Determining a carburetor throath diameter

[tornitore45]

What criteria drive the dimension of a carburetor throath diameter?

Is it fair to assume that a carb correct for a 1 cylinder 4 strokes is good for up to 4 cylinder since they intake one at the time?

I suppose a minimum air speed is required to atomize and suck the fuel but too much restriction can starve the engine.

What happen to the fuel mixture ratio when the throttle closes?
Less air at higher velocity lower pressure? Would that pull more fuel making it rich?

In other words I have no idea how the Physic of a carb works.

 

[Cogsy]

I'm not too good with carbs either, but I know a carb set up for good high speed operation will be too rich at low throttle/idle. For this reason, they often have an 'air bleed' which is basically a hole to allow extra air into the mix at low throttle settings. Of course this hole has to be correctly sized as well, and can be replaced with a needle for fine adjustment. At higher throttle settings it is effectively closed off and had no effect.

 

[tornitore45]

OK we got one thing settled. Unless some compensation is used a plain carburetor goes rich as the speed slow down.

 

[Hopper]

Generally, a carb for multi-cylinders will be bigger than the carb for just one cylinder of the same size. But not twice as big for a twin cylinder etc. While only one cylinder is sucking at a time, the total cubic centimetres of air flow per minute is greater with two cylinders than one. So the carb has to accommodate that without getting air flow speed too high. It's a dark art and there are no easy answers. Best to look at what has already worked on established designs as a starting point.

 

[gbritnell]

You're correct in your assumptions about the carb that's why they have air bleed carbs. They are about the simplest form of throttling carb there is. When the throttle barrel is wide open it forms the venturi part of the carb (smaller diameter than the throat) for increased velocity, lower pressure through the carb. The needle valve is adjusted for this wide open operation. When the throttle barrel is rotated toward the closed position it basically creates a much smaller venturi opening which increases the velocity and lowers the pressure even more. This in turn pulls more fuel from the needle valve. To compensate there is what's known as an air bleed hole drilled at right angles to the throttle bore. As the throttle barrel closes it uncovers this port which goes directly to the atmosphere. It's basically an air leak which reduces the amount of vacuum being formed in the carb and thereby pulls less fuel.
To answer your question about carb size I can only give you a comparison. I have a V-8 engine with a 1.00 bore and .90 stroke. It idles about 1200 rpm and will rev conservatively to 7000 rpm. The carburetor bore is around .220 with the throttle barrel (venturi size) around .180. Generally with small engines less is more when it comes to carb bore size. As far as multi cylinder to single cylinder size comparisons I don't think anyone has ever figured out a number for that. They find a carb size that will work on their specific engine and let it go at that.
gbritnell

 

[tornitore45]

Very informative Gbritnell, just for comparison

I finished getting my LYNX engine running

Is 0.9 cuinch single, the throttle hole is 0.275"

Your 8 cylinders has 0.7 cuinch displacement per cylinder
Roughly 2 cylinders are intacking at every one time.
We may say that is comparable to 1.4 cuinch with a throttle hole of 0.18"

Quite a difference but supporting of your statement

Generally with small engines less is more when it comes to carb bore siz

I am pondering this because I am preparing to build the Hoglet.

 

[lohring]

Below is a formula for Walbro carbs that might help. It's probably for two stroke engines so use half the rpm for a four stroke.

[FONT=&quot]D = K x Square Root(C x n)
where:
D=venturi diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000

Lohring Miller
[/FONT]

 

[WOB]

There are two basic types of model carbs, air bleed( as described above) and two needle. Air bleed works fairly well and is easier to manufacture. That's why they are often found on model airplane and car glow-plug engines. Two needle carbs have independent mixture adjustments for both high and low speed. This is especially good for 4-stroke multicylinder engines which often have a wide speed range and a good steady idle is desirable. They usually cost more, but the benefits are worth it IMHO.

WOB

 

[tornitore45]

Revised and disregard the spreadsheet it contains conceptual errors.

Now we are getting somewhere

I am attaching a spreadsheet comparing data collected so far.
The air speed is in Mach because m/s is hard to visualize but knowing how close to the speed of sound give some idea and is just for comparison.

I have also added what the Walbro formula would give for each engine.
The Walbro formula yields a Constant Air Speed of 0.22 Mach
when K = 0.75. Basically is says keep the air speed around 300 m/s across the jet.

View attachment Carbs.xls

 

[Cogsy]

[FONT=&quot]n=RPM at peak power/1.000 [/FONT]

This is a weird definition unless maybe the 1.000 is bore size or something? Otherwise that 1.000 is just redundant.

 

[Mechanicboy]

Formula to calculate the gas velocity, venturi diameter, piston speed and vacuum. This is use both for 4 stroke and 2 stroke engine.

V= Gas velocity in meter/second
d= Venturi diameter in millimeter
Vm= Piston speed in meter/second
Pu=Vacuum in bar

 

[Cogsy]

V= Gas velocity in meter/second
d= Venturi diameter in millimetre

Your gas velocity at the point marked 'd' is going to be very different to the point right behind the restriction, and will revert to its previous velocity after the restriction as well. At what point is it supposed to be taken from? And how does this help carb sizing when gas velocity and piston speed vary greatly with RPM, and venturi size is an unknown?

 

[tornitore45]

Cogsy I do not know about Australia but the dot is not a decimal, in Europe they use a comma an decimal. The dot is a "thousand separator" so you must read as RPM/1000 which is what I use in compiling the spreadsheet other wise the Venturi throat comes out huge.

 

[lohring]

It might be the two stroke or four stroke factor. Like I suspect, the formula is for two strokes so the number would be 2 for four strokes. It's not my formula, and I don't have an explanation to go with it. The Mach number of the flow is probably very relevant. Also, you need enough flow to provide low pressure in the venturi to induce fuel flow. Walbro carbs need that to activate the flow regulating diaphragm.

Lohring Miller

 

[tornitore45]

Mechanicboy Are you sure the Venturi Vacuum result is in Bars?

Placing Air Velocity to 300m/s The results is 562 which make sense if interpreted as milliBars

I interpret this as Vaccum, the Absolute pressure should be 1000-562=438

 

[Mechanicboy]

This formula is for the engine who are running at max revolution and find out how fast is gas velocity and find how big is the venturi will be selected to atomize fuel best as possible without weaking the engine. A larger venturi = less gas velocity create bad fuel atomizing and vice versa. tornitore45, there is not wrote milibar, bar in vacuum only. The formula is from the book at german language Josef pfyl: Formeln für Auto Technische berechnungen

 

[tornitore45]

Some more research:

On Wikipedia there is a formula fro the Venturi pressure:

Pressure Drop = 0.5 x Air Density (ConstrictedVelocity ^2 - InletVelocity^2)
This is based on Energy conservation

Mechanicboy formula: VenturiVacuum = (VenturiVelocity^2 /160 yeld the same result for InletVelocity = 0 That is why is approximate.

Using a middle range K=0.75, the Walbro formula basically gives a Venturi hole to obtain 302 m/s
Meaning: Keep air speed from exceeding sonic at top RPM, same advice for airplane propellers tip speed.

Observing the spreadsheet I notice
The LINX engine I built has the lowest air speed and has a crappy Idle
The Dulcos engine I built has the second lowest air speed and has a crappy Idle.
The Upshur engine I built has the second highest air speed and has good idle.

Gbrittnell V8 seems to have the highest air speed, is actually above sonic. Gbrittnel reports a good running from 500 to 7000 RPM. 500 is an excellent idle RPM.

There is something to think about Gbrittnell preference for smaller carburetor throat.

Back when I was flying U-control Airplane models the fashion was to bore the Venturi larger to increase RPM, possibly it was a bad idea but nobody care about Idle we wanted all the RPM.

View attachment Carbs.xls

 

[mu38&Bg#]

Carb sizing is based on the engine's actual air consumption. A Holley is rated in CFM. An engine that "idles" at 7000 RPM running no load on the shaft doesn't need a large carb. If you don't want such an engine to overspeed, the venturi essentially throttles the engines to max RPM at wide open throttle. At sonic velocity, the orifice is choked and cant flow any more, which is why the engine won't overspeed. If the engine is to output power, the carb will have to be larger to allow enough air.

 

[Mechanicboy]

Think at the model diesel engine has smaller venturi than glowplug engine due viscosity in fuel who are difficult to atomize by air velocity.

 

[gbritnell]

Over the years I have met and talked with many builders about carburetion. That being said no one ever said that they used a formula for calculating the throttle bore/venturi size. In my own experiences it was an experimentation until I found something that worked well and then refined what I had learned.
The principles of carburetion hold true no matter what size you're working with but without hard numbers for air flow, cam timing etc. it's very hard to make a comparison from large to small. When working with a full sized manifold with a smooth bore for example there is a wetting action whereby the fuel clings to the runner walls hence the need for turbulence to reduce this physical condition. Now take a model manifold with a runner diameter of anywhere from .188 to .312 and you get an entirely different set of conditions. The reason I use this example is because of what it is, small scale, a different set of circumstances.
As with a lot of conversations about small engine performance it's best to have a working knowledge of the full sized components to at least understand what is taking place.
A full sized engine can only ingest so much air, given what I already stated, cam timing and flow numbers. If you put a small carb on the engine and adjust the fuel requirements for that carb the engine will run wide open at a specific rpm. Now upsize the carb to what the engine can use and the rpm's will increase until other factors limit them, engine timing, valve float etc.
Automotive carbureton has 5 levels of fuel management, idle, part throttle, wide open throttle, acceleration and load. Each system of the carb is constructed to somewhat maintain the proper air/fuel ratios for these varying conditions, of which we generally only have 2 on our small carburetors, idle and wide open.
It's funny how you don't really pay attention to some things, case in point. When I worked on car and motorcycle engines I never really paid attention to the position of the throttle plate or carb slide when the engine was at idle. I just knew that you would go between the idle speed setting screw and the idle mixture screw until the engine ran well. Ok you're good to go but I never actually visually looked at the throttle plate and said Wow how can the engine run with that little tiny opening.
It all kind of came to light when I built my Holt engine. The carburetor sits horizontally and has no cover over the throttle inlet so one day when it was idling at about 600 rpm (1.00 bore and 1.25 stroke) I looked into the inlet and could hardly see any opening between the throttle barrel and the inlet bore. Only when I used a bright light could I see a tiny crack. It was at this point I decided that if the engine would run on that little air, I know it was at idle, that I would make a carb with a much smaller throttle bore and venturi size. When I did the engine ran so much better than the larger carb I had on it initially. That's why I stated what I did in my previous posting.
So where is this all going? In my opinion if you're building and engine of a given displacement just pose a question on the forum and someone can give you a good starting point from their experience. There are too many variables in these small engines to use hard numbers for carburetor sizing.
gbritnell