Not Sure if it's been done...

[SPG]

Fnord, what kind of volume did you have in your piston hybrid behind the piston?

I was thinking about air-springs yesterday (and after all that's basically what you've got) and then of course Boyle's Law struck me as being useful in air-springs.

Boyle's law gives us:

p1 x v1 = p2 x v2

So by altering the size of the chamber behind the piston you can change the rate of the air-spring.

We'll call the distance the piston moves the "swept volume". It's the ratio of this swept volume to overall voume which is important. If the swept volume is a high proportion of overall volume the air-spring will have a very strong rising rate, as air pressure behind the piston will increase quickly with volume decrease.

However if it's a very small proportion then the air-spring gets closer to having a constant rate, the air pressure wont change much.

I reckon this second type is what you need for a hybrid, that way you'll get a large movement in the piston, and so maximum airflow into the barrel.

I wonder if this makes any sense?

[Brian the brain]

It makes perfect sense.
I believe I may have invented the idea of a piston hybrid ( or not) back in spudtech days.
It felt like a logical idea, although I've never even tried a hybrid myself.

My suggestion at that time was to use either a pop-off, or a large pilot space, just like you said.

I don;t mean to claim being the creator of piston hybrids, as I was just thinking out loud without really understanding hybrids at the time..
I had never thought of using it for multiple shots though..

[SPG]

Brian, I think the way to phrase it is:

"I believe I might have lead the brain-storming"

I only cited Fnord's cos he's built one and was commenting in this thread, so thought he might have experience. Did you build one too? My occasional disappearances mean I don't quite know who did what, just that it's been done.

[Brian the brain]

No never built one.
And you are right, I invented brainstorming...

Oh no...I mean...well..

I believe I posted a diagram of the general idea..

Care to share your thoughts on the dieseling option?

To get auto ignition you're generally talking compression ratios on the order of 20:1. I doubt you'll see that in the dead space on a pneumatic.

My brother managed to do it in a CO2 pistol, using a drop of pure ( medical) ether.
So compressed air should be easier..
I think 45 bar should be enough...

[Fnord]

Fnord, what kind of volume...
...boyle's law... ...other stuff.

Yeah, I deliberately put a lot of piston travel in my hybrids because of this. I think about the same as the porting diameter should work well.


However there is something else I need to point out.
If you use a high port diam to piston diam(for high opening pressure), your piston needs a good seal. In my newest version I used a rubber piston that fit tightly, but it did not have o-rings.

It did not work correctly like this. I had about a dozen 4x tests where the piston did not actuate. Instead it bulged out of the seat in a somewhat scary fashion(This was a 2" thick chunk of rubber, forming a 1/4" high bump across only a 1" space ) until the gasses cooled off.
The pressure was leaking around the piston too fast, and not applying enough force to the small area around the port to open it.


Then I cut a floating oring groove and it worked perfectly.