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Engine basics => Engine set up tips => Topic started by: Joe Messinger on August 13, 2006, 08:59:24 PM
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I don't really understand what a sprinkler venturi is all about. I see them offered on Ebay for Super Tigre engines. Were/are they ever used on other engine makes?
Are there any advantages to a sprinkler venturi?
I don't have any plans for refitting my engines with sprinklers, just would like to know what the reasoning is regarding their design.
Thanks,
Joe
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Randy Smith's P.A. engines all come with sprinklers. I don't really understand the dynamics but they are supposed to be more efficent
and are called true venturies.
Walter
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TD engines have sprinkler venturis. Basically the NVA assembly is offset and feeds into a ring around the venturi. The fuel enters the venturi through several small holes. Understand the best is what George Aldrich called a 'schnozzle'. Like a sprinkler, but instead of small holes the fuel goes through a tube to the center of the venturi. That puts the fuel where the airflow is the fastest and suction greatest. Lost out in Slow Rat one time because the schnozzle tube came out and was lost. :-[
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Randy's PAs have a single hole in the venturi where the NVA goes through the back of the case. The Super Tigre engines had a series of holes around a groove cut in the venturi. The NVA sits where the carb hold down screw fits at the back of the intake stack. You have to point the hole of the spray bar straight into the "hole".
It's called a "true venturi" for several reasons, not the least of which is that there is nothing passing through the opening (like the spray bar) to obstruct it.
Mr. Smith will expound and/or correct me if I'm wrong. y1
Bill <><
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thats anular discharge.
for some reason makes more h.p. and is illegal in scca auto racing.supposed to be more even mixing but i bet all suspended fuel condeses when crankcase presurerizes.
made a large dia one for a s.t.g21 .35 combat engine(in place of the normal combat side discharge venturi)and it did turn more revs--and looked cool with the pollished brasstrumpet flair.
David
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Just to add a few comments---I don't guartantee what I say is completely coprrect, but it makes sense to me! ???
The main purpose of the "venturi" is to provide a pressure drop to let fuel flow from the tank to the engine. It can do it like a "true" venturi==faster moving air has a lower pressure, or simply by providing an obstruction (like a spraybar) which when the piston moves upward causes a small vacuum--again this will let the fuel flow from the tank to the engine. I assume that there is no other pressurization in the tank, any positive pressure will also force fuel towards the engine.
One advantage of a "true" venturi is that the pressure drop across the venturi (where the air now enters the crankshaft opening--not just where the fuel enters) will be less than the obstruction type. This means the piston has less work to do drawing the air into the engine against this overall pressure drop. This should increase the power of the engine.
I am not sure how the mixing of the fuel into the air plays out with the different styles of venturis--spraybar, single point or ring true venturis. Naively, the better the fuel is mixed up the better I would expect the engine to burn the mixture. However the airflow in a 2 stroke engine is a pretty tortuous affair by the time it gets up into the combustion chamber.
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Alan,
On a quick skim of your post, a very nice statement of the conditions...
I would add that the "armpit" problems with a spraybar across an open venturii are all that worsen the area change, which, in turn, determines the pressure drop across the venturii minimum area.
The 'spigot' (single jet in a clean venturii) type has an advantage: It can be positioned so that the "annular fuel ring" carved into it doesn't switch from under to over the fuel level within it.
A while back, Frank Williams, as the Power Train columnist in a certain publication, ran some tests on clean venturii, and spraybar and 'spigot' configurations. ASIR, a spigot, properly positioned, drew fuel better than the others.
Frank ran several bench tests to find that oiptimum position, so, no rules of thumb from me on that...