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Hybrid fueling Help/advice required

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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Hybrid fueling Help/advice required

Unread postAuthor: daniel0663 » Tue Jan 17, 2012 9:09 am

hello there,
i've made a mini hybrid based on probably the most simple design available to make, and i am still having trouble with the basics.

I'm using syringe to fuel my chamber.

I measured all my chamber volume. (used a syringe)

Image

Image

and my spark plug gap should be o.k?, gap is around .4/.5ml piezo ignitor from portable gass grill (Slighter more juice than lighter piezo)

As you can see,
Total volume (Chamber 63ml + hose DV 12.5ml)= 75.5ml.

so,
normal- (75.5*0.032)
2X- (75.5*0.032)*2, +14.7psig (1bar)
3X- (75.5*0.032)*3, +29.4psig (2bar)
4X- (75.5*0.032)*4, 44.1psig (3bar)
5X-(75.5*0.032)*5, 58.8psig (4bar)
etc.

So with those calculations,
i got prepared for a 2X combustion.

2X = 4.8ml butane

and did the steps as following. (Fresh chamber, fresh air etc)

1. fill syringe to 10ml with lighter re-fill butane

2. take away the excess fuel untill 4.8/5 roughly (Syringe only has lines of so i had to roughly estimate.)

3. Squeezed the syringe into the front of the union (At this point the rear ball valve is open)

4. Burst disk is in place and tighten the union.

5. connect pump, and pumped the onboard pump gauge to 1bar (No leaks )

6. closed rear ball valve

7. now the chamber volume is closed down to 56mls, since the ball valve is now closed, the dead volume behind the ball valve is now insignificant and is vented with the hose bleed valve.

8. Ignite.

9. No ignition.

Take consideration the actual combustion chamber is only 56ml
as i closed the ball valve after adding 1bar so the mixture doesn't escape while disconnecting the pump hose.

Am i doing something wrong?
I've been trying for 2days now, and it's starting to get on my nerve.
re-measured my chamber volume and i get the same calculations as before.

Any help would be greatly appreciated.
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Last edited by daniel0663 on Tue Jan 17, 2012 4:20 pm, edited 1 time in total.

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Unread postAuthor: DYI » Tue Jan 17, 2012 11:38 am

I don't like the sound of "tightened the union as quickly as I could" (or your poor grammar and creative mixing of tenses, but I digress... :lol: ). You'd be better off filling through the air inlet (I can't tell what your inlet valve is - if it's a Schrader, switch to a male quick connect, seeing as you already have a ball valve on there).

Other than that, the potential problems I see are ignition placement (your fuel may end up concentrated closer to the breech), leakage at the rupture diaphragm (you never specified the leak rate through the union, so I can't tell for sure), and leakage during air filling with the pump, either at the pump or at the connection to the launcher.

I'm also curious about your choice of fuel mix. 3.23% will work fine, but how did you come to that choice? Considering the displacement of air from the chamber, a 2:13 butane:oxygen ratio would require 3.13% of the chamber volume in the syringe per X. Due to two factors (loss of some butane during fueling and somewhat higher deflagration pressure for a given volume of air with slight overfueling for hydrocarbons) your chosen mix will perform better than the "correct" stoichiometric mix, but I wanted to clear up any misconceptions you may have had about the fueling calculations; the 2:13 ratio and 21% oxygen gives 3.23% of the chamber volume in butane per X. The chamber fraction of butane is thus 0.0323/1.0323 = 0.0313.
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Unread postAuthor: dewey-1 » Tue Jan 17, 2012 1:30 pm

Your "spark plug" may be the problem.Which electrode of spark gap is hooked to brass fitting?

One electrode should be ground, to the brass fitting.
The other should be centered in the diameter of the fitting.

Here is what I am referring to.
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Unread postAuthor: Technician1002 » Tue Jan 17, 2012 3:26 pm

A piezo is a low current device. Often the added capacitance of the insulator in the spark plug can be enough to reduce the voltage preventing a spark. With the higher chamber pressure you need a higher voltage to get a spark. The shunt (parallel) current through the insulator combined with the higher voltage requirement may be preventing ignition.

The single center electrode design adds more distance between the grounded shell which reduces the shunt capacitance leakage so I do recommend the center electrode design. Another way to reduce capacitance is to reduce the diameter of the electrode so it has less surface area where it goes through the insulation. Use as small of a diameter for this as practical. Sewing needles may help.

Your other option is to use a higher voltage and higher current ignition source such as an ignition coil or stun gun.
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Unread postAuthor: SpudBlaster15 » Tue Jan 17, 2012 4:13 pm

As I told you in my reply to your PM, and as DYI mentioned in the first paragraph of his post, you problem is almost certainly related to fueling losses during removal of the syringe and loading of the burst disk. Figure out a way to fuel through the air inlet valve, and you should achieve better results.

The issue is not likely related to your ignition system, as the spark plug appears to have sufficiently thick insulation, as well as reasonable electrode spacing for 2X. I have used a similar design at 10X with a relatively low voltage source (~12kV), and achieved reliable ignition with no complications.
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Unread postAuthor: Zeus » Tue Jan 17, 2012 6:37 pm

SB, the problem isn't voltage, it's source current. As Tech said, capacitance can effectively short out the spark.

A higher current source will circumvent that problem.
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Unread postAuthor: SpudBlaster15 » Tue Jan 17, 2012 8:05 pm

The capacitance of the insulator is not a significant concern here, and therefore neither is the source current. The hybrid spark plug design I have been implementing for several years uses only a few millimeters of relatively high dielectric strength insulation, and it works fine with a piezo igniter at up to 8X or so. 2X will be no problem at all for the OP's setup to handle.
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Unread postAuthor: mobile chernobyl » Tue Jan 17, 2012 8:05 pm

As Jeremy Clarkson would say, MOAR POWA!!! :)

Also I'm siding with Dewey on his refined "spark plug" proposal. Yours does appear to allow many alternate paths for stray spark breakdown-age. Maybe this is not the case... but I made sure I had no risk of that in my hybrid's "spark plug", but I also made it a little differently.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 18, 2012 12:48 am

SpudBlaster15 wrote:As I told you in my reply to your PM, and as DYI mentioned in the first paragraph of his post, you problem is almost certainly related to fueling losses during removal of the syringe and loading of the burst disk. Figure out a way to fuel through the air inlet valve, and you should achieve better results.


If that were an issue, none of my hybrids would have ever worked.

Does it work at 1x?

Dewey's diagram is a very good representation of what the "spak plug" ought to look like...
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