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Fuel Injection and Air Mixing....

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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Unread postAuthor: D_Hall » Thu May 01, 2008 9:21 pm

jimmy101 wrote:Basically, you're talking about a mixing manifold or carburator. The tricky part is getting the mix correct. In a true mixing mainifold you would regulate the pressure of both gases. Pretty darn tricky for propane + air since one is 25x the volume of the other. Small errors in pressure will kill the accuracy of the mix.

Actually, not difficult at all when dealing with gaseous fuels. Regardless, that is NOT what I'm discussing.

How to precisely control the air / propane ratio. Are you thinking of having the eductor injest say all the propane plus half the air to get good mixing then "topping off" the chamber with the rest of the air?

No, I'm putting 100% of the fuel through the eductor then 100% of the air. Again, the job of the eductor is NOT to *measure* fuel, but simply mix it. Again, from an equivelence ratio perspective, it changes absolutely nothing.

How bout this instead...

1. Evacuate the chamber down to a moderate vacuum, perhaps 0.1 ATM.

The cost of suitable vacuum pumps is part of what made me move away from the evacuation option. While it's possible that I'll have a large budget, it is in no way guarenteed.

3. Now inject air at high pressure until the chamber reaches the desired final pressure. The air could be injected simultaneously from several ports.

Agreed. But why just inject when you can inject through an eductor? That's pretty much all I'm doing.
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Unread postAuthor: Lentamentalisk » Thu May 01, 2008 11:16 pm

how big of a cannon are you thinking of making? why not just stick a ball valve near the back of the chamber, and then put a shopvac into the barrel after firing and vent like that? I dont get why you want to create a complete vacuum unless you are going to replace the air with O<sub>2</sub>
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Unread postAuthor: D_Hall » Thu May 01, 2008 11:22 pm

Lentamentalisk wrote:how big of a cannon are you thinking of making?

See other thread: Pipe Dream.
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Unread postAuthor: THUNDERLORD » Thu May 01, 2008 11:26 pm

@Lentamentalisk , when it said that DHMO affects the DNA I was interested for a DNA virus I am interested in (because at this point we just won't have enough bullets)
And sure enough DHMO will be useful in that!!! 8)

EDIT: Trust in the lord
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Unread postAuthor: Lentamentalisk » Thu May 01, 2008 11:50 pm

DHMO actually has been used in spudgun ammo before clicky
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Unread postAuthor: THUNDERLORD » Fri May 02, 2008 12:07 am

Lentamentalisk wrote:DHMO actually has been used in spudgun ammo before clicky


Yeah, I used it for ammo yesterday and it was so much fun I was late to work.

I just stuffed a plastic bag (from Giant) down the barrel and filled it full of DHMO and shot that up at a tree. What fun!!!! 8) :wink:
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Unread postAuthor: jimmy101 » Fri May 02, 2008 2:19 pm

Dave

You're right. It would take a massive vacuum pump to evacuate a chamber this big down to only a modest vacuum (say 1 PSIA). To bad, with a decent vacuum system mixing the fuel would be trivial.

The ejector (eductor, aspirator) sounds like it might work. Are you thinking of putting the ejector at one end of the chamber and piping gases from the other end of the chamber back to the eductor's intake? (As in the upper drawing)

If you are still considering a U-shaped chamber then the eductor could be plummed across the top of the legs of the U. Injection of air or propane would setup a flow around the U (which is now a donut since the tops of the U's legs are connected). A couple ball valves could be used to isolate the eductor leg from the combustion envelope.

The second drawing below might work as well. I think this is basically what you described previously. The injectors are just inlet port restrictions. They insure that the injected gas/air is at high velocity and that all the injectors are actually delivering gas into the chamber instead of just the ones closest to the main gas source line. Multiple input points means the affective diffusion distance is greatly reduced.

I wonder if you could just use a set of automotive fuel injectors as the injectors? The type of injectors used in direct injection engines and most diesels, where the injector is designed to inject directly into the cylinder (instead of into the intake manifold). One set of injectors for the propane and another for the air. Fuel injectors can be controlled pretty darn precisely. You wouldn't even need to know what the injection rate is. You're injecting so much gas, even during the propane injection, that you could just use the rise in pressure of the chamber to measure the amount of gas injected.

Turn on power to the propane injectors and wait for the chamber pressure to rise ~6 PSI. Turn off the propane injectors, turn on the air injectors and wait for the pressure to rise to ~155 PSIG. Gives you a ~10X mix. With a pair of injectors every pipe diameter (or two) along the chamber, I would think the mixing time would be pretty quick. Since direct injection fuel injectors are basically designed for use in a "10X (or higher) Hybrid" (i.e., a diesel) they shouldn't have any problems at all surving the combustion process.

Injectors are kind of pricey, $50 to $100 each? Heck, I wonder if a fuel injector could be used with a regulator and propane tank on a standard combustion? You might be able to measure fuel injected by time instead of using a meter pipe. (Hmmm, I wonder if I could pick up a cheap fuel injector from the junk yard ...)
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Unread postAuthor: D_Hall » Fri May 02, 2008 3:11 pm

jimmy101 wrote:The second drawing below might work as well. I think this is basically what you described previously. The injectors are just inlet port restrictions. They insure that the injected gas/air is at high velocity and that all the injectors are actually delivering gas into the chamber instead of just the ones closest to the main gas source line. Multiple input points means the affective diffusion distance is greatly reduced.

Now, replace your fuel injectors with eductors and you're on the same page I am.
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Unread postAuthor: Lentamentalisk » Fri May 02, 2008 7:43 pm

If you put the eductors on pipes of different lengths or at different rotational positions around the chamber, and then pointed them in random directions, you could get a ton of turbulence to mix up the fuel better than if you just ran them all in one direction.
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Unread postAuthor: D_Hall » Fri May 02, 2008 8:17 pm

Lentamentalisk wrote:If you put the eductors on pipes of different lengths or at different rotational positions around the chamber, and then pointed them in random directions, you could get a ton of turbulence to mix up the fuel better than if you just ran them all in one direction.


Bingo!

Somebody finally gets it.
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Unread postAuthor: Lentamentalisk » Fri May 02, 2008 8:22 pm

I should get a prize, maybe a video of the destruction once you make it ^_^
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Unread postAuthor: jimmy101 » Sat May 03, 2008 3:26 pm

D_Hall wrote:Now, replace your fuel injectors with eductors and you're on the same page I am.

What is the source of the makeup gas for the eductors? Are you going to put the entire eductor in the chamber and just plumb in the high pressure gas?

I think you should just fill the chamber 10% with packing peanuts and use a crane to invert the gun end-to-end a couple times.
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Unread postAuthor: D_Hall » Sat May 03, 2008 4:21 pm

jimmy101 wrote:What is the source of the makeup gas for the eductors? Are you going to put the entire eductor in the chamber and just plumb in the high pressure gas?

Yes. (although I'll most likely pull the makeup gas from the far end of the chamber)

Again, the purpose of the eductor is not to measure gas, but simply to mix it.
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