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.

Tuesday, September 2, 2008

Prototype status update - plugging the leaks

It has been a while since I have updated my progress.

LEAKS, call the plumbers!
I feel like president Nixon.
My main problem has been the leaks. I setup a pressure test gauge to check out each cylinder section. It seems the bags are getting damaged, but not where you would expect it. The leaks mainly occur near the top of the piston. The problem is that the lateral forces on the push rods is causing the piston to rub on sides of the cylinder bore.

I am hoping that adding the push rod cam follower guides is going to relieve much of this sidewards force. This should also reduce the friction at the bearing where the push rod enters the cam case.

Still looking for a few good bags
I bought a thousand polyethylene bags and am going through then. It may be a blessing that they are so soft, since I can identify potential ware problems early.

I will have to start sending out more requests for samples. These plastic people are not very professional. I had one guy say he was sending me some nylon bag samples and I haven't heard from him again. The company was KNF. I have also left messages with local suppliers but
I still haven't heard back from any of either.

I was trying to get the engine running without the cam follower guides, but now that I know it is going to cause more bag wear, I am going to take the whole thing apart and rework it.

To do list:
  1. Make cam follower guides
  2. Adjust each piston to make them more uniform in height and maybe diameter*
  3. Adjust length of pushrods to make them more uniform
  4. Drill hole in each push rod for follower pin and glue in place
  5. Drill hole in shaft for cam set screw to stop the slippage
  6. Make motor simple mount
  7. mount heat exchanger water tubes to heads
* There are different diameter piston shells, originally I was shooting for 0.015" gap between cylinder bore and piston, the first two pistons have ~ 1.97" diameter.
Later when I realized that schedule 40 pipe has an inside diameter of 2.067, I have been shooting for a piston diameter of 1.930". Since I have a 2" ID clear acrylic cylinder, I have to turn down 2" OD tube to make the equivalent gap.
Since I don't have a lathe, I use a Dremel tool to "turn" the piston shells. I will post a picture of the setup. The results are quite rough, with gouges and variations of diameter.

Early technique of turning the piston shell using die grinder. The plastic tends to melt and create a clump of plastic. So I could only turn 1/3 before it gets in the way.

{Picture to be taken}

Newer approach, I use a piece of cylinder material and use it as guide. It is slightly less stable, but with the extra space between the holder and piston, I can turn continuously. It does bind every once and while, causing gouges about 4 mils deep. So if we were trying to make a perfectly smooth and piston, this is not the way to do it. The beauty of the bag seal system is that the piston and cylinder surfaces do not have to prefect.