Introduction: Start a Fire With Air

About: I'm an Emu. As a young chick my parents use to feed me watermelon and I loved it so much everyone nick named me, you guessed it, watermelon. Now that I have moved away from home I rarely get to eat any waterme…

To do this we need to make a firepiston, which is a device for starting cook, camp and signal fires with air.

Firepistons seem so high tech that it's hard to believe they were not developed at MIT or Carnegie Melon under a top secret DARPA contract with unlimited funding. Since the firepiston is also given credit for Rudolf Diesel's invention of his Diesel engine it is quite surprising that the firepiston was not invented by Diesel himself or by one of his contemporaries or the likes of Ben Franklin and other European and American pyro based device inventors of the 16th, 17th and 18th centuries.

What historians note is that the firepiston was most likely invented in conjunction with the invention of the blow gun by prehistoric South East Asians since firepistons are normally found in the possession of those who use blowguns. The need to dislodge the internal partitions at each connecting joint inside a length of bamboo by ramming a rod or piston through the membrane, and in the course of doing so, rapidly compressing the air thereby setting dust particles or the membrane alight, is the reasoning behind co-invention.

I was so amazed at the technology and the science behind it that after building my own firepiston to prove to myself that the science and technology was real I could not resist sharing the science, technology and construction of my first firepiston with you by creating this instructable.

Here then is a description of how to make your own working firepiston to see first hand for yourself that the science and technology do in fact work. I left lots of room for improvement in materials, workmanship and degree of completeness for possible use in the field, if going beyond a demonstration is what you want to do.

Step 1: So How Does a Firepiston Work, Anyway?

A firepiston can set an ember to burn as a result of energy being concentrated into a smaller and smaller space by compression of air resulting in a corresponding increase in the air's temperature.

The principle of increasing temperature by method of compressing air is explained by the Adiabatic process in which the internal energy of a gas must increase when a mass of air is rapidly compressed (or the volume of space containing a mass of air is rapidly decreased). The resulting increase in internal energy results in a rise in the temperature of air sufficient to light an ember, just like the pressure produced by an ice skater's blade is sufficient to increase the internal energy of the ice, which turns ice under the blade into water.

You can envision the effect in a 2D graph (below) with pressure as the ordinate (vertical) and volume as the abscissa (horizontal) and the adiabat or curve of constant entropy as the inverse relation curve. (Black lines are the curves of constant entropy.)

Rapid compression by a factor of 25 to 1 produces sufficient internal energy to send the air temperature to 800 degrees Fahrenheit. This temperature is sufficient to ignite a piece of char cloth for use in kindling a fire.

After several tries and some fine tuning (better sealing with more lubricant) I was able to produce embers with char twine using the apparatus I constructed.

Step 2: Materials and Tools

In general you want a material that will allow a cylinder to be bored smoothly with approximately an 1/4" ID which is around 6" deep and sealed on one end. Incidentally, If you do the math then you will see that it is the ratio of the height of the uncompressed space to the height of the compressed space that determines the height of the cylinder. In other words if you want .25" of char cloth to be ignited and you need a compression ration of 32 to 1 (instead of 25 to 1) to do it then the height of the cylinder will need to be 32 times .25 or 8", plus a little extra room to insert the piston.

The piston is a rod that fits snugly inside the bore of the cylinder using a gasket or seal of some type to make it air tight. The bore and the piston must be perfectly straight and smooth to achieve and to maintain an air tight seal while it is being operated to produce an ember.

Natives might collect woods and cutting tools and keep at least one firepistons under construction in various stages of completion to be able to go from start to finish within minutes, hours or days.

To speed up some of the process required for construction requires use of a 1/4" by 6" brass nipple with an end cap filled with JB Weld and an aluminum or oak dowel to construct two test pistons. The pistons require a 1/4" OD O-Ring (or slightly larger diameter that can be filed down to fit) to act as an air tight seal. Natives would use fibers of some sort and perhaps anything from animal fat to earwax as a sealing lubricant. We can use lip balm or Petroleum Jelly.

I used a 1/4" power drill and a piece of dowel wrapped with an inch long sheet of tissue paper soaked with a solution of water and Bar Keeper's Friend (Oxalate acid) to polish the inside of the brass nipple to a mirror finish. (On the larger 3/8" nipple I'm working with now the inside of the nipple is much less smooth and straight requiring various grades of sandpaper to hone it smooth before polishing.)

I used a hack saw blade, a glass file and the threads on a bicycle wheel spoke to fashion the seat for the O-ring in both the aluminum and in the oak dowel. A 1mm bit was used to drill the hole in the char twine box. (Commercial units have a much deeper cylindrical hollow to hold more char cloth better.)

Originally I used a temporary piston stop made by using a pair of vice grips clamped around the piston shaft where the handle would be attached to the piston shaft to keep the char twine from striking the bottom of the bore. This worked but it appears that commercial makers of firepistons hollow out the end of the piston much deeper to provide greater room for more char cloth and allow the end of the piston around the hollow to serve as the piston stop, rather than using a shaft pin or the handle as the piston stop. BTW - commercial versions can run upwards of $50, but some come with all of the amenities such as char cloth and lubricant.

Step 3: Polishing the Cylinder

Do not use cylinders with a weld seam left inside but only nipples that have been bored smooth at the factory. Otherwise you will have to bore out the inside of the pipe to remove the excess material remaining after the seam was welded.

I used a 1/4" power drill and a 1/4" oak dowel with an inch wide piece of damp tissue wrapped around it for polishing. I used various grades of sandpaper for honing first since the nipple was dented or had internal high or low areas resulting from the equipment used to keep the nipple from slipping while the threads were cut.

The tissue was dampened with a solution of water and Bar Keeper's Friend (oxalate acid), inserted into the nipple and spun by the drill. The nipple can become quite hot so wear rubber or leather gloves to prevent pain and give a better grip on the brass. Also, stop frequently to add water or more solution to help keep things cool. Adjusting the width and amount of tissue wrapped around the dowel will help to optimize the fit and lower the degree of friction. The more you polish the better, but 10 minutes or less should be enough if your last job of honing was with extra fine grit sandpaper.

Step 4: Putting on the End Cap

JB Weld is used to fill in the space where the nipple does not reach the bottom of the end cap. If the space is not filled the O-Ring will expand in this space and release the air pressure too soon as well as get caught in the space.

The end cap is set down like a cup and filled halfway with "steel" and half way with "hardener." They are mixed with a matchstick or toothpick. Then the nipple is held upright and the bottom end threaded into the end cap and tightened. Once tightened the nipple and end cap are either hung by a string or stood up so the JB weld will set with a flat and horizontal surface on the inside to stop the piston and to eliminate all unintended air space.

The hard part comes next - waiting the full 16 hours for the JB Weld to set completely.

Step 5: Cut the Hollow and the Seat

On the very end of the piston a hollow is cut using a rounded or squared end drill bit. The hollow may also include 1mm or 1/16" hole, drilled in the center for a total depth of no more that 1mm or 1/16" to help hold the char cloth or twine better. Commercial versions may use a slightly deeper (3/16") hollow with no center hole and with the O-Ring set back behind it for practical use in the field.

For construction of this firepiston I just wanted to see if it the limits of the science and technology that might keep it from working. I tried several other configurations. You can experiment with all sorts of arrangements, including a double O-Ring configuration. Feel free to experiment and learn everything you can in case you ever really need to make one of these out in the field!

I cut the seat for the O-Ring next by placing the other end of the piston in a 1/4" power drill and using a hack saw blade, glass file and the threads of a bicycle wheel spoke as lathe chisels.

The O-Ring is and seat are next lubricated and the O-Ring is slid on and the O-Ring fitted to the inside diameter of the cylinder.

Step 6: Test the Fit of the Piston and O-Ring

While it should not be difficult to insert the piston and O-Ring into the nipple there should be obvious resistance from air pressure and the piston should spring back when released. If not try a little more lubricant.

If the piston can not be inserted then remove all lubricant from the shaft and O-Ring, slide the O_Ring up onto the shaft past the seat, put the piston back in the drill and spin the O-Ring on some sandpaper to reduce the outside diameter of the O-Ring a very small amount each time.

Re-lubricate and re-seat the O-Ring and try again. Repeat this process if necessary.

Step 7: Add Handles

Since I was not planning to use this in the field (just yet) but rather testing compression ratios I used vice grips as an adjustable piston stop and for the piston handle and a towel to hold the cylinder. For field use you can used pieces of wood with a hole drilled in them the size of the outside diameter of the piston and the outside diameter of the cylinder and secure them with epoxy.

Step 8: Insert Char Cloth and Test

Char cloth is cotton or denim cloth that has been placed inside a closed container, like an Altoids tin or capped nipple, with a small hole drilled in it about 1mm or 1/16" in diameter to allow gases to escape. The container is heated in a fire to drive off water and other volatile compounds. I found also that setting a piece of twine alight after inserting it in the hole and letting it burn down and then capping it just before the flame went out produced enough char twine on the very end to be set alight by compression, although for a much shorter period of time.

( Here is an instructable for making char cloth in the field for use with a firepiston...)

A piece of char cloth is inserted in the recess of the piston head and the piston is inserted into the cylinder. A rapid motion is used to drive the piston forward with great force and then withdraw immediately (but with some slight delay).

When withdrawn a small ember should be found which must be nurtured if used to set a piece of tinder alight in order to build a fire.

Do a Google search on the keyword "firepiston" and you can find hundreds of sites on the internet that discuss how to use a firepiston and find videos that show the glowing ember and how it is used to light tender and start a fire. If you have an Adobe Flash Player or add-in you may also be able to view this.

Mine works but until I put on handles I'm not going to develop an operating technique. I have learned that a slight delay after plunging rather than a sort of very rapid bounce worked much better. Experiment.