Carb talk for Beginners

[Peter Twissell]

The electrical analogy is great as far as it goes, but it has it's limits.
Electricity has no effective mass, so it doesn't exhibit the square law change in kinetic energy with velocity which is seem in fluids and is fundamental to Bernoulli's principles.

 

[Steamchick]

Sorry if this is the wrong thread for this comment.
There is no analogy between "electrical energy flow" versus gas flow.
Electrical energy flow is compared to water - an incompressible medium, whereas gases are compressible so a totally different set of Mathematics is required. "PV=RT"... - which is a thermodynamic way of saying gases have potential energy comprising a "pressure" and a "temperature" and the relationship of these can be altered by changing the volume. What is happening in the venturi is that the volume is changing, but not the temperature, so the pressure must change as the inverse of the volumetric change. (P is proportional to 1/V... or flow). In the River context, from a point upstream to a point downstream the mass flow of the river is constant, but when the river narrows it flows faster. The venturi in the carburettor does this with air, but as it is a gas, not water, the speeding air also has a pressure that drops as the velocity changes, because the velocity relates to a Volume per time. and higher velocity of gas is a higher volume per time within the flow, so the pressure must fall (Inverse of volume).
Maybe this helps a bit? - I can expand further, but don't want to waste time/space for others on the thread. (And it makes my brain hurt!).
I am sure a mathematician can explain really what I mean... "Help?"
(Notwithstanding, electrical "flow" is not the movement of electrons, but the "shock-wave" change of the electrical field transmitted as a wave. A bit like a slow moving queue of crowded people when struck at the back by something, so each falls against the one in front... when the wave of energy travels much faster than everyone falls over, the speed of each hitting the one in front, which is much closer than the floor. I.E. at the speed of light for electrons).
Sorry to waste space on the thread with some fundamental Physics...
K2

 

[Klrskies]

I vaguely recall from the ancient days (pre-1985) that I studied non-electrical topics in Physics I & II, and as I recall we were forced to take Thermodynamics, which I hated.
We called it the dreaded "Thermo" class, and it seemed more like Chinese water torture than anything useful for EE's.

I am sure I never had any fluid dynamic classes.

I remember PV=mRT

As far as Bernoulli's principle, I have never been able to wrap my head around that.

I can understand electrical, basically because it cannot be compressed, and it acts like water flowing in a river, with the branches all summing to equal the flow in the main river channel.

Voltage is analogous to pressure, amperage is analogous to current flow in a river or a conductor.
Resistance is like dikes and obstructions in the river.

Electrical transformers typically have a higher voltage (pressure) on one side, and a lower voltage (pressure) on the other side.
Reducing the voltage (pressure) increases the current (flow).

I visualize the Bernoulli image as a large pipe (wire) on one side, which intuitively means high current, low pressure (voltage) in the electrical world, and the small pipe (wire) would be high voltage, low current.
But this is the opposite of the actual Bernoulli lower pressure on the smaller pipe size.

So this is the opposite of what visually is seen in the electrical world, and thus difficult for me to wrap my head around.

I don't think you actually change the speed of electrons in the electrical world, and so there is no velocity change.

I can visualize the effect of power factor in the mechanical world, which is a surge tank.
A surge tank acts exactly like a capacitor or inductor, ie: they all store energy.

If I crimp the end of a garden hose, I can spray water a long distance.
The same water flows out of the hose (I guess) whether I crimp the end or not.

If I crimp/bend an electrical wire, the same current flows through the wire, so no analogy there.

It is a bit of a culture shock to go from the electrical to the fluid dynamic world.
Sort of like driving your car, but turning the steering wheel left moves the car right, and vice versa, or stepping on the brake pedal makes the car go faster, and stepping on the accelerator pedal slows the car.
Its all mixed up in the fluid world.

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Please allow me to try and share how I try to visualize the process for the sake of extended discussion:

From within the cylinder, as the piston begins traveling away from cylinder head and increasing the cylinders volume, and reducing pressure within the cylinder. And, with the now open intake valve exposing the cylinders low pressure signal to the attached intake tract and the attached carburetor. The higher atmospheric at the entrance of the carburetor begins forcing air from the high-pressure atmosphere to toward the low-pressure cylinder.

As the piston's movement creates increasing volume within the cylinder, an increasing and accelerating, low pressure signal is communicated to the mass within the intake tract, with the higher atmospheric pressure from the open entrance of the column pushing the air and its mass toward the low-pressure area of the cylinder.

It's important we realize that the cross sections all along the length of column, including the carburetor, vary in cross sectional area, and because the mass is a compressible fluid (air) it must conform to the varying cross sections by accelerating its mass in lower cross sections and deaccelerating its mass in larger cross section areas. This is why the intake needs to steadily decrease in cross section as the mass approaches the entrance into the cylinder, increasing its velocity to fill the cylinder as much as possible.

When we introduce a venturi into the intake tract, we force the mass to speed up within the venturi, with the narrowest cross section of the venturi having the highest velocity. What forces this mass to speed up in the venturi to occur, wrather than slow down the entire column is due to the greater volume of mass in the of entire cylinder and intake column compared to very small volume of mass within the relatively short, narrower venturi. The pressure differential between atmospheric and within the intake tract being the greatest at the point of highest velocity, in this case the venturi.

This last part of the dynamic process is easier in my mind to visualize when I think of the intake column being exposed to vacuum wrather than pressure from the atmosphere. I guess it's all the same, but it's hard to imagine a restriction causing increased flow into the column when thinking in terms of pressure.

 

[GreenTwin]

I am going to keep pondering the Bernoulli principle, and I will have to re-read everyone's comments several times to try and soak it all in.

Thanks much for everyones feedback/comments.

I am not there yet with getting some sort of good feel for it, but it took me a long time to get comfortable with electrical concepts too, and I never gave up on that.

Edit:
I have heard various theories about electron flow (or lack of it).

What is important to me is that I have the formulas I need to accurately predict what a clamp-on ammeter will read, regardless of exactly what that means on an eletron level inside the wire.

I distinctly recall taking a power lab, and we connected a large capacitor at 480 volts, and also connected a power meter.
The power meter read zero, but the clamp-on ammeter read 100 amps.
I went to my professor and showed him our setup, and asked him if something was broken.
He did a huge facepalm on his forehead, rolled his eyes, and said "OMG, that is such a dumb freshman question".
He was right, it was a very dumb freshman question.
I finally got it figured out enough in the real world to be functional with electrical power designs.

I understand the concepts behind electronics design, but since I don't do that every day, I have never become proficient at it.
One of the problems with designing electronics is that there is basically an infinite number of ways to design it, and I consider it several levels of magnitude more difficult than power design.
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[GreenTwin]

So in foundry work, I use a slurry sprayer, which is very similar to an airbrush used for painting.

There is no venturi, and yet the ceramic slurry is drawn up the vertical tube from the container below, and then joins and atomizes in the airstream.

So even without a venturi, the velocity of the airstream can create a siphon.

I would think the same thing could happen in a carburetor, without a venturi, if the velocity of the air was high enough.
Unlike a slurry sprayer, the air velocity in a carburetor is relatively slow, and thus perhaps the venturi must be used to bump up the velocity a bit to allow siphoning of the fuel.

I suppose that if an automobile had a source of compressed air, then you could use an arrangement like a slurry sprayer, which I guess would be a form of fuel injection, and spray into the intake plenum.

And perhaps the increased velocity in a venturi also acts to atomize the fuel, which may otherwise be coarse droplets or even a liquid stream without the venturi.

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[Klrskies]

Some of the early, single barrel Solex carburetors had a removable venturi. Early drag racers removed the venturi, leaving the straight bore without a venturi. As they didn't care about anything except wide open performance and maximum cfm flow rate. It was an acceptable loss of the normally improved performance and drivability from the enhanced fuel atomization offered by using a venturi with it's stronger vacuum signal and higher air velocity within the venturi. As the bottom of the smaller venturi booster, where fuel is introduced to the air stream, sits right at the most restricted portion of the main venturi where air velocity and vacuum signal is highest.

Another example of a vacuum, or a low pressure signal being created and pulling liquid out of a container without a venturi is to hold the open end of a straw sticking out of fast-food soft drink cup with a lid, with the open end of the straw parallel to air flow direction while driving down the road as you observe the liquid being drawn out of the cup and atomized into the air stream.

 

[Jasonb]

Pat as I said back in post 38, you don't need a venturi.

Go look a the small weedwackers and Walbro type carbs they are straight through.

Two engines I'm working on at the moment have parallel holes no converging tapers.

 

[GreenTwin]

You don't actually need the classic taper in- taper out shape. The end of the gas inlet protruding into the air inlet will reduce the cross sectional area and cause the air to speed up thus creating a vacuum which will draw in the fuel.

I read this statement, but I think it is more a matter of having sufficient velocity through a straight venturi, ie: there is enough air velocity to draw fuel up through the fuel tube, just like the slurry sprayer.

I don't think the tube in the center of the venturi is large enough to have any material affect on the air velocity.

All mere speculation on my part; just thinking out loud.

Edit:
I would think that the pickup tube could be at the bottom of a straight venturi.
With the pickup tube at the center of the venturi air flow, you would get a more even distribution of atomized fuel in the air, but I would bet an engine would run with the pickup tube flush with the bottom of a straight venturi, perhaps not as well as a centrally located pickup tube.

I have played around with a lot of different types of foundry burners, and tried kerosene with a paint sprayer.
If you try kero with a paint sprayer, use great caution outside, with full face shield and safety glasses, since you will have a BIG flamethrower.

So I will build a little straight venturi carburetor, with an adjustable height pickup tube, and try it with a slight amount of compressed air through the venturi, and see if I can get a flame going.
Then I could vary the pickup tube height and see how that affects the flame.

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