Liquid Fueling Made Easy - Adv. Injectors For Liquid Fuel

[Eric]

I have been getting a significant increase in questions about liquid fueling lately, it seems like the world is ready to progress beyond propane :)

Since I have been selling plans with these types of injectors for quite some time I have decided to post lots of details and eventually specific highly tested and dimensioned injector plans. I am going to be using this thread to share the various designs and dimensions.

The standard infinijectors work excellent for propane, and they also work well with liquid fuel, but there is a relatively simple way of enhancing both liquid fuel and vaporous fuel performance.

It might be too difficult for the beginner to build, but by simply putting a spring in the design, instead of the manually adjustable screw thread, the mixing performance can be pushed to the max, for perfect mixing of any fuel over the entire throttle range.

The spring simply keeps the injector nozzle closed by default, and as pressurized fuel is pumped in, the injector will open up to an orifice size that will keep the fuel spray pattern at optimum conditions regardless of fuel flow rate.

The injector can be made to only open to a preset limit, or can even be adjustable if you want to make things complicated, this can help limit the upper flow rate to prevent accidental over fueling.

This system works with any pressurized fuel source, and the design can be tailored to specific fuel types and pressures in order to get the most performance out of your engine.




To highlight some of the performance gains from using an infinijector type fuel injection system:

Dramatically improved ease of starting, almost any engine can be started from a single quick blast of compressed air, or no air at all depending on design.

Greatly improved throttle range, throttle down lower than you have ever dreamed of, and run the engine higher than it could before.

Huge thrust increase compared to using unoptimized injector designs, and also a significant decrease in TSFC.




Anyone have any specific engine designs / sizes that they would like to see some injector plans for first?

Eric

[larry cottrill]

Eric -

Might as well start out with your Advanced FWE. Several people have been interested in that one, and if there's a reasonable increase in thrust to be expected, it might even turn out to be a flyable engine.

L Cottrill

[Eric]

I should have expected that one :)

The problem is with small engines is that the injectors would have to be small, with the normal infinijection gap system it would be nearly impossible to make. I would have to go with something more like an air brush nozzle, but its entirely doable. I wouldnt want to produce them, but a reasonably equiped pulsejet enthusiast should be able to make one.

I'll sketch up a quick picture of something I have been thinking of for the RC sized engines and we can all work on refining the details.

Eric

[Eric]

Ok heres a sketch up of what I am thinking of. I think injection after the bend would work well, especially for people who would run the intake on the bottom.

Basically moving any adjustment / spring that would increase diameter to the outside of the engine.

Eric

[milisavljevic]

Hello Eric --

Since I have been selling plans with these types of injectors for quite some time I have decided to post lots of details
and eventually specific highly tested and dimensioned injector plans. I am going to be using this thread to share the
various designs and dimensions.

Cool! I hope that the black flies will let you be, so that you may share your injector developments with the forum!

Huge thrust increase compared to using unoptimized injector designs, and also a significant decrease in TSFC.

What you are not making clear here, is that superior injectors make it possible to extract the full thrust potential
from pulsejets, but not to attain extraordinary thrust levels. For example, consider a fully-optimised pulsejet with
a combustor ID of 73 mm: the theoretical maximum thrust for a self-aspirating duct of this caliber is almost 63 N
(NOC method, alkane-based fuel); inferior injector(s) and/or poor injector placement may reduce observed thrust,
while superior injector(s) and/or proper injector placement will permit full thrust saturation - but not beyond 63 N.

Your own TP-180-18 is an excellent example of what I am trying to convey: using your advanced injector(s), this
motor developed 125-129 N - thrust levels well beyond simple injectors, but still within the theoretical thrust limit.

Best regards,
M.

[Eric]

Yes thats a good distinction, although with the infinijectors set up for max entrainment, and using high pressure propane vapor, the engine can be made to run as if it were getting a supply of forced air, sort of like an over powered rosscojector, that would be the only applicable case where you get more thrust from a combustor than it can achieve on its own under static conditions just by the fuel injector gains.

But you are right, that no pulsejet will make more thrust than is possible to make :)

Eric

[marksteamnz]

Minor point but the second injector sketch, not the cad rendering, appears to have the spring on the wrong side. ie spring loaded open not closed.

[Eric]

I probably should have put some arrows indicating the direction of pull, I slopped that sketch together in about 5 minutes, the spring should pull the core innward, not push it. I drew the other segment of the injector break open instead of closed. I should probably stick to CAD and leave MS Paint to Larry :)

Eric

[milisavljevic]

With the infinijectors set up for max entrainment, and using high pressure propane vapor,
the engine can be made to run as if it were getting a supply of forced air, sort of like an
over powered rosscojector, this would be the only applicable case where you get more
thrust from a combustor than it can achieve on its own under static conditions.

Hello Eric --

This is a good distinction that you have made. Normally, we need not consider the momentum added
by fuel injection, nor the effect of mass entrainment; however, certain injector configurations will add
measurable momentum to the system, and significant mass entrainment. This 'supercharging effect'
may be handled as an increase in static pressure (above local conditions), and will allow thrust levels
beyond normal rated static thrust. This is not "cheating" after all, and in fact it should be encouraged.

Keep up the good work! =)

Best regards,
M.

[NickC]

what is the weight of that spring in your first sketch?

[marksteamnz]

I probably should have put some arrows indicating the direction of pull, I slopped that sketch together in about 5 minutes, the spring should pull the core innward, not push it. I drew the other segment of the injector break open instead of closed. I should probably stick to CAD and leave MS Paint to Larry :)

Eric

Doh! A tension spring of course, my mistake.

[Graham C. Williams]

Dear Eric.
Do you have any spring resonance issues with this design and how do you tune them out?

Regards
Graham.

[Eric]

I havent observed any resonant issues, the distances these things open up is so small. If there is any resonance its causing the core to open and close only slightly, and at an extremely high frequency, its nothing pronounced enough that fuel flow shuts off or you can hear the enigne running poorly.

Eric

[NickC]

how much are they moving?

[Eric]

Depends on the engine size and the type of fuel, but you can get a rough idea based on a current injector design, say your using propane at 75 psi through 6 1/8" holes. Now since its a radial gap, it doesnt need to open 1/8" to get the same area and flow.

When you figure liquid fuel is much more dense than propane and you need a lower volume flow rate, the injector really doesnt have to open or close a lot.

I'll get some specific examples and plans posted up soon, something that will help you with the TP-180 pocket bike project.

Eric