Stirlingengineforum.com

Hi there!

As the story goes, I was building a small alpha stirling engine, but it turned out to be too hard to accomplish. It had so great compression ratio, that the temperature difference required to run was astounding.

I'm currently rebuilding it to gamma engine. I'ts ok this far, but I have problems with the displacer sleeve. While the displacer is moved by hand (by pushing and pulling the connection rod), It moves freely, and there's no rubbing noise. When it is attached to the crank mechanism, however, there is notable, vibrating hum at several points of the cycle. It seems the mechanism twists the rod slightly, but this shouldn't be if the sleeve vas as carefully built as it is.

The connection rod is made of very hard, polished steel rod of 3mm in diameter. The sleeve is made of 4mm seamless brass pipe just large enough to accommondate the rod. It moves very freely even without oiling. But when the sleeve is horizontal and the crank mechanism pushes and pulls it, the displacer shakes and hums or "beeps" at a low frequency. What can cause this and how can it be fixed? How are the sleeves generally done in gamma engines?

Ok, the problem was too short sleeve and too heavy displacer. The sleebe being too short allowed the rod to vibrate a litle. Too heavy displacer twisted it downwards, where it made rubbing contact with the sleeve pipe. This caused the overall vibration that robbed away power.

I got the problem fixed today, or at least for the rubbin part:
I had to build another displacer that would be lighter. I first took 30mm (1,5mm wall) steel pipe ant put it into a lathe. Then I drilled it with 28mm drill piece, so it now had 1mm walls. After drilling I carefully turned it from the inside with a small cutter blade until the wall thickness was about 0,8mm. I then cut it into right length, and welded 1mm steel sheet to one end (That's about finest MIG weld I have ever done). I now had half of a displacer done.

I didn't like the idea of destroying this part by welding, so I did not weld the other end. instead, I took a piece on 30mm aluminium shaft and turned a cap for the displacer. It was a tight fit, and didn't even require any glue. The alu cap also had a hole for the displacer rod which was glued in place. The displacer was now ready. I also desoldered the sleeve pipe and installed longer one to prevent any looseness. The new displacer seemed to move very easily. The new displacer was, however, larger in diameter, so I needed to turn some metal off the cylinder and displacer. The displacer wall thickness ended up to some 0,5mm, the thinnest thing I have ever machined in a lathe. After the displacer was fittig the cylinder nicely, I polished it and assembled the cylinder.

The only thing left to do is to build some sort of connection tube between the displacer and power cylinders. I quess this can be accomplished by some pieces of brass piping, but stay tuned for further updates

I got the engine assembled today with some of the gaskets in place. I was anxious to test the engine, so I lit up the spirit burner, and started to heat it up. At first nothing happened so I detaced the power piston con rod: When turning the flywheen the power piston really moved slightly. I then attached it again.While the burner heated the engine it turned aesier and easier every try until it spinned almost 20 secconds after giving it a good swivel to the flywheel.

At this point it was obvious what was the cause of no-go: I could hear air hissing out of the power cylinder stating that the piston was actually little too loose. I sprayed in some weapon oil, swiveled the flywheel, and it spinned. The spinning now didn't seem to stop. After some more oil it started spinning faster, under it's own power. However, at this point the spirit burner had consumed most of its fuel and the flame started to go out. It was the end of the first, successfull test run.

I discovered several problems during this test. Firstly the power piston was too loose in the cylinder. Seccondly, there were several leaks in the displacer cylinder due to no gaskets at all :(.
Thrirdly, The heat transfer from the hot side to the cold side in unnacceptable at its present rate. I made the hotcap out of carbom steel, thus it's heat transfer is too great. The aluminium cold side was designed for stroke of 30mm, so it is too long compared to the short hotcap. There are also problems with the screw mounting of the hotcap. I have machined it in a way that only a tiny part of the thing actually touches the cold part of the cylinder. This, however, isn't very good seal against leaks. I will need some heat resistant gasket material for this...

There are also several minor problems with the crank mechanism. The main crank is somehow twisted so it makes the yoke setup wobble sideways. This puts extra strain on the conrod bearings, and can even prevent the engine from running properly. I will have to look into this issue further...


But now i do have a really working engine of my own design. I can only say this has been a colorfull learning process in the world of metal machining. I will improve my engine to make it run better and also try to post in some pictures so do stay tuned.