SpudFiles

as a kid i always toyed around with the basic combustion cannon, nothing special. but after discovering spud files a few weeks back and seeing where these things have gone over the years i knew i had to build a bigger and better one

it consists of
metered propane injection, meter pipe is cut exactly to length to get a perfect 4.02% in my chamber (@ 90psi)
dual ignition via bbq grill ignitor
80mm pc chamber fan powered by a nine volt and a momentary switch mounted on the forward grip
so far the thing has been working great, fires every time. i had at first started to paint it but after scratching the hell out of the paint during assembly i decided to just get it together and worry about paint when the summer gets here

You do need pictures to go in the showcase. It sounds good, but I'd like to see that myself.

I should also point out that the perfect propane percentage is 4.19%, not 4.02% or 4.03% - which I see hanging around a lot.

Then that should be fixed in the wiki and the stiociometric calcultions should be redone

Spudfiles Wiki wrote:The actual percentage of fuel for a stoichiometric mixture will depend on whether the fuel is added to the air in the chamber or the fuel displaces some of the air in the chamber. If the fuel is added to the air in the chamber then the fuel volume is 4.2% of the chamber volume. If the fuel displaces some of the air in the chamber then the fuel volume is 4.02% of the chamber volume.

Most fuel meters do not displace air in the chamber with fuel (the pressure in the chamber rises slightly when the fuel is injected), so for a meter the fuel volume is 4.2% of the chamber volume. If the fuel is injected with a syringe, and the fuel displaces some of the air in the chamber, then the fuel volume should be 4.02% of the chamber volume.

It's already in the Wiki and can be found here.

wow......... very nice gun. pretty clean and not to complicated, i like it.

pretty old topic but can you tell what size the chamber and barrel are?

Ragnarok wrote: I should also point out that the perfect propane percentage is 4.19%, not 4.02% or 4.03% - which I see hanging around a lot.

Actually, I would like to point out that both are correct. For an injection system which injects propane into a closed chamber, (i.e. the gas displaces no air from the chamber), 4.2% is the correct percentage. For an injection system which displaces air as it injects gas, 4.03% is the correct ratio to use in you calculations to simplify things, instead of having to solve a linear system of equations to get the correct meter pressure.