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Maybe, the problem with vacuum, flame eater, atmospheric engines?

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John Hill:
Maybe, the problem with vacuum, flame eater, atmospheric engines?

Atmospheric engines under various names and configuration are popular home shop projects and many seem to be a real challenge to get to run.

One of the popular types is the 'flame licker' where a port opens and hot gases are drawn into a cylinder then the port closes, the gases cool and while contracting atmospheric pressure pushes the piston down.

So lets look at this for a moment, the port opens at or near bottom dead centre and hot gases are drawn in as the piston goes 'up', the port closes near the top of stroke and the gases cool to produce the 'power' stroke.

But hangonaminute! As soon as those gases start to get drawn in they contact the cool cylinder walls they start to contract, ditto for the gases that contact the piston head.  Therefore by the time the piston gets to top of stroke we have cold, contracted, gases all around the cylinder and near the piston, around the cylinder head too if it is cooled.  Obviously a large proportion of the gases in the cylinder are already cool and take no part in the power stroke.  If the engine was turning slow enough presumably all the gas would be cool at top of stroke and there would be no power to be had at all.

So how to fix this?  Maybe a partial fix would be an insulating sleeve or liner to the cylinder geared to move and cover or uncover the cylinder walls according to crank position.

Bottom of stroke, sleeve right down, gases are starting to be drawn in.
Near top of stroke, most of the gas is still hot, sleeve moves up to expose cool cylinder walls, gases contact cool cylinder walls and contract for the power stroke.
Near bottom of stroke, sleeve moves down.

Now it might appear that this scheme would have a few drawbacks, and I can see a few.

For example the piston crown and cylinder head are not shielded, maybe it would be better if they had insulated surfaces and take no part in the cooling at all?

Friction, there would be an unavoidable increase in friction.

Something to think about?

tomrux:
any additional friction is a killer in these atmosheric engines.

most of the difficulties getting these to go in the first place comes down to excess friction.

Tom R

John Hill:
Excess friction or lack of power, the result is the same.  I think there is a lot of power wasted because too much of the gas is cold before the port closes.

Stilldrillin:
Hmmm....... Yes!

But. How cold is cold?   :smart:

The likkers I own, must be warm before they will run.......   :thumbup:

Defor.

BillTodd:

--- Quote ---But hangonaminute! As soon as those gases start to get drawn in they contact the cool cylinder walls they start to contract,
--- End quote ---

It's important to remember that flame lickers don't run on gas as such. The flame sucked into the cylinder is a plasma a different form of matter to the gas (like gas is to a liquid)

 A plasma is a super heated gas in which the molecules have sufficient temperature to break apart into ions - When the plasma cools, it under goes a rapid phase transition to a gas and its volume dramatically decreases. (i.e. it condenses as the molecule snap back together)

A free Plasma like a flame  loses most of its energy (heat) by radiation - energetic photons fly out of the plasma taking their energy with them -  i.e.  the light you see and heat you feel from a candle flame*.  In an encclosure, like the cylinder of an engine, the best way to keep the plasma warm is to reflect the photons back into the plasma by polishing the internal surfaces to a mirror finish - (incidently, when the cylinder is hot, it helps to keep the plasma warm by adding its own infra-red photons)

Like a gas, a plasma  will also lose energy by conduction at its surface. The plasma has very low density and very little heat capacity, and it internally turbulent (well stirred) , if it touches anything , even a good insulator, it will cool very rapidily.


Bill



*watch a candle flame  and the smoke that rises from it - the smoke and flame is moving at, what?, about  a metre a second (guess) the flame, plasma, part is say 30mm long so the plasma has cooled in about 30mS (and that is with a continuous supply of energy from underneath)

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