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Hot air turbine prime mover


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Hello, I am new to this forum. I got tired of waiting for Engineering Forum.Org to come back on line. I used to ask questions there.


In my mind, I equate a stirling cycle engine to a tripple expansion steam piston engine, as compared with a steam turbine. I think that there must be a way to have rotor(s) and stator(s) replace the multitude of parts used in a stirling piston engine. I have thought about this for several years now. I have reached the limits of my mental ability, and need help. Since no one that I know personaly is familar with the stirling cycle, I have come here.


My ideas so far; 1. I am considering an open as oposed to a closed system. This greatly simplifies construction. Since cold air is constantly being introduced, the recuperator and regenerator are not needed. The theoretical loss of efficiency does not bother me, as my primary goal is co-gen, the "lost" energy will be used anyhow. My primary question in this regard, is whether the additional force generated by the cold end in the typical (alpha, beta and gamma) engine is needed.


2. I am aware that the difference in volume between water and steam is a ratio of 1-1700 IIRC. This makes steam engines powerful, there is a lot to work with. I imagine that the rate and volume of expansion of air, given a delta T of perhaps 600 degrees F to be dramaticly less. It is possible that I could increase the delta T to 1000 F or thereabouts, max, given my present inability to work with advanced ceramics. With my present goal of 3-5 K.W. electrical generation, could a hot air turbine fit in a house, considering a firebox of at least 24" height must be incorperated beneath the turbine? How do I calculate or determine the increase in volume of air for a given temp rise?


3. I surmise that it may be necessary to compress the cold air which enters the "hot end" or expansion chamber in order to create an effective engine. Do others agree?

If compression of the cold air is required, I would like, if possible, to incorperate the compresser into the (hopefully) single rotor. I asume that the parasitic drain would be overcome by added effiency. I am completely unconcerned, at this point, with fuel effiency.


4. I am aware of the operation of centrifical pumps. They draw fluid into the center of the rotor and expel it though the outer circumference. This seems to me to be more effective than impulse or reaction style. In this case, the nozzles would be integral to the turbine. I realize that this is the oposite effect of a Tessla turbine, however,the air should be accelerating, as opposed to slowing, as in the Tessla turbine. The other possibility is a Tessla turbine, but perhaps they require greater pressure/ flow rates than could ever be acheived. I seem to recall that Tessla's turbine needs at least 35 P.S.I. to work at all.


5. It occurs to me that due to the low amount of working energy available, a wide turbine would be needed, many turbines on one rotor. I also occurs to me that a larger diameter would compensate for the weak work force. I don't have the knowledge to differentiate between the two concepts. Help in this area would be especialy benificial.


5. I feel that I am overlooking something so basic as to be embarrassing. I cannot put my finger on it, which is why I am posting here. In my mind, I cannot see the entire enterprise at once, only segments at a time.


I am not an engineer. I am a competent machinist, fully equipped to manufacture a prototype. I appreciate any and all responses. I will take offence at none, however, kindness is always appreciated.

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I am afraid that I haven't really understood what you want to do. So, while I give you some existing examples below, I realize that this may not be what you are trying to do. Maybe you can provide us with a schematic? Pictures work better than text usually.


If you compress cold air which is then heated, you're essentially building a gas turbine engine... although gas turbines are often heated directly (by the injection and combustion of fuel)... I don't know how you intend to heat the air.


If you also utilize the waste heat from the combustion process, you create a Combined Gas and Steam power process (which runs a gas turbine engine, uses the waste heat to make steam, for a second turbine + generator).


But there are many gas turbine engines. For example, airplanes have completely different gas turbine engines. They often use a turbofan jet engine, which has a large fan at the front to move extra air in a bypass.


If you draw up a schematic (your first plan), don't lose yourself in the details. All we need to know is which flow of air/fuel/exhaust is going where. We need to distinguish between hot and cold, and high pressure and low pressure. The types of compressors and turbines, and the choice of materials will come only after that.

Edited by CaptainPanic
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Thankyou for your reply Captain. I apologise for my incomplete description. I am attempting to design an external combustion engine utilizing wood (firewood) as fuel.


My goal is to entice those already habituated to heating with wood, to also generate electricity though the availability of said engine, a move towards residential Co-generation http://en.wikipedia.org/wiki/Cogeneration


I will create a schematic as quickly as possible.

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This is my first attempt at paint. The quality is poor. Hopefully it is complete enough. I think a pulse sequence may be needed.

In this picture, I am not sure what the turbine is connected to... And I am not sure what the color green represents.


If the color green would be indicating water/steam, and the "heat emitter" is a cooling device, then you have just reinvented the steam cycle. :)


What would the pulse be able to achieve?

Edited by CaptainPanic
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In this picture, I am not sure what the turbine is connected to... And I am not sure what the color green represents.


If the color green would be indicating water/steam, and the "heat emitter" is a cooling device, then you have just reinvented the steam cycle. :)


What would the pulse be able to achieve?


The turbine is connected to a generator in typical fashion.


The color green repesents the coolant, which circulates through passages in the turbine housing and is not part of the engine cycle. It does not turn to steam or even boil, but removes heat from the system. The heat emitter is one of the following, radiant floor, baseboard radiator or water to air heat exchanger depending upon the original design of the home's heating system.

It seems to me to be more of a modernized open erricson cycle.


Pulse would allow the compressed air more time to develop pressure in the expansion chamber, the area at the bottom of the turbine/top of the fire box.


The complete cycle is as follows: Cold outside air is forced into the expansion chamber by the compresser. (compression)


Air is heat in expansion chamber. (heat input)


Air expands, propeling turbine. (expansion)


Turbine housing is cooled by water (heat rejection)

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Whoa! You want to build a pulse-jet powerstation :)


The V1 flying bomb was the first large scale implementation of this type of propulsion. Please note that rather than creating a propulsion, you can also drive a turbine with it... so it can work. I think that your marketing department may have to find a catchy name for the invention (The V1 Powerplant probably won't make it popular), but that is something that can be solved later.


I would suggest you seriously consider a continuous operation. Pulsed systems will just make your life harder than necessary (you already have enough trouble trying to invent something new in a multi-billion field of research that has been around for 3 centuries).


Here you have a link (yes, again wikipedia) about turbochargers, which may do exactly what you need: compress air for a combustion process. Turbochargers have the added benefit that they're powered by the exhaust gases. If you want to build this for a 3 - 5 kW electricity generator, a small car's turbocharger may be just the right size (or even a bit oversized - yes, cars are that powerful - for example 100 hp equals about 75 kW in power).


Finally, some funfacts (completely unrelated to this topic): Here is something about how to add two liquid fuels into a combustion chamber (like kerosene and oxygen). You need a so called "turbopump" to get the fuels in... but once it's in, it really moves.

Edited by CaptainPanic
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Thankyou Captain. I would much prefer non pulse operation, I only considered it as a possible nesessity.


I am not sure if I have been clear enough in my descriton for you to thoroughly understsand my proposal. The goal is an external combustion engine, utilizing the expansion of air only as the driving force, similar to a stirling cycle engine. I do have an alternative plan which may be required, but I would like to follow the hot air as working fluid plan first, as to pursue the alternative seems to be taking the path of least resistance, at the cost of greater complexity, especialy in relation to the quality of the fuel.


I have considered turbochargers many times, however, the power density of the exhaust of an internal combustion engine is much greater than the power density of a wood stove exhaust.


Certainly the configuration need be similar,i.e. two turbines sharing a shaft, however, I think that the compression turbine must be much smaller than the expansion turbine.


Again, I apreciate your input.

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  • 1 month later...

Hello 'Dune',


In answer to your question this may be of help.


It is a hot air rngine using a hydro turbine.


Also far greater work forces are produced if air is replaced with Co2.

Let me start by saying "sorry", because I am about to be very blunt.


I have no idea how your contraption works. I cannot understand your picture, and I cannot understand the text. The words in the text ('saddle tank', 'short pipe' and a couple more) are nowhere to be found in the picture. You don't explain the heat source, and you don't explain the cold sink. You use all kinds of arrows and V-shaped pointers to show where gas or liquid is flowing. You use the same lines for arrows and pipe-walls.


I'm not gonna say that it doesn't work... I am only saying that you are probably still the only one who understands it. Unless you give a better explanation, I advice everyone to ignore it.

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Hello 'CaptainPanic'


So right you are grabbed the wrong picture and the new on is not much better.


To help out the tree verticals are water pipe. The inverted V are one way valve.


The heat source can be any.


Cooling is ambient.


How it works. Looking at the picture see one vertical pipe is shorter than the other two.


An air pocket in top of the short pipe expands pushing water before it.


The pushed water goes through a hydro turbine into a catchment known as a saddle tank

as it sits astride the turbine, or can be side tank as shown (easier to draw)


The water filling the saddle tank compresses the air inside it into the pressure reseviour.


When the air pocket reaches the end of the short pipe it crosses into the pipe next to it the tall pipe.


The air rising up the tall pipe dislocates some water leaving the float nothing to float on and drops away

from blocking the exhaust hole.


The air now having escape point chooses to do so and in so doing allows the water to flow back from the saddle tank and fill both pipes full of water, plus vaccums fresh air into the saddle tank.


When both pipes are again full of water the float is blocking the exhaust hole.


Now the air pressure inside the pressure reserviour trickles into the short pipe and rises the surface where it is heat expanded beginning the process again.


If Co2 is used in place of air cooling is done the -40*C Dry-Ice formed by the gas pressure is used to cool the exhausting Co2 into the saddle tank.


As for the drawing some people get some people dont, cant be helped thats life.

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  • 2 years later...

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