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On Flow-Based Fuel Metering for Hybrids/Combustions

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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On Flow-Based Fuel Metering for Hybrids/Combustions

Unread postAuthor: mark.f » Mon May 26, 2008 12:41 pm

I put this in the hybrid section, but the principle could also be used for atmospheric combustion launchers as well.

Basically, the way I see it, there are currently two types of fuel meters in operation: volumetric (traditional meters), and manometric (pressure-based) fuel meters like those successfully used by DYI. Both have their advantages and disadvantages, and this isn't a thread about those, so let's just forget about those for now.

I am interested in a flow-based fuel metering system. It would work off the principle of adjusting air flow and propane flow into an evacuated chamber, to achieve an ignitable mixture.

There are a few questions I need to answer which are hindering the progress of such an idea.

Obviously, as the chamber fills, pressure across the flow-control device changes. My question is how does flow rate change in respect to pressure differential across a flow-control device? For this thread's purpose, I will simplify the flow-control device to a simple orifice. Does the flow change linearly, quadratically, cubicly, logarithmically, etc., in respect to pressure across the orifice? If so, would this mean that if the pressure changed at the same rate across both the air and propane orifices, would the proportions of fuel still be correct?

This is sort of a complicated question, but I guess I could simplify it to the following question: how much of a difference is there between the flow of air and the flow of propane, and will it make a significant difference in fueling? If you had to gases near the same specific gravity, then the pressure change would not affect the proportions, because the flow would change at the same rate proportional to the pressure, but propane has a different specific gravity than air... would it make that much of a difference in fueling?

If the change in pressure as the chamber fills does not throw off the fueling proportions, I would buy two flow regulators from Home Depot. These control flow by adjusting orifice size through the device, and have a pressure gauge connected to the device. I'd then remove the pressure gauge and add a flow-level to measure flow rate at a fixed pressure to atmospheric pressure. After calibrated to the correct flow-rate at a certain pressure, the propane and air inputs would be regulated to a certain max. pressure and connected through these flow devices into the test chamber. The chamber is evacuated using a hand vacuum pump like those sold at AutoZone, and then filled to the desire mixture pressure by opening the valves for air and propane at the same time (this can be done electronically or by using a synchronization rod of some sort on later models).

Thanks for your time in reading this thread, and I hope I can get some answers out of it.
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Unread postAuthor: jimmy101 » Mon May 26, 2008 1:43 pm

markfh11q

I think you probably can get this to work. It might be a bit fiddly but should be possible.

Are you thinking that both gas sources are regulated to the same pressure? Then controlling the mixture by the relative orifice areas? It might be tough to get the orifices correct since the ratio of air to fuel is so large. Using a "zeroth order" model, the air orifice has 25x the area of the propane orifice. There will be an affect caused by the differences in the molecular weight and density of the gases.
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Unread postAuthor: D_Hall » Mon May 26, 2008 2:00 pm

I'm not sure I follow what you're proposing but I will say that there's an 8X hybrid where I work that basically functions thus:

1) Air pressure and propane pressure regulators are set to identical output pressures.

2) Each regulator dumps into a small plenum (two plenums, one for each gas).

3) Each of those plenums dump into a third small plenum. The air is dumped through a fixed oriface. The propane is dumped through a needle valve (the needle valve allows for adjustment of the fuel to air ratio).

4) The gases from the third plenum go to the chamber in a pre-mixed, stoichiometric state.
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