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Fuel injector

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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Fuel injector

Unread postAuthor: daccel » Tue Jun 30, 2009 1:19 am

Working on the injector part of a liquid fueling system now. The plan is to use a spool valve to meter a very small amount of liquid. Reservoir air will then blow this into the chamber as well as additional air through one or more separate lines.

This second line will help to fully atomize any fuel that doesn't as it enters the chamber. Built a quick test piece to try the idea out. Water comes out the bottom tube and air the top. But I think there may have been a bit of water in the top one because I didn't make them separate enough.

This is at 300psi, with an epoxy/hot glue nozzle cast around .014" piano wire (subsequently removed).

[youtube]http://www.youtube.com/watch?v=Oa6rHgd_OTA[/youtube]

I thought about using a nozzle off a spray paint can, but that would be difficult to place in my small chamber, and the plastic might melt over time.

Thoughts?
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Unread postAuthor: D_Hall » Tue Jun 30, 2009 8:38 am

I used to do a fair bit of fire supression tests for the military. Obviously, to put out a fire you must first HAVE a fire. Suffice to say that atomized jet fuel was very important to us.

Our mechanism of choice?

Nozzles for paint sprayers. Not those off a spray paint can; those designed for actual professional paint sprayers. Cheap. Professionally designed to atomize liquids. Available in a WIDE variety of capabilities. Very difficult to beat.
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Unread postAuthor: daccel » Tue Jun 30, 2009 9:11 am

HVLP or airless? Airless needs more pressure than I have on tap to properly atomize, although come to think of it I've never used those fine finish ones with a smaller orifice (usually use .017, they go down to .008), maybe they would work at lower pressure. And HVLP would probably be too big to fit the small chamber. But maybe I could make some sort of extension nozzle that could connect to it.

Good suggestion though, I will put some thought into adapting those.

Edit: mixed up terms..what i get for posting without morning coffee.
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Unread postAuthor: jimmy101 » Tue Jun 30, 2009 11:30 am

I wonder if instead of just using the paint nozzle if it would be better to go ahead and pretty much use the whole sprayer setup. Obviously not the full size sprayer, but all the basic components; A metered fuel resevoir (which is going to be tiny, perphaps just a couple inches of small id pipe), a valve between the metered fuel resevoir and the bulk fuel resevoir, an atomizer/aspirator tip (basically the head of an air brush or spray gun), the main air supply and it's control valve. The "paint" is your fuel, the pressurizing air is the spray gun's air supply. You are "painting" the inside of the chamber with fuel instead of simply injecting it.

In the upper layout you have a basic paint sprayer with a modified "paint" injection line. The modified line consist of a pair of valves and a meter pipe. You fill the "Meter pipe", then close the "fuel meter fill valve" and set the "Fuel injection valve" from "waste" to the "Mixer/ Atomizer". When the main "Air injection valve" is opened you are injecting your pressurizing air and sucking / atomizing / boiling the fuel out of the "Meter pipe". The "Mixer / Atomizer" should act as an aspirator and lower the pressure in the the "Meter Pipe" significantly, that'll help suck the fuel out of the "Meter Pipe" and also helps vaporize/boil it.

The second drawing is a simpler system where the amount of fuel injected is controlled by how long the "Fuel meter control valve" is opened. Simpler plumbing and fewer valves but you would probably need an electrically operated valve and some type of timer to control how much fuel is injected. And, the injection valve would have to synchonized with the "Air injection valve". In this setup, when the "Air injection control valve" is opened a timer is started which opens the "Fuel meter fill valve" (I should have labeled the valve the "Fuel Injection Vavle" or sometheing :} ). When the timer times out the valve is closed. When the chamber reaches the targeted total pressure the "Air injection control valve" is closed. This setup is kind of like an automotive fuel injection system though it uses an aspirator to move the fuel instead of a pressurized fuel line.

You have to make darn sure though that there is now way that the gun can be fired when the fuel supply valve is opened. You don't want the chamber flame propagating back into the main liquid fuel resevoir. A very small ID (like less than 0.05") fuel supply line will act as a flashback arrestor but I wouldn't count on it (though it is a nice backup just in case the other ignition interlock system fails). A true flashback arrestor between the "Fuel meter fill valve" and liquid fuel resevoir might also be a good idea.
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Unread postAuthor: Technician1002 » Tue Jun 30, 2009 11:32 am

I have toyed with hitting the junkyard and trying an automobile fuel injector. Spray, adjustable precise metered shot, atomized, What more could you ask for? Other than an electronics timer/driver to operate it. :D

Automobile injectors seem to come in two varieties. Peak and hold,
These type of injectors and drivers may also be called current sensing or current limiting. They are more expensive and complex than saturated circuit drivers, and are not generally used with domestic production ECUs. They are primarily used in aftermarket high performance systems. Most high flow injectors are low resistance (2-5 ohms) and use a peak and hold driver to activate them. The Peak current is the amount required to quickly jolt the injector open, and then the lower Hold current rating is used to keep it open for as long as the ECU commands. These require the extra kick from the higher current to keep the opening and closing time of the injector stable at the higher fuel flow rate. With this type of driver, 12 volts is still delivered to the injector, but due to the its low resistance, the current in the driver circuit is high. How high? Using Ohms’s Law we can calculate the current rating (12v/2 ohms = 6 amps). This is substantial current flow and a Saturated Injector cannot handle it. The drivers also come in two values; 4 amp peak/1 amp hold, and 2 amp peak/0.5 amp hold.. Even though 6 amps may be available to operate the injector, the maximum it is allowed to reach is 2 or 4 amps, depending on the driver’s current limit.


And Saturated.
Most domestic OE production EFI systems use an ECU with 12 volt Saturated Circuit drivers. These are very inexpensive, simple, and reliable. This type of driver works by supplying 12 volts to the injectors and the ECU turns it on and off to establish a fuel injector pulse. In general, if an injector has a high resistance specification (12-16 ohms) the ECU uses a 12 volt saturated circuit driver to control it. This means that the current flow in the driver and injector circuit stays low keeping the components nice and cool for long life. Conversely, a downfall of a Saturated Circuit driver is that it has a slower response time (and closing time) than a peak and hold type. This slower time can somewhat decrease the usable operating range of the injector energized by this driver. An injector operating on a saturated circuit driver typically has a reaction time of 2 milliseconds while a peak and hold driver typically responds in 1.5 ms.


Injector drive info gleaned from here;
http://www.robietherobot.com/Storm/fuelinjectorguide.htm
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Unread postAuthor: daccel » Wed Jul 01, 2009 12:52 am

Jimmy, my plan was basically what you had in the first diagram. The bolt action will have a spool valve inline so that when you fully open the bolt it vents the chamber, move it half way closed, fuel is metered into a hollow or recess in the piston of the valve, fully closed and air is injected. Using pressure feed instead of siphon feed will ensure that all the fuel makes it into the chamber.

Air will be injected above chamber pressure with a regulator on the chamber to cut flow once the target pressure is reached, ie. inject at 400 psi but only bring it up to 100. This should help with atomization and mixing.

Might pick up a cheap spray gun to use. The challenge is integrating it with size and layout restrictions. But maybe I could separate the tip from valve somehow and use the handle as the handle on the cannon.

On the flashback issue, I can understand not wanting high pressure to transfer back to the fuel reservoir which may be lower strength, but if there is no air, then I don't understand how there is a risk of the reservoir igniting? At any rate, the valve won't ever be open to both the chamber and reservoir at the same time.

Tech, auto fuel injectors did cross my mind, but just didn't want to deal with electronics. Though I suppose I could just use the tip without the valve.
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Unread postAuthor: jimmy101 » Wed Jul 01, 2009 1:08 pm

daccel wrote:On the flashback issue, I can understand not wanting high pressure to transfer back to the fuel reservoir which may be lower strength, but if there is no air, then I don't understand how there is a risk of the reservoir igniting? At any rate, the valve won't ever be open to both the chamber and reservoir at the same time.

I figured that the fuel resevoir wouldn't be collapsable as fuel is withdrawn. If the resevoir sucks in air to replace the volume of the fuel removed then there is air in the resevoir and combustion is possible.
Might pick up a cheap spray gun to use. The challenge is integrating it with size and layout restrictions. But maybe I could separate the tip from valve somehow and use the handle as the handle on the cannon.

You might try a small diameter compression T in place of the guts of the paint gun. You can always add a flow restrictor to the paint/fuel leg of the T.

The drawing below is an edited mechanical drawing of a 1/4 compression T (for a selector for a semi auto BB gun, never worked quite right). Ignore the spring, moving selector, BBs etc. I think the compression T might be useable as the spray mixer. Flip it over and have the fuel ("paint") be sucked in where the BB magazine is. Cheaper than gutting a real sprayer, perhaps more flexible in how it is incorporated into your design.
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