Bundubasher

Skeeter,
I've done the calculation - if the fuel tank is 3" high, standing level and is un pressurized, the hydrostatic head of a full tank of fuel will be 0.09 psi. That is insignificant - fuel can siphon out slowly, but that is as much as it gets. What makes a difference is if the fuel tank and carburettor is not aligned - tank too high or too low.

The pressure is always equal in all directions in a contained vessel - the fuel tank and pipe to the muffler is a "vessel". I'm not sure what you mean by "restriction" when the tank is full.

What would you then recommend to be the best ?: a large flat tank or a narrow high fuel tank?

skeeter_ca

I'm no hydro engineer but i have learned a few things over the years, maybe some of them wrong! lol.

The restriction i talked about was the back pressure, however slight, in the submerged muffler line. It would actually take a minute amount of extra pressure from the muffler to overcome that extra .09 psi, you calculated, to clear and keep the line clear to add pressure to the tank. Come to think of it. It would take .09psi. As the level goes down it requires less pressure to keep the line clear. And more pressure would go into the tank.

The pressure would not be equal in all areas of the fuel tank. The fuel tank contains two forms of pressure. One - the pressure created by the muffler and two - the pressure created by the depth of the fuel. Example - suppose(and i'm just pulling these numbers out of my bum)the tank is pressurized by the muffler to 1psi. The top of the tank where there was only air, if you want to call it that, would be 1psi on all areas of the tank. If you go to the bottom of the fuel tank it would be 1psi plus the weight of the fuel. Understand or am i just off my rocker? I do realize that you would be talking about very small numbers and probably would not make a noticable difference to 99.999% of the people.

As for the shape of the tank. All other things being equal a large flat one would be more optimal as the fuel level would change less than a tall one. And in turn less pressure change to the carb which means less mixture change.

Hope i got this right. Please correct me if not.

skeeter

Bundubasher

Skeeter,

Thereis something called "pressure delay"... if the tube from the muffler to thetank is excessively long,very thin or pinched/kinked somewhere, then the pressure in the tank will build up/bleed offslowly - butit is due to wrong installation.With a correct installation, this pressureequalization (i.e.going from full speed to idle) is instantaneous.

The densityof 80/20 methanol/castor oilis around sg 0.83 or 6.7 lb/gal. Around 0.35 psi/vertical ft pressure gradient if I'm not mistaken.

The pressure variation due to height of fuel in the tank in rc planes are insignificant. Tanks are too small. What is critically important is correcttank installation and that the muffler that came with the engine have that "baffle plate" in.

This plateserves to disperse the sonic boom/pressure wavefrom the exhaust gas coming out of the engineport,but more important, it creates a certain amount of back pressure, sufficient to pressurize the fuel tank so the fuel can reach the carburettor ata certain pressure.

Each enginemuffleris designed around the carburrettorpressure requirement of that specific engine.The size, shape andvolumetric placement of that baffle plate in the muffler (and size/shape of muffler)isengineered to feed the carb a contant flow of fuel and at certain pressure range regardless of the level of the fuel in the tank. Had fuel levels been that important, engine manufacturers would have specified tank sizes/dimensions.

If you take the baffle plate out, the muffler cannot pressurize the tank/carburrettor, and trouble starts.Other modifications (too long extention pipe etc)may increase the pressure too much, causing too high fuel pressure to the carb.

I keep it simple. All myglow engines with their unmodified mufflers are running trouble free for many years andnever had problems with engine performance due to low fuel in thetank. These engines can run flatout until all the fuelis used, then cut out.You get specialized mufflers for engines - they are specially designedand can improve performance.

Iknowpeople who take out this baffle plate, believing the engine will run better, this is where thecircle of trouble starts.

skeeter_ca

Well Bundu,
Just to let you know i was not advocating using this "Uni-Flow system". I was merely trying to figure out how it works and the justification for using it above others. Jaka was the one saying it should be used above all. I was not i any way saying it is a better system than any others out there. I personally use just the two line system as it simple and fool proof. I have not had to use a three line system yet.

You did state however-

"The pressure variation due to height of fuel in the tank in rc planes are insignificant. Tanks are too small. What is critically important is correct tank installation and that the muffler that came with the engine have that "baffle plate" in."

So the pressure variation due to height of fuel in the tank are insignificant, but what is critically important is correct tank installation. Doesn't correct tank installation include making sure the fuel level will be at the proper height in relation to the carb? Just making sure i read you correctly.

skeeter

Bundubasher

Hi Skeeter, no problem.
You're right, the tank needs to be at the correct height with the carburettor. The way I did with mine is to install the engine/tank in such a way that the carb is the same height as the rubber bung of the tank (where the brass tubes go through),

The pressure line from the muffler goes into the tank and curl up to the highest point without it blocking off by accident. It's been working for me for close onto 40 years.

Cheers Mate

aerofly0610

I tried the great planes fuel valve.  All kinds of grit gets into it no matter what you do.  I switched everything over to the hanger 9 fuel dot.  Simple enough.  I don't do any stunts or 3ding so I don't have a need for the uni flow, but a fuel dot would work with it.

pura

My Feedback: (5)

Hi,
I came across this thread while looking for solutions to my Moki 1.8 and its problems in getting a consistent run. It is installed in an Aeroworks Extra where everything is pretty much set as per the ARF design- i.e. the tank is some distance away from the engine. Anyway, I thought I'd try the Uni-flow set up so I pulled the tank and set it up ( I think) according to the JAKA recommendations:
Clunk #1 to engine
Clunk #2 ( somewhat shorter) to the muffler
Overflow line to a Hangar 9 fuel dot.
So I started to fill the tank through the fuel dot and shortly afterward fuel was pouring out of the muffler! Seemed like it reached about half full and then started running out of the tank.
So what am I doing wrong?
Bill


Hi!
No ,No! Use the the Uni-flow system!
The line from the silencer is Not, I repeat is Not going up in the tank!!!! It should be going down same as the clunk line.That way you have two clunks running in parallel (side by side).That's the Uni-flow system.

The other lines is the over flow line which is going up in the top of the tank!

In a plane where you cannot get to the to the fuel lines when fueling (cowled in engine) you have the over flow line hanging down under the cowl and fuel trough the T-fitting line connected to the same fuel line that goes through the carb.
The overflow line is plugged with a 3mm screw after you have fueled up (same goes with the fueling line )!

skeeter_ca

I believe with Jaka's setup with an enclosed engine you need to add a tee in the line from the tank to the carb with the line sticking out below the cowl and use a fuel dot to plug it. When fueling or defueling you remove both fuel dots and fuel into the teed line you just added. Also make sure the throttle is closed all the way. Hope this helps.

skeeter

GoNavy

My Feedback: (15)

Attached is copied from the Owners Manual for the Fox Eagle 60 and 74 (two stroke glow).

This seems similar to be what has been called the "uni-flow" system; (I see no difference between two clunks and soldering the vent line to the fuel clunk).
However, I wonder why Fox was instructing to leave the "vent" line open? Their engines came with mufflers that had pressure taps.

flhyr

Uniflow downside- chop your throttle really fast from full, watch fuel surge into your muffler.... and in my case, dead stick... over and over and over.

GoNavy

My Feedback: (15)

I want to be sure I understand the fueling systems discussed in this thread. Referring to the attached sketches:


2A is the typical 2 line setup. We detach the line to the carb when fueling or emptying the tank.

In 2b, to fuel or empty the tank, we use either 1) a valve in the carb line, 2) we install a fuel "T" in the carb line and run a line from the "T" to a fuel dot to seal the filler line during flight.

3A is the "uniflow" three line system described by Jan (Jaka); The line to the top of the tank is a vent line during filling or emptying the tank; It is plugged during flight.

3B is the three line system described by MinnFlyer and Bundubasher; The second clunk line is used to fill and empty and it is sealed by a fuel dot during flight.

I do not include systems which do not use muffler pressure, such as the Fox three line (post 34).

Do I have the systems correct on these sketches?