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New fuel/air injection method?

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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New fuel/air injection method?

Unread postAuthor: whoa044 » Thu Sep 23, 2010 10:44 pm

Has anybody thought of a design in which we don't use one meter pipe (for fuel), but we use two meter pipes? One for air, and one for fuel. The fuel meter pipe will be the same as any ordinary one. Let's say we are using propane. A (near) perfect fuel/air ratio would be 4 percent. Or 1:25

Meter pipes we could theoretically use for a 100ci chamber: (sch 40)
1in (ID) x 3in pipe has 1.72 cubic inches (fuel) @ 35 psi
1.5in (nominal size) x 6in long pipe has 12 cubic inches (air) @ 120 psi
The fuel meter will inject 4.2 ci each injection.
The air meter pipe will produce 100 cubic inches of air.


Let's say you use 240psi in a 6ci meter pipe (1.5 x 3in), you can achieve a maximum of a 16x mix, until chamber pressure is equal to meter pressure.
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Last edited by whoa044 on Wed Sep 29, 2010 3:49 pm, edited 2 times in total.

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Unread postAuthor: Lockednloaded » Thu Sep 23, 2010 10:47 pm

how is that different from the tank on the compressor or the bike pump? its pretty much just a portable tank
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Unread postAuthor: whoa044 » Thu Sep 23, 2010 10:54 pm

It's different in the sense that you don't need to stop pumping at a certian pressure, or that you need a pressure guage to tell when to stop filling from a compressor.

This makes it a lot easier to go up in mix levels.
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Unread postAuthor: D_Hall » Thu Sep 23, 2010 11:00 pm

To answer the original question: Yes, it's been thought of before. If by nobody else, by myself. One of the earlier concepts for Vera included such a set up. From my perspective the advantage was avoiding the requirement for remote instrumentation since all gas measurement could be done before the test site was evacuated.

whoa044 wrote:It's different in the sense that you don't need to stop pumping at a certian pressure, or that you need a pressure guage to tell when to stop filling from a compressor.


Except you do need to stop pumping at a certain pressure. You said it yourself... You're assuming that your air metering pipe is filled at 120 psi. Not many compressors stop *AT* 120 psi.
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Unread postAuthor: whoa044 » Thu Sep 23, 2010 11:35 pm

I meant as if you wanted 120psi, you would need to regulate the pressure somehow down to 120, or stop filling (closing the valve) at a certain pressure.

I think this method would be the most efficient with a high pressure paintball tank, a 70ci 4500 psi tank. If the gun is made to stay in one place, there is no question that compressors make the easiest job. But don't forget you can get up to 600 from the tank's regulator
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Unread postAuthor: Technician1002 » Thu Sep 23, 2010 11:59 pm

I have considered drilling metering orifices for proportional flow rates of both air and fuel. It is based on the same way a soft drink soda fountain works where the syrup and carbonated water are delivered and mixed on delivery at the nozzle.

A variant would be two metering chambers that would be pressurized to the same pressure, but volumetrically proportional to the mix. The two gases would be delivered through one orifice so as the mix is delivered, the expansion in both metering chambers would deliver each component in proportion as the system pressure is drawn down. This could provide rapid multi shot capacity. After the burst both metering chambers would be refilled to the same pressure. A pair of check valves would prevent cross contamination of the chamber contents. An error in pressure balance would soon equalize after the first dispense until both check valves opened.

A variation of the first would be an airline feeding a metering orifice. A tracking pressure regulator set to zero (tracking the air pressure) would dispense fuel through another metering orifice at the air supply pressure. This is another mix on dispense metering variant. It would be good for hybrids as it could deliver the proper mix regardless of the chamber pressure as long as it was below the component delivery pressures.
The regulator would work a lot like a car fuel injection system where the fuel rail is regulated at a fixed pressure above the manifold pressure.

These metering systems would not require venting the chamber between shots as the new charge of air and fuel displaces the spent gas. The spent gas may dilute the mix, but not change the ratio. This is much how a 2 stroke gas engine operates. The exhaust cycle and intake happen at the same stroke. Fuel air mix displaces the spent exhaust. In a 2 stoke the exhaust tuning helps scavenge the chamber of spent gas, but that is not significant in spudding.

Some day I may actually build a combustion cannon.
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Unread postAuthor: D_Hall » Fri Sep 24, 2010 8:54 am

whoa044 wrote:I meant as if you wanted 120psi, you would need to regulate the pressure somehow down to 120, or stop filling (closing the valve) at a certain pressure.

I think this method would be the most efficient with a high pressure paintball tank, a 70ci 4500 psi tank. If the gun is made to stay in one place, there is no question that compressors make the easiest job. But don't forget you can get up to 600 from the tank's regulator


Then the question goes right back to "what have you accomplished?"

You fill your meter pipe to 120 psi.

You fill your combustion chamber to 50 psi.

Why is the first any easier than the second? It isn't.
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Unread postAuthor: POLAND_SPUD » Fri Sep 24, 2010 9:03 am

I've got a better question (sorry for hijacking this topic :wink: )...

DYI and spudblaster15 have been using manometric fuelling for some time but they claim they it doesn't work right with small volume chambers

Any idea why ?
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Unread postAuthor: D_Hall » Fri Sep 24, 2010 9:05 am

Technician1002 wrote:I have considered drilling metering orifices for proportional flow rates of both air and fuel. It is based on the same way a soft drink soda fountain works where the syrup and carbonated water are delivered and mixed on delivery at the nozzle.

It doesn't even have to be that difficult.

I've done metering with two similar sized orifaces running different pressures to deliver the correct fuel/oxygen mixture. Very easy to do.

Admittedly, in my system it was for an ambient pressure system so that made things easier but it's still a thought.
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Unread postAuthor: whoa044 » Fri Sep 24, 2010 3:14 pm

If the chamber is so small, you have to be very accurate with how much volume the meter pipes dispense. And you cant use a calculator, because the calculators don't take into account of the volume of the meter pipe.

Let's say the chamber is 50ci already at 75 psi, you want to go up another mix. You fill the 10ci meter pipe to 90 psi, and open the valve. The pressure equalizes between the meter pipe and the chamber. Leaving approx 80psi in each chamber. A way to solve this problem is to use some sort of plunger, forcing all the air from the meter, to the chamber
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Unread postAuthor: Fnord » Fri Sep 24, 2010 4:12 pm

A way to solve this problem is to use some sort of plunger, forcing all the air from the meter, to the chamber


JSR just uses syringes to meter small amounts of fuel. I don't think you can get more accuracy than that for the same amount of money.
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Unread postAuthor: whoa044 » Fri Sep 24, 2010 4:15 pm

Never thought of syringes. Maybe a large volume syringe, with a flexible tube leading from a hole in the side towards the back for fuel injection, and a hose at the "needle" going into the chamber.

Would have to account for tube volume, if you really want to get accurate.
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Unread postAuthor: SpudBlaster15 » Fri Sep 24, 2010 11:35 pm

I'm wondering why anyone would think that dealing with a secondary air metering tank and the P<sub>1</sub>V<sub>1</sub> = P<sub>2</sub>V<sub>2</sub> equation is somehow easier and/or more simple than directly filling a fixed volume chamber. Separate volumetric meters are susceptible to the issues of pressure equalization and gas diffusion, so it's likely that you would be making your fueling less accurate.

I have a secondary tank on my hybrid, but using it to meter the air charge would be ridiculous. It's designed only for seating the piston, filling the pilot chamber, and venting the meter pipe of residual fuel.

DYI and spudblaster15 have been using manometric fuelling for some time but they claim they it doesn't work right with small volume chambers

Any idea why ?


I'm not entirely certain why my design failed, and I didn't put much effort into diagnosing the problem, since my integrated volumetric metering system works so well with identical chambers.

It was probably some combination of residual volumes in hoses, poor air/fuel mixing, and the questionable accuracy of the inexpensive pressure gauge used in the setup.

I won't be attempting it again until I begin to pursue very large, high mix designs, at which point trapped volume metering will no longer be practical. Until then, I maintain that the air through meter setup is the best for most applications.
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Unread postAuthor: POLAND_SPUD » Sun Sep 26, 2010 8:17 pm

I'm wondering why anyone would think that dealing with a secondary air metering tank and the P1V1 = P2V2 equation is somehow easier and/or more simple than directly filling a fixed volume chamber. Separate volumetric meters are susceptible to the issues of pressure equalization and gas diffusion, so it's likely that you would be making your fuelling less accurate.

Dude, you've just said that volumetric metering sucks :wink:
Sure, the increase in pressure is minimal for low mixes... but that's a technical problem
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Unread postAuthor: SpudBlaster15 » Sun Sep 26, 2010 10:10 pm

Notice how I said seperate volumetric meters. The standard volumetric metering system design is notoriously issue prone when used on hybrids. That's why I created the air through meter design.
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