Saturday, March 19, 2011

Updated Sketch Up model and piston assemblies

I have shared the model in Google and will upload it to the web site. The pistons now have 4 nylon screws that protrude through the cores and shell to keep its base centered. This should prevent the piston crown from scraping the plastic seal, which is just a polyethylene bag, because the pressure on it should center the front.

The seal bag is not included in the model, I am still trying to figure out how to represent it.

The current heat exchangers are also shown attached, but without the hoses for the hot and cold water.

Added the piston core caps and the push rod guide's bearings. Push rods have cutouts to allow snapping them on and off the wrist pin, to make changing the bags easier.

It now pretty much represents the current state of the real prototype. I still have to rebuild 2 more of the pistons to install the new shells with 50 mil clearance and the Nylon spacer screws.

I hoping once back together this time, it should be able to hold pressure long enough it to get it run. It has a low displacement cam, so should run with a pretty low temperature difference ~40 degrees F. So far I only have one pair of heat exchangers, which I pump ice water through. Still need a heating means for the hot cylinders. I was just heating the heads with torch, but since most of other parts are plastic or rubber, that is not a good idea. I may just use some high wattage resistors, it would be less messy than using hot water.

Monday, February 14, 2011

New insight into bag seal failures

I looked at my collection of leaking bags. The hole were close to crown of piston or located where the crown of piston hit the bag.

Plan: Make the piston outer shell smaller to allow more clearance between the piston and cylinder.
Previous version had 20 mils. Turning new set with 50 mil gap.

The pressure on the bag centers it in the cylinder but the piston can tip causing the crown to scrap the bag. To prevent that I am adding some diameter to the tail end of piston. For now I am going to use nylon screws that would also attach the piston outer shell to core. Later may use Teflon ring fitted to a groove at the end of the piston.

Status: Turned shells, still need to drill the holes for the attachment screws. Going to make another piston assembly for my piston test fixture. That way I can see if the piston twisting problem has been fixed.

Friday, August 21, 2009

Status update, still looking for a good seal

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.

Thursday, October 2, 2008

Alpha prototype - ordered piston cap seals

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.

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.

Monday, September 29, 2008

Alpha prototype - Quick temperature testing

Fixed leaks and started to do temperature testing. I attached the heat exchangers to the cold cylinder heads and circulating ice water using a pump. Also heated the hot cylinder heads with a torch. It seems that the cam is too eccentric, so the engine would need a higher temperature difference then I feel comfortable applying. The polyethylene bags have max working temperature of ~160 degrees F. One pair of cylinders lost pressure and the other pair has a slow leak. See follow up section below for temperature measurements and failure analysis.

Fixing Leaks
I found one causes of the gas leakage, the tire valve stem was loose. Tightening it stopped most of the leakage. I also replaced another seal bag. It split near the head gasket and tore at a wrinkle in the bag near the piston. After replacing the bag, the 3-4 cylinder held pressure.

Cooling the cold cylinders
I made 2 heat exchanges out of 1" square aluminum tube, by sealing the ends of 5" sections with scrap acrylic squares. Drilled and tapped 1/4" NPT treaded holes into the side of the tube at both ends and screwed in hose barb adapters.

I was intending on using a rectangular shaped tube instead, to provide more surface area to transfer heat the flat head. But the square section tube was more available.

The heat exchanger is attached to the cold cylinder heads using plastic straps. A small 12V bulge pump is connected to both cylinder heat exchanges by 1/4" hose through a tee. The other side of the exchangers are connected to another tee and a return hose. The container is filled with ice and water.

Heating the hot cylinders
I used a propane torch with a very small flame to gently heat just the head, avoiding the rubber hose connections. I heated them until hot to the touch. Since I was in a hurry, I didn't take any temperature measurements. I could have used more ice and gotten a lower cold temperature.

Cylinders 3 and 4 spring a leak
After a few minutes into the test. Luckily 1 and 2 still had a pretty good seal, so I could continue.
One of the advantages of this engine configuration is that each pair of cylinders are essentially separate engines. Because of the cam drive, when a pair looses pressure, the pistons don't press down on the cam and just stay mostly out of the way.

This was a quick test performed, before I was to BBQ fish for dinner, so I didn't have time to hook temperature probes.
I could feel the engine want to run a little, even with only the 2 cylinders. When I calculated the cam ratio, I assumed shorter push rods then I currently have. I wanted to do a real test before committing to shortening them. I still have to check out the leaks that have developed in both cylinder pairs, just one is much worse then the other. I bought a 0-30 psi presure gage, so to make leak detection more sensitive, old gauge was 0-200. I am only pressurizing the cylinders to 10 psi right now. If I go higher it makes it to hard to turn the engine over. The plastic shaft starts to deform. I am also afraid the case my come appart from the forces. After I get the cam size nailed down, I will switch to steel. The 10 psi translates to ~30 lb of force for each piston, pushing on the cam. The pressure increases even more when the both pistons are near the top of their travel. I also need to add a gauge to the pair when turning it over, to compare the actual pressure against my calculations, which ignore the volume of the hoses.

Pistons 3 and 4 still have the original wrist orientation that is parallel to the main shaft. So I will shorten their push rods and drill the wrist pin hole perpendicular to the main shaft. This way push rod can slide if there is lateral forces on the push rod and not push against the bags as much. The other 2 pistons already have this enhancement, but will also need shortening. I am only going to shorten 3 and 4 for now and see if it is enough so engine can run on the current temperature difference 35 to 140 degrees F.

I will redo the cam/temperature difference calculations based the current configuration and see if it agrees with my observations.

Do an other temperature test on just the pair with the shortened push rods and take accurate temperature measurements.

I still have to find a better sealing bag material, one that is tougher and has a higher working temperature. Still trying to find a plastic supplier for nylon bags. The polyethylene bags keep breaking.

Follow up
I measured the temperature of the head using a thermocouple probe that came with my multimeter. Without a thermal pastes the head was 8 degrees C. Cylinder #3 was the one that sprung the leak. A hole in the bag as usual. I used the new 0-30 psi pressure gauge to check #4 and it held 10 psi for over 2 hrs, while I watched Chuck and Heroes.

I am going to change the push rod length and wrist pin orientation. Also the piston core had some cracks where the set screw is threaded through to hold the piston shell in place. Luckily no damage to the shell. I am going to flip the core over and drill and tap two new holes.

The new push rod length from center of wrist pin hole to cam end will be 3". Full length will be 3.5" That should give a 1/4" clearance from the piston inner cap. The bottom of the stroke will have the bottom of the piston a little less then 1/4" from case side. The greater volume will reduce the required temperature difference without having to change the cam. Right now the cam is using the same 1-3/4" diameter Plexiglas solid rod stock as the piston caps. These solid rods near 2" diameter are quite pricey so sharing the stock for both pieces keeps the material cost lower. The circumference of these rods are smooth as glass, so it beats cutting circles form sheet stock. My band saw is out of alignment so I am having trouble cutting in a straight line, so most of the piston cap pieces have a tapered thickness. I try to arrange the high side of the inner cap with the low side of the outer cap, which presses the end of the bag into the piston cavity. This is quite snug, so I haven't had one pop out yet. The pressure should hold them in place anyway.

Thursday, September 25, 2008

Pushrod guides done, still working on the leaks

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.

Monday, September 8, 2008

Adding push rod guides, need to replace 1 case piece

I could see from the push rods bending when I turn the engine over, that there was a lot of lateral force from the curved edge of cam. I believe this was causing the pistons to rub too hard against the sealing bags. After cutting the guides to center of where the rods travel, I discover that one of the case pieces has its push rod shaft slight off center. So now I have to make a new one. I may just drill the guide pin hole in push rod to compensate instead.

I also realize now that the the direction of wrist pin should be parallel to the shaft rather then perpendicular like it is in a crank based engine. With the cam driving the piston, the lateral forces from the curve of the cam will translate to pressure against the wall of cylinder. The push rod of this design can slide along the wrist pins, if the pistons are rotated 90 degrees. This along with the guides should reduce wear of the piston seals.

I just got found another older digital camera, so I can keep it handy and try to take more pictures. I apologize that I haven't taken too many.