My Piston Hybrid

[chronic_fabricator]

I am a new member and I would just like to say, thank you for posting this! I have been building combustion launchers for the past 20 years and with your help, I've completed my first hybrid. This build has been a blast. My first question is, where I might I find info on piston valve psi? I have yet to have a successful test fire... due to what I'm thinking is a sticky piston. Last night I fired up my lathe and turned a new piston out of solid nylon. Now it moves much easier while still holding pressure at the back and at the barrel. However, when I try to fire it pneumatically(no fuel), the piston still doesn't retract when the back pressure is released. Then, with no piston back pressure, if I pull on the spool I can feel suction holding it forward. Like somehow the chamber pressure is holding the piston face to the barrel instead of pushing the piston back. This seems physically impossible... any ideas on what could be keeping it from retracting? I've tried up to 130 psi in the chamber and about the same behind the piston.

[Brian the brain]

ChronicFab: Please tell us the diameters of the piston and port...pictures help too!

[chronic_fabricator]

I'll upload pics and info when I get off work. Thanks for the quick reply!

[chronic_fabricator]

Piston is 1" OD. Port is 1/4" ID. I assume there is a desired ratio that I am not aware of. I had to build this on a budget so most of the materials were already on hand. I'm planning on machining a new brass port to replace the brass fitting shown in the pic. Any input is greatly appreciated.

[jrrdw]

This deserves it's own topic, rename it anything you want to...it's your cannon.

Welcome to Spudfiles!

[MrCrowley]

When you say the port is 1/4", do you mean the barrel seat or the pilot valve port? If the barrel seat really is only 1/4" diameter, it's hard to see why the piston wouldn't be pushed back. If that's not the barrel seat diameter, what is?

I think the problem might be with your design. See how the sealing face of the piston is exposed on two sides? On the front side, what is exposed to the chamber pressure is the area not taken up by the barrel seat the piston is sealing against. On the back side of the sealing face, the chamber pressure acts on the entire black surface area (and little 'lip' of the white body). This would make it seem that any valve with a design like this wouldn't open, as the back surface area is greater than the surface area on the front that is not covered by the barrel seat.

However, most valves of this spool variety have a third surface area exposed to chamber pressure. This is usually the front face of the body of the piston. In my original piston (not the aluminium one), it is the nut/washer part that holds the first o-ring. Now this surface area is very large, it is nearly the diameter of the piston itself in my design. The chamber pressure is pushing back on this entire area. On the other hand, the chamber pressure is pushing forward on only the little bit of area exposed on the back of the sealing face. This means the spool piston valve can still work as, when the chamber pressure is high enough and overcomes the pressure in the pilot area pushing the piston forward, it will force the piston back.

On your design, there really isn't this third large surface area that has the chamber pressure pushing the valve back. Instead, your valve changes diameter as the main body of the valve rides along in a smaller diameter length of pipe. This means that only the small 'lip' of the body of the valve is exposed to the chamber pressure and the area it represents is smaller than the area represented by the back of the sealing face. In other words, the chamber pressure is holding the valve forwards when it should be trying to push it back.

This problem could be overcome if the barrel seat is quite a lot smaller than the sealing face. You need to work out how much area is exposed to chamber pressure on the 1st 'area' and 3rd 'area' and then combine the values. This area needs to be larger than the 2nd 'area' exposed to chamber pressure.

*When I say surface area, I mean cross-sectional area. Saying surface area just helps when explaining things.

Here's a diagram to help, let me know if you have more questions!

[ToasT]

Simply put, your pilot is too small relative to your barrel.

MrCrowley said it all if you want less simply. However I think it is easier to analyse by simply comparing the size of the barrel to the side of the piston housing to the size of the rear vent.

[MrCrowley]

Simply put, your pilot is too small relative to your barrel.

MrCrowley said it all if you want less simply. However I think it is easier to analyse by simply comparing the size of the barrel to the side of the piston housing to the size of the rear vent.

Nope! That's not what I said at all

I'm not even talking about anything to do with the pilot, at the moment. That would be next on my list of things to check. For now, it seems like the design of the piston itself is the culprit and not a lack of flow through the pilot vent.

Also, the pilot size is never required to be relative to the barrel, rather it is relative to the volume of the pilot area and the piston valve.

edit: what suggests that the pilot vent is not at fault is the 'suction' OP described when trying to pull the valve back when the pilot was empty. This suggests there is a problem with the force differential acting on the piston.

[ToasT]

Nope! That's not what I said at all

Well I think it is

It seems the issue is that I have decided upon a naming convention that isn't standard. Sorry for the confusion.
What I mean by pilot is the size of the piston (or rather piston housing) and what you call the pilot, I call the vent.
I will call the pilot the piston for the rest of this post.

You need to work out how much area is exposed to chamber pressure on the 1st 'area' and 3rd 'area' and then combine the values. This area needs to be larger than the 2nd 'area' exposed to chamber pressure.

This can be simplified to just comparing the barrel bore and the piston diameter(at the o-rings), where the barrel needs to be smaller than the piston in order to get the favourable force balance.

[MrCrowley]

It seems the issue is that I have decided upon a naming convention that isn't standard. Sorry for the confusion.
What I mean by pilot is the size of the piston (or rather piston housing) and what you call the pilot, I call the vent.
I will call the pilot the piston for the rest of this post.

Ah yeah that's definitely the problem. The "pilot" has always meant either the pilot valve (exhaust/vent, etc) or the area between the piston and the pilot valve (pilot volume). The piston has always meant the piston and the piston housing the piston housing. At least on this forum.


Re: the other thing, I think it's better if you think think in terms of all the different cross-sectional areas and force directions from the start as you'll have to work out that stuff later on when it comes to calculating the pressure required behind the piston for a given mix. It's also important to differentiate between the bore and the outside diameter of the barrel, the latter dimension is the more important one when it comes to calculating piston forces.

[chronic_fabricator]

Thanks to everyone for taking the time to weigh in. MrCrowley: that makes perfect sense! I think I was just staring at the problem for too long. Seems obvious now.

Anyway, last week I increased the piston housing/upper tee to 1 1/4" to accommodate a bigger piston and... Success!!! I got it to throw the piston when firing pneumatically. But wasn't so lucky when I tried mixing in the propane. Until today I successfully got a 2x mix to ignite! I think I just need a little more practice with manometric metering.

I'll post more pics tonight when I'm in my shop.

I'm so glad this place exists! Thanks again!