Lopsided spraybar

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Lopsided spraybar

Post by tritile » Thu Nov 21, 2013 1:10 am


I built a valved engine with lopsided fuel spraybar. It means that the spraybar is perpendicularly inserted in the throat section of air intake. In my case, for convenience, the spraybar is directly connected to a needle valve responsible for controlling the mixture ratio.
The best point of this configuration is that it is pretty easy to build. However, by other hand, it shows not good homogeneity of spray. It means that the spray of fuel does not meet all holes in the same amount. This causes problems of local overheating in petals, once there is no fuel to evaporate and drop the temperature or intake air, only dry air.

This next picture shows one petal as example of this non homogeneity of petals cooling. As it is not cooled the tip of petal overheats and fails prematurely.
I experienced dozens of petal with the same systematic behavior.
I tried changing the spraybar length beginning from the center of throat, until only 5 millimeters long (next picture… really short!!).
What I noticed was a better petals cooling, however the engine was substantially unstable and harder to start (maybe because it was too close to wall).

I made one picture showing what looks like the overheating profile of petal based on the damage found after some tests.
Sem título.jpg
It looks like that the spray bar causes a “shadow of fuel” on intake. As the fuel comes out of spraybar and insist to go straight and do not come back to the back of spray bar.

What is a fact is that I have never seen a popular engine using the configuration of spray bar. The closest I could find are the plans of the German “Pulso 1” witch uses a similar concept.
Sem título.png
The feeling says that it may fix the problem, once there is an obstacle (the needle itself) that organizes the fuel spray, and so it is easily spread to each hole.

Has anyone here who has worked with this type of fuelling and could share results? I would be very glad.

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Re: Lopsided spraybar

Post by IRONMAN » Tue Mar 11, 2014 8:09 pm

The mini pulsejet I made to a 1956 "Mechanics " magazine design uses a spray bar with two cross holes across the centre line of the venturi, originally the needle was set in the spraybar but it got broken off so I blanked off the broken end and used a remote mounted needle valve from an old Kavan carb. this seems to work just as well but is very sensitive. The reed is a single 0.006" spring steel leaf 0.5" wide covering a single hole in the valve face which is cut to a slant to increase the effective area, it is retained with a single screw and a brass plate. had one reed failure where the reed indented itself into the valve port slightly and the free ends curled up. Guess that with only one petal it is bound to get all the fuel/air mix! This motor was designed to run on petrol but I only got it to go on 80% methanol 20% nitromethane.
The overall spraybar effect looks similar to the Dyna-jet Flojector with its two cross holes
There appears to be a modern version of this motor flying a small foamie on U-Tube under the "foamie Pulse jet " title

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Re: Lopsided spraybar

Post by Mechanic » Wed Mar 03, 2021 10:12 am

Hi ironman!
I also built a copy of the1956 mechanics magazine mini pulse jet, this was after I got a Hobby King Pulsejet .
(which I could not get to run on petrol, but got it going on 80%methanol 20% nitromethane, and bored the jet out to 1mm)
(did not the Dynajet come with a no. of different jets?)
Any how this encouraged me to try and make the Mini pulse jet.
Got some stainless tube for the tail pipe and combustion chamber but as I dont have any welding gear i machined the cone bit out of a piece of mild steel bar!. Figured it would last long enough to give me a run.
Again no luck on petrol but it ran well on 80/20 mix.
My plan was to fly the hobby king and built a big delta with twin fins, tried it out with a 70mm EDF on 6 lipos flew great, problem was where to fly with pulse jet as the noise was a problem could not use my model club field, insurance and noise again.
Eventually gave up for several months but found the baby one amongst my bits and pieces and thought i would try it out (rusty cone and all!) amazed as it started without any air , just primed it with some fuel and turned on the sparks and away it went! (never managed that again though) Will try a small delta like the one on Foam mini pulse jet video, think I might get away with the noise hopefully.

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Re: Lopsided spraybar

Post by Mark » Wed Apr 07, 2021 6:48 pm

One time I sprayed some methanol into a Dynajet and then screwed the head on, holding the jet in one hand and lighting it with a Bic lighter in the other at the tail to see it it would rev up. I also sprayed some alcohol in the intake too with a finger pump sprayer bottle. After 2 or 3 tries it roared to life briefly which was quite startling. I've never been able to get it to rev like that again.
I made a primitive pulsejet from plumbing pipe though where I sparked it to start it, it either started or I had to air it out and then attemp to spark it again if it didn't catch. I never used starting air but sometimes it would also start by lighting the tail with a Bic. If it was hot and humid out the fuel/air mixture would only create a weak pop but if I corked the tall with a stopper, the starting impulse from more compression often worked. It was like a potato gun bang starting method.
Presentation is Everything

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Re: Lopsided spraybar

Post by Mechanic » Mon Mar 28, 2022 11:18 am

Back to my mini pulse jet from 1956 popular mechanics, Having got it to run on 80/20 methanol nitro mix I tried an electronic candle lighter which I fitted with 2 croc clips for the 1/4 " spark plug, and was amazed when it started up after a prime into intake with no air supply! tried it several times in case it was a fluke.
Found that it was easy to start with a prime and blowing across the intake with a bit of plastic tube about 3/8" dia. (short puffs rather than a steady blow.)
Now I can carry all the starting kit in one pocket, so must get stuck into a model , perhaps the fast foamboard Delta?

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