crank angle
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brett
crank angle
if running this engine design with the power piston upwards and a crank system will it work and what phase should the crank be at?
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brett
i found the answer 90 deg.
but my nxt question is what is the relation in size between the displacer and the power piston?
i have all the information relating to the displacer piston and displacer cylinder but how does this relate to the power piston and power cylinder?
I think this is a question a lot of people search for and will spend a long time searching for and not finding the info, seems no one posts the basic design principles
but my nxt question is what is the relation in size between the displacer and the power piston?
i have all the information relating to the displacer piston and displacer cylinder but how does this relate to the power piston and power cylinder?
I think this is a question a lot of people search for and will spend a long time searching for and not finding the info, seems no one posts the basic design principles
The displacer is about three times as long as the diameter of the displacer cylinder, and about 3/4 of the length of the cylinder.
As it moves from hot end to cold end it displaces about 1/3 of its volume of air from the cold end to the hot.
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brett
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jay
Swept volume of the displacer
Brett-
First, I am not an expert. But I am interested.
If the objective of the displacer is to move the air away from the heating or cooling source, it seems reasonable that the more the better. Perhaps the goal however should not be 'volume', but surface area of the displacer.
Take the extreme case of the displacer being flat (large surface area with small volume). Maybe a cylinder with no top or bottom, like a can with the top and bottom cut off. As it moves in, the air is no longer able to heat because the heat source is 'shielded' from the air by the displacer. All this type of displacer does is 'shield'. Since it has very little volume, it actually moves very little air as it moves in and out. Is this good or bad?
On the good side, the less air you move, the less energy is used moving it.
On the bad side, perhaps an important feature of the displacer is to push the air across the heating and cooling source. This would be an interesting test.
If this is true, that an important requirement of the displacer is to move the air across the heating and cooling elements, then obviously a higher volume would mean more air moved.
So maybe a good analogy would be a fan blowing across a heating element. A larger displacer is like a larger fan blade. If what you want is to heat more air you could use a larger fan blade, that is true. But you would spend more energy moving the air. If instead you blew the air across a larger heating element -- one with a larger surface area -- you would use less energy and heat more air.
Anyway, I have to go. Sorry I have no answers. I just wanted to share a thought or two about it.
What do you think?
First, I am not an expert. But I am interested.
If the objective of the displacer is to move the air away from the heating or cooling source, it seems reasonable that the more the better. Perhaps the goal however should not be 'volume', but surface area of the displacer.
Take the extreme case of the displacer being flat (large surface area with small volume). Maybe a cylinder with no top or bottom, like a can with the top and bottom cut off. As it moves in, the air is no longer able to heat because the heat source is 'shielded' from the air by the displacer. All this type of displacer does is 'shield'. Since it has very little volume, it actually moves very little air as it moves in and out. Is this good or bad?
On the good side, the less air you move, the less energy is used moving it.
On the bad side, perhaps an important feature of the displacer is to push the air across the heating and cooling source. This would be an interesting test.
If this is true, that an important requirement of the displacer is to move the air across the heating and cooling elements, then obviously a higher volume would mean more air moved.
So maybe a good analogy would be a fan blowing across a heating element. A larger displacer is like a larger fan blade. If what you want is to heat more air you could use a larger fan blade, that is true. But you would spend more energy moving the air. If instead you blew the air across a larger heating element -- one with a larger surface area -- you would use less energy and heat more air.
Anyway, I have to go. Sorry I have no answers. I just wanted to share a thought or two about it.
What do you think?
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Guest
I am currently building my first stirling tin can engine and have also started pondering the cylinder and volume relationships.
Interesting idea Jay. My very limited understanding is that you want to alternatively cool and heat the volume of air in the displacer cylinder. I believe the shielding you propose may prevent further heating of the air, but I don't see how any active cooling would occur?
As an analogy, consider a vessel over a heat source such as a burner. While the burner is lit, the temp in the vessel will rise. Remove the burner, and the temp will stabilize and slowly return to ambient. The rate of this change to ambient will depend mostly on the air temp in the room. Now consider moving the vessel to a freezer. The rate of change will of course much faster and over a larger temp range.
I guess the point I am trying to make is that if you don't move the volume of air (solid piston), you won't get much of a temperature differential between the hot and cold sides. Seems to me that the larger the differential, the better.
Happy to debate, as I say I am just getting into this and would like to learn more.
Cheers, springman
Interesting idea Jay. My very limited understanding is that you want to alternatively cool and heat the volume of air in the displacer cylinder. I believe the shielding you propose may prevent further heating of the air, but I don't see how any active cooling would occur?
As an analogy, consider a vessel over a heat source such as a burner. While the burner is lit, the temp in the vessel will rise. Remove the burner, and the temp will stabilize and slowly return to ambient. The rate of this change to ambient will depend mostly on the air temp in the room. Now consider moving the vessel to a freezer. The rate of change will of course much faster and over a larger temp range.
I guess the point I am trying to make is that if you don't move the volume of air (solid piston), you won't get much of a temperature differential between the hot and cold sides. Seems to me that the larger the differential, the better.
Happy to debate, as I say I am just getting into this and would like to learn more.
Cheers, springman
Been just eyeballing it myself. I would think that the volume relationship determines compression as the flywheel is going "over the top." More compression would mean power lost to the flywheel itself cuz it would need to be bigger but usuable torque should be there somewhere. As soon as we mention torque there's stroke arm and blah blah blah to consider. Forgive me, I'm eyeballing again. Less compression I would think it would run with less to virtually no "thumping" when the flywheel goes over the top but torque is gonna be minimal. Changing the timing (angle) must have an influence on torque just like advancing the ignition on an internal combustion engine.
My opinion on surface area is that area is a factor of the volume and has a great relationship with efficiency (of changing temperature) and then we get back to the torque mess. Hmmmmmmmm
Any physicists out there?
My opinion on surface area is that area is a factor of the volume and has a great relationship with efficiency (of changing temperature) and then we get back to the torque mess. Hmmmmmmmm
Any physicists out there?
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SScandizzo
- Posts: 66
- Joined: Tue May 16, 2006 5:06 pm
- Location: California
Hi guys,
I'm new to the forum and the hobby of hot air engine building but my most recent design has given me quite a bit of confidence in addressing the questions generated in this thread. Take it (or leave it) with a grain of salt, and please don't hesitate to fire out more questions!
Oh boy did you all open a can of worms! There are several different issues to consider when designing the dimensions of your engine. First, just to correct a bit from the quote at the beginning of the thread: the displacer should be 2/3 the length of the displacer cylinder not 3/4 or else it could not displace 1/3 the volume of air. Also, the displacer cylinder should have a length 3 times its diameter, not the displacer itself.
Most of the books I have read also suggest the 3 to 1 length to diameter ratio for the displacer cylinder as well as the 1.5 to 1 displacer to power stroke volume ratio. BUT there are other aspects to consider as well. Keep in mind that these numbers are considered to be ideal for simple Stirling engines but may not be practical.
Here are some other aspects to consider. The power stroke length versus the power cylinder's diameter. The longer the stroke length, the more power produced each cycle but the shorter the stroke, the faster it moves. A long stroke must overcome more friction (the piston is touching the cylinder walls) but a short stroke may not utilize all the force created from the change in internal pressure. Then there is the subject of the cylinder configuration. Using two cylinders often requires an air passage between them which is filled with air that is neither being heated or cooled (this is known as dead space). But a single cylinder with both the displacer and the power piston is more complicated to make.
Most of the "tin can" designs on the internet have a design in which the displacer volume is far more than the power stroke volume (that is, its ratio is bigger than 1.5 to 1). This makes them more inefficient, but allows them to run on much smaller heat sources, such as candles.
So what are the best dimensions? I think it depends on your application. Figure out what you want to build in terms of general size of the engine, type of heat source, and types of materials. Then design the configuration and dimensions accordingly.
-Stefan
I'm new to the forum and the hobby of hot air engine building but my most recent design has given me quite a bit of confidence in addressing the questions generated in this thread. Take it (or leave it) with a grain of salt, and please don't hesitate to fire out more questions!
Oh boy did you all open a can of worms! There are several different issues to consider when designing the dimensions of your engine. First, just to correct a bit from the quote at the beginning of the thread: the displacer should be 2/3 the length of the displacer cylinder not 3/4 or else it could not displace 1/3 the volume of air. Also, the displacer cylinder should have a length 3 times its diameter, not the displacer itself.
Most of the books I have read also suggest the 3 to 1 length to diameter ratio for the displacer cylinder as well as the 1.5 to 1 displacer to power stroke volume ratio. BUT there are other aspects to consider as well. Keep in mind that these numbers are considered to be ideal for simple Stirling engines but may not be practical.
Here are some other aspects to consider. The power stroke length versus the power cylinder's diameter. The longer the stroke length, the more power produced each cycle but the shorter the stroke, the faster it moves. A long stroke must overcome more friction (the piston is touching the cylinder walls) but a short stroke may not utilize all the force created from the change in internal pressure. Then there is the subject of the cylinder configuration. Using two cylinders often requires an air passage between them which is filled with air that is neither being heated or cooled (this is known as dead space). But a single cylinder with both the displacer and the power piston is more complicated to make.
Most of the "tin can" designs on the internet have a design in which the displacer volume is far more than the power stroke volume (that is, its ratio is bigger than 1.5 to 1). This makes them more inefficient, but allows them to run on much smaller heat sources, such as candles.
So what are the best dimensions? I think it depends on your application. Figure out what you want to build in terms of general size of the engine, type of heat source, and types of materials. Then design the configuration and dimensions accordingly.
-Stefan
Hi Brett. In regards to your question about the crank angle, I would like to tell you what I have found and I don't know if it holds for all Strilings or just this one I have. But I just recently completed a vertical tin can engine ( http://boydhouse.com/stirling/ ), having seperate cranks for the displacer and power pistons. With that configuration, I was able to experiment with the angles. I was able to increase the speed significantly by increasing the angle past 90 degrees! (See my video.) I don't have an accurate means of measuring the angle, but with a protractor I estimate it is about 120 degrees.
Hope this helps.
Donnie
Hope this helps.
Donnie
Last edited by DonnieB on Sat Feb 24, 2007 5:03 pm, edited 1 time in total.
Crank angle does have a considerable effect on speed and power. It also depends greatly on your engine. My first engine will only run if it's displacer leads by 90 degrees. but my second engine performs much better at a bit over 100 degrees. The disigns are very simular. My point is that making your engine so it can be adjusted can be very useful. A 90 degree phase will likely make any Stirling engine "run" and is the right place to start but may not be what you actually want after you have tweeked and experimented with it a bit.
90 deg. phase angle :
http://www.logicsys.com.tw/wrkbas.htm
(chapter A4)
relation in size between the displacer and the power piston :
http://www.logicsys.com.tw/ltdeng.htm
http://www.logicsys.com.tw/wrkbas.htm
(chapter A4)
relation in size between the displacer and the power piston :
http://www.logicsys.com.tw/ltdeng.htm