How a Venturi works video

[Bob Reeves]

Hope this link works, still figuring out how to copy and paste on this tablet. This is really a video on carburators but it also does a great job of showing how a Venturi works.

[Claudio Chacon]

This is a very enlightening video, Bob!

Thank you very much for posting it!

[frank williams]

Hwy Bob
Way Cool ... enjoyed watching ... thanks for postng

[Chris Wilson]

So any form of restriction before the jet richens the mixture and any form  of restriction after the jet leans it out?
Following that it would seem that layers of nylon stocking or a filter over a model engine Venturi acts as a choke and by default richens the mixture.
Chris.

[Jim Hoffman]

Chris, that would be correct if the stocking material were added AFTER the mixture had been set by the needle valve.  In our models the mixture is set with the stocking in place.

Great video

[Serge_Krauss]

Thanks, Bob. Great video!

[RandySmith]

great video on how  a  carburetor  works !

Randy

[Chris Wilson]

I particularly like when it shows that a single peripheral jet that is flush with the ID of the venturi actually ejects the fuel deep into the airflow and then gets atomised (literally smashed) by the incoming airflow.
To wit, the fuel is not dribbled down the throat and it is not atomised as it leaves the jet.
It's purely shredded mid stream by the air pulse when the valve opens.
Kinda puts to bed theories on spigots being better on the basis of atomisation.
Chris.

[Bob Reeves]

Someone needs to build it with the ability to swap in different Venturi configurations. Problem is coming with the high speed camera.

[Tim Wescott]

So any form of restriction before the jet richens the mixture ... ?
Following that it would seem that layers of nylon stocking or a filter over a model engine Venturi acts as a choke and by default richens the mixture.
Chris.

Kind of a different take on what Jim Hoffman said:

If you get your engine all adjusted without a filter and then put a filter on -- yes.  But you generally tweak the needle after such a mod, so things work out OK.

... and any form  of restriction after the jet leans it out ...

In theory, no, because more restriction after the venturi means less airflow, means less gas pulled from the bowl.  In practice -- have you ever rebuilt a carburetor?  Get one apart, or look at a diagram of the main jets, the auxilliary jets, the power pump, etc.  There's a lot going on in an automotive carburetor that makes sure that it does a good job of delivering the correct mixture at any given time, and even then a carburetor doesn't do nearly as good as a good electronic fuel injection at calculating just exactly the right amount of gas to put into the airstream for any condition.

Simple carburetors for lawn mowers or for model airplane engines get by with simpler arrangements because, first, you just run the thing a bit rich, and, second, at least for much of control line usage you're not changing the engine speed much.

[Tim Wescott]

Apparently if things had worked out a bit differently three years ago at my current job, I'd be working with that guy.  He worked at the parent to the company I work at, and was seriously considering coming on as a founding member -- then he got other work.

[Brett Buck]

I particularly like when it shows that a single peripheral jet that is flush with the ID of the venturi actually ejects the fuel deep into the airflow and then gets atomised (literally smashed) by the incoming airflow.
To wit, the fuel is not dribbled down the throat and it is not atomised as it leaves the jet.
It's purely shredded mid stream by the air pulse when the valve opens.
Kinda puts to bed theories on spigots being better on the basis of atomisation.

   It is not a theory, we have observed the "dribble" effect on flush inlets on multiple occasions.


This particular demo has the same problem as all static runs/tests, in that there isn't a 60 mph relative wind 1/2" from one end hitting the inlet at various angles. I think everyone agrees that in the conditions of the experiment it works as shown, but it has the same simplifications that ignores the real issues in real stunt venturies. In fact, if you test it on a flow bench it will work better, no one disputes that. Neither the theory nor the tests address the problems we are trying to solve.

   I have no stake in what venturi you run, but if looking at one video of one highly-stylized case is enough to induce you to naysay literally decades of careful experimentation, and effectively call people like David, Frank, and I fools, then I don't care to address it any further.

   It *is* a classic case of someone with a small amount of information jumping to a wild conclusion, but that it hardly a new phenomenon in stunt.

    Brett

[Chris Wilson]

Brett, I am keen to see video evidence of a 60mph wind at all angles demonstrated.
You also have no idea of the amount of a information that I have seen, you have based that conclusion on one reply in one thread but as you say it's nothing new.
Chris.

[Chris Wilson]

Pretty much what I witnessed with an MVVS 40 that had 4 peripheral jets.
The fuel stream needed convincing to leave its central position.
Chris.

[Chris Wilson]

Absolutely correct.
Condensation is an anethma to atomisation.
As soon as a measure of fuel leaves the jet it wants to ball into the smallest possible area, then it gets hit with the atomic sneeze of intake, and  impacts onto the spinning surface of a 10,000rpm centrifuge called the crank shaft, then thrown at high speed at the back plate, then whirled around again in an even bigger centrifuge called the big end and finally shuttled up the transfer ports .
Every one of those actions dilutes the intention of jet atomising the fuel as condensation from each of those events is far more vicious.
I believe that most of the atomisation in model two stroke is done when the transfer first cracks open under high pressure and that the jets true role is one of a simple metering device.
Chris.