SpudFiles

Hi all,

This is one of my first posts in the combustion section, mostly I just do pneumatics, so excuse my ignorance. I'm looking to build a propane powered system with a high ROF (several per second ideally). I realize there are multiple challenges involved here but the thing that bothers me the most is ignition. I have little experience with electrical setups so I would prefer to avoid some kind of complicated high-voltage switch arrangement. I was thinking a piezoelectric or battery-powered BBQ sparker might work, since I would be much more at home with getting a mechanical setup to automatically operate the push-button at appropriate intervals to work.

So my question is, could these types of devices provide a high frequency of ignition reliably over a reasonably long period of time for this to have a chance of working? If not, other suggestions? Thanks.

Use a 9-12v battery hooked up to a car coil/driver circuit, or a stun gun. This will provide instantaneous and reliable ignition, at least until the battery runs out.

How are you planning to get such a high rate of fire? How will the fuel be injected quickly (propane tanks and regs have low flow), and how will new rounds be loaded? Also, if this rate of fire is actually possible, you shouldn't use PVC, it will melt within the first minute of firing.

You should take a look around before posting

The Sparker Reference Guide

@kjjohn: I am aware of the other problems associated with high ROF combustions. I don't have any finalized design at the moment, but these problems don't seem insurmountable and my skill set is sufficent to allow me to have a shot at dealing with them. The ignition issue is outside my experience and hence was what I thought I should ask for help with. And of course I will be using metal and not PVC.

Also, how difficult is it to construct said circuit?

@rcman: I did in fact do some searching prior to posting, and that guide was one of the first things I read. It was quite helpful but didn't provide much information on the suitability of the various options for rapid fire, so I decided to ask for more information. I'm sure I'm not the first one to consider the problem of repeating ignition, but I couldn't find much existing stuff on it.

@kjjohn: I am aware of the other problems associated with high ROF combustions. I don't have any finalized design at the moment, but these problems don't seem insurmountable and my skill set is sufficent to allow me to have a shot at dealing with them. The ignition issue is outside my experience and hence was what I thought I should ask for help with. And of course I will be using metal and not PVC.

Also, how difficult is it to construct said circuit?

No prob, I just wanted to make sure you knew what you're doing, since you said you mostly do pneumatics.

The circuit required to drive a car coil is very basic. All it needs to do is pulse on and off very quickly (I don't know the exact frequency). The pulsing induces a higher voltage in the output of the coil. They can typically boost the voltage by a factor of 100, and have a rated input of 12v, although a slightly higher or lower voltage will work the same. I believe there is an article in the wiki on how to construct such a circuit, just do a search for "car coil" or "ignition coil".

I look forward to seeing if you succeed in this attempt. A full auto combustion would be a first on Spudfiles.

I think that you will have a hard time purging the chamber after each shot in time.

kjjohn wrote:How will the fuel be injected quickly (propane tanks and regs have low flow),

Trivial to overcome if you simply buy industrial items (not "BBQ stuff").

Your best bets, in terms of cost, size and practicality, are probably a battery operated BBX sparker (~$15) or a stun gun (~$15).

Ignition coil based systems require a pretty beefy (read heavy) battery and the coil itself pretty big and heavy.

Whatever system you go with there's a good chance you'll need to add some auxilary cooling. Stun guns don't like to be fired for more than a cuple seconds at a time. Pretty easy to rip open the case and add a significant heat sink to the relevant parts.

It took about 5 months to develop my repeater. It was originally going to be a semi, but I failed at finding a solution that fit my parameters (portablility, price less than $1000).

If you can figure out how to go propane full auto, you are the man.

Full auto would be sick. You would be showered with props. You would need a props umbrella.

Moonbogg wrote:Full auto would be sick. You would be showered with props. You would need a props umbrella.

You'd have to constantly spin the propbrella like a propeller to keep the props from piling up. You'd get so tired of holding it, you would have to prop it up on something.