[hw97karbine] has made a pretty cool tennis ball cannon. While making a cannon of this sort is nothing new to us, we were impressed by the effort taken to get a stoichiometrically ideal mixture of 3.2% butane and air in the combustion chamber.
[hw97karbine] filled a syringe with butane and then dosed exact amounts into the chamber using a hole in the back. To control the air mixture he marked lines on the outside of the cylinder with magic marker. Simple but effective.
More rewarding than the methods was the cool slow-mo videography of the explosions in the chamber. You really have to check it out. [hw97karbine] shows clearly the difference between a well-balanced fuel to air mixture and a poorly balanced one. It’s one thing to say that more fuel does not mean better combustion, as we all remember from our personal potato cannon experiences, but it’s another thing entirely to see it.
[Via Reddit]
In normal usage, a stoichiometric ratio IS ideal.
In practice, the mixture needs to be a little on the rich side. A rich mixture gives you more energy per shot. The stoichiometric mixture is supposed to be perfect, but the actual combustion is always uneven and some of the fuel is always left unburned as the ball exits the barrel – a richer mixture manages to burn more fuel and release more energy before the ball exits.
A lean mixture it’s hard to ignite and you get less energy per shot, but it can actually give you much higher peak pressure and more speed out of a shorter barrel at the risk of detonation and shattering a foolishly chosen PVC pipe.
The output from a butane soldering iron/torch is usually quite perfect. I once made a miniature spudgun out of an aluminium bottle. The barrel was inserted through the cap, and there was a hole on the side of the bottle that would let gas flow in freely from the torch without any back-pressure. Once the gas was ignited, the pressure would close a leather flap on the vent hole from the inside, much like a reed valve in a two-stroke engine.
I bet if I had sized the barrel correctly, and fed it with enough gas from a hose, it would have started to fire like a pulsejet engine.
Well wanting a flaming ball flying through the air I wonder how an alcohol soaked tennis ball would effect the amount of butane needed, if any at all?
Why stop there? Use a hypodermic and fill the tennis ball with alcohol, or butane, or a mixture.
Better restrict testing to your own back yard.
It would mess up your A/F ratio by evaporating.
And it shoots too fast to stay lit. Of course you can dial back the amount of fuel to make it slower, but then you might as well use a slingshot.
A typical butane torch works very well as a carburetor…
Of course, the image at top is for “incorrect combustion chamber to barrel length ratio”, not the combustion ratio being off :)