A New Type of Hot Air Engine

[Tom Booth]

..., what I mean is that the hot air is completely cooled before BDC in a cold soaked engine. Therefore there is no contraction towards TDC, but instead compression.

OK. Still seems contradictory to me.

If the cylinder is hot enough not to absorb heat on the expansion stroke, how is it then suddenly cold enough to absorb heat on the "contraction"? And at that, after the gas has expanded and cooled to some degree, logically, I think.

Anyway, the slow mo video, I think, tells the tale. It isn't cold enough, the pressure relief valve, when free to do so stays open. In other models the intake valve opens early to let out the pressure during the supposed "contraction".

Admittedly, hot air molecules driving the piston, or even a jet stream of hot air, with no "pressure" and the valve wide open is pretty hard to imagine or accept, so I think some testing is needed to confirm this, but the conventional theory or assumption that heat is being absorbed by the cylinder so quickly makes no sense to me. It makes even less sense if the cylinder has to be heated up before it can cool down the gas.

I think the gas is cooled by the very meger adiabatic expansion, against all odds, and a miniscule amounts of adiabatic contraction just after BDC (expansion) before the pressure begins to build.

As for the standing wave theory, reference the 2 stroke tuners handbook

Thanks, I'll try to locate that and take a look.

[Tom Booth]

OK, got it, but a quick word search of the content turns up "0" zero references to the phrase "standing wave".

Resize_20230322_105106_6805.jpg (87.28 KiB) Viewed 5012 times

Do you have a page number? Chapter heading or something?

I find scant reference to "pressure wave", but they don't seem relevant.

Logically, IMO no "standing wave" is possible in a cylinder containing a rapidly reciprocating piston taking in and exhausting it's contents. There is terrific turbulence.

[VincentG]

I thinks its a bit of semantics here. If you study the purpose of the two stroke expansion chamber, you will see that the effect it has can occur in a perfectly round tube, or even just an open port, though less controlled. The same holds true for 4 stroke intake and exhaust tuning. The point being, we can use this to our advantage, or it will fight us.

https://link.springer.com/chapter/10.10 ... -0749-2_13

As for the other question, even a hot flame licker engine is much cooler than the flame it is ingesting, so there is still a large temperature difference. If we look at the forces trying to cool the flame, we have the cold surfaces of the engine, and the expansive cooling effect of the piston traveling toward BDC. The piston can flow much more air than the small intake valve lets in so there is still expansion here. Also, the gas can be fully contracted before BDC and a slight lag can explain how there is still a vacuum powered stroke towards TDC, which quickly turns back to compression as again, the piston flows much more air than the port allows. This can be seen on a more basic flame licker with just a slide valve, the flame gets blown away from the port before it is sucked back in.

You made me realize this may show how perfecting that design may be a good path forward to understanding the Stirling cycle.

[VincentG]

And yes, I am implying too, that there can be pressure when the relief valve is closed, and vacuum while it is open. This is why performance 2 strokes have ultralight and responsive reed valves, and why perhaps the pinnacle of their performance is achieved with the rotary valve design, which eliminates a lot of these valve inertia and pressure wave issues.

[Tom Booth]

I thinks its a bit of semantics here. If you study the purpose of the two stroke expansion chamber, you will see that the effect it has can occur in a perfectly round tube, or even just an open port, though less controlled. The same holds true for 4 stroke intake and exhaust tuning. The point being, we can use this to our advantage, or it will fight us.

https://link.springer.com/chapter/10.10 ... -0749-2_13
...

So,... you can't locate any reference in the previous book?

Sorry but I find the entire "thermoacoustic" theory in general completely nonsensical and non-productive in relation to actual engines.

This for example, seems like nonsense to me.

https://youtu.be/DEF0KplocEo

I think the resemblance between a Hofler tube (and similar heat driven acoustic devices) and any actual heat engine is purely superficial and constitutes a red herring as far as any kind of actual engine performance or "tuning" is concerned.

[VincentG]

So,... you can't locate any reference in the previous book?

I maybe had chosen the wrong phrase and should have just said "pressure wave". The latest link i posted references "standing waves". The 2 stroke tuning guide doesn't use that term directly but it can easily be inferred I think.

Regardless, this is a bit of a tangent anyway. We can agree to disagree on the matter. Its only a small part of the operating theory of these heat engines.

[Tom Booth]

So,... you can't locate any reference in the previous book?

I maybe had chosen the wrong phrase and should have just said "pressure wave". The latest link i posted references "standing waves". The 2 stroke tuning guide doesn't use that term directly but it can easily be inferred I think.

Regardless, this is a bit of a tangent anyway. We can agree to disagree on the matter. Its only a small part of the operating theory of these heat engines.

Well, if there were any evidence of some actual truth behind what you seemed to be alluding to here:

viewtopic.php?f=1&t=5514&sid=7482e7523e ... =60#p18826

regarding " - waves" I would certainly be interested in any such phenomena and you used some pretty strong language: "... proven,... significant forces associated with the standing waves... These have been taken advantage of to great success in modern 2 stroke design theory... poorly designed engine will be fighting these pressure waves endlessly"

A bit of an overstatement perhaps?

Your apparently null reference could possibly cost me several hundred dollars for a rare out of print book, if there did not happen to be a PDF available.

Now you say: "The 2 stroke tuning guide doesn't use that term directly but it can easily be inferred".

Can you cite some actual passage? Page number? Paragraph or statement from that book to that effect?

You said: "As for the standing wave theory, reference the 2 stroke tuners handbook"

I've pretty thoroughly skimmed through the chapter headings, used search terms on the PDF etc.

I can't honestly "agree to disagree" regarding this apparently extremely important and vital topic every engine designer really needs to know or face total failure without my having some actual concrete data point to evaluate one way or the other.

I have assumed you are not just talking out your a--- and sending me on a wild goose chase for non-existent "proven" information.

[Tom Booth]

There is a Google books preview of the relevant chapters of the second book here:

https://books.google.gy/books?id=ekvtCA ... &q&f=false

No offense but the book reads, (to me), worse than merely theoretical. Entirely vacuous, incoherent academic babble without substance.

Yes, there is some brief mention of "waves" of various sorts, but the context just seems like words thrown together by someone with no practical or personal experience with the topic (actual engines of any kind) whatsoever.

The only reference I see to "tuning" manifold pipes has to do with fluid mechanics, not literal tuning as in, or of "waves" pressure, sound or other, just air flow through pipes, mainly in relation to high performance race cars.

[matt brown]

https://youtu.be/oxxmzfQEC5M

Typically it is said that these engines work by cooling and vacuum. In this slow motion video though, it is apparent that the "free" pressure relief valve is open for nearly the entire return stroke.

If the engine operates in the manner supposed, how is it possible to have a vacuum with the pressure above atmosphere?

It appears, though a bit difficult to judge, I suspect, the intake valve begins to close well before the gas is fully expanded and is probably closed completely by BDC if not before. With "contraction" the pressure relief valve opens a long long way before TDC.

Very interesting video. Note piston throws are returning piston towards TDC when the "relief" valve opens as if the piston is exhausting cylinder thru this valve. I have no problem with that except: (1) surely not the typical flame eater 'pitch' (2) if so, this would make exhaust isobaric like Lenoir, but how could there be greater pressure inside cylinder then ambient after inlet valve closes unless cylinder walls add a little heat after inlet valve closes ?

[Tom Booth]

To be fair, that engine looks to be a home machinist model and may be atypical, I am now more interested in these engines and am looking for a model to buy for closer inspection and general tinkering with.

[VincentG]

Tom, start on page 65 of the 2 stroke tuners handbook. I made a mistake with the book title. I'm still looking for the book I'm thinking of that goes into MUCH more detail. But this is a start. If you have ever ridden a high performance 2 stroke dirt bike you will understand the mind blowing surge that is the "power band" where the pressure wave based tuning of the expansion pipe and port timing turns the engine from a turd to an absolute animal. If you have not experienced that, I can understand how hard that may be to believe.

TUNED LENGTH
We may start by determining the proper length through the entire system back to
the expansion chamber's closed end. That task requires that we know the speed at which
sonic waves travel within the chamber, and therein lies a great difficulty. As noted
previously, the velocity of these waves is determined largely by the temperature of the
gases through which they are propagated - and that factor, temperature, varies
continuously in the course of a single operating cycle. Exhaust gases emerge from the
cylinder at about 1200° F and have very nearly (about 800o
F) the same temperature back
in the outlet pipe. But expansion within the chamber itself cools them (prior to
recompression and reheating back in the baffle cone) to perhaps 500° F., or less, in the
midsection, and a wave does not move as rapidly through those cooler gases. It is
possible to calculate fairly exactly the temperatures at all points throughout the system,
but that is a very complex thermodynamic problem and certainly beyond the capabilities
of the layman. Indeed, honesty compels me to admit that it is not a problem I would like
to face without a computer and the assistance of someone experienced in that kind of
work.
Happily, in this instance it is possible to arrive at a satisfactory solution to the
problem by determining wave speed -by starting with the answer and working back. In
short, you can measure a lot of existing expansion chambers known to be effective, and
by comparing their lengths, exhaust port timings and the speeds at which the engines
develop their power, eventually come up with a figure for wave speed representing a
workable average for a whole range of high-output engines. My own research, conducted
along the pragmatic lines just described, was begun in about 1960 and I arrived at a
conclusion in 1964 that has required only slight modification over the succeeding eight
years. That conclusion was, and is, that one may use a wave speed figure of 1700 ft/sec
in combination with the anticipated engine speed at maximum power to arrive at a system
Two Stroke TUNER’S HANDBOOK
56
length (measured between the exhaust port window and the point of mean reflection in
the cone that constitutes the closed end of virtually all expansion chambers). That figure
provides an excellent starting point for the system, as it represents a high average and any
error will merely result in a lower-than-projected power peak. Actually, the addition of
more examples to my charts in recent years make me inclined to think that something like
1670 ft,/sec is more accurate, but I still use the 1700 ft/sec figure as a starting point, and
subsequently shorten the system slightly, perhaps an inch, if tests indicate that the power
peak obtained with the chamber is too low.
Using that high-average figure for wave speed (or indeed any figure your fancy
dictates, if your findings contradict my own) you may establish the exhaust system's
tuned length by means of the following formula:
N
E V L o s
t
× =
Where Lt is the tuned length, in inches
Eo is the exhaust-open period, in degrees
Vs is wave speed, in feet per second
N is crankshaft speed, in revolutions per minute
For example, in an engine with an exhaust-open period of 180-degrees, and a
power peak at 7000 rpm, and using the 1700 ft/sec figure for wave speed, then,
7000
180 1700 Lt
× =
L 43.7 t = inches
That length is, I must again stress, measured from the exhaust port window back
to a point slightly more than halfway down the baffle cone at the end of the system. The
exact point, and how to find it, will be dealt with shortly, along with an explanation of
why we use a cone to close the system instead of a flat plate - and how the taper of that
cone influences an engine's power curve. First, we'll consider the size and taper of
diffusers.

[matt brown]

To be fair, that engine looks to be a home machinist model and may be atypical, I am now more interested in these engines and am looking for a model to buy for closer inspection and general tinkering with.

It has me thinking too.

https://en.wikipedia.org/wiki/Vacuum_en ... nktion.gif

The wiki gif of vacuum engine is how I normally consider a flame licker.

Flame licker_1.png (108.21 KiB) Viewed 4960 times

and wiki's lead paragraph is correct.

Flame licker_2.png (28.92 KiB) Viewed 4960 times

However, wiki's paragraph on "Ideal Thermodynamic Process" for flame licker does NOT match their first paragraph.

To clarify, per wiki's gif, I have typical flame licker working this way: (1) cylinder intakes hot air from TDC to BDC at ambient pressure where force on each side of piston is equal BUT density in cylinder is less than ambient due to greater temperature (2) cylinder gas is cooled at BDC whereupon cylinder pressure drops and ambient pressure is now greater than cylinder pressure (3) piston returns from BDC towards TDC with adiabatic compression until cylinder pressure equals ambient, whereupon relief valve opens 'near' TDC. The trick here is that the lower density of the hot air games PV=nRT since the ambient 'n' is greater than the cylinder 'n' during intake.

[matt brown]

TUNED LENGTH

Using that high-average figure for wave speed (or indeed any figure your fancy
dictates, if your findings contradict my own) you may establish the exhaust system's
tuned length by means of the following formula:
N
E V L o s
t
× =
Where Lt is the tuned length, in inches
Eo is the exhaust-open period, in degrees
Vs is wave speed, in feet per second
N is crankshaft speed, in revolutions per minute
For example, in an engine with an exhaust-open period of 180-degrees, and a
power peak at 7000 rpm, and using the 1700 ft/sec figure for wave speed, then,
7000
180 1700 Lt
× =
L 43.7 t = inches

Love it, pour yourself some more hard cider !!! I'm glad that in my slow-speed low-temp world I don't have to worry about such things.

[VincentG]

Matt, to be clear, I am obviously not inferring that a flame licker or Stirling engine has the same pressure wave tuning effects as a 2 stroke otto. My point was merely that these waves occur any time you have a reciprocating piston and air flowing through a port.

As for the flame licker is concerned, can we not simply explain the fact of the relief valve flying open by a few things...first would be pressure waves as I have stated already. Second may be due to some strange resonance causing the metalic and unsprung valve to "bounce" off of the engine, at least in part.


Third seems the most likely to me, that the volumetric efficiency of this engine is far below 100%, so it is not nearly ingesting a full volume of hot gas, and therefore it is always shuttling some amount of cold gas back and forth, and so has a compression period towards TDC.

[Tom Booth]

Tom, start on page 65 of the 2 stroke tuners handbook. ...

Thanks for the response. I was worried I might have scared you away by being so critical. The reference helps me to understand where your coming from.