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Fully Automatic potato cannon project.

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: Technician1002 » Wed Jul 14, 2010 11:49 am

With regulators set to 50 PSI and the flow ratio set, the rate of fall off of both flows due to back pressure should be proportional. 1X, 2X 3X etc should work well.

The time dwell can be set to purge and fill the chamber with it open, then go to 2X or more after the chamber valve to the barrel is closed.

If you know what you are doing, another regulator mod to regulate flow instead of pressure will set the regulators up as mass flow controllers. These regulators flow a regulated mass of gas. The outlet pressure from vacuum to the max the regulator will function with provides the same units of gas mass per unit of time. As a commercial unit, they are expensive, but modification to BBQ Propane regulators is not difficult to change them to a mass flow controller.

A calibrated ( or adjustable ) orifice is needed. The pressure drop on the orifice is regulated so the flow remains the same even with various delivery pressures. For those filling hybrids, this may be an option.

Trust me, you don't want to buy a precision one sold to the industrial community with full digital control unless you have an insane budget. A home built can be useful for fueling.
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Unread postAuthor: ramses » Wed Jul 14, 2010 3:47 pm

I'm trying to think how to regulate the pressure drop across an orifice. Would one put the regulator in parallel with the orifice?

Assuming identical orifices, could the pressure ratio be used to determine the air/fuel ratio? Something tells me that turbulence will mess with that.

I suppose needle valves would tend to have less drift than regulators, and if the regulators were ganged together, the pressure would be the same, and the ratio would be the same, but the absolute mass flow rate would change. Feedback isn't cheap, either.
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Unread postAuthor: Technician1002 » Wed Jul 14, 2010 5:48 pm

I had it drawing up once, but I don't think I kept the drawing. The basic configuration is a low pressure regulator with the vent port tied to the output pressure instead of atmospheric for the reference pressure. The regulator output feeds directly into the flow metering orifice or needle valve. If you use a BBQ regulator for example, one side of the orifice will be 11 inch WC higher than the outlet pressure on the orifice. This remains true from zero PSI all the way up to say 50 psi. The feedback loop is the output (delivered pressure) fed back into the pressure regulator vent so the regulator tries to regulate 11 inch WC pressure above it's own output. A flow restriction between the regulator output and the delivered output provides the rate of flow delivered. In the expensive controllers, the orifice is a modulator valve that adjusts the delivered gas mass from 0% to 100% of the valve capacity.

The mass of gas that will go through an orifice is based on the orifice size and pressure differential across the orifice. A modified regulator is used to provide a regulated pressure differential on the metering orifice even when the delivered pressure changes.
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Unread postAuthor: Conrad2468 » Wed Jul 14, 2010 7:42 pm

I cant suspend the reload time :( it uses the pressure of the gas not a servo or anything. The fuel:air mixture wont be a problem. Ill rig up my BASIC stamp to a Potentiometer to control the timing until it gets right. My only problem so far is purging the cylinder without shooting out a golf ball.
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Unread postAuthor: ramses » Wed Jul 14, 2010 9:44 pm

Okay, I understand now. I have no idea what I was thinking when I said put the reg in parallel with the orifice. It surprises me that propane regulators are self-relieving.

When I spoke of feedback, I meant pressure transactors, a micro-controller, and a solenoid valve.

@conrad - If you're going to mess with a potentiometer, you may as well use a $0.20 555 timer. Are you using a piston valve? if so, filling the pilot to a given pressure separately and not allowing it to escape would close the valve immediately after the shot. My piston hybrid actually pulls vacuum in the chamber afterwords from the cooling gas. Purging is tricky, could you fill to some pressure with air, then bleed it out with some solenoid valve? That would have to take the high pressure spike unless it was protected.

If you were using a high mix, I would say screw it, you're dealing with a mere 10% (relative, for the stat nerds) less oxygen at 10x, comparable to the air being only 18% O2, rather than the ~20% typical.
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Unread postAuthor: Conrad2468 » Wed Jul 14, 2010 11:46 pm

can i run 12 volts through a 555 timer? Ive ran 6 volts....http://www.youtube.com/watch?v=s_EhqXYH8X8
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Unread postAuthor: c11man » Wed Jul 14, 2010 11:51 pm

555 can handle up to 18 volts typicly, 12 will be just fine
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Unread postAuthor: Conrad2468 » Wed Jul 14, 2010 11:55 pm

Im not using a piston :( how about...hooking a HV (taser) to fire constantly....once the cannon fires, i release the propane and air into the chamber (pre mixed), and before the ball even starts moving (in theory) the mix will ignite boosting the golf ball out the barrel. How about that?
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Unread postAuthor: Technician1002 » Thu Jul 15, 2010 12:15 am

If high speed purge vent fuel is desired, the simplest way is to pre stage the air in one meter pipe and fuel in another meter pipe, with each pipe proportioned in size to the charge needed. For example air at 15 PSI in a meter the same volume as the chamber will vent the chamber when released and metered shot of fuel dispensed together mixing on delivery.

The meter pressure can be increased to use smaller components and higher pressure delivery into the chamber for a quick shot cycle time.
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Unread postAuthor: Conrad2468 » Thu Jul 15, 2010 12:26 am

why not just stick them in the same pipe?
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Unread postAuthor: c11man » Thu Jul 15, 2010 10:20 am

because that is a explosive mix
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Unread postAuthor: Technician1002 » Thu Jul 15, 2010 10:56 am

It also defeats the metering. When you fill it, it is no longer metered by volume. With 2 volumes, the air and propane are both measured in proportion for the correct mix when delivered.

A single pipe will meter a charge into the chamber, but the meter may have a very poor proportion because it was not a metered air and propane volumes.
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Unread postAuthor: Conrad2468 » Thu Jul 15, 2010 7:05 pm

Can i not just go off of time? like open the propane solenoid for a couple seconds then open the air for a couple seconds or both?
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Unread postAuthor: Technician1002 » Thu Jul 15, 2010 10:16 pm

Conrad2468 wrote:Can i not just go off of time? like open the propane solenoid for a couple seconds then open the air for a couple seconds or both?


You can do timed releases of both gasses with a known flow rate to deliver a known volume of both.

A soda fountain at the local convenience store does this with liquid to dispense carbonated water and syrup. They both dispense at the same time and mix on delivery. Duration is just long enough to fill the cup. The dispense rate is 6X faster for the carbonated water for 6 parts water to 1 part syrup.

A timer can be set to fill the cup to the top every time. Many fast food places do this. Set down the cup and push a button for small, medium or large, and the cycle time fill the appropriate size cup. Delivery pressure is regulated.
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Unread postAuthor: Conrad2468 » Thu Jul 15, 2010 11:08 pm

everything is going to be regulated. so i hope this will work!
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