Hammer and tennis ball idea

[rich59]

I have an idea for fire by compression that certainly does not work in its current form. If anyone has some ideas then it might progress to something useful, who knows.

The original vision was that a half tennis ball would be put on a firm and flat surface with a bit of charcloth under it. All you would then do would be to hit it with a heavy object. In theory the air under the half ball would would be compressed enough to ignite the charcloth - just like the fire piston. The downward pressure would seal the edge of the ball.

I am sure the main reason this does not work is that the diameter of the air pocket is huge and the force needed too great. Also the circumference would expand so again not allowing sufficient compression.

This could lead on to a question of what is the widest bore fire piston that could be made to work if one were allowed to use a hammer (or back of an axe) as the force and not just your own hand and arm.

Then again one might come up with a "one time piston" made of simple material that will crush just the once, trapping air that gets compressed.

Any experiments should probably need goggles as high pressure can lead to high velocity and tiny shards.

Isn't there somewhere a rock with neat cylinders cut into it in Asia that it has been suggested may have been used as a fire piston?

 

[Nemisis]

What about an old bike hand pump fitted with one of those needle adapters used for inflating a football fitted into a small wooden block with the char cloth in the bottom?
Dave.

 

[Lifthasir]

Hitting half a tennis ball with a hammer would never work. Not enough compression
by miles. It's important that the super heated compressed gases are focused where the tinder is. There will an optimum compression ratio which can either
be calcuated or arrived at by trial and error. This is why pistons of all kinds exhibit
similar characteristics i.e. long and narrow.

 

[leon-1]

Surely the superheated gasses if you could get them that hot would change the structure of the tennis ball i,e melt it.

The other problem is that the walls of fire pistons are rigid, there is no give, the elasticity of rubber would not allow you to achieve enough compression in the space provided because its very nature would allow the walls of the chamber to expand before the gasses could achieve a critical temperature.

It tends to make me think of car tyres reaching temperature due to friction (rather than compression), but the gasses inside the tyre do get warm and expand, the walls of the tyre get warm over extended periods of time, however they do not combust even with the increased pressures and heat.

Only a couple of thoughts

 

[Ogri the trog]

Rich59,
The skeptic in me says why expend so much energy creating a coal when a spark derived by similar means migth be more viable.
The optimist however, likes your style and is on the look out for something that would take the punishment.

ATB

Ogri the trog

 

[bloodline]

unrigid dieseling what a concept :You_Rock_

 

[rich59]

Thanks for the various ideas.

I had an idea for a slight change of setup - not half a tennis ball but 7/8th of a tennis ball. Small hole at the bottom to seal on the base only. Then the ball deforms loads in its natural way. It can expand sideways but would still get pretty well flattened by the impact. Might explode, but who knows. Most likely the nice seal at the bottom edge would be broken by the edges curling in. So it might need a non hairy ball an a smooth, oiled surface as wide as the ball.

As to the heat damaging the ball - it is only a flash of heat I suspect so anything bulky would not be affected. Only your delicate and well aerated char or tinder would get hot.

 

[match]

You'd also have to make sure that you can compress the air into a small enough space - i.e if you hit the tennis ball with a hammer smaller than the width of the ball, you'll get quite a big 'uncompressed' gap round the head.

I've used a bike pump for fire by friction before, but it wasn't a one-off pump that did it - I basically got the bike pump, pushed it down into a large wad of flax and pumped repeatedly - the small amount of compression on each stroke causes a slight increase in temperature each time, which eventually got the flax smouldering (took about 2 minutes of pumping though )

 

[Lifthasir]

assuming you got an ember - wouldn't the action of flattening the tennis ball
also flatten the ember...A fire-piston is designed such that the tinder is never
subjected to an impact.

 

[rich59]

I've used a bike pump for fire by friction before, but it wasn't a one-off pump that did it - I basically got the bike pump, pushed it down into a large wad of flax and pumped repeatedly - the small amount of compression on each stroke causes a slight increase in temperature each time, which eventually got the flax smouldering (took about 2 minutes of pumping though )

Hey Match tell us more about that one. Sounds interesting. Any pictures?

 

[rich59]

assuming you got an ember - wouldn't the action of flattening the tennis ball
also flatten the ember...A fire-piston is designed such that the tinder is never
subjected to an impact.

Good point. I guess though that if the tinder doesn't crumble into dust then it will be OK to burn afterwards as the flattening would presumably only be for an instant. I also reckon you could never completely crush the space to nothing as the pressure goes up enormously as the space gets smaller - tending to repell the force.

What's the betting the ball splits under the pressure before getting to ignition point?