#21: FIRE PISTON
The fire piston (also known as a slam rod) is a very unique method of starting a fire using only air. That makes it another great party trick, if you’re collecting tricks to impress your friends. It operates under the principle that pressure creates heat. However, whereas a gasoline engine compresses at a ratio of 7:1 and a diesel engine compresses at a ratio of 20:1, a fire piston needs about 25:1.
To make a fire piston, you’ll need a piece of copper or brass tubing. Typically, tubing of about 1/4 inch diameter is used, although slightly larger tubing will work as well. The thing you have to remember is that the length of the stroke needs to be 25 times the diameter of the tubing; so a larger diameter means you’ll need a longer stroke. The piston can be made of any non-flammable material, or a flammable material that is tipped with a non-flammable material. It needs to fit snugly inside the tube or have seals around it to make it fit snugly. If a good seal is not obtained, there is no way that you are going to get the compression necessary to get ignition. This is critical. Without a good seal, the air will escape around the piston and won’t heat up.
The piston needs to be cut so that it will stop just short of the end of the tube (about 1/4 inch). It also needs something to use as a handle, so that you can press down on it with enough force to compress it rapidly. A notch in the face of the piston can be used to hold the tinder in place.
To use the fire rod, attach a piece of tinder to the face of the piston; char-cloth works well. Place the end of the fire rod on a surface which will allow it to seal off the air escaping. Push down rapidly and firmly on the piston handle, compressing the air inside the cylinder as fast as possible. This should cause it to heat up sufficiently to start the tinder burning. That tinder need to be immediately put in contact with a larger piece of tinder, as the small amount of tinder which can be put in the slam rod won’t burn long.
If your fire rod doesn’t work, it is most likely because you aren’t pushing the piston down rapidly enough or the air is escaping from the cylinder. For this method to work, you need to be able to generate the heat faster than it can escape. That can only happen with rapid compression.