fire piston help

enix

enix

Joined
Mar 21, 2007
Messages
452
After reading about fire pistons and looking at some pics and watching some videos I asked my dad if he would make one at the machine shop where he works. I showed him some videos and examples and he said he would come up with something in his spare time. Fast forward a few months and he calls me and says he has 'this piston thing'. So I go by the parents house after work and get handed this:

100_1015.JPG

(Sorry about the blur)This thing is a behemoth milled from unhardened A2. The piston is 6'' long and the chamber is a bit longer.

100_1016.JPG

This is the diameter of the chamber.

100_1019.JPG

And this is the length of the piston assembled and 'at rest' 10''

The piston has a nice spot drilled out for tinder, an o-ring for seal and a super snug fit. But I can not seem to be able to get any type ember whatsoever. I am usuing char cloth for tinder and have experimented with more and less amounts. Compression seems to be good because I have not been able to get the piston in all the way so that it 'bottoms out' at the top, where the cap is.

Never mind carrying this, its too huge, but would still like it to work for fun if nothing else. So any ideas on how to improve it? Just make a smaller version? Any input appreciated.

BTW, while my old man has no problem making these for me he thinks I am 'being taken' or that these are a joke type of thing, so I definably want to prove him wrong.:D
 
I have one made by another forum member. Always works for me with charcloth, havent tried any other tinder yet. Keep practicing. put the plunger end on the ground and push straight down like your doin cpr. You should get a bit of smoke. Blow on the tinder, and you should have a good ember. It sounds like you have a good seal, try and put a dab of wheel bearing grease on the o ring.
 
I've got a concern that this piston won't work for you because it's made of metal, which is a great heat conductor. I've seen videos of working fire pistons made of plexiglas or wood, both of which are poor heat conductors, but I've never seen a hand-operated metallic fire piston. The cylinder may be drawing off the heat just enough to prevent combustion.
 
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Thanks for the reply fellas, LittleHairyApe: I was thinking that the metal might have been a problem too.
 
Metal works great! Heat flow properties aren't significant since the whole thing happens so fast. The problem here is size. The smaller the piston is, the better it will work. Nearly 3/4" bore is MASSIVE. I made one from some simple metal plumbing fittings, a bolt, and an o-ring from the hardware store. Bore is ~3/8" and the piston part is ~3" long. Make everything as small as you can. Also, when first learning to use it, try backing one end against a board or something.

I can't log in to my photobucket account right now, but here is a video of it on the Wilderness Wiki:
http://funditor.110mb.com/wiki/index.php/Fire_Piston
 
CanDo that is a great link, I have a firepiston made out of aluminum and I have never had any luck with it. Though it does not have a recessed hole to hold the tinder in. I just thought I was not putting enough pressure on it, but I slam it against concrete and its still not getting an ember.
 
CanDo said:
Metal works great! Heat flow properties aren't significant since the whole thing happens so fast. The problem here is size. The smaller the piston is, the better it will work. Nearly 3/4" bore is MASSIVE. I made one from some simple metal plumbing fittings, a bolt, and an o-ring from the hardware store. Bore is ~3/8" and the piston part is ~3" long. Make everything as small as you can. Also, when first learning to use it, try backing one end against a board or something.

I can't log in to my photobucket account right now, but here is a video of it on the Wilderness Wiki:
http://funditor.110mb.com/wiki/index.php/Fire_Piston
Click to expand...

Good Point. 3/4" is 4 times the cross-section of 3/8". The one thing we don't know is what the volume is of the fire piston cavity is when the piston is fully inserted. Can someone who has a working fire piston guesstimate what the initial and final volumes are? It would be interesting to find out what is a good compression ratio is for a fire piston. Also as CanDo said, when it comes to bore size, less is more.
 
LittleHairyApe said:
Good Point. 3/4" is 4 times the cross-section of 3/8". The one thing we don't know is what the volume is of the fire piston cavity is when the piston is fully inserted. Can someone who has a working fire piston guesstimate what the initial and final volumes are?
Click to expand...

As you note, the cross section is 4 times the area. While numbers are relatively unimportant to me as it is a "works or doesn't work" issue, here they are:

Piston shown in first post:
2.65 in^3
My piston
.331 in^3

The first has 8 times more volume uncompressed. Compressed, the volume of mine approaches 0; while it is cushioned it feels as though the piston may just touch the end of the chamber.


Photos:
FirePiston012.jpg


FirePiston013.jpg
 
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I can't do the math, but I agree that it is a compression issue due to the size of the bore. You probably cannot generate enough force to compress the chamber fast enough to get fire.
 
CanDo said:
As you note, the cross section is 4 times the area.
Piston shown in first post:
Click to expand...

(Awkward) Since you attributed a slightly inaccurate statement to me, for the record, what I said is:

Assuming a (circular) cylinder of uniform diameter cut through by a saw perpendicular to the axis of the cylinder

cross section = area = (pi * radius^2) = (pi * diameter^2) / 4

A(3/4") = (3.14 * 3/4 * 3/4) / 4 = (3.14 * 9) / 64 = 0.4415625

A(3/8") = (3.14 * 3/8 * 3/8) / 4 = (3.14 * 9) / 256 = 0.110390625,

A(3/4") / A(3/8") = 0.4415625 / 0.110390625 = 4 :)

That's where I got the number 4.

CanDo said:
...Compressed, the volume of mine approaches 0; while it is cushioned it feels as though the piston may just touch the end of the chamber.
Click to expand...

Knowing that it touches the bottom is qualitative rather than quantitative data. And although I prize quality over quantity in knives, I work with numbers, so numbers, i.e. quantitative data, are important to the final answer.

And, yes, for a working fire piston I'd expect the volume to "approach" zero, but that's not sufficient for calculating compression ratio. If you use that approach, then you have to conclude that the compression ratio approaches infinity. :eek: So you need both numbers. :) You need the uncompressed volume (easy), and you need the compressed volume (difficult). And it's a very small volume, to be sure, so that makes it interesting to measure accurately. That's why I used the word "guesstimate."

Sorry for the long post. Apologies to the BF members who are doing this:
:mad:, :jerkit:, :grumpy:,:barf:, or :confused:
 
Thanks to everyone for their input.

CanDo: I think I showed my old man the video you made of yours, and he turns areound and hands me this giant thing. Jeez.:rolleyes:

Also I think I will make a run to the hardware store this weekend and see what i can put together.

Thanks again everyone, input much appreciated.
 
enix said:
Thanks to everyone for their input.

CanDo: I think I showed my old man the video you made of yours, and he turns areound and hands me this giant thing. Jeez.:rolleyes:.....QUOTE]

I'm with ya on that one! My Pop did 40+ years as a Tool & Die Maker and I am VERY careful about what I mention in front of him. His fire piston would be a re-worked 6-ton Hydraulic jack with a new ram of Visount tool steel polished to a .0005" clearance and I would have to stick a charcoal briquette in it, attach it to the bumper on the Jeep and ram it into a tree.:eek:

I JUST picked up the May/June Backwoodsman tonight and, whaddya know, an article on making a fire piston using a stub-out from the hardware store.:D
Click to expand...
 
Jeff H said:
I'm with ya on that one! My Pop did 40+ years as a Tool & Die Maker and I am VERY careful about what I mention in front of him. His fire piston would be a re-worked 6-ton Hydraulic jack with a new ram of Visount tool steel polished to a .0005" clearance and I would have to stick a charcoal briquette in it, attach it to the bumper on the Jeep and ram it into a tree.:eek:
Click to expand...

That would be a sight. hmmmm wonder if it could work. Someones gotta try!:p

LMAO thats funny.................
 
LittleHairyApe,

I'm sorry I misspoke. What I meant is that the area of the cross section of one is four times that of the other, but I see now how it could look like I'm just hideously confused. I'm not sure that quantitative data will really do anything useful. We know what works, we know what doesn't work - assigning numbers is nice, but in the end the device is a terribly imperfect way of starting a fire compared to a firesteel or bic, so the details are pretty pointless. What were you planning on doing with the compression ratio? It would be cool to try to calculate the temperature or something, but since the gas and the system are imperfect, it's quite impossible.
 
CanDo said:
LittleHairyApe,

I'm sorry I misspoke. What I meant is that the area of the cross section of one is four times that of the other, but I see now how it could look like I'm just hideously confused. I'm not sure that quantitative data will really do anything useful. We know what works, we know what doesn't work - assigning numbers is nice, but in the end the device is a terribly imperfect way of starting a fire compared to a firesteel or bic, so the details are pretty pointless. What were you planning on doing with the compression ratio? It would be cool to try to calculate the temperature or something, but since the gas and the system are imperfect, it's quite impossible.
Click to expand...

You seem kind of sensitive on this issue... This is pointless. I feel like I'm torturing a puppy. No offense intended. I will pursue the question in other venues.
 
LittleHairyApe said:
You seem kind of sensitive on this issue... This is pointless. I feel like I'm torturing a puppy. No offense intended. I will pursue the question in other venues.
Click to expand...

I've taken no offense until this. My question is not rhetorical; I am wondering what you want a compression ratio for. Is it just for the fun of finding out, or are you planning on applying it to something? I personally can't see a single practical application for it since at the end of the day, the fire piston is basically just a toy. This isn't some novel principle - if you're looking to build an engine or something, look up Diesel engines on wikipedia.

Do not be so condescending; people on this forum are not only more intelligent, but better educated than you assume. You may be perfectly intelligent, but plugging a number into PIr^2 demonstrates seventh grade mathematic ability. Before acting like an asshole, maybe you should do something remarkable - like find this compression ratio you seek so much. Or how about making a fire piston to start things?

EDIT:

To be constructive, allow me to add a proposed experiment for the maximum compression ratio. Make an arcryllic fire piston (they can be seen on trackertrail.com) and add graduated markings. Mount the tube in a solid area. Depress the plunger while having a high speed camera trained on the piston. In case they aren't the same thing, you'd be able to find both the comp ratio at the point of ignition and the maximum ratio.
 
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Well, if we know the compression ratio, we can calculate how fast the Jeep needs to be going when it hits the tree. 88 miles an hour, anyone? :D

It's good to know that something works, and also good to know why it works. Then you can figure out whether your piston is too big, too loose, or just ornery.
 
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