Re: Spraybar hole position

Marleysky Sun Nov 13, 2016 11:24 pm

KariFS wrote:Well, here's what NASA's rocket surgeons have come up with:

Case #3 shows an oscillating airflow, and that is the case with our engines. The airflow starts, stops, bounces back etc, so maybe the best place for vacuum is not so easy to predict. It also varies according to engine speed, ambient temperature and probably a lot of other things.

NASA had the best and brightest working on a multitude of engineering problems. I ( in my humble opinion) think the NASA diagrams don't factor in the fact we are trying to introduce a fluid into the airstream. I think if the airflow is laminar (diagram 5) and the fuel is introduced at the low pressure point (180 deg from incoming air ) the fuel will fill the void created by the airflow as it splits apart around the cylinder (spraybar). We have two high velocity air streams with a hole introducing fuel which should atomize and mix with the air. The "best" position of the hole where the fuel is introduced into the airstream is dependent in the length of the intake, the turbulence in the airstream and the pressure of the fuel being sucked ( vacuum) or injectioned (pressure) with the Cox engines we have a limited number of variables i.e.: spray bar OD, hole size in spray bar and the length and ID of the Venturi. Too bad we weren't in college and had access to a engineering lab to run tests on various combinations to find the ideal position and hole size and pressures.

Oldenginerod Mon Nov 14, 2016 5:45 am

RknRusty wrote:I didn't find anything in my search except repetitive guessing. But here's my question:
Other than possible ease of cranking, does it really matter?

The needle can be adjusted anywhere from too lean to too rich. The engine runs best, which for a tach race means fastest, with its optimum F/A mixture. If the spray bar hole is moved slightly in any direction, the needle may need to be turned in or out to compensate. So it seems like the same amount of fuel will flow with the same amount of air flow at the same RPM, once the needle is adjusted for the desired speed... :head scratching emoticon:
Rusty

The same volume of air & fuel won't always produce the same results. If that were the case then automotive fuel injectors wouldn't need to produce an ultra-fine atomized spray- you'd just need to squirt in the right amount of fuel. Those of us with mechanical experience know that if an injector spray pattern is interrupted, even though it injects the same volume of fuel, the combustion of the fuel/air mixture is generally incomplete, causing power loss. With this understanding, that's why I suggested that a slight offset in the spray bar may aid atomization, due to the higher airspeed traveling past the side of the restriction (spray bar). It may be that the slowing of the airspeed as it comes around the back of the spray bar probably creates excessive turbulance as seen in the NASA pictures) causing the fuel to partly re-condense, causing a "wet" mixture.
This is all only speculation, but it certainly gets you thinking.