OK, Engine Gurus -- here's a question

Because the fuel doesn't spray directly into the airstream, perhaps it is designed to stop any blow-back of fuel out of the intake due to reed flutter. Just a thought.
Also, maybe it just helps to increase the pressure depression in the vicinity of the spray orifice.

Rod.

The drawing makes perfect sense, except that for my explanation/guess, I need to change some names. Lets call the hole in the backplate the "air intake," whereas the conical tube in the tank extending to the reed opening is the "venturi."

Maybe they were experimenting with keeping the intake air accelerated for a longer time before entering the venturi. Technically, in this odd case, that makes my so-called air inlet function as a venturi too. Maybe they hoped it would pull a stronger vacuum on the fuel inlet with less time to expand before entering the venturi nipple.

I wonder if you bored the backplate inlet to Black Widow size if it would take this flashing off.
Rusty


EDIT:

Oldenginerod wrote:...Also, maybe it just helps to increase the pressure depression in the vicinity of the spray orifice.
Rod.

Yeah, that's about what I said. Rod beat me to it.

RknRusty wrote:
I wonder if you bored the backplate inlet to Black Widow size if it would take this flashing off.
Rusty

Rusty, to answer your question, yes, I think it would.  I considered that, but because it wasn't my engine and it is an older model with the C-clip rod retainer, I'll leave it.  I also considered coming in from the tank side with a larger bit and removing just the flashing (it's fairly thin) -- this would leave a slight chamfer on the inside of the air inlet and expose the fuel inlet to direct air flow.  Fuel draw on most of our normally aspirated engines is due in part to manifold vacuum and partially to the venturi effect of accelerated air flow over the fuel orifice.  The way the protrusion is positioned negates the venturi effect.

Maybe someone here has an engine disassembled with a metal backplate and can check to see if their backplate has the same design.

I have a bunch of partial engines I can take a look at.

Bandit and Rod --

It may have been an experiment to increase fuel draw, but I don't understand how the physics would work in this case.

However, it could as Rod noted, reduce fuel blow back out the intake --- perhaps fuel remained in the cup formed by the  venturi body and the protrusion.  There is likely some positive pressure in the venturi tube either from reed flutter or from the reed simply re-seating during normal running.

andrew wrote:Bandit and Rod --

It may have been an experiment to increase fuel draw, but I don't understand how the physics would work in this case.

However, it could as Rod noted, reduce fuel blow back out the intake --- perhaps fuel remained in the cup formed by the  venturi body and the protrusion.  There is likely some positive pressure in the venturi tube either from reed flutter or from the reed simply re-seating during normal running.

That very well could be it kinda would act like a type of "catch can" to reduce the spittle out the intake. (Bandit)

If your tank back plate does not have a screen you can see the reason for the design.  
When the tank and back plate are mated together you will see a gap between the edge of the back plate hole and the end of the venturi.  The fuel now has a 360 degree channel to atomize entering the venturi, hence the annular discharge concept. there is no needle or spray bar disrupting the air flow which is why the inlet is so much smaller than a sure start style back plate. Trying to enlarge the inlet too much will ruin it.  
Mahalo  
Agustin Jr.

I think I have seen this on metal backplates too, but haven't checked if it is there also on the plastic ones. I think the purpose is to create a primitive velocity stack and it is easier to see if you include the rest of the tank in the drawing.

The venturi effect relies on laminar flow, and you would not get a good fuel draw right after the intake aperture, where there is turbulent flow due to the sharp edge of the hole. Ideally the needle should be positioned much closer to the reed, but then the whole construction of the tank would be much more complicated.

I guess the explanation is much less sophisticated than the difference between laminar and turbulent flow. With the protrusion/rim on the backplate the fuel leaving the needle valve orifice is more uniformly distributed around the intake tube perimeter and allows for a more efficient mixture with air. Without this cylindrical protrusion the intake tube would slurp in the fuel at its point nearest to the needle valve orifice, i.e not symmetrically.

Remember that the venturi body of the TD-s also includes 3 small holes on its perimeter to admit the fuel into the airflow in the most even way possible.

OK guys -- there have been a number of good explanations proposed.  I'm going to play the Devil's Advocate --- not disagreeing with anyone, but just throwing out some more questions.

Rusty had asked earlier if the protrusion would be removed on the larger intake opening on the BW backplates.  I suspected it might, but did not know for certain.  I pulled a backplate from a stunt tanked BW this AM and it was gone.

Surfer Kris suggested that it could be a crude velocity stack.  The has merit, but then fuel flow would be based on manifold vacuum only and not aided by the venturi or Bernoulli effect of increased air velocity over the fuel orifice.  However, in support of Kris, the fuel inlet orifice on the BW is not in the direct path of air being pulled into the intake since the intake diameter is less than the shoulder diameter on the end of the tank venturi tube, so the orifice actually sits above the intake hole.  There would be a lot of turbulent flow in the area, but I don't know if it would mitigate the Bernoulli effect.  Because these engines were designed to run flat out and the BW was a better performer, the designers may have thought that the increased intake airflow on the BW did not need any additional boost.

I also wondered if the ring would improve vaporization.  Balough made the same point and noted that the TDs use 3 fuel orifices on the NV ring.  This protrusion is very small and fuel/air mix travels some distance down the venturi tube in the tank -- the tube is hot during normal running and there is a lot of turbulence when the fuel/air mixture passes around the reed, so I'm not sure that fuel mixing was part of this design.

Leroy Cox was a manufacturing genius with most of his production being done with screw machines.  However, his castings were rough and had to be machined to final tolerances, unlike the injection molding today.  I considered that the protrusion was perhaps drill or punch flash, but it's thickness and smooth edges tend to indicate that it was part of the machining process, so it seems that he had a function in mind.

The cylinder, BTW, is a single bypass cylinder without a boost flute.

What about capillary action forming a continuous sprinkler ring?

I suspect that ring is in the right gap range to rather actively move fuel.

Phil

andrew wrote:OK guys -- there have been a number of good explanations proposed.  I'm going to play the Devil's Advocate --- not disagreeing with anyone, but just throwing out some more questions.

.....................I also wondered if the ring would improve vaporization.  Balough made the same point and noted that the TDs use 3 fuel orifices on the NV ring.  This protrusion is very small and fuel/air mix travels some distance down the venturi tube in the tank -- the tube is hot during normal running and there is a lot of turbulence when the fuel/air mixture passes around the reed, so I'm not sure that fuel mixing was part of this design........................

My comments:

1. The protrusion is short and the gap between the protrusion and the venturi tube is even shorter. Hence the fuel will fill the space around the rim and thus will enter the venturi along its full perimeter, helping the efficient mixing in the airflow. In reedy engines the high-frequency throbbing/vibration of the of the reed will further enhance vaporization.

2. The fuel-air mix travels much longer in a TD than in a tanked Bee (length of venturi plus that of the crankshaft, which is spinning fast, and is really-really hot, hence maximizing mixing and vaporization) still the design of the venturi includes 3 holes spaced equally around its perimeter to help improve the mixture of fuel and air...

So I still believe the impact of centrally symmetrical admission of the fuel into the airflow (with the rim in the BB backplate shown above, and with the 3 holes in the TD) on maximizing the fuel-air mixing should not be played down.

andrew wrote:
Rusty had asked earlier if the protrusion would be removed on the larger intake opening on the BW backplates.  I suspected it might, but did not know for certain.  I pulled a backplate from a stunt tanked BW this AM and it was gone.

There are two effects that will give you a lower pressure, one is a venturi effect (from gas flow speed alone) and the other is a real pressure difference that gives the flow of gas in the first place (this is from the "suction" of the engine).

Using both gives you the best fuel draw, but using only the effect from a restriction is perfectly alright too. See the illustration below.

In a "typical" CL engine the spray-bar crosses the whole intake tube and there is one fuel-jet hole facing straight down the intake tube. This will only use one of the two effects (the "vacuum" or "suction" one), you get a better fuel draw from a hole which is slightly off center in the spray bar, as there you also have laminar flow and a venturi effect. This is utilized in PAW engines (two holes in the spray-bar, none facing straight down) and Super Tigre engines (rotatable spray-bar for midrange adjustment of the RC carb).

Kris, I believe apart from the normal depression created by the suction of the engine, the Bernoulli effect may also have a boosting effect on the fuel draw in engines where the spraybar is crossing the air-intake: air will accelerate in the air-intake where part of the free-flow section is redued by the spraybar. Hence the higher air velocity in the spraybar area will reduce the static pressure of the air and add to the suction effect of the engine in drawing the fuel.

The venturi effect is described by Bernoulli's principle (or equation), so they are one and the same effect. It only works for ideal, non-turbulent, flow.

In the wake of a regular spray-bar there will be turbulent flow, so the lowest pressure point is slightly off-set from straight down the venturi, where there is still some laminar flow.

I believe this is also used on the Medallion engines, but it has been a while since I had one appart, the hole in the spray cannot (or shouldn't at least) be mounted straight down.

There is a good discussion about Venturi's (and many other things) here; http://www.clacro.de/ 
(click on design in the left menu, then Venturi)

Kris --
Thanks for the link --- lots of good information there. The positioning of the spraybar hole was always a hot topic at the field.
andrew