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Fuelling thoughts for Combustions/Hybrids.

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Fuelling thoughts for Combustions/Hybrids.

Unread postAuthor: SPG » Tue Feb 19, 2008 10:59 am

Following from various people saying that a precharged tank of Propane/Air on my BB Hybrid is a mad idea, I got to thinking about how to do it with two seperate tanks, but avoiding the clunky metering systems.

Anyway, here's the thoughts. You'd need two tanks, both pressurised one for your propane one for your air. On each of these is an adjustable regulator. With a bit of calculation it's easy enough to work out the pressure difference caused by the addition of the 4.2% propane. Then it'd be a matter of setting the regulators to the two corresponding pressures, and voila, instant fueling.

Obviously for a straight combustion you've only got a pressure differential of 4.2%, but as you go up the mixes in a hybrid you get 8.4%, 12.6%, 16.8%, 21% which are quite substantial differences and should be easy enough to measure.
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Unread postAuthor: jackssmirkingrevenge » Tue Feb 19, 2008 11:55 am

Looks like it would work assuming reliable and reasonably accurate regulators, and a lot safer that a premixed hybrid bomb close to your face ;)

Perhaps a couple of check valves to protect the regulators would not go amiss either.

The more ideas you throw at the auto-hybrid concept, the more plausible it sounds, I for one am optimistic that it can be done :)
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Unread postAuthor: Ragnarok » Tue Feb 19, 2008 12:58 pm

I think it will need fairly accurate regulators (the difference will only be 0.6 psi per X), and you'll need to find a way to control the propane one, or when chamber pressure falls, you'll get flow from both regulators (and a poor mix) - not the air one, then the propane one, which is what you'd need.

Perhaps I should show the forum the full plans for my auto hybrid design, as I'm unlikely to build it for some time (unless I can find a machinist that's reasonably cheap).

It's bloody complicated, and I've never drawn it up in full... but I think it's probably the single fastest automatic combustion/hybrid concept out there at the moment - because I reckon it should be able to complete a full cycle of: ignite, fire, vent, reload, reseal the valve, refuel (to 2x) then mix... in under 50 milliseconds (good for 1200 rpm with a good ignition source), and with a perfect mix every time.

The refuelling is performed by a fully mechanical set-up, and triggered by chamber pressure falling below a set limit after all combustion is complete (and the projectile has left the barrel), so there are minimal delays between firing and the start of the process.

It's all designed for 6mm airsoft pellets (although it uses a rotary minigun-esque loader), which will mean the basic principles would need little adaptation for your design.
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Unread postAuthor: SPG » Tue Feb 19, 2008 1:46 pm

I had imagined in this set up you'd need valves at the trigger as well, so it'd fill with air first, then top up with propane...

Go on show us yours, I showed you mine.
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Unread postAuthor: Ragnarok » Tue Feb 19, 2008 3:43 pm

SPG wrote:Go on show us yours, I showed you mine.

Right, a drawing could take a little while, but here's a text explanation.

The design is based around three custom spool valves.

The main spool valve is linked to the piston of a piston hybrid. On ignition, this is forced back, which "latches" the refuelling mechanism (latched by the minor spool, as it's known).

When the main valve comes forwards again and reseats, this vents the pilot of a QEV valve, which exhausts a volume of air through the chamber. Some of this is for venting, and exits through an exhaust port opened at the back by the secondary spool valve (pushed open by the air flow from the QEV). Most of the air is then trapped in the chamber by the 2ndary spool valve which closes when the chamber is at the air pre-charge pressure.

When the 2ndary spool valve is pushed open, it connects a small meter volume to the propane source. When the valve re-closes, this meter volume is then connected to the chamber - it's a bit like using a three port push button valve for metering, but controlled by pressure.

The QEV setup is refilled simultaneously by flow from - the minor spool, IIRC. The operation of the minor spool isn't really mentioned here, mostly because I can't remember exactly how it works...

It's all mechanical, and can be built relatively small. The main spool is also connected to an external "cocking" lever to manually trigger the refuelling process, in case of misfire or at the start of a shooting session - or just to look bad-ass.

It's so fast the only loader I think would work will be a minigun design, although a bolt operated by a pneumatic cylinder hasn't completely been ruled out.

I don't imagine that immediately makes much sense, but I'm here for questions until I get this drawn up.
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Unread postAuthor: Jared Haehnel » Sat Mar 01, 2008 1:03 pm

Sounds like a very nice design. Well thought out. I look forward to seeing the blue prints.


Something that can fire that fast will need to have some means of cooling or things are going to get really hot really fast and then you could have a danger of pre ignition of the fuel and air in the chamber...may be not but things would still get very hot.
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Unread postAuthor: Ragnarok » Sat Mar 01, 2008 4:06 pm

Jared Haehnel wrote:Sounds like a very nice design. Well thought out. I look forward to seeing the blue prints.

Still working on them, but it's a bit like trying to draw Gravelly Hill Interchange, which is generally better known as Spaghetti Junction (one of the many) - it's more than a little complex, but I think it's the fastest design possible which doesn't use a constant air supply like a BBMG.
It's quite a claim, I know - and one I'd very much like to be able to verify. I don't know when I'll be able to build it, it's going to be a very considerable task, and not cheap either.

Something that can fire that fast will need to have some means of cooling or things are going to get really hot really fast.

Yes, that's one of the main problems with it - my calculations show it's going to be putting out as much as an electric fan heater.

It's going to need some form of cooling, and I've had to do a large change in design to incorporate water cooling.
If I were doing a minigun, it would likely to be powered off a modified air drill, and some of that power could be directed to a drill pump. I've found one considerable which should be able to force 60 litres a minute through the system, which should be able to draw away enough heat for the job.
Truthfully, I could probably do it with much less flow than that, but hey.
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Unread postAuthor: Jared Haehnel » Mon Mar 03, 2008 8:29 am

Thats a lot of flow :)

I would think there would be a danger of the combustion cylinder or a valve or something getting hot enough to act as a glow plug even with water cooling...
I don't know what you plan on using as a trigger but the gun could "runaway" or cook of rounds even after the trigger is let go of. You'd have to have a fuel shut off. Probably that would be the best way to trigger it...

You've probably have thought of this already...but just somethings I thought of...


Like I said its a fascinating concept and I hope it doesn't cost you to terrible much to build cause I want to see it built 8)
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Unread postAuthor: Ragnarok » Mon Mar 03, 2008 1:43 pm

Well, in theory, a litre of water a second could draw away enough heat to keep the chamber walls within a few degrees of their original temperature, so I'm not hugely concerned about the design running away, but I'm always one for safety, so I will add a fuel shutoff valve just to be on the safe side - so good idea, thanks for that one.

However, running the system at full flow will possibly draw enough heat to detract from power, so I really need to find a medium between too little cooling and too much. I know propane autoignites at 480<sup>o</sup>C, and butane at 420<sup>o</sup>C (it's possible I'll use propane/butane mix, although unlikely), so obviously, I have to keep all parts safely below that.

Main triggering is probably going to be done by a design based around a flyback, with some timing arrangement based around the operation of the loader (whichever design I eventually choose - although I'm leaning back towards the pneumatically operated bolt design).
I was considering having a thermosistor mounted on the chamber, all linked to a system that would cut the main ignition in the event that the chamber began to reach unsafe temperatures - a bit like you get on video games with weapons that will stop firing if you use them for too long.

Cost might well be moderately nasty, it's going to need some bits machined and although these aren't strictly complex parts (I could easily do it myself if I had the equipment), that doesn't necessarily make them cheaper.
But, it might well be worth it to have the most awesome BBMG ever - my maths puts it at roughly 12 ft-lbs, and at least 20 rps.
It might not have the rate of fire of a vortex BBMG, but it's up for churning out at least twice as much muzzle energy per second.

I reckon I could get it to feed from basic airsoft magazines, so it would be able to use some premade parts.
In theory, it could be made to fire airgun pellets, but that's going to mean more trouble.
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Unread postAuthor: Jared Haehnel » Mon Mar 03, 2008 1:56 pm

I'm going to school for engineering too. And I can't wiat to get into some of the math and science that will let me figure all that out.

If you advertize enough you might find someone on the forum that has the equipment you might be able to get them to do it and pay them to ship it to you.

Not that I can but its a thought....
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Unread postAuthor: Skywalker » Mon Mar 03, 2008 2:25 pm

I've been toying with the idea of using a carburetor for fueling a small pump-action combustion (nerf gun possibly). The chamber would contain a piston that would pull air in thru the carburetor. I don't know whether the airflow would be great enough to actually operate something like that. Anybody on here know anything about small engines? It would obviously have to be a liquid fuel, but I think that would simplificate things alot compared to using a propane reg and a volume-metering system.
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Unread postAuthor: Jared Haehnel » Mon Mar 03, 2008 2:38 pm

Its a cool idea and if you have a small carberator like one used on the large model airplane engines it could work. Its an idea that has been suggested before.

I think it would be a fun Idea to test for an automatic design if say you had a chamber fan that drew in enough fresh air and fuel to simulate a two stroke combustion engine. As long as there was an vent for exhuast gases I wouldn't see why it wouldn't work.

You might have troubles with your fuel to air but you should be able to adjust that... the carberators use for model aircraft are not that expensive, and are very small and light wieght.

It wouldn't be worth with to attatch a large lawn mower carberator or and thing bigger since the air flow would be harder to simulate with a fan. You'd have to have a check valve to control when the air was let in since the carberator would be a sizable hole and air pressure would certinatly blow past it.

I would have to think about it to give a god answer to weather or not it would work but I say its probably worth a try.
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Unread postAuthor: Skywalker » Mon Mar 03, 2008 2:48 pm

No point in adding a fan: if you build some check valves in, the piston will act like a pump to pull fresh air into the gun. What I was curious about is whether I could get enough airspeed to pull fuel out via the venturi effect, and whether this would be consistent. I'm afraid it wouldn't be consistent, because the amount of fuel drawn out might depend on how hard you racked the piston forward. Maybe a better option would be incorporating some kind of sprayer, like from a hairspray bottle.

gads, dude, spelling! "give a god answer to weather" lol!
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Unread postAuthor: Ragnarok » Mon Mar 03, 2008 3:08 pm

Jared Haehnel wrote:If you advertize enough you might find someone on the forum that has the equipment you might be able to get them to do it and pay them to ship it to you.

That would be nice, but shipping to the UK would probably wouldn't be.

I've yet to look for a quote for the parts, as I've yet to create a full set of plans, but I imagine it will come out looking like a telephone number (well, not quite that bad).
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Unread postAuthor: Jared Haehnel » Tue Mar 04, 2008 7:11 am

Maybe a better option would be incorporating some kind of sprayer, like from a hairspray bottle.



Thats an idea or how about a fuel injector of a car? It would mean you'd have to pressurize the fuel line going into it but it would do the trick. A small tank of gas with compressed air in it would do fine for pressure.
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