Turns out the piston cap seals I bought are too tight. Not only do they cost too much, but they have too much friction.
I have been distracted lately and haven't done much work. I need to get bags that are made of different materials. I have been in contact with 2 plastic suppliers that promised sending me Nylon and Teflon bags, but neither of them came through.
I will be sending out requests for quotes to more suppliers for 3" wide, 6" long plastic bags made from tougher material then polyethylene.
Showing posts with label stirling engine. Show all posts
Showing posts with label stirling engine. Show all posts
Friday, August 21, 2009
Thursday, October 2, 2008
Alpha prototype - ordered piston cap seals
Abstract
After shortening the piston rods and changing the orientation of the wrist pin to allow pistons to slide; I am still having problems maintaining the seal. The bags are just too flimsy. So until I can find a better seal material, it have decided to just use a piston cap seal, like used on a bicycle pump.
They cost more $7 each vs less then $0.01 for the bags.
Terminology Change
In all my previous posts, I called the rods that are connected to the piston and ride on the cam as "push rods". Since in a internal combustion engine, the push rods also ride on cam I thought it was an appropriate name. On further thought, since these rods actually connect to pistons, I think "piston rod" is a more descriptive and less confusing term. Since these rods are not actually attached to the cam, the traditional term "connecting rod", I thinks would also be confusing.
Reworked pistons rods
I shortened the rods on cylinders #3 and #4. In the process changed the orientation of the wrist pins to be perpendicular to main shaft and piston rod guide pins. I was hoping this would reduce the lateral forces on the piston, which I thought was causing the bag failures.
Reduced volume ratio
Shortening the piston rods should reduce the volume ratio. This ratio is the maximum volume: when one piston is at the bottom of its stroke and the other is half of its stroke, and the minimum volume, when one piston is at the top of its stroke and the other is half.
Using an equation I found on the web, from this ratio and the temperature of the cold cylinder, I can calculate the temperature at the hot cylinder if the engine is in equalibrium, where not energy is added or lost from the system, adiabatic cycle.
Before shortening the rods and assuming no additional gas volume for the hose connecting the cylinders together, the calculated volume ration was 1.44 after shortening 1.26.
Assuming the temperature at the cold cylinder is 8° C then in theory a temperature higher then 52.21° C and 35.49° C respectively should have produced some power.
Taking the volume of the gas in the hose further reduces the ratios and temperatures respectively.
Similar formulas predict the pressure, so once I connect a pressure gauge, I should be able to verify the formula.
Temperature test of shorter piston rods
Since I only shorted 2 of the 4 piston rods, in may latest test, I am only pressurizing 1 pair of cylinders. Because I don't have piston forces countered by the opposite pair of pistons, I am using only 10 psi of pressure. Otherwise it is very difficult to turn the engine.
After cooling and heating the cold and hot cyliner heads, I couldn't tell if the engine wanted to run. After turning it over for a few minutes, I could tell that the bags were leaking and the pressure dropped.
It takes around 20 minutes to remove the head, pull the guide pin, take off the old bag, leak test it, mark the damaged areas, turn the new bag inside out, prestretch the top ~1 -1/2" of the bag opening, stretch the 2" diameter bag over the 2-1/4" cylinder wall until it is the proper distance ~1-1/2", fit the piston through the cylinder and bag, fold the excess bag end into the opening at the end of the piston, smooth out the creases, push the piston cap to hold the bag in place, pull the piston back down so the bag overlaps itself, blow into the cylinder opening while pulling the piston further down so piston rod can be inserted into case side bearing, line up the guide pin bearing with the rod guide pin and push it into place, put the head back on top of the cylinder without moving the piston, tighten then torque the head screws.
This time it took only 2 minutes of turning to spring a leak. Not fun.
Alternate piston seal
I really want to see the engine run, at least have half of it feel like it wants to run. Then I can commit to the current cam size and esentricisty. While changing the cam is easy during assembly, I would have to take the engine 1/2 apart to change it and I am not sure what ratio I should shoot for. I am limited on both the hot and cold temperatures. I would prefer not to add salt to the ice and don't want to overly weaken the plastic with high temperatures.
The solution was to order piston cap seals for the 2" bore cylinder. It will take over a week for them to be delivered. They only cost over 2000 times the cost of the bag, but will allow me to not to have to worry about the seal as much. I am sure they will also leak slightly, so I may have to add a way of maintaining the gas pressure, if I get the engine running for a long time.
The seals will also need lubrication, which I have been avoiding, because of the mess. So far all I have used is teflon based stray. The seals will probably want oil.
While the pressure range for the seals is very high, up to 5000 psi, their temperature rating is only 100°C. I think the acrylic plastic can go up to 140°C so the high temperature is still limited by seal. If I have to, I will push the temperature until I can get it to run.
Plans
Do a failure analysis on the last set of bags.
Consider using a die and liquid soap to fill the gap in the bag, so I can see where how well it centered in the cylinder and where it is being pushed together. The soap may help lubricate bag so it will last longer and would be easier to cleanup then oil.
Do some more pressure and temperature tests using all 4 cylinders.
Add more weight to the flywheel by filling the V grove with fine chain
Fix my band saw so it cuts straight.
Make new piston caps with hole for mounting the new seal.
Make an engine mount, so it doesn't keep falling over, because the flywheel is so heavy.
Post some new pictures with the heat exchangers attached.
After shortening the piston rods and changing the orientation of the wrist pin to allow pistons to slide; I am still having problems maintaining the seal. The bags are just too flimsy. So until I can find a better seal material, it have decided to just use a piston cap seal, like used on a bicycle pump.
They cost more $7 each vs less then $0.01 for the bags.
Terminology Change
In all my previous posts, I called the rods that are connected to the piston and ride on the cam as "push rods". Since in a internal combustion engine, the push rods also ride on cam I thought it was an appropriate name. On further thought, since these rods actually connect to pistons, I think "piston rod" is a more descriptive and less confusing term. Since these rods are not actually attached to the cam, the traditional term "connecting rod", I thinks would also be confusing.
Reworked pistons rods
I shortened the rods on cylinders #3 and #4. In the process changed the orientation of the wrist pins to be perpendicular to main shaft and piston rod guide pins. I was hoping this would reduce the lateral forces on the piston, which I thought was causing the bag failures.
Reduced volume ratio
Shortening the piston rods should reduce the volume ratio. This ratio is the maximum volume: when one piston is at the bottom of its stroke and the other is half of its stroke, and the minimum volume, when one piston is at the top of its stroke and the other is half.
Using an equation I found on the web, from this ratio and the temperature of the cold cylinder, I can calculate the temperature at the hot cylinder if the engine is in equalibrium, where not energy is added or lost from the system, adiabatic cycle.
Before shortening the rods and assuming no additional gas volume for the hose connecting the cylinders together, the calculated volume ration was 1.44 after shortening 1.26.
Assuming the temperature at the cold cylinder is 8° C then in theory a temperature higher then 52.21° C and 35.49° C respectively should have produced some power.
Taking the volume of the gas in the hose further reduces the ratios and temperatures respectively.
Similar formulas predict the pressure, so once I connect a pressure gauge, I should be able to verify the formula.
Temperature test of shorter piston rods
Since I only shorted 2 of the 4 piston rods, in may latest test, I am only pressurizing 1 pair of cylinders. Because I don't have piston forces countered by the opposite pair of pistons, I am using only 10 psi of pressure. Otherwise it is very difficult to turn the engine.
After cooling and heating the cold and hot cyliner heads, I couldn't tell if the engine wanted to run. After turning it over for a few minutes, I could tell that the bags were leaking and the pressure dropped.
It takes around 20 minutes to remove the head, pull the guide pin, take off the old bag, leak test it, mark the damaged areas, turn the new bag inside out, prestretch the top ~1 -1/2" of the bag opening, stretch the 2" diameter bag over the 2-1/4" cylinder wall until it is the proper distance ~1-1/2", fit the piston through the cylinder and bag, fold the excess bag end into the opening at the end of the piston, smooth out the creases, push the piston cap to hold the bag in place, pull the piston back down so the bag overlaps itself, blow into the cylinder opening while pulling the piston further down so piston rod can be inserted into case side bearing, line up the guide pin bearing with the rod guide pin and push it into place, put the head back on top of the cylinder without moving the piston, tighten then torque the head screws.
This time it took only 2 minutes of turning to spring a leak. Not fun.
Alternate piston seal
I really want to see the engine run, at least have half of it feel like it wants to run. Then I can commit to the current cam size and esentricisty. While changing the cam is easy during assembly, I would have to take the engine 1/2 apart to change it and I am not sure what ratio I should shoot for. I am limited on both the hot and cold temperatures. I would prefer not to add salt to the ice and don't want to overly weaken the plastic with high temperatures.
The solution was to order piston cap seals for the 2" bore cylinder. It will take over a week for them to be delivered. They only cost over 2000 times the cost of the bag, but will allow me to not to have to worry about the seal as much. I am sure they will also leak slightly, so I may have to add a way of maintaining the gas pressure, if I get the engine running for a long time.
The seals will also need lubrication, which I have been avoiding, because of the mess. So far all I have used is teflon based stray. The seals will probably want oil.
While the pressure range for the seals is very high, up to 5000 psi, their temperature rating is only 100°C. I think the acrylic plastic can go up to 140°C so the high temperature is still limited by seal. If I have to, I will push the temperature until I can get it to run.
Plans
Do a failure analysis on the last set of bags.
Consider using a die and liquid soap to fill the gap in the bag, so I can see where how well it centered in the cylinder and where it is being pushed together. The soap may help lubricate bag so it will last longer and would be easier to cleanup then oil.
Do some more pressure and temperature tests using all 4 cylinders.
Add more weight to the flywheel by filling the V grove with fine chain
Fix my band saw so it cuts straight.
Make new piston caps with hole for mounting the new seal.
Make an engine mount, so it doesn't keep falling over, because the flywheel is so heavy.
Post some new pictures with the heat exchangers attached.
Thursday, September 25, 2008
Pushrod guides done, still working on the leaks
Abstract
Added push rod guide to help reduce binding and lateral forces on the push rod bearings and pistons. Changed orientation of piston wrist pin to be perpendicular to the guide pin. Seal leaks remain a problem. Posted pictures of guide and assembled engine without heat exchangers.
Push rod guide
I finally made the push rod guide. It consists of 2 pieces of 4.5" square 1/8 " thick Plexiglas sheets with slots cut in it. There are 4 separators that are ~1.2" square, 5/8" thick.
As I descibed in my last post, the holes in case sides, were not perfectly centered, so some of slots do not line up perfectly with the center of push rods. So rather then making new case sides, I just offset the hole for follower guide pin that I drill into push rod end.
The pin is 1/8" diameter steel stock, the bearings are copper inserts used for plastic tubing compression joints. The slots are slightly wider then the OD of these inserts. The inserts have a flare on one side, so that keeps them from sliding off the pin, because they are inside the guides.
The pins are just long enough so that don't hit the case ends when centered. If the pins do get loose at the push rod ends, then the case will prevent them form going too far.
The side pieces have had there push rod hole enlarged to 5/8", so a nylon bearing can be inserted. They are 5/8" long, so stick out through the 3/8" thick case side. Pushing the bearings so they are flush with outside of the case, centers the guide and for now holds it in place for drilling the holes for mounting screws to the case sides. For now I am letting the guide float.
I cut the slots before drilling the 1/2" hole for the shaft. This was a mistake. One of the guide plates cracked when enlarging the pilot hole. The shaft hole should be drilled first.
Assembled engine
Below you can see the assembled 4 cylinder engine. I am using car tire valves for pressurizing the cylinder pairs. I have the parts for the heat exchangers but haven't installed them yet, because I have to keep pulling the heads to replace the leaky bags.
The plastic bag in the foreground is one that I replaced. The leaks are circled, but not really visible in this picture.
Changed wrist pin orientation
Because the pistons are being driven from a cam rather then a crankshaft, they hardly pivot.
When the do, because of lateral forces on the push rod when it rides the cam, it would transfer these forces to the piston and make the bag seal rub against it self. By changing the orientation of the wrist pin hole so it is perpendicular, shaft and guide pins. This will allow the push rods to slide along them. Hopefully the pressure in the cylinder will keep the piston centered, so there is an gap all around the piston.
Added push rod guide to help reduce binding and lateral forces on the push rod bearings and pistons. Changed orientation of piston wrist pin to be perpendicular to the guide pin. Seal leaks remain a problem. Posted pictures of guide and assembled engine without heat exchangers.
Push rod guide
I finally made the push rod guide. It consists of 2 pieces of 4.5" square 1/8 " thick Plexiglas sheets with slots cut in it. There are 4 separators that are ~1.2" square, 5/8" thick.
As I descibed in my last post, the holes in case sides, were not perfectly centered, so some of slots do not line up perfectly with the center of push rods. So rather then making new case sides, I just offset the hole for follower guide pin that I drill into push rod end.
The pin is 1/8" diameter steel stock, the bearings are copper inserts used for plastic tubing compression joints. The slots are slightly wider then the OD of these inserts. The inserts have a flare on one side, so that keeps them from sliding off the pin, because they are inside the guides.
The pins are just long enough so that don't hit the case ends when centered. If the pins do get loose at the push rod ends, then the case will prevent them form going too far.
The side pieces have had there push rod hole enlarged to 5/8", so a nylon bearing can be inserted. They are 5/8" long, so stick out through the 3/8" thick case side. Pushing the bearings so they are flush with outside of the case, centers the guide and for now holds it in place for drilling the holes for mounting screws to the case sides. For now I am letting the guide float.
I cut the slots before drilling the 1/2" hole for the shaft. This was a mistake. One of the guide plates cracked when enlarging the pilot hole. The shaft hole should be drilled first.
Assembled engine
Below you can see the assembled 4 cylinder engine. I am using car tire valves for pressurizing the cylinder pairs. I have the parts for the heat exchangers but haven't installed them yet, because I have to keep pulling the heads to replace the leaky bags.
The plastic bag in the foreground is one that I replaced. The leaks are circled, but not really visible in this picture.
Changed wrist pin orientation
Because the pistons are being driven from a cam rather then a crankshaft, they hardly pivot.
When the do, because of lateral forces on the push rod when it rides the cam, it would transfer these forces to the piston and make the bag seal rub against it self. By changing the orientation of the wrist pin hole so it is perpendicular, shaft and guide pins. This will allow the push rods to slide along them. Hopefully the pressure in the cylinder will keep the piston centered, so there is an gap all around the piston.
Thursday, August 7, 2008
Alpha Stirling engine protoype status
I was using my personal blog to post the status of this project, but now we have a site just for these Stirling engine projects. Also please go to the new project web page at http://sites.google.com/site/openstirlingengine for more detailed description of the project and a 3D Sketchup model. When the plans are finalized, I will also post a spreadsheet with the full parts list and some formulas for scaling the design for more cylinders.
The web site doesn't seem to support comments, so that is why I made this blog.
Please feel free to add comments and info on any other designs or engines the you know about. This site is not just for my design, but all open source designs and even to discuss commer
This one is an alpha configuration engine that is designed to be low cost and use mostly off the shelf materials. One goal of going open source is to create standard parts, which as the design is improved, replaced with better performing ones.
The pistons are 2" diameter in a 4" long cylinder. There is a small gap between the piston and the cylinder where a flexible seal is fitted. For now it is simply a plastic bag the fits over the piston, folds down and then back up to the top of the cylinder where it held in place by the head.
So far the only bags I can find are polyethylene, so the bags are not very tough and develop leaks, mainly where the bag exits the piston cap, which holds it to the top of the piston. The bags are just plain packaging grade, 3" wide by 6" long. It turns out that they are slightly wider so do not fit the piston as snugly as it should, so it gets creases. The bag is 4 mil thick and the gap between the piston and cylinder is 35 mils. This is because schedule 40 pipe has an ID of 2.069" and the piston can be exactly 2" diameter. Since I want clear cylinders, I am using 2" ID acrylic and have to "turn" the piston down from 2" to make the equivalent gap.
The push rods are just going through a 1/2 hole in the acrylic engine case and this is causing binding, because of the lateral forces from riding on the cam. I am going to add nylon sleeve bearing this weekend. Hopefully that reduce the friction enough so the current flywheel can provide enough inertia to keep the engine spinning.
Here is a picture of the 4 cylinder prototype without the flywheel, so you can see the cam and push rods.
Wednesday, August 6, 2008
Created new blog just for Stirling engine projects
I have been using my personal blog for posting the status of my Stirling engine project, but it was getting awkward. First I checked to see if stirlingengine was free, but it is being used for another project. So I used the same name as the project web site on Google sites, http://sites.google.com/site/openstirlingengine.
I am not sure how to move my existing posts and comments from my personal blog
use the preceding link to get to it so see my previous posts, until I start making more progress here.
Please visit the project web site if you want any of the design information, because that will be where I will be posting the project documents. So far all I really have is the SketchUp 3D model, which does not include the piston seal details. It was too curvy for me to figure out how to include it. All the other parts are there. There is a very rough sketch of the piston seal on my old blog, but I will post some pictures and more details soon.
Please leave any comments about the web site here, since Google project site doesn't seem to have that feature.
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