I wound the solenoid today with whatever wire I could find. It was pretty heavy wire (#20) so the coil ended up being 1 ohm. At 6 volts this is 6 amps which caused the coil to become very hot very quickly (in about 30 seconds it was too hot to hold) The iron core does not pose a problem as far as I can tell. The coil had lots of power, sucking the piston in instantly.
At this point I have 3 options:
1) make a bigger coil to get more turns on - not my first choice really. It would mean major design changes to my engine, and I wanted to keep a "scale" looking cylinder size.
2) run it on 3 volts - I could live with this if I have to.
3) find a lighter gauge wire. I think this is the best option. This gives me more turns in the same physical space. Also the smaller wire has more resistance and would be longer, causing it to have more resistance again. This would reduce the current.
I will try option 3 as soon as I can find the wire.
What are the dimensions (ID, OD, length, & Flange diameter) of your cylinder? Also, how much of the cylinder length did you use for the coil? I'd like to do some similar experiements with an iron cylinder.
According my calculations, if you use a #25 wire, you'll need about 120 feet which will give you a resistance of about 3.8 ohms. This would limit the amps to 1.573. #25 wire is rated at a maximum of .64 amps for continuous duty.
I think you would be OK as long as you didn't run the motor 24 hours a day. You could drop down to #26 wire which is rated at about .5 amps and would require about 150 feet. Resistance would be 6 ohms.
The power in watts (which is all heat) equals the current squared times the resistance. This means if you keep the same winding, but reduce the current (you can only do this by reducing the voltage), the heat will be reduced by the square of the current.
So for my coil:
at 6 volts I had 6 A and 1 ohm:
Power = (current squared) x resistance = 6x6x1 = 36 watts
If I reduce the voltage to 3 volts, coil is still 1 ohm:
Power = (current squared) x resistance = 3x3x1 = 9 watts
That is 300% less hot. Still too hot? maybe.
Chuck, where do you get your current carrying data from? Do you have a link? #20 wire is good for 11 amps in free air, but I have no idea what it's good for in a coil.
I uploaded an excel spreadsheet that contains a wire table I found on the internet. I added the calculations and formulas. It shows that number 20 wire is good for about 2 amps, continuous duty, presumably in coil form. So far, my experiments seem to validate the numbers in the spreadsheet.
Ok, so I only have two sizes of wire. #20 and #33. The 20 is too heavy and the 33 is too small. I am going to run the #20 and try various voltages once I get is assembled. I think the #20 would be ideal on a much larger engine. Maybe the next one.
Here are some pics of the coil parts. This engine will definately run, but probably only for a few minutes before it gets too hot.
I will run it as a 2 cycle. I can play with the timing one it's running to minimize the "on time" of the coil and still allow it to run. I find an engine that just ticks over more interesting anyways. Performance is not what I am after here.
Sorry that the progress is slow. Life has been interfering again. :-\