Choosing ALPHA Cycle Ratios
Choosing ALPHA Cycle Ratios
Am wondering what experiences others have had when choosing displacement volume ratios on hot and cold pistons in Alpha motors?
Re: Choosing ALPHA Cycle Ratios
I understand that the ratio is 1:1, but the one on this forum that may know more is GeoffV, he has built more complex motors than I have. Ian S C
Re: Choosing ALPHA Cycle Ratios
Yea, have been reading his posts. that engine of his is a 'sweet ride'.
Geoff wrote -
"My Alpha on the other hand has a fixed phase angle (kinematic) of 90deg but a maximum volume ratio of 0.64. So that there is no misunderstanding I am refering to the ratio of the maximum volume swept by the displacer and the maximum volume achieved between the top of the piston and the bottom of the displacer."
So i think this refers to the hot piston side ratio, and its relative dead space (i could be wrong). When you start to break it down there are a lot of ratios one can play with. Will have to join his topic and get some ideas..(unless he comes bye this one)
One design i'm looking at making, uses caps on both hot and cold sides (i think geoff (oops sorry) refers to it as a displacer in his statement)
Dead space is the main concern i have, as some of my own tweaks may increase dead space. Ian, I've read one of your posts about how this may not be a concern due to certain experiments carried out. But I have a fear that the greater the dead space the greater the loss in torque due to increased 'squishiness'.
vamoose
Geoff wrote -
"My Alpha on the other hand has a fixed phase angle (kinematic) of 90deg but a maximum volume ratio of 0.64. So that there is no misunderstanding I am refering to the ratio of the maximum volume swept by the displacer and the maximum volume achieved between the top of the piston and the bottom of the displacer."
So i think this refers to the hot piston side ratio, and its relative dead space (i could be wrong). When you start to break it down there are a lot of ratios one can play with. Will have to join his topic and get some ideas..(unless he comes bye this one)
One design i'm looking at making, uses caps on both hot and cold sides (i think geoff (oops sorry) refers to it as a displacer in his statement)
Dead space is the main concern i have, as some of my own tweaks may increase dead space. Ian, I've read one of your posts about how this may not be a concern due to certain experiments carried out. But I have a fear that the greater the dead space the greater the loss in torque due to increased 'squishiness'.
vamoose
Last edited by vamoose on Thu Jul 05, 2012 11:46 pm, edited 4 times in total.
Re: Choosing ALPHA Cycle Ratios
Vamoose, all I've ever made run yet is a little toy, so know this is just my theory. The sizing and whatever dead space you come up with determines the compression ratio. I read somewhere that the compression ratio should only be about half of the heating ratio. That makes sense to me since there needs to be "room" for heat to get in above the compression heating. So for instance, if your gas runs at 100 F cold-side and 250 F hot-side, that's approximately a 1.3 heating ratio, (measured from absolute zero) so the optimal compression ratio would be about 1.15. For a 1.4 heating ratio the compression ratio should be about 1.2, etc. After choosing your swept volume ratio you could still fine tune compression ratio by varying the exchanger/regenerator volume. I'm rebuilding my current project because I finally realized I had way too much compression.
Re: Choosing ALPHA Cycle Ratios
Vamoose
Unfortunately you have lifted part of a previous 'post' of mine such that it now reads out of context.
The displacement ratio on an Alpha is the same as the swept volume ratio because there are two pistons each controlling their respective volumes. The Beta is quite different, the expansion volume is controlled by the displacer stroke yet the compression volume is controlled by both the compression piston and the displacer. Because of the necessary phase angle, the displacer is descending towards the compression piston before the compression piston reaches TDC. The only way therefore to accurately calculate the displacement ratio on a Beta is to measure the maximum gap between the bottom of the displacer and the top of the piston x (the area of the bore) before calculating the displacement ratio. If the dispacement ratio is too near unity (and/or the heat exchangers are inadequate) the working gas will not expand enough to maintain a pressure difference across the power piston as it descends and the engine will at best 'just run' or at worst won't run at all.
Compression ratio, many engines will not run if the compression ratio is too high, I have seen several examples where the compression ratio has been lowered and the engine will then run. The probable reason is that unlike most academic theory, which considers the heat transfer to be isothermal, in practice, due to the speed of the cycle, the heat transfer is almost certainly adiabatic, an experiment carried out by John Archibald demonstrates this to my satisfaction at least. If the adiabatic rise in temperature is too great because the compression ratio is too high it overloads the cooler which cannot remove the heat fast enough.
Keep your design sequence simple.
1. Heat exchangers are the heart of an engine, if you can't get the heat into and the waste heat out of the engine the power output will be very small.
2. The displacement ratio ( and the ability of the heat exchangers) primarily dictate the temperature at which the engine will run.
3. Choose a crank mechanism that can be balanced or you will use what little power you have driving the vibration. Try shaking a house brick for a while and see how much power you have to input!
3. Design the engine to be pressurised, this is where the power really comes from, not the temperature difference.
Geoff V
Unfortunately you have lifted part of a previous 'post' of mine such that it now reads out of context.
The displacement ratio on an Alpha is the same as the swept volume ratio because there are two pistons each controlling their respective volumes. The Beta is quite different, the expansion volume is controlled by the displacer stroke yet the compression volume is controlled by both the compression piston and the displacer. Because of the necessary phase angle, the displacer is descending towards the compression piston before the compression piston reaches TDC. The only way therefore to accurately calculate the displacement ratio on a Beta is to measure the maximum gap between the bottom of the displacer and the top of the piston x (the area of the bore) before calculating the displacement ratio. If the dispacement ratio is too near unity (and/or the heat exchangers are inadequate) the working gas will not expand enough to maintain a pressure difference across the power piston as it descends and the engine will at best 'just run' or at worst won't run at all.
Compression ratio, many engines will not run if the compression ratio is too high, I have seen several examples where the compression ratio has been lowered and the engine will then run. The probable reason is that unlike most academic theory, which considers the heat transfer to be isothermal, in practice, due to the speed of the cycle, the heat transfer is almost certainly adiabatic, an experiment carried out by John Archibald demonstrates this to my satisfaction at least. If the adiabatic rise in temperature is too great because the compression ratio is too high it overloads the cooler which cannot remove the heat fast enough.
Keep your design sequence simple.
1. Heat exchangers are the heart of an engine, if you can't get the heat into and the waste heat out of the engine the power output will be very small.
2. The displacement ratio ( and the ability of the heat exchangers) primarily dictate the temperature at which the engine will run.
3. Choose a crank mechanism that can be balanced or you will use what little power you have driving the vibration. Try shaking a house brick for a while and see how much power you have to input!
3. Design the engine to be pressurised, this is where the power really comes from, not the temperature difference.
Geoff V
Re: Choosing ALPHA Cycle Ratios
Bumkin,
You say all you've made run is a little toy... well your one up on me mate. I've played with a few bought ones, but am still to make one (i'm working towards fixing this soon). Have to get my head around a few things before i can progress. Good luck with your rebuild, will be interested in how it goes...
Thanks for those papers you sent me Geoff..
Quality stuff. Your explanations of whats occurring are very helpful. Am still having trouble getting around ratios, but i will post something more in-depth soon when my noggin is a little more clear (am waiting for an epiphany).
1. Heat exchangers are the heart of an engine, if you can't get the heat into and the waste heat out of the engine the power output will be very small.
-This is the possibly the biggest problem i may have with the design i may pursue. I am potentially going to create large amounts of dead space in the quest for heat exchange. How much should i fear dead space? How can i combat this necessary evil?
2. The displacement ratio ( and the ability of the heat exchangers) primarily dictate the temperature at which the engine will run.
-I am hoping to design an engine that operates with an input temperature ranging from around 90 - 180 deg Celsius. This is my heat source supply range, (in liquid form). Considering that i may have a "theoretically" effective heat ex-changer (with a lot of dead volume), Do you have any thoughts on what engine cycle type i should lean towards.
3. Choose a crank mechanism that can be balanced or you will use what little power you have driving the vibration. Try shaking a house brick for a while and see how much power you have to input!
-Leaning towards a ross yolk style, What others are worth considering?
4. Design the engine to be pressurised, this is where the power really comes from, not the temperature difference.
-Totally agree.
Question - Does a gas at 2bar have twice the thermal conductivity as the same gas at 1bar? (seems like a stupid question, of course yes, (but i am often wrong)).
Glad to hear any thoughts anyone may have..
vamoose
You say all you've made run is a little toy... well your one up on me mate. I've played with a few bought ones, but am still to make one (i'm working towards fixing this soon). Have to get my head around a few things before i can progress. Good luck with your rebuild, will be interested in how it goes...
Thanks for those papers you sent me Geoff..
Quality stuff. Your explanations of whats occurring are very helpful. Am still having trouble getting around ratios, but i will post something more in-depth soon when my noggin is a little more clear (am waiting for an epiphany).
1. Heat exchangers are the heart of an engine, if you can't get the heat into and the waste heat out of the engine the power output will be very small.
-This is the possibly the biggest problem i may have with the design i may pursue. I am potentially going to create large amounts of dead space in the quest for heat exchange. How much should i fear dead space? How can i combat this necessary evil?
2. The displacement ratio ( and the ability of the heat exchangers) primarily dictate the temperature at which the engine will run.
-I am hoping to design an engine that operates with an input temperature ranging from around 90 - 180 deg Celsius. This is my heat source supply range, (in liquid form). Considering that i may have a "theoretically" effective heat ex-changer (with a lot of dead volume), Do you have any thoughts on what engine cycle type i should lean towards.
3. Choose a crank mechanism that can be balanced or you will use what little power you have driving the vibration. Try shaking a house brick for a while and see how much power you have to input!
-Leaning towards a ross yolk style, What others are worth considering?
4. Design the engine to be pressurised, this is where the power really comes from, not the temperature difference.
-Totally agree.
Question - Does a gas at 2bar have twice the thermal conductivity as the same gas at 1bar? (seems like a stupid question, of course yes, (but i am often wrong)).
Glad to hear any thoughts anyone may have..
vamoose
Re: Choosing ALPHA Cycle Ratios
Vamoose
Dead space is not a 'Killer' but does dilute the pressure pulse, if the rest of the design is uncompromised, some can be tolerated, unfortunately your proposals seem to have other compromises (low max T)!
Regarding which configuration of SE to use, I cannot advise, as I have far too little information. Operating with a max T of 90-180 can be made to work, LTD engines manage to run with less than one degree delta T, but as to the best displacement ratio, you guess is as good as mine at this stage.
Which kinematic configuration to use? none are perfect, not even the dear old Rhombic. If good balance is high on the priority list then the Rhombic or the 90deg vee twin Alpha would be a logical choice, for a compact mechanism, try Ross Yolk or four bar linkage, but both are difficult to balance and there are many, many more, each with their own compromises. Once again I can't advise from this distance.
Pressurisation, the power output is proportional to the rate at which an engine processes the working gas, large turbo jets are probably processing around a ton every second! Increasing the pressure (mass) from 1bar absolute (atmospheric) to 2bar absolute will double the mass being processed each revolution (less any losses due to increased friction and pumping losses).
I hope these thoughts are of use and I'm sorry I can't be more specific, but every aspect of a SE is interrelated and has to be treated as such during the design process.
Geoff V
Dead space is not a 'Killer' but does dilute the pressure pulse, if the rest of the design is uncompromised, some can be tolerated, unfortunately your proposals seem to have other compromises (low max T)!
Regarding which configuration of SE to use, I cannot advise, as I have far too little information. Operating with a max T of 90-180 can be made to work, LTD engines manage to run with less than one degree delta T, but as to the best displacement ratio, you guess is as good as mine at this stage.
Which kinematic configuration to use? none are perfect, not even the dear old Rhombic. If good balance is high on the priority list then the Rhombic or the 90deg vee twin Alpha would be a logical choice, for a compact mechanism, try Ross Yolk or four bar linkage, but both are difficult to balance and there are many, many more, each with their own compromises. Once again I can't advise from this distance.
Pressurisation, the power output is proportional to the rate at which an engine processes the working gas, large turbo jets are probably processing around a ton every second! Increasing the pressure (mass) from 1bar absolute (atmospheric) to 2bar absolute will double the mass being processed each revolution (less any losses due to increased friction and pumping losses).
I hope these thoughts are of use and I'm sorry I can't be more specific, but every aspect of a SE is interrelated and has to be treated as such during the design process.
Geoff V
Re: Choosing ALPHA Cycle Ratios
Just my two cents on pressurization if you haven't settled on a design yet. What matters is not how much air that can be moved, but how much heat that can be moved. Pressurized air can obviously move more heat, but as the pressure goes up, the exchanger area per equal unit of air or heat is compromised. It's wonderful if you have the technology to build efficient exchangers, but technology is a compromise too. I lean towards increasing displacement rather than increasing pressure, the burner and exchangers take the most space anyway. Bumpkin
Re: Choosing ALPHA Cycle Ratios
Dear Bumpkin
I've been dreaming for years of a way to build a Hot Air engine that produces usefull power with plain heat exchangers running at atmospheric pressure on air. Have you discovered a way of circumventing the problem of reducing surface to volume ratio (hydralic radius) as the engine is scaled up to a larger swept volume? Please advise as I spent 30 hours machining my last two heat exchangers and I would love to see a design that overcomes the difficulties of getting heat into and out of these engines.
This is probably not the right place to go through the measurements and calculations that I have made over the last 32 years but I would be willing to do so if you like, in a new thread. Sadly the above is just a dream and in the cold light of day, unless one is content with a few tens of watts from a mechanically huge engine, comprehensive heat exchangers and a regenerator are indispensible. My best advice to all reading this thread, is, if you are not able or not willing to make good heat exchangers, stop now and save yourself a lot of time and expense, unless you just want to watch something running.
Geoff V
I've been dreaming for years of a way to build a Hot Air engine that produces usefull power with plain heat exchangers running at atmospheric pressure on air. Have you discovered a way of circumventing the problem of reducing surface to volume ratio (hydralic radius) as the engine is scaled up to a larger swept volume? Please advise as I spent 30 hours machining my last two heat exchangers and I would love to see a design that overcomes the difficulties of getting heat into and out of these engines.
This is probably not the right place to go through the measurements and calculations that I have made over the last 32 years but I would be willing to do so if you like, in a new thread. Sadly the above is just a dream and in the cold light of day, unless one is content with a few tens of watts from a mechanically huge engine, comprehensive heat exchangers and a regenerator are indispensible. My best advice to all reading this thread, is, if you are not able or not willing to make good heat exchangers, stop now and save yourself a lot of time and expense, unless you just want to watch something running.
Geoff V
Re: Choosing ALPHA Cycle Ratios
Well gosh, Geoff, since you ask; one way to increase internal surface area is a big-bore short-stroke design. My power diaphragm is 15'' stroking 1". It's a Beta directly over a 12'' dia. displacer chamber - which I wish I'd made bigger for a shorter displacer stroke and to lower the compression ratio. I might glue some steel wool or such to the ends, but as with another of Vamoose's concerns, I don't know if the insulation effect of the glue would counteract the added thermal transfer of the wool. Diaphragms have their own problems, but I don't remember ever saying I had THE answer. I tried to represent a different view without being adversarial to you and if I was, I apologize. To everyone else, of course the same laws of physics apply to us all - my project engine is Approximately 150 cubic inches and I'd be happy to get 100 watts from it - if it ever runs at all. I never said otherwise. This forum history is full of people who live on alternate power, heat with woodstoves, and who would be, as Geoff says, "content with a few tens of watts from a mechanically huge engine." If I get mine going I'll post it, but don't hold your breath.
Vamoose, I'll leave your thread at that and I apologize for going off topic. Bumpkin
Vamoose, I'll leave your thread at that and I apologize for going off topic. Bumpkin
Re: Choosing ALPHA Cycle Ratios
Vamoose
Forgive me for replying to Bumkin on your thread, but as it is pertinent to your project and I can't think of another way of doing it, here goes.
I genuinly hope Bumkin is succesfull in his approach to obtaining some power from a Hot Air engine but there seems to be a general misunderstanding as to the process by which air accepts and rejects heat. In one of the articles I sent to you there is a graph of heat transfer vs gap for a solid to air, I will try to incorporate it in this reply. It is this problem which caused me to waste over 30 years of work and why I wish to share my findings with you all so you will hopefully not waste your time.
Air is a very good insulator, which is why our clothes work to keep us warm.
When air is flowing over a surface there is a boundary layer such that the air molicules next to the surface are stationary even though the free stream may be travelling at several hundred knots. A dirty aircraft departs from A and flies through clouds and rain to B where it arrives still dirty, why? boundary layer.
The amount of heater/cooler surface area is only part of the problem because unless the air is forced through a very small gap (inside the boundary layer) most of the air will travel down the middle, insulated by the boundary layer with little change in temperature. I have seen a report of an Italian engine where the heater was at 800c yet the average air temperature inside was only 400c , I have made similar measurements myself.
A conventional Hot Air engine with a displacer running in a plain tube heater responds well to the gap being reduced but suffers from the pass area decreasing with the gap such that the pumping losses rise dramatically. Slotted heat exchangers over come these pumping losses and increase the surface area whilst still forcing the working gas close to the surface of the heater and thereby realising good heat transfer.
Just in case I can't incorporate the graph, heat transfer through a gap of 0.5mm will give about 150watts/sq metre/deg K, increasing the gap to 1.2mm halves the heat transfer to nearer 75watts/sq metre/deg K. Home built engines will at best have an internal thermal efficiency of 10% and may well be lower 5% or less.
My Alpha with 90 slots, 0.5mm wide and 46mm long at normal operating temperature transmits about 1800watts to the working gas, at 10% thermal efficiency that should give 180 watts power output, the best to date 135watts. A plain tubular heater at the same temperature, 60mm diam and 46mm heated length with a 0.5mm gap will transfer about 327watts, at 5% thermal efficiency (due to increased pumping losses) gives 16watts power output!
Please don't be disheartened but please be realistic about the difficulties.
Geoff V
Forgive me for replying to Bumkin on your thread, but as it is pertinent to your project and I can't think of another way of doing it, here goes.
I genuinly hope Bumkin is succesfull in his approach to obtaining some power from a Hot Air engine but there seems to be a general misunderstanding as to the process by which air accepts and rejects heat. In one of the articles I sent to you there is a graph of heat transfer vs gap for a solid to air, I will try to incorporate it in this reply. It is this problem which caused me to waste over 30 years of work and why I wish to share my findings with you all so you will hopefully not waste your time.
Air is a very good insulator, which is why our clothes work to keep us warm.
When air is flowing over a surface there is a boundary layer such that the air molicules next to the surface are stationary even though the free stream may be travelling at several hundred knots. A dirty aircraft departs from A and flies through clouds and rain to B where it arrives still dirty, why? boundary layer.
The amount of heater/cooler surface area is only part of the problem because unless the air is forced through a very small gap (inside the boundary layer) most of the air will travel down the middle, insulated by the boundary layer with little change in temperature. I have seen a report of an Italian engine where the heater was at 800c yet the average air temperature inside was only 400c , I have made similar measurements myself.
A conventional Hot Air engine with a displacer running in a plain tube heater responds well to the gap being reduced but suffers from the pass area decreasing with the gap such that the pumping losses rise dramatically. Slotted heat exchangers over come these pumping losses and increase the surface area whilst still forcing the working gas close to the surface of the heater and thereby realising good heat transfer.
Just in case I can't incorporate the graph, heat transfer through a gap of 0.5mm will give about 150watts/sq metre/deg K, increasing the gap to 1.2mm halves the heat transfer to nearer 75watts/sq metre/deg K. Home built engines will at best have an internal thermal efficiency of 10% and may well be lower 5% or less.
My Alpha with 90 slots, 0.5mm wide and 46mm long at normal operating temperature transmits about 1800watts to the working gas, at 10% thermal efficiency that should give 180 watts power output, the best to date 135watts. A plain tubular heater at the same temperature, 60mm diam and 46mm heated length with a 0.5mm gap will transfer about 327watts, at 5% thermal efficiency (due to increased pumping losses) gives 16watts power output!
Please don't be disheartened but please be realistic about the difficulties.
Geoff V
Re: Choosing ALPHA Cycle Ratios
Although I have my own thoughts and experiences for discerning information, I have an open mind.
I'm glad to see different ideas, and I don’t know enough to agree or disagree with anyone so far. I haven’t formed my own perspective as yet. There really are a lot of different ideas out there. I'd like to think that some of the best insights can come from a difference of opinion.. Please feel free to participate in banter with any of my topics, I am grateful for anyone’s genuine input.
I'm not trying to be a kiss ass or some kind of conciliator.
But I do enjoy a good stoush from time to time.
Bumpkin, in regards to your statement towards displacement rather than pressure, I came across this youtube video, and although I cant verify what they are claiming, to look at the engine operating, really does spike my interest. I'm assuming its a gamma style engine (or possibly a beta??).
http://www.youtube.com/watch?feature=pl ... uuk_r--lqU
I have seen a few 'large style' displacement engines on youtube, they give the impression of a lot of potential oomph (is that even a word). They generally seem to be LTD with low RPM. I think there is a lot more about fluid dynamics that I need to learn.
Geoff, i really like the scientific approach and knowledge you have. Would it be possible to put some kind of link to the papers you sent me (I hope this is not too presumptive). I liked them a lot, and think others might too. Have found It really hard to get concise descriptions and information about stirling engines.
Regarding the videos I’ve seen on your engine. Thanks for the visual breakdown. Is your engine a pure alpha type engine or could it be considered a kind of alpha beta hybrid (this might be a misinterpretation). I can see why its called an alpha, but it seems like you have 'tricked it out' with another 'mix'. I can see in the test footage that it has good thrust (I've made a couple 'electric' boat motors, paddle and prop driven). I think you should get some respectable speed using the twin displacement catamaran hulls.
It does seem that some of the posts are getting off topic, including mine, but am enjoying it.
Regarding ratios. The more I read the less I know. I'm still mulling over stuff. If anyone wants to help increase my confusion then please post your ideas, or your ideas on those ideas...
or also any questions. My Grandpa once told me "the dumbest question is the one you don't ask"
(and to this i responded "why?")
vamoose
I'm glad to see different ideas, and I don’t know enough to agree or disagree with anyone so far. I haven’t formed my own perspective as yet. There really are a lot of different ideas out there. I'd like to think that some of the best insights can come from a difference of opinion.. Please feel free to participate in banter with any of my topics, I am grateful for anyone’s genuine input.
I'm not trying to be a kiss ass or some kind of conciliator.
But I do enjoy a good stoush from time to time.
Bumpkin, in regards to your statement towards displacement rather than pressure, I came across this youtube video, and although I cant verify what they are claiming, to look at the engine operating, really does spike my interest. I'm assuming its a gamma style engine (or possibly a beta??).
http://www.youtube.com/watch?feature=pl ... uuk_r--lqU
I have seen a few 'large style' displacement engines on youtube, they give the impression of a lot of potential oomph (is that even a word). They generally seem to be LTD with low RPM. I think there is a lot more about fluid dynamics that I need to learn.
Geoff, i really like the scientific approach and knowledge you have. Would it be possible to put some kind of link to the papers you sent me (I hope this is not too presumptive). I liked them a lot, and think others might too. Have found It really hard to get concise descriptions and information about stirling engines.
Regarding the videos I’ve seen on your engine. Thanks for the visual breakdown. Is your engine a pure alpha type engine or could it be considered a kind of alpha beta hybrid (this might be a misinterpretation). I can see why its called an alpha, but it seems like you have 'tricked it out' with another 'mix'. I can see in the test footage that it has good thrust (I've made a couple 'electric' boat motors, paddle and prop driven). I think you should get some respectable speed using the twin displacement catamaran hulls.
It does seem that some of the posts are getting off topic, including mine, but am enjoying it.
Regarding ratios. The more I read the less I know. I'm still mulling over stuff. If anyone wants to help increase my confusion then please post your ideas, or your ideas on those ideas...
or also any questions. My Grandpa once told me "the dumbest question is the one you don't ask"
(and to this i responded "why?")
vamoose
Re: Choosing ALPHA Cycle Ratios
Actually now i think about it Geoff, I'm probably wrong about the hybrid statement but am still intrigued by the close proximity of the heating and cooling ex-changers to each other and the hot piston.??
Bumkin, i see vile_fly mentioned that youtube link in a post on the topic called "Large stirling engine"
http://stirlingengineforum.com/viewtopi ... mera#p3701
he provides a little bit of info about the maker and device.
Even though its a completely different concept to whats been going on in my head i would like to know more.
Might join in and see if he or someone has got any real data on it..
Bumkin, i see vile_fly mentioned that youtube link in a post on the topic called "Large stirling engine"
http://stirlingengineforum.com/viewtopi ... mera#p3701
he provides a little bit of info about the maker and device.
Even though its a completely different concept to whats been going on in my head i would like to know more.
Might join in and see if he or someone has got any real data on it..
Re: Choosing ALPHA Cycle Ratios
OK (hoping for some feedback here),
Have been refining my Alpha design and is definitely becoming focused around minimising gap size and air boundary layer and maximising heat exchange. I'm worried that I might be creating too much resistance (back pressure) and dead space (gulp!).
I am thinking for the moment to keep my hot/cold piston displacement ratio at 1:1 for my first engine, for design simplification and ease of component duplication. I hope this is not too much of a compromise, please stop me if you think its a possible terminal mistake. My pistons are going to be side by side, not a v configuration, this is based on my heat exchanger and regenerator design. Will be using a 90 deg phase angle.
Geoff
I think I read somewhere else that you referred to the pressure transfer as a pulse, I hadn’t thought of it like that previously, is this why the heat transfer could possibly be considered adiabatic.
Vamoose..
Have been refining my Alpha design and is definitely becoming focused around minimising gap size and air boundary layer and maximising heat exchange. I'm worried that I might be creating too much resistance (back pressure) and dead space (gulp!).
I am thinking for the moment to keep my hot/cold piston displacement ratio at 1:1 for my first engine, for design simplification and ease of component duplication. I hope this is not too much of a compromise, please stop me if you think its a possible terminal mistake. My pistons are going to be side by side, not a v configuration, this is based on my heat exchanger and regenerator design. Will be using a 90 deg phase angle.
Geoff
I think I read somewhere else that you referred to the pressure transfer as a pulse, I hadn’t thought of it like that previously, is this why the heat transfer could possibly be considered adiabatic.
Vamoose..
Re: Choosing ALPHA Cycle Ratios
Vamoose
I'm beging to wonder if you have paid any attention to what I've been saying, you seem more interested in dreaming about water vapour and nano bucky balls or whatever, please consider yourself chastised.
You stated early on that you have a heat source with a maximum temperature of 90-180 degrees, F,C or K? and you would like to know what the displacement ratio needs to be for an engine to run with this peak temperature.
Lets go back to basics, for a SE or a hot air engine to run, there must be a pressure difference maintained across the piston to apply a force to the crankshaft to impart enough momentum to the flywheel to sustain the motion through the rest of the cycle. This pressure difference is created by heating a volume of gas such that there is sufficient expansion to exceed the volume increase as the piston descends during the down stroke (power stroke). If the rise in temperature is too small or the volume of gas being heated is too little this pressure difference will not be realised and the engine will not run, let alone produce a surplus of power for external use. It is well established that LTD engines (with a displacement ratio of 100:1 typically) can be made such that the power produced equals the total losses and the engine will continue its motion with a temperature difference of less than 1deg C. Clearly, as the available temperature difference (delta T) is increased the displacement ratio can be reduced such that with very good heat exchangers and a very high heater temperature (600-700 deg C) one can run with a displacement ratio of unity or even less. With your heat source of 90-180 deg C, I'm assuming C, it should be clear that a displacement ratio of 1:1 will not work at all and you will need to select a D/R somewhere between 100:1 and 1:1, exactly where will depend on the ablity of you heat exchangers to raise and lower the mean temperature of the working gas.
Regarding my reference to 'the pressure pulse', if an undamped pressure gauge is connected to the working gas, when the engine is running the rapid rise in pressure during part of the cycle looks like a pulse and therefore I find it logical to refer to it as such. I do not wish to imply anything more than that and do not consider it is an indication of adiabatic or isothermal heat transfer.
Every aspect in the design of a SE or HAE must be in harmony as they all interreact, having conrods the same length for the convenience of component duplication alone, will very likely result in disappointment on the test bed.
Geoff V
I'm beging to wonder if you have paid any attention to what I've been saying, you seem more interested in dreaming about water vapour and nano bucky balls or whatever, please consider yourself chastised.
You stated early on that you have a heat source with a maximum temperature of 90-180 degrees, F,C or K? and you would like to know what the displacement ratio needs to be for an engine to run with this peak temperature.
Lets go back to basics, for a SE or a hot air engine to run, there must be a pressure difference maintained across the piston to apply a force to the crankshaft to impart enough momentum to the flywheel to sustain the motion through the rest of the cycle. This pressure difference is created by heating a volume of gas such that there is sufficient expansion to exceed the volume increase as the piston descends during the down stroke (power stroke). If the rise in temperature is too small or the volume of gas being heated is too little this pressure difference will not be realised and the engine will not run, let alone produce a surplus of power for external use. It is well established that LTD engines (with a displacement ratio of 100:1 typically) can be made such that the power produced equals the total losses and the engine will continue its motion with a temperature difference of less than 1deg C. Clearly, as the available temperature difference (delta T) is increased the displacement ratio can be reduced such that with very good heat exchangers and a very high heater temperature (600-700 deg C) one can run with a displacement ratio of unity or even less. With your heat source of 90-180 deg C, I'm assuming C, it should be clear that a displacement ratio of 1:1 will not work at all and you will need to select a D/R somewhere between 100:1 and 1:1, exactly where will depend on the ablity of you heat exchangers to raise and lower the mean temperature of the working gas.
Regarding my reference to 'the pressure pulse', if an undamped pressure gauge is connected to the working gas, when the engine is running the rapid rise in pressure during part of the cycle looks like a pulse and therefore I find it logical to refer to it as such. I do not wish to imply anything more than that and do not consider it is an indication of adiabatic or isothermal heat transfer.
Every aspect in the design of a SE or HAE must be in harmony as they all interreact, having conrods the same length for the convenience of component duplication alone, will very likely result in disappointment on the test bed.
Geoff V