Stirlingengineforum.com

Hi,

I'm currently trying to build a Free Piston engine like Attila Blade large model on YouTube.

I'm having problems getting this engine running. I've followed the general scheme but have made the cylinder with a 1.4mm stainless tube with an 8mm thick aluminium end cap. The rest is similar to Attila's, except it doesn't have a water jacket or wire wool regenerator.

I've set the power piston to have the resonant frequency as the displacer, 6.5Hz, using a 1.4kg weight. Can anyone think of anything I've missed?

Also, I'm interested to know what Attila' stainless steel cylinder was made from - it's 2 separate items in his scheme with a flange joint in the middle and the cold end has a water jacket - it looks like it might be an automotive part. Can anyone tell me what was used?

Thanks,

Steve

Can't help much, but here is my GAMMA type FPSE. The alternator produces about 6 volts, with enough current to run a transistor radio. Ian S C

DSC00839 (640x480).jpg

Hi Ian,

Thanks for your post but I can't see the attachment - could you try re-attaching?

Steve

Okay, so some progress made. I'd terminated my cylinder with a pressed aluminium cap - when I tested this, I found that heat transmission was extremely slow (a delta t of 120 degrees Celsius took 120seconds when heated over a gas burner) so I replaced it with a pressed stainless cap and bingo the engine ran. I've now replaced the cylinder with a one piece stainless job and it seems to be working well but not much power yet. I'm going to look at reducing dead volume first and make sure the expansion space and compression space are well matched in volume.

You are on the right track now, now it might take adjustment of weight on the power piston. I take it that you don't have the load of an alternator, or pump yet. What sort of springs are you using, if ordinary coil springs, this is where a lot of time and (if you have to buy them), money starts to be used up.
The motor in the photo uses two magnets with their like poles facing each other as springs for the power piston, the displacer uses a large diameter rod as in a Ringbom motor. The first attempt at a free piston motor was a BETA. It was only after I built the GAMMA motor that I found the reason for it not working. The original motor was built of steel, and with the magnet in the alternator there was too much magnetic drag for the motor to overcome, ie., a steel cylinder and a cast iron piston for a start, that got changed to a bronze cylinder wit a cast iron piston.
Ian S C

I measured the k of the Displacer Spring (a rubber bellows and flat Spring combination) and the power piston (a larger rubber bellows) and matched the fundamental frequencies at RTP. Interestingly the fundamental should be 5.38Hz but the engine runs at 6.4Hz so I think there could well be some mileage in tuning the power piston mass. The frequency change direction is a bit counterintuitive - I'd expect the spring constant to drop with increasing temperature.

I'll post a photo when I can work out how!

Aim for the highest frequency you can.
Ian S C