I now have the piston/cylinder assembly ready for testing. On Friday I found a distributer of plastic bags near where I work and got 3 samples, 3" x 6" flat polyethylene bags:
1- 2 mil thick
1- 4 mils thick
1- 6 mils thick.
The actual size of the bags varied and the thin one split down the side as I tried to fit if over the piston. The 4 mil bag was oversize so while it easily fit over the piston, it was too loose to properly fit into the cylinder with creating creases.
The 6 mill bag was tight and had to be stretched over the piston. Get it into the cylinder was easy, but getting it to fold back over the piston was tough. The final step of stretching it over the outside of the cylinder took hours. I think I have a better technique now, but with the bag already stretched out it may be just easier.
The bag got damaged near the sealed end of the bag, so I had to patch it with scotch tape.
This bag seems too thick, because it would rather slide in cylinder rather then roll. It seems to work OK when under pressure, but the seal is still not good. It could be more leaks through the bag or the top cylinder to head seal.
Conclusion: The current bag is both the wrong material and not the right size. But it does seem like it could work. Still need to find a seamless 2" diameter Nylon bag, which would 3" across when flat. I man need to do some custom reforming of the bag to get to be easier to install.
Monday, May 26, 2008
Monday, May 19, 2008
Open source Stirling engine project
I started to research Stirling engines. There are several different configurations. Some have quite complex gearing and levers arrangements. One version I saw on the Stirling Engine Wikipeda page was called an alpha configuration. It seemed like a much similar design, with two pistons that are stoke 90 degrees apart. The crankshaft is very similar to a internal combustion engine.
One of the problems with creating a powerful Stirling engine is that the working gas has to be under pressure usually several atmospheres. The working gas also should be very light, like hydrogen, helium or methane. While air and nitrogen can work, but it reduces the amount of power that can be extract per cycle. Hydrogen is the best, but it is difficult to contain since it can escape through tiny gaps around the piston seals and diffuse right through metal.
I think I have come up with a sealed arrangement that uses a flexible membrane to provide a more positive seal. I made a small model today using old plastic cylindrical medicine jars. It was somewhat difficult to assemble it because the diameter of the plastic bag I used was slightly smaller then the piston. So it took a lot of fiddling to get bag into position. It seem to work.
The area of the plastic that is not supported by either the cylinder or the piston is very small. Like skinny balloon, it takes a lot more pressure to get it to stretch.
Now that it looks like I can get an inexpensive sealed piston, that doesn't require precision machining. I think I can make a very simple and cheap engine.
My goal is to make plans for several different engines using standard parts made from easier to obtain sources like aluminum or steel pipe. Since the piston actually has to be somewhat smaller then the cylinder the tolerances are quite large.
The current design I am working on is more a demonstration unit. So it will be made mostly from clear plastic tubes, rods and sheets. My plan is to use steam to as the heat source and ice water for the cold side cooling. This will limit the thermal efficiency, but it will allow me to see the how well the sealing membranes are holding up.
In initial tests I will use air as the working gas at only a few atmospheres. But my plan is switch first to natural gas, methane then hydrogen. Any gas that diffuses through the membrane seals would be captured in the sealed crankcase, it could be used to contribute to the heat source.
In this design, both hot and cold pistons and cylinders can be made identical so as to reduce number of different parts. This would allow some economies of scale.
The hot and cold cylinder pair could be ganged to make longer engine. When ganging the piston pairs they should be set to balance the forces from the opposing piston set. Because the pistons are always under pressure, the force from one set should counter act the force from the other set. In a four cylinder engine, they should 180 out of phase from each other.
The current design is based on sketches I found on the web and I am scaling it so the cylinder has a 2 inch diameter. I chose this size because the cost of the components is not to high and so the cylinder walls can be thin. There are metal tubes and aerosol cans with similar dimensions that can be used in production unit.
The heads will be made from a flat piece of metal. Either aluminum or brass about 1/4 inch thick. The cylinders are simply pressed between the head and the plastic crankcase housing, with long bolts. Since the heads are where the heat or cold is applied, it pretty much has to metal. Since I am planning on directing flame at the hot heads, I am going to make them horizontal cylinders so the head would be vertical even with a horizontal shafted engine. For eight and twelve cylinder engines, it may have a vertical shaft and the head would be a single unit for of each adjacent cylinder in the quadrant.
I am also thinking of modulating the burners, so it is off during the "exhaust" stroke of the hot cylinder. Why heat it, just to send it to the cold cylinder?
The heads will have a standard threaded fitting that connects to the re generator pipe. It will also be plastic for now and filled with a series of short bundles of small diameter metal tubes. These should be assembled so the holes align, to improve gas flow.
Since making a crankshaft for more then 2 cylinders is non-trivial, I am thinking of using a cam instead. Initially it will just be an eccentric circle mounted on the flywheel. I chose this because the each of cylinders are under pressure and will press against the crank shaft making it very difficult to turn. With the cam arrangement it is easier to add another set of pistons 180 degrees out of phase from the first pair. The forces from these pistons will cancel out. The cam allows all the pistons to be in the same plane. That way we can still add more sets without a complicated crankshaft. Just slide more cams onto a straight shaft. The down side is now the flywheel has to be at least twice the diameter of what the crank would have been. Since the forces on the cam are quite large even with moderate pressure, I am planning on pressurizing the cam case. This will require changing cam case round and having to make the bases of the cylinders curved so they seal properly.
Because the demonstration unit will be made mostly of clear plastic, it will not have very much power, because heating and cooling will mainly be done by the heads and not the cylinders.
Since all the parts will have standard dimensions, the plastic parts could be swapped out an replaced with metal ones.
I still have to make some scaled drawings of what to build, so I can buy the plastic parts from Tap Plastics. I also put a request for a quote for a Teflon piston seal. Sort of a big condom for the piston.
I am hoping to make this project be open source, so we can each share improvements for each of the components. Since the parts will mostly be a standard sizes, we should be able to swap out components when they ware out or when a better one becomes available.
One of the problems with creating a powerful Stirling engine is that the working gas has to be under pressure usually several atmospheres. The working gas also should be very light, like hydrogen, helium or methane. While air and nitrogen can work, but it reduces the amount of power that can be extract per cycle. Hydrogen is the best, but it is difficult to contain since it can escape through tiny gaps around the piston seals and diffuse right through metal.
I think I have come up with a sealed arrangement that uses a flexible membrane to provide a more positive seal. I made a small model today using old plastic cylindrical medicine jars. It was somewhat difficult to assemble it because the diameter of the plastic bag I used was slightly smaller then the piston. So it took a lot of fiddling to get bag into position. It seem to work.
The area of the plastic that is not supported by either the cylinder or the piston is very small. Like skinny balloon, it takes a lot more pressure to get it to stretch.
Now that it looks like I can get an inexpensive sealed piston, that doesn't require precision machining. I think I can make a very simple and cheap engine.
My goal is to make plans for several different engines using standard parts made from easier to obtain sources like aluminum or steel pipe. Since the piston actually has to be somewhat smaller then the cylinder the tolerances are quite large.
The current design I am working on is more a demonstration unit. So it will be made mostly from clear plastic tubes, rods and sheets. My plan is to use steam to as the heat source and ice water for the cold side cooling. This will limit the thermal efficiency, but it will allow me to see the how well the sealing membranes are holding up.
In initial tests I will use air as the working gas at only a few atmospheres. But my plan is switch first to natural gas, methane then hydrogen. Any gas that diffuses through the membrane seals would be captured in the sealed crankcase, it could be used to contribute to the heat source.
In this design, both hot and cold pistons and cylinders can be made identical so as to reduce number of different parts. This would allow some economies of scale.
The hot and cold cylinder pair could be ganged to make longer engine. When ganging the piston pairs they should be set to balance the forces from the opposing piston set. Because the pistons are always under pressure, the force from one set should counter act the force from the other set. In a four cylinder engine, they should 180 out of phase from each other.
The current design is based on sketches I found on the web and I am scaling it so the cylinder has a 2 inch diameter. I chose this size because the cost of the components is not to high and so the cylinder walls can be thin. There are metal tubes and aerosol cans with similar dimensions that can be used in production unit.
The heads will be made from a flat piece of metal. Either aluminum or brass about 1/4 inch thick. The cylinders are simply pressed between the head and the plastic crankcase housing, with long bolts. Since the heads are where the heat or cold is applied, it pretty much has to metal. Since I am planning on directing flame at the hot heads, I am going to make them horizontal cylinders so the head would be vertical even with a horizontal shafted engine. For eight and twelve cylinder engines, it may have a vertical shaft and the head would be a single unit for of each adjacent cylinder in the quadrant.
I am also thinking of modulating the burners, so it is off during the "exhaust" stroke of the hot cylinder. Why heat it, just to send it to the cold cylinder?
The heads will have a standard threaded fitting that connects to the re generator pipe. It will also be plastic for now and filled with a series of short bundles of small diameter metal tubes. These should be assembled so the holes align, to improve gas flow.
Since making a crankshaft for more then 2 cylinders is non-trivial, I am thinking of using a cam instead. Initially it will just be an eccentric circle mounted on the flywheel. I chose this because the each of cylinders are under pressure and will press against the crank shaft making it very difficult to turn. With the cam arrangement it is easier to add another set of pistons 180 degrees out of phase from the first pair. The forces from these pistons will cancel out. The cam allows all the pistons to be in the same plane. That way we can still add more sets without a complicated crankshaft. Just slide more cams onto a straight shaft. The down side is now the flywheel has to be at least twice the diameter of what the crank would have been. Since the forces on the cam are quite large even with moderate pressure, I am planning on pressurizing the cam case. This will require changing cam case round and having to make the bases of the cylinders curved so they seal properly.
Because the demonstration unit will be made mostly of clear plastic, it will not have very much power, because heating and cooling will mainly be done by the heads and not the cylinders.
Since all the parts will have standard dimensions, the plastic parts could be swapped out an replaced with metal ones.
I still have to make some scaled drawings of what to build, so I can buy the plastic parts from Tap Plastics. I also put a request for a quote for a Teflon piston seal. Sort of a big condom for the piston.
I am hoping to make this project be open source, so we can each share improvements for each of the components. Since the parts will mostly be a standard sizes, we should be able to swap out components when they ware out or when a better one becomes available.
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