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| Another version of how not to build a model engine |
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| Darren:
I think the problem was too much fuel and not enough control ? |
| AdeV:
A venturi causes a drop in air pressure (swapping pressure for velocity). This serves two purposes: It draws fuel into the airstream by the difference in atmospheric pressure; and, as a happy coincidence, the faster-moving air helps to atomise the fuel. I think that the "venturi" effect in the Schroeder engine is created by the tapered cut (creates a large surface area at the end of the tube), the air would be accelerated as soon as it's in the main body of the tube. The effect will be quite small & as a result inefficient, but I suppose on an engine that size, you can trade efficiency for simplicity. With regards to this engine, I suspect if you cut a slight taper in both sides of the main carb body and open out the fuel/air mix passage into the engine, as per the diagram below, it'll work better. Go easy on the size increases & test often, that way you shouldn't overshoot the sweet spot. There's a pretty good explanation here: http://www.explainthatstuff.com/how-carburetors-work.html This page may also help you out a bit: http://www.hooked-on-rc-airplanes.com/nitro-engine-tuning-tips.html I see Bogs has come up with a better explanation of the Schroeder carb than the one I'd stumbled around at. |
| bogstandard:
You have to be very careful with angles in a venturi, if they are the wrong combination, you can actually get the air stalling and causing all sorts of problems, with dead air in places you don't want I have just made a venturi for my spraymist system using a couple of basic angles that I know work. That is 22 degrees for going in and 6 degrees coming out, both inclusive, but I did away with the 6 degree one because of the way I will be using it. That should provide a steady pressure drop and hence suction once the engine is turning over. Bogs |
| AdeV:
--- Quote from: bogstandard on December 02, 2009, 06:11:43 PM ---You have to be very careful with angles in a venturi, if they are the wrong combination, you can actually get the air stalling and causing all sorts of problems, with dead air in places you don't want --- End quote --- I should point out that IANAFD (I Am Not A Fluid Dynamicist), so yes - if the OP can find the correct angles for a basic carb, that would be better. Or use the spraybar, which is probably easier to machine anyway. |
| bogstandard:
I was picking up on what you had said Ade, not criticising, but trying to be of assistance I have stated before, I am no super expert on it, but I do tend to remember bits of two years of aerodynamics that I was force fed many many years ago. As you suggested, in this instance, a spraybar would be the easiest way to go. Bogs |
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