Ignition Sources
There are many ways to ignite a fuel/air mixture. Anybody who's
anybody who's ever looked into accidental fires knows this.
There are several reliable ways to ignite the mixture inside of a
spudgun. We'll list them in order of complication.
Touch-Hole
Arguably the most crude and simple way of igniting a spudgun,
and possibly responsible for the most burns on people's
fingers, hands, and arms.
The construction is simple: drill a small (anywhere from 1/8"
to 1/4") hole in the cannon's combustion chamber. After
fueling, touch a match or long-nosed lighter to the hole, and
leaking fuel/air will ignite and the flame will travel into the
chamber, igniting the rest of the mix.
Aside from a very small decrease in performance due to pressure
loss, this is mildly dangerous as the pressure increase in the
chamber will create a jet of hot gases a respectable distance
from the touch-hole, capable of burning skin quite severely.
For the above reasons, it's not something I'd recommend.
Lantern Sparker
These are one of the oldest methods along with piezoelectric
igniters for igniting a spud gun.
They are flint igniters designed to be mounted inside of a
propane or camp fuel lantern. By drilling a 1/4" hole through
the chamber (a double layer of piping/fitting preferably), the
igniter can be disassembled and threaded into the hole, and
reassembled to result in the knob outside of the cannon and the
flint and file-wheel inside.
AdvancedSpuds
has a decent page on installation. In addition, I've included a
simple diagram below:
Piezoelectric Igniter
These come in a number of sizes and shapes. By far the most
commonly used are the big "red button" types used on propane
BBQ grills for ignition.
However, they can be pulled from a number of butane lighters,
ranging from long-nosed lighters for lighting grills and fires
to "electronic" Bic's.
Each has two contacts. When the button is pressed, a spring is
compressed until it is released after a certain pressure. This
strikes a small quartz crystal with a hammer, generating a
large voltage across it's face, which is conducted to the two
electrodes on the exterior of the igniter.
Small igniters for use in butane lighters can generally jump up
to a 1/4" air gap at atmospheric pressure. The larger ones used
for grills may extend this up to 1/2" but a good rule of thumb
is to use a single gap totalling no more than 1/4", or multiple
gaps equalling the same.
Stunguns, Electronic Igniters, etc.
These are circuits based on a voltage multiplier and capacitive
energy storage to generate several pulses of high voltage (up
to 50,000 for most mediocre stunguns), capable of jumping
relatively large air gaps.
Stunguns can be bought in varying qualities. Reliable ones will
have a completely potted circuit which simply get supplied with
~6 volts and outputs anywhere from 50 kV and up. Cheaper ones
will have a board with components soldered on it, and a small
air-gap which controls the capacitor's discharge through the
output transformer.
Electronic igniters are usually more expensive than their
piezoelectric counterparts, and bulkier as well, with
completely potted circuits for the most part.
Recently, these
little circuits (similar) have been flooding the hobby, and are
extremely cheap and decently reliable, providing a
high voltage output for a 5V input. You should know that I
recently bought five of these on Amazon and not a single one of
them worked. Take it for what it's worth.
Ignition Coils and Other More Advanced Solutions
I won't go into too much detail here, but many people enjoy
integrating their electronics hobby with spudgunning. Car
ignition coils, flyback transformers, and even hand-wound
transformers all find themselves jammed into project boxes or
clamped to a cannon for ignition.
The simplest ignition coil driver is a switch, a small
capacitor, and the ignition coil itself, along with a 12V
source. Going up from there, drivers can be built using an
astable multivibrator such as a 555 timer for switching voltage
to the coil repeatedly, or drivers with feedback utilizing
hall-effect sensors to switch current through the primary side
of the coil at the most opportune time.
Camera flash circuits can be utilized either by themselves or
in conjunction with an output transformer such as an ignition
coil or flyback transformer. The simplest way to use one of
these boards is to mimic the flash tube (in open air) inside
the chamber by having three screws converge very closely
together, wiring two to the leads of the capacitor on the
board, and the third to the output of the trigger transformer
on the board. The high voltage pulse from the trigger ionizes
the air between the other two screws, allowing the relatively
low (~300V) voltage of the capacitor to discharge across the
gap, resulting in a very hot spark.
