typical' ICE prior Otto (4) if carefully studied, a closed cycle version with regen appears possible (5) edu 'science' is industrial dogma with a master (6) Carnot hash has severe limitations, but rock solid within it's 'box'.

The Lenoir Cycle is almost beyond living memory, except for a handful of antique iron in museums. The only other place you hear Lenoir is referencing pulse jets, from WWII V1 thru some current UAV stuff.

PV (above) is to expose the Lenoir Cycle (vs Lenoir 'engine') and to this end, it required a lofty example with 300k >>> 800k isochor. A similar cycle with typical LTD thermal ratio would be a mere dot on this PV plot !!! but with the same 3 proportional processes. The typical LTD model has a continuous mech, so it deviates from any 'ideal' cycle, just as any real SE (usually) deviates from a Stirling Cycle when using a continuous mech. The output from any LTD is so small that it won't support a discontinuous mech, and likely why LTD use similar SE phasing (practice approximating theory).

So Tom, here's my pitch on how LTD work...hot gas is displaced to cold space whereupon piston expands adiabatically before cold space 'cools' gas (think thermal lag in reverse) then piston 'compresses' gas in cold space before displacer shuttles cold gas back to hot space. The compression force doesn't come from a flywheel, but from the atmosphere on back side of piston while gas is contracting during meager isobaric cooling. There's lots of details in this scheme that require careful consideration or this bugger won't work. Senft & Kolin wasted a ton of time on an academic toy that's merely a novel distraction.

[Nobody]

.in regard to the opening post only.


Nope.

[Tom Booth]

Yes, isobaric compression from pt 3 @ 600k/2v to pt 2 @ 300k/1v, thus 2:1..

So by 2:1 you actually mean 3:2.

That makes more sense.

[matt brown]

No Tom, above PV has 2:1 isobaric compression, from 2v @ 600k to 1v @ 300k.

[Tom Booth]

Aside from your cycle going in reverse or some such, which confuses things enough, I don't personally believe the compression in a Stirling engine is at constant pressure.

Does that even make sense? Compression without a change in pressure?

[matt brown]

Indeed, the reverse arrows suck, as does the blaring Wneg. The best way to think of isobaric compression in an engine is to compare it to a flame licker. Assume that a flame licker has a total vacuum inside cylinder during part of its cycle. During this time, the atmospheric pressure will attempt to force the piston in until the pressure equalizes on both sides of the piston (limited by mech). Now, consider same flame licker with PARTIAL vacuum inside of cylinder during part of its cycle, and atmospheric pressure will also attempt to force itself in until pressure on both sides of piston equalize. By definition, this vacuum would require removing some gas, but lowering the pressure inside cylinder can be achieved via removing some gas or lowering the temperature of initial gas. Both schemes could be described as atmos engines, since both derive any 'work' from atmospheric pressure. Now, if this crude engine had a large clearance vol at TDC (requires initial gas vol to expand) and we can control heating & cooling the gas inside the cylinder at the right time, we can use atmospheric pressure to move the piston in ('compressing' gas inside cylinder) without any input from a flywheel ;)

Tom, you're right, no Stirling is constant pressure (that's an Ericsson feature) but most guys call all hot air engines Stirling, However, the real confusion comes from growing up in an ICE world, so some of this stuff requires a major reassessment.

[Tom Booth]

Indeed, the reverse arrows suck, as does the blaring Wneg. The best way to think of isobaric compression in an engine is to compare it to a flame licker. Assume that a flame licker has a total vacuum inside cylinder during part of its cycle. During this time, the atmospheric pressure will attempt to force the piston in until the pressure equalizes on both sides of the piston (limited by mech). Now, consider same flame licker with PARTIAL vacuum inside of cylinder during part of its cycle, and atmospheric pressure will also attempt to force itself in until pressure on both sides of piston equalize. By definition, this vacuum would require removing some gas, but lowering the pressure inside cylinder can be achieved via removing some gas or lowering the temperature of initial gas. Both schemes could be described as atmos engines, since both derive any 'work' from atmospheric pressure. Now, if this crude engine had a large clearance vol at TDC (requires initial gas vol to expand) and we can control heating & cooling the gas inside the cylinder at the right time, we can use atmospheric pressure to move the piston in ('compressing' gas inside cylinder) without any input from a flywheel ;)

Tom, you're right, no Stirling is constant pressure (that's an Ericsson feature) but most guys call all hot air engines Stirling, However, the real confusion comes from growing up in an ICE world, so some of this stuff requires a major reassessment.

Well, I agree that the piston returning (without the momentum of a flywheel to push it back) is due to a drop in internal pressure below that of the "outside" pressure, (atmospheric or buffer).

So yes, something like a flame licker.

The pressure reduction is due to lowering the temperature.

Where I'm not sure we are in agreement is, what causes a (sudden) lowering in temperature.

I would say it is due to the engine being held at full compression, with simultaneous heat input until the crank turns enough to allow a sudden expansion and release in pressure, due to the piston being able to move.

The high temperature and pressure is converted to velocity. The transformation of heat into work results in a drop in temperature. (Not because of heat being "removed" to a "sink")

[matt brown]

Academics get hung up with optimization buzz, but fail to acknowledge approximation issues. I often stare at legacy designs (think antique iron) and ponder exactly what the designer was trying to achieve, then whether he came close (on paper or reality). I look at it this way...consider a mythical engine B which has no known cycle B, but there are well known cycles A and C. The question (and study) becomes whether engine B is approximating cycle A or cycle C, and this is where PV plots are very helpful, since they're hard to cheat.

Yep, Tom, a heat sink has nothing to due with an expansion PROCESS, but some type of heat sink is req'd to complete the CYCLE. The cycle concept is what made Carnot famous, since with prior heat engines (gas & steam), mankind had only non-cyclic 'engines' (wind mills, waterwheels) that harnessed natural inputs. Without modern thermo it's impossible to know when a car with 10 mpg 'somehow' achieves 20 mpg, whether this car went (for ex) from 5% to 10% eff or from 10% to 20% eff.

[Tom Booth]

..., but some type of heat sink is req'd to complete the CYCLE...

OK,

I disagree. Or perhaps you would like to elaborate on where and when the alleged "heat rejection" to "some type of heat sink" actually takes place.

You say: " ...a heat sink has nothing to due with an expansion PROCESS..."

Then, do you agree with mr 'nobody' (Carnot cycle) that heat rejection to "some type of heat sink" takes place during the compression stroke?

If so, that would seem to be a reversal in causality

The cooling that causes the piston to stop dead in it's tracks and reverse course takes place after the piston has already decided, for some reason, to stop and reverse course.

[matt brown]

This is where process vs cycle is paramount. Prior heat engines, work was via a single output 'process' from windmills and waterwheels without 'cycles'. Then steam engines added a heating process (boiler), Watt added a cooling process (condenser), and steam became a 4 process cycle (feed pump is a compression process). However, these steam guys missed most cycle concepts due to small backwork of feed pump and a dance card full of other issues: engine format, metallurgy, regulation (valving, cutoff, governor), combustion & condenser eff, insulation, etc.

If a steam engine has a condenser, the condenser is obviously the heat sink. However, when a steam engine has no condenser, it still has a 'heat sink' (the atmosphere) just not as obvious as a hunk of hot metal. I didn't see Noboby's orig comm, but if referring to LTD then I agree that "some type" of heat sink takes place during compression stroke, and with everything near microscale, I'm sticking with isobaric cooling from top plate...

My favorite cycle is Otto (tho little interest in ICE) due to all DISTINCT processes which compliment each other (the secret sauce). Anyone studying ECE would due well to study the history of ICE, and a good place to start is with Lyle Cummin's "Internal Fire".

[Tom Booth]

However, when a steam engine has no condenser, it still has a 'heat sink' (the atmosphere) just not as obvious as a hunk of hot metal. I didn't see Noboby's orig comm, but if referring to LTD then I agree that "some type" of heat sink takes place during compression stroke, and with everything near microscale, I'm sticking with isobaric cooling from top plate...

A lot about steam engines.

Your earlier suggestion was the "compression" stroke was the result of "suction" caused by a flame licker type reduction in internal gas temperature/pressure. Your choice to describe any of this as "isobaric" (without any change in pressure) seems strange.

I would likely not be contributing to this thread aside from the fact that my name was mentioned repeatedly in your OP:

Tom's thread on high eff probably has any of the lurking engineers thinking Tom is full of **it. Well, Tom may be lacking in engineer speak, but a bigger problem is ....

......Tom's reasoning is not daft. Once you get your head around .. .

I think your imagination is working overtime as far as "lurking engineers" busily evaluating how daft or full of shit I am, nevertheless, since I'm not able to make much sense of what you post, generally, I thought I should make my actual position clear:

In converting heat into work, (the purpose of a heat engine), heat/energy is "lost" resulting in an actual lowering in the temperature of the working fluid.

The greater the engines efficiency, the more complete the conversion, the less "waste heat".

This is, or should be obvious.

Quite often, in explainations of how heat engines operate, however, this fact is entirely neglected or ignored, or even denied, as if it were necessary that all the heat going into an engine needs to be removed in order to "complete the cycle".

This is wrong.

Heat that is converted into work by the engine does not need to be "rejected" to a "cold reservoir".

[Tom Booth]

In most cases, a great deal of emphasis is placed upon the importance of, or the actual necessity for the removal of "waste heat", with ever larger cooling fins, water cooling jackets, auxiliary fans and so forth, with virtually no attention given to reducing or eliminating "waste heat" by improving engine efficiency.

In fact, it is alleged that improving actual engine efficiency is "impossible". The rate at which heat can be converted to work has some definite limit.

This assertion has no factual basis. There never was any experimental evidence that ever established such a limit, nor any subsequent empirical evidence to support it throughout history.

The only support for it, harkens all the way back to Carnot and the Caloric theory, where it was imagined that all the heat from the "hot reservoir" necessarily had to flow through the engine to the "cold reservoir".

The "efficiency limit" still promulgated by our educational systems today is based entirely on this same fiction, the so-called "height of the fall" or temperature difference.

This is completely wrong headed and entirely detrimental.

Rather than working towards making heat engines more efficient, emphasis is on compensating for inefficiency by introducing yet more inefficiency.

Bigger and bigger cooling systems while blasting the engine with multiple flame throwers, it is a kind of self-fulfilling dictum that inefficiency is unavoidable, "waste heat" inevitable.

That attitude is not just counter productive, it is also just plain wrong.

[Nobody]

In regard to Matt Brown's last post, yep, thou I haven't read Internal Fire.

Tom seems to think that an atmospheric engine, such as a free piston Stirling, produces power on both the power stroke and the compression stroke.

Atmospheric engines use up, absorb, some power on both strokes. Not through the whole stroke.

Caloric Theory was discarded years ago. Why does it keep coming up! It being wrong makes it useless, hence nothing can be proven from it's mentioning.

Heat rejection has nothing to do with power out. It is a power absorber process. It takes power to move during the heat rejection/compression stroke. Rejecting heat just makes that power-loss less.

An atmospheric engine's stroke changes from power out, to power in, when it crosses the point where internal pressure is the same as the external atmosphere's pressure. Thus causing the piston to slow after that point. That is true for both the compression and expansion strokes.

P.S., It is blatantly obvious from Carnot's theory that heat flows in the hot side, and out both the cold side and out as power, proving Caloric Theory wrong.