VincentG wrote: ↑Tue Mar 21, 2023 12:48 pm
I have been guilty of oversimplifying in the past.
Though, in favor of simplicity, I would like to offer the flame licker engine as an example.
Just since you mention it, I have an unconventional theory about these engines, aside from not really agreeing they are really an example of simplicity, particularly in comparison with other types of heat engines, nearly all of which are simpler.
Anyway, simplicity is very good, if it is true. A simple
explanation of how something works that is,
if true.
My issue with the Carnot theory generally is not it's simplicity so much as it's apparent falsehood.
It was a bit difficult to find, but I finally located a pretty good slow motion video of a flame eater engine. It helps to reduce the playback speed even more to 25%
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?
Well, the efficiency of these engines is considered very very small. Almost non-existent, but probably more than other Stirling type engines, timing is hyper-critical. There are valves, cams, etc.other Stirling type engines don't require.
Anyway, back to my heretical theory.
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.
Considering molecular theory, it is the actual local impact of hot, high velocity air molecules impacting the piston head that actually drive the piston, so does it actually even matter that the valve is open while the flame is being "eaten". There are still hot high velocity air molecules being generated as they are drawn in past the flame.
Another anomaly is that some YouTubers mention that the engine cannot operate until the cylinder warms up.
What? Really?
https://youtu.be/Zh1JZ16HBaI
If the mode of operation is actually cooling and contraction of the hot gas, common sense would seem to suggest that the colder the cylinder the better, right?
So how does a hot cylinder cool the gas? The conventional explanation makes no sense in many ways.
I'm guessing the efficiency and performance of these flame lickers could be greatly improved by recognizing how they actually operate. Basically the same as other heat engines.
The expanding hot gas drives the piston, at least those hottest of the gas molecules impacting the piston head and "
hitting a moving target". If you've seen this explanation:
https://youtu.be/PMKPZuCj9a0
Is that possible with the valve open and drawing a vacuum? If the kinetic theory is accepted, I would say yes. Any gas consists of a mixture of high speed ("hot") and low speed ("cold") molecules. It is only the very hottest high speed molecules that ever do any work in any engine.
The engine will have a hard time running while the cylinder is cold because the energy of the high speed air molecules is absorbed by the cold cylinder walls before they can pact the piston.
Because of the wrong theory of operation, the potential hat air expansion of these engines is not realized, the gas has little opportunity to do work and cool, so the pressure relief valve has to stay open during the supposed "contraction" of the gas that is allegedly driving these engines.
If the cylinder is hot, the hot gas molecules drawn in will ricochet off the side walls and still hit the piston.
From all these facts and observations I'd say it is reasonable to assume that these engines don't actually operate the way generally believed.
Probably a quick opening and rapid closing at the start of intake allowing more time for the trapped hot gas to expand and do work on the piston, actually cooling the gas by a full work output would result in a much stronger running engine, without any need for a pressure relief valve.
Possibly some model engines of this flame licker type are designed more along the lines I am suggesting, but these are somewhat uncommon, I don't have one to play with and slow motion videos that show the actual timing are hard to find.
So is there really "heat rejection" in these engines? Do they really run on a vacuum? I'd say hardly. They don't do enough real expansion work to actually produce much of a vacuum, therefore, the necessity for relieving pressure on the return stroke due to the less than perfect expansion work.
I think I'd try having the flame heat up a metal mesh "regenerator" or heat accumulator at the intake. Make a cam that opens the valve very briefly to let in a volume of very hot air, so that then the hot air can fully expand, doing work to drive the piston and therefore completely cool so as not to build up any pressure on the return stroke.
Maybe a bit of metal mesh "regenerator" type material just inside the cylinder could continue to heat and expand the gas more after the valve closes.