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.

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