Giant Piston

[Mike Schreiber]

Originally Posted by Dan_Karol

Something to think about:

if you have a 2 inch bore and 20" long piston and want it to impart just as much energy when it is almost completely deployed as when it starts you need to maintain the 60psi inlet pressure the entire time. During the operation of a piston used to shoot the ball it can be assumed that there is going to be no effective contribution from the compressor, it just isn't fast enough to keep up.

so, the ideal gas law tells us:
P*V = nRT

For argument's sake we will ignore the change in temperature. This tells us the pressure times the volume must be equal to a constant, so:

P_1*V_1 = P_2*V_2

where:
P_1 is the pressure in the tank (120 PSI MAX)
V_1 is the total volume of all tanks
P_2 is the working pressure after actuation (60 PSI MAX)
V_2 is the volume of all air tanks + the volume of the cylinder

so, with your setup (using my assumed cylinder)
120 PSI * (574 ml *4) = ((Pi*(1in^2)*20n )+(574 ml *4) * P_2
P_2 = ((120 PSI) * (574 ml *4) )/ ((Pi*(1in^2)*20in ) +(574 ml *4) ) = 82.85PSI

so,
unless you are experiencing an airflow problem.

Using several smaller diameter cylinders in tandem would solve this problem because it would allow higher flow rates into the combined cylinder-system.


Good Luck,
Dan

Didn't check your math, but I'm concerned about your units. Storage in ml and cylinder size in inches^3, did you include the conversions to add these?

Two side questions that this made me think of:

1)
Is this air expansion adiabatic or isothermal? You've calculated it using the isothermal assumption. I would argue that it is adiabatic since no time is allowed for heat transfer with the outside to occur. Not sure though.

In the case that it is adiabatic the equation would become P*V^gamma = constant, where gamma for air is 1.4

2)
Using the ideal gas law, don't you need to calculate with absolute pressure not gage pressure? This would mean adding atmospheric (14.7 psi) pressure to the working and high pressure side. 74.7 and 134.7?

Likely doesn't make a big difference, just curious which is correct.