Liquid fueling.

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Chadly33
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Liquid fueling.

Post by Chadly33 » Sat Aug 23, 2008 12:39 am

I was still having trouble with the liquid fueling of my Go-kart despite having almost having 6 feet of 3/8 stainless and braided line between the bottle and the engine. I know a mixture screw may help this by offering finer tuning rather than taking it straight off the bottle, but I would lose the initial pressure to induce enough air for starting. What I was thinking was putting an expansion chamber in after the bottle to allow the gas some more area before running through the line to the pre-heat wrap around the exhaust. I was wondering if anyone has done this and to what success?
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larry cottrill
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Re: Liquid fueling.

Post by larry cottrill » Mon Aug 25, 2008 12:14 pm

Just a couple of things:

You should make sure that the liquid pipe into the expansion chamber is the tightest restriction in the system. If you don't, there is the possibility that the tank will pressurize under high flow conditions, there will be no expansion, and you'll be inadvertently delivering liquid downstream. As you can imagine, the liquid flow in will be much slower than the vapor flow out, even though the mass flow is the same, so a somewhat smaller line for the liquid is quite acceptable.

You need to make sure that the expansion tank and vapor tube out to the engine don't form a resonant system at the engine operating frequency. This will be completely different from the frequency they have when empty (meaning, full of air), because the density of propane is very different from that of normal air. Of course, I have no idea how to accomplish this, other than careful observation as it's running -- but, the chance of this happening are not great, it's just something that has come in discussions as a potential problem. I think it was suggested that rubber hose tended to damp wave action while metal tubing allows pressure waves to propagate along the line, helping to create the problem.

You must keep in mind that any valving you do on the liquid side will have a delayed action on the vapor side, because of vaporization and expansion into the tank, and because the tank will have some positive pressure (though much less than the propane cylinder pressure, of course). The smaller the line from the tank to the engine is, the more noticeable this will be -- another reason to make the line to the engine significantly larger than the line from the propane supply. Remember, too, that it isn't just a matter of the size of the piping -- if your stinger is a fairly restrictive, high velocity nozzle, you WILL have significant pressure in the line; if it's very "open ended", you'll have little more than ambient pressure in the line. In any case, you will not have the "instantaneous" throttle action with the chamber in place as you would have with a straight pipe from valve to stinger, because you've introduce a "capacitance" into the "circuit".

If you are throttling at the propane cylinder (and assuming your fuel system is adequately sized to actually flame out your engine at high flow), your chamber will do nothing at low (throttled) flow -- the vaporization will take place right at the throttle valve, and the whole system beyond that point will be at low pressure. That valve will get mighty cold at low throttle settings, because practically all the expansion will take place right there. At moderate to high flow, the expansion chamber will be the "cold coil" in your system.

You should have some fun verifying or refuting my predictions as you test this scheme -- believe it or not, propane has actually been used as a refrigerant in some air conditioning systems!

L Cottrill

Eric
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Re: Liquid fueling.

Post by Eric » Mon Aug 25, 2008 5:40 pm

A tank is a bad idea, the bigger the tank, the more throttle lag you will have, and if it did fill with liquid the engine could keep running for some time, or spewing flames for some time after the engine quits.

A 1/16" ID fuel tube flowing liquid propane would be the equivalent of a 1/2" ID tube running vapor.

Make a restriction in the small diameter liquid fuel line, then have it expand out into a larger diameter fuel line which then gets sent to a preheat coil. The more you preheat the fuel the higher the effective resistance of the nozzle will be, so you have to make the injector bigger. Also the more you preheat the fuel the more work you can make it do once its injected.

Eric
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Graham C. Williams
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Re: Liquid fueling.

Post by Graham C. Williams » Tue Aug 26, 2008 10:01 am

Eric wrote: Make a restriction in the small diameter liquid fuel line, then have it expand out into a larger diameter fuel line which then gets sent to a preheat coil.
Eric
Dear Eric.
Expand the fuel Gently or Rapidly, similar to how a fridge works, before adding the heat?

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Chadly33
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Re: Liquid fueling.

Post by Chadly33 » Fri Aug 29, 2008 8:45 pm

Shouldnt have a problem with the gas running on as the shutoff valve is located just before the fuel line goes to the preheat line so all throttling can be done there. That way if the engine does flame out or anything goes wrong it can be shut down and the remaining liquid gas can be released once the bottle has been shut off and everything is safe to do so. I will make the chamber smaller though so as to have less waste. I think the biggest problem will be tuning the amount of liquid that goes in as to the amount gas that comes out. Might have to put a drink holder on the chamber to keep my beer cold! :D
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Ghrey
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Re: Liquid fueling.

Post by Ghrey » Sat Aug 30, 2008 7:25 am

Frozen Beer Hmmmm......
In the process of moving, from the glorified phone booth we had to the house we have.

No real time to work on jets, more space, no time.

Life still complicated.

larry cottrill
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Re: Liquid fueling.

Post by larry cottrill » Tue Sep 02, 2008 12:14 pm

Chadly33 wrote:Shouldnt have a problem with the gas running on as the shutoff valve is located just before the fuel line goes to the preheat line so all throttling can be done there.
Chadly -

Of course you're keeping in mind that if a shutoff is placed downstream of the cylindrical chamber, it will need sufficient strength to take full propane cylinder pressure. All the piping, too.

Yeah, OK -- I knew you'd think of that.

L Cottrill

Chadly33
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Re: Liquid fueling.

Post by Chadly33 » Wed Sep 03, 2008 8:10 am

LOL! of course I have larry, My fueling system comprises of solid stainless lines and stainless braided hose! all aircraft grade of course even the valves, joiners, fittings etc. As I have said before"I love working in the aviation industry!" Hehehehe! :twisted: We do full rebuilds so everything cost me say, oh... nothing! :D
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Zippiot
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Re: Liquid fueling.

Post by Zippiot » Sat Sep 06, 2008 3:13 am

What ever happened to inflating rubber surgical tubing inside a bottle?
The stuff I bought for 5 bux per 15 feet is amazing inflates around 25 psi. Find a nalgene bottle and fit sum vales to it; a really easy way to refill is to build a pvp pipe that can hold the tanks max and attatch a bike valve to the top and a way to connect to the tank to be filled. A simple bike pump pressurizes the fuel and forces it into the tubing tank. Pressure is pretty consistant but no guarentees.......
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Mr.B
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Re: Liquid fueling.

Post by Mr.B » Tue Oct 21, 2008 9:24 am

My large pulse-jet runs on liquid propane and runs pretty stable once the preheater coil has warmed up.
I have the throttle located just before the pre-heat/evaporater coil. Putting in a pre expander defies the purpose
of the evaporator, It will freeze and you'll end up trying to run your engine on low pressure, cold propane.
What is important to know is that your pressure behind the evaporator will be about the same as in your bottle!
If you put, say 20Bars of nitrogen inside your propane tank (above the liquid propane) and you have enough heat in your evaporator,the gas pressure after your evaporator will be 20 Bars!?? Think about this one. In my system the bottle-neck is in the bottle and ofcourse the nozzle. Seems to work well. Starts up on gas by opening the bottle a little (start setting), then as the throat of the jet turns white from heat it becomes throttleable and gets very aggressive, even a little scary... The bottle never freezes and you can run it completely empty.

larry cottrill
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Re: Liquid fueling.

Post by larry cottrill » Tue Oct 21, 2008 12:35 pm

Mr.B wrote:In my system the bottle-neck is in the bottle and ofcourse the nozzle. Seems to work well. Starts up on gas by opening the bottle a little (start setting), then as the throat of the jet turns white from heat it becomes throttleable and gets very aggressive, even a little scary... The bottle never freezes and you can run it completely empty.
I have always thought that evaporating the propane OUTSIDE the cylinder was the way to go, in terms of minimizing supply chilling and maximizing the propane you can use, and you seem to be confirming this. It is exactly like mechanical refrigeration (and in fact, propane has been used as a refrigerant!): the heat absorption takes place at the point of greatest pressure drop. That's why a refrigeration loop has a compressor on one side of the loop and a TINY tube (called the "capillary tube") on the other side. The refrigerant on the output side of the compressor is liquid and hot; the refrigerant coming out of the little tube is vapor and cold. The mass flow is always the same (so, the flow speed is quite low in the liquid side (the "condenser") and fairly high in the vapor side (the "evaporator").

In a practical system, the REAL bottleneck shifts around as you do "throttling". Say you have the tank valve, then a needle valve (your "throttle"), and then a pinched fuel pipe (like a Rosscojector, for example), and say you run with the cylinder "inverted" for liquid feed. At a very low throttle valve setting, you will have all the pressure drop at the throttle, and the hose coming out of the valve will contain cold vapor. Liquid turns to vapor at the needle valve. As you open the needle valve, you reach a point where the injector is no longer "free flowing" but starts to become a restriction. At that point, there is an initial pressure drop at the needle valve and a secondary pressure drop at the nose of the injector; there is still vapor in between, but much warmer. If you open the valve enough, there will be liquid reaching the injector, with evaporation occurring after the liquid sprays out (i.e. a high pressure drop) into the chamber. Of course, if you throw in an evaporator coil around the chamber or some such on the way to the injector, that adds to the complexity of what happens.

On the other hand, if the valve were large and the injector were an extremely "open" design and the flow became high enough, the valve at the cylinder would become the bottleneck and all evaporation would happen there, and you'd get quite a bit of chilling at the cylinder. If you use the cylinder "right side up" (vapor feed) all the evaporation usually takes place at the surface of the propane within, and you have very effective chilling of the cylinder contents, with extremely poor utilization of the fuel as its vapor pressure quickly drops.

One thing that would usually help would be to put "cooling fins" on the throttle valve or any other designed "pinch point". That would keep heat exchanges localized; for example, lessening the heat "drain" on the cylinder by the valve. I remember seeing pictures of Victor CO2 cylinder regulators that had large cooling fins to prevent internal icing of the CO2 as it flows past the valve seat inside.

L Cottrill

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