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I know it's a long long.. looong post, but I hope you read it through before asking "how it works" and "what it does"..
Some of you know me and many does not, but I have been member of spudgun forums for over 10 years. I have used RDX name earlier here and also in spudtech and others that don't even exist anymore.. So hello everyone
I had a problem when building a large piston valved hybrid that needed to be fueled fast and with a "push of a button", without too complicated electrical stuff or programming. First I did try manometeric fueling but I had problems with low mixes, like the 4x mix that I had been using. Most problems I had was because of large chamber and low mix = I didn't get the air and propane to mix well.
I did not want to put chamber fan inside, I don't trust them to work long time in that use
Then I searched for gas mixing devices and found thing called "Static gas mixer". It's a tube filled with helical elements that mix two gases when they flow through the tube. I was like.. that's what i need to use ! ...But then I ran in to next problem, how to fill low pressure propane and relatively high pressure air at the same time and make sure that the air pressure doesn't fill the chamber before all propane is fed in ? ..only suitable thing that came to mind was to force that propane in the chamber with higher pressure, doing it by pump, little bit like syringe fueling.
Many weeks after and I had a "Hybrid gas meter pump / static mixer" setup ready test.. and BANG..it worked the first time !
I think this is not best suitable for small hybrids (because it needs lot of space), but it's very well suitable for larger hybrids like mine that have problems with mixing the gases.
I wanted to get this here fast, so a little of MS Paint and that's it
It works so that you only have to fill it with air and it "loads" simultaneously air and propane in chamber through the static mixing tube. Then you remove the air source (or vent the pressure) from the pneumatic cylinder inlet and it automatically fills the pump again with propane and cylinder moves back to normal position. The flow adjusting valve is there for adjusting the airflow to be in proportion to propane flow so that both are fed same time in the mixing tube. Propane does not need any other valves than the check valve and the poppet valve.
You should also be sure to have some kind of safety valve at the low pressure side, if the isolation valve leaks and flame ignites the mix in the static mixer ! Or you could use flame arrester, but those are not cheap. I used a 100bar rated high quality stainless steel valve with PTFE seals for isolation and it works well.
Pressures mentioned in the picture are of course only for my setup and mix, you would have to calculate different pressures for yours. Selecting right size cylinder is same as selecting right size of metering pipe.. know your volume and pressure and then few calculations and that's it. (pneumatic cylinder can be also replaced with any kind of pump tube, even home made one).
And the real setup.. (sorry for the bad picture)
There you can see the pneumatic cylinder that is used as a propane meter and pump.
All feed back and questions are welcome If you have ideas for improvement that's even better
Ah, now I know what I've been looking at in that Spudfiles whatsapp group the past few months
Really cool idea, would love to see a video of the cannon from fuelling to firing.
Few more pictures from the SF Whatsapp group.
Thanks for uploading I think I should also do a post in hybrid showcase soon.. but I said the same thing about a year ago when it was "ready" the fist time.. but I have made lot of small and big modifications to this.. for example the piston has been made three times now, earlier two got bend
EDIT.. OK.. I don't want to make this a hybrid showcase post, but here is one better picture of the cannon in it's stand that can be tilted up with a huge pneumatic cylinder.. oh and If somebody did not know it's a vortex cannon, that's why the weird barrel.
But let's keep the topic in the fueling setup, I will do showcase post later..
The "Pump" is not needed at all. Your fuel and air are both supplied under pressure. The fuel and air can be "Metered" using either metering by flow meter and timed dose cycle, or with a pair of volumetric related traditional metering tubes, fuel and air can be metered in 2 metering volumes at the same pressure, say 4 bar, then valved together through an orifice (to keep the pressure drop on dispensing valves near 0) into your chamber. This would have both the air and fuel metering tubes drop in pressure at the same rate and in so doing would dispense metered amount in proper proportion. This would eliminate the mechanical friction and resulting dispense pressure fluctuation that would result in uneven dispense doses.
The physical schematic would be a simple 4 valve and 2 metering volume arrangement. A pair of valves would operate in tandom to fill the two metering volumes to the same dose pressure, but volumetricly propertional. The first cycle is the charge cycle.
When the charge cycle is completed (Simple pressure switch to end the fill cycles), the two metering chambers go to a ready state, at which time the logic goes to fuel status "ready"
The next cycle is the first part of the fire sequence. Both dispense valves are opened together and discharge through a mixing tube. The mixing tube with it's small size ensures both metering volumes decrease in pressure together resulting in proportional volumes of fuel/air into the mixing tube and chamber. After a timed cylinder charge, the valves are closed and ignition commenses. Your dispense cycle can be varied in time to on the fly adjust the power for fine tuning for best fuel effeciency or power adjustment.
A simple timing circuit can fully automate this as needed. Charge and wait as needed. Dispense and fire.
Remember in your original design, the pump is not a positive displacement liquid pump or metering volume. You pressure will vary as well as the volume. With 2 fixed volumes, environmental factors will not change your ratio. Changes in barometric pressure, temperature, etc will change the gas mass of both air and fuel at the same time leaving the metered portions in the same ratio.
The most common method for this type of metering simply uses two dispense valves with 2 flow meters and flow regulators. A timed dispense at fixed flow rates will dispense both together in proportion. This is sometimes seen online with the idiots filling oxyacy balloons. A welding torch is adjusted for a proper flame, extinguished by smacking it on a plate, then using the adjusted torch to fill balloons. This method is very dangerous as static electricity can cause and unexpeced ignition during fulling, but the metering principal is sound.
A second metering practice is the use of carborator. Fuel is provided at slightly below atmospheric pressure (like a propane buffer carborator and pressure regulator) and when air is dispensed, it draws in the fuel in proportion in the venturi. You can buy commercial venturi's for this applicaiton. This requires only one selenoid valve for operation.
Here is a link to a Air and LPG or CNG gas venturi. http://www.lgc.hk/Products1025.html It will be needing the companion low pressure regulator to provide the needed volume of gas at low pressure to meet the flow rate of your dispense air flow.
I don't quite get what you mean that the pump is not needed in this setup, I do understand the idea of two volumeteric chamber but.. Lets say I have a 6 liters chamber, and I want a 6X mix in it.. Wouldn't that mean that I should have about 12 liter volumetric chamber to fill the hybrids chamber with air to 5bars.. If I would have 10bar compressor... Also, where I live it's cold outside, in winter it can be -20 celsius, how can I use propane in same pressure as the air if propane vapor pressure drops to about 2-3bars in cold weather.
The other two methods of flow regulation or timed fueling would work, I thought of both those methods and even asked help here for PLC controlled timed fueling, but both of those methods need some electrical and programming skills that many of us don't have. Also cost of flow regulators and flow meters is quite much more then a simple pneumatic cylinder and few check valves. Cylinder that I used costed 10 euros used.
I know my setup has it's limits also and I'm not saying that it's best for every type of cannon, but for my use in somewhat large hybrid with mixes of 4x up to 6x, I think it is a good and quite easy way to get fueling and mixing done same time.
Last edited by matti on Wed Nov 12, 2014 5:15 am, edited 2 times in total.
Vapor pressure will be a cold weather issue with a pump resulting in condensation, or low vapor pressure on delivery. Only a timed flow meter or low pressure venturi has no elevated vapor pressure in cold weather. A cold weather option is switching to higher vapor pressure fuel such as CNG.
When working with volumetric metering, you work with Absolute pressure.
If for example you want to charge a 1 volume chamber (noninalized values) to 2 atm, It has one atm already in it. If my combined total of air and fuel metering is 1 volume total, charging to 2 atm would mix the one voume in the chamber with 2 volumes in the meter into a space of 2 volumes resulting in 3 volumes of gases in 2 volumes of space resulting in a charge in the sytem of 1.5 ATM pressure. 3/2 atm.
Working backwards if you want a 2X charge, you start with 1 volume in metering and 1 volume in chamber as you begin with 1 ATM. You desire an end result of 2X in the chamber and 2X in your combined metering volume. If the chamber starts at 1X, the meter will need to contain 3ATM.
The combined metering volume is simply the Air volume added to the gas volume. For example for a 3 parts air to 1 part gas, would have an air volume of .75 volume and gas meter of .25 volume. (3/4 + 1/4)
In the above no compensation is made for the initial 1 atm of air or spent exhaust. If the chamber is vented with fresh air between shots, a 3rd meter tube or cycle can be added to dose the existing air, or the chamber can be flushed with fresh fuel/air, then charged to the desired mix without complicated compensation calculations.
Compressing Propane above it's vapor pressure in cold weather will result in condensation. Heated parts could be used.
Cycle timers are off the shelf items. No programming is required. Set the desired cycle time and you are done. They can be cascaded. For example a press of a fire butten can start the fuel valve timer. When it times out, it can toggle power to an ignition cycle which would turn on a stun gun and time out after say 1/2 second, which then drops power to the first timer causing a reset and repeat as long as the trigger is held on.
Pressure regulators and flow meters/regulators are off the shelf items. Adjusting flow can permit fine tuning of the system.
For cost containment, go used. Don't try to get an exact match flow meter. Use common gas flow meters such as Oxygen or Argon. They will be out of calibration, but using gas densities, a correction factor can be calculated. For simplicity, instead of real values say Propane is twice as dense as Argon and we are using an Argon meter. Porpane would read 2X it's actual flow, so a reading of 10 LPM would really be 5 Liters per Minute Actual. I picked up a bunch of air, Argon and Nitrogen flow meters as medical surplus very cheaply.
You are unlikely to find a Propane flow meter.
Re: New fueling system for hybrids (tested and works)
Ok I get what you mean, thanks for the clarification. I might need to make some heating system around the pump tube to make sure it doesn't condensate the propane to liquid.
I might try to build a second version of the fueling setup using a timed flowmeter or low pressure venturi.
Interesting and innovative.
Racked my brain over a smoke break on the liquid propane issue. Would agitating it work, like with the static mixing tube? Something tells me no.
Tech introduced the time dose meter idea a while back and I've kept it in mind. Idea is still far away in fantasy land, but I always thought of coupling it with an automated burst disc loader. Reason being is I think the rubber sealing components in a burst disc vs piston valve would fair better against heat damage, ultimately higher ROF.
For timing I thought of using electrically actuated valves, stun gun (cause it has a switch) and placing them around a slow moving cam shaft.
Venturis work off magic right? Like an automatic transmission. They have always scared me as a concept for some reason.
Static mixer tube, eh? I figured steel wool confined in a tube.
The whole idea of the pneumatic cylinder is to slave the fuel side off of the air side? Pretty sure I get it, but damn hard enough to conceptualized with it all explained. I can only imagine how long that idea took to stumble across.
Well cool! Thanks for posting, this makes my boring existence a little more exciting.
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