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A place to ask general spud cannon related questions.
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Unread postAuthor: Spedy » Tue Jun 26, 2007 10:37 pm

I've read about these and seen videos, its basically a clear glass cylinder with the top open and an aluminum/brass piston. You put a bit of tissue paper inside, se the thing on a hard surface, insert the piston and press down the piston's plunger really fast and the paper ignites spontaniously due to the heat from pressurization.
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Unread postAuthor: jimmy101 » Wed Jun 27, 2007 12:25 pm

_Fnord wrote:I've never seen this before. The paper doesn't ignite by itself does it?

Yes it does. Google "firepiston fire piston" for what limited info there is on'm. Here is a bit of info. Apparently south east asian islanders have been using them since before about 1800 to light there fires.

Because it takes a tremendous amount of force to get to compression ratios of greater than 15:1 a fire piston is usually a pretty small diameter, like 1/4". The generate very high temperatures but a fairly small amount of heat. For best results people will use a tinder that ignites easier than paper, like charcoal or various kinds of dried moss.

You can use GasEq to calculate the air temperature for various compression ratios. Or, IIRC, the equation is
T<sub>2</sub> = T<sub>1</sub>*(V<sub>1</sub>/V<sub>2</sub>)<sup>(gamma-1)</sup>
where gamma = 1.4 for air.

Some temperature at various compression ratios (assuming an adiabatic isentropic process);
Code: Select all
   Ratio      T2         T2          P
  (v1/v2)  (Kelvin)    (F)     (atm)
     1         295        71         1.0
     3         458       364        4.7
     5         562       551        9.5
     7         642       697       15.2
     8         678       760       18.4
     9         710       819       21.7
     10       741       874       25.1
     15       871      1109      44.3
     20       978      1300      66.3
     30      1150     1610     117
     40      1290     1863     175

Paper ignites at ~451F, so theoretically a ~4:1 compression ratio is enough. But the gases have such low heat capacity that at 4:1 there is really very little heat energy. Therefore, most fire pistons are probably operated in at least the 15 to 20:1 compression ratio range.
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