SpudFiles

In a closed chamber combustion when ignited is there just a flash of pressure or is there a constant pressure? I am asking this because I am wondering if you could make maybe a 2'' bore pneumatic that would use 90psi from combustion so that you wouldn't need to pump it. This is just a idea you could have it portable with a better rof because of no pumping.

A pneumatic powered by a combustion would be pretty dismal I'm afraid.

The power in a combustion is from one short pressure spike and the gases temporarily being very hot, the cold gases afterwards will be under very little pressure.

90psi is more like the pressure directly from a decent combustion. There will be nothing like that left in a cooled down sealed chamber.

Well I was wondering if there was a constant pressure or a pressure spike well you obviously answered my question.

I get closed chamber ignitions when my burst disks are too thick. What's amazing is that the shot is almost totally silent...it seems a lot like a misfire. However, when you open the air vent a light burst of warm air rushes out and the chamber is a little warm to the touch. I give it a minute to cool down before another shot.

This would be a poor way to "pump up" a pneumatic. The huge energy pulse has been mostly absorbed into the chamber as heat. However, fit with a properly sized burst disk, you can transfer that energy in a real big way to the projectile...speed you can never get from a simple pneumatic.

The pressure comes from the heat, and when the heat is gone, so will the pressure.
The temperature drops within the same second, so it is not feasible to hold it in a chamber and release it later.
Only the combination of a burst disk may improve an combustion cannon. It will hold the pressure back while it is still building up, and then releases it near the top pressure, allowing for maximum performance.

An interesting thing to see, is that when you've got a really long barrel or so, the gases cool down so much, that vacuum is created and the projectile is sucked back into the chamber.

Pressure versus time graph for the combustion of 1X propane in air in a closed combustion chamber. (time=0 is the point when combustion is complete.)

(From here)
The pressure has dropped by 50% within ~1/8 second (or so) after combustion is complete. The pressure is essentially all gone one second after combustion is complete.

Remember that there are more moles of gas after combustion than before (a 6 : 7 before to after ratio), so the pressure will never drop back to ambient unless there's a leak. The tiny pressure increase isn't really useful for anything though. The only simple way to capture some of the combustion pressure is to have the combustion drive a piston which forces air through a check valve. And even that isn't very simple...

DYI wrote:Remember that there are more moles of gas after combustion than before (a 6 : 7 before to after ratio), so the pressure will never drop back to ambient unless there's a leak.

Almost correct.

Propane + air (roughly);
C3H8 + 5 O2 + 20 N2 = 3 CO2 + 4 H2O + 20 N2
So, there are 26 reactant molecules and 27 product molecules (you have to take into account the nitrogen) if the water doesn't condense. So, the pressure increases by 1/26th not by 1/6. That's about 0.6 PSI which you probably can't even measure with a pressure gauge that'll survive the 50~120 PSIG pressure spike.

If the water condenses then the pressure drops by 3/26ths (26 to 23) relative to the starting pressure, that's 1.8 PSI below atmospheric.

So the pressure can drop below the starting chamber pressure. In practice, it'll get so close to the starting pressure that you can't tell the difference without lab grade instrumentation.

Ah yes, forgot about the nitrogen. Yet another of the many points in the whole "optimal percentage of buffer gas" conundrum that I'll soon be looking in to...

DYI wrote:Ah yes, forgot about the nitrogen. Yet another of the many points in the whole "optimal percentage of buffer gas" conundrum that I'll soon be looking in to...

I only remembered it because of the recent thread about maximum pressure in a hybrid before the fuel condenses.

Its the old "partial pressure" issue. The partial pressure of the fuel+oxygen goes from 6 to 7 but most of what's in the chamber is boring old N2.