External valve grid

[GRIM]

Heres a video and some pics of a small valved engine I have been working on lately ,
I am having fuelling issues , it is running on gasoline in the vid , (tried methanol , same deal)

I cant get it to sustain without compressed air , the air is necesary to maintain fuel flow , (just at the tip of the injector )

I have searched , but not been able to find detailled drawings of the shape and sizes of the dynajet fueling device, or any others for that matter,

Any pointers would be much appreciated ,

Thanks to Forrest D Eckstein (Webpilot ) for all his help with the math and his excellent worksheet ,

[Mike Everman]

Man, Grim, you're nothing if not prolific! I like it!
Two observations.
I think since you are using that curved surface as valve retainer, it should be grooved so you do not briefly make a vacuum between petal and outside wall. I do know that this can screw things up.
Also, the intake looks long to me. If the venturi intake was designed for a motor with a flat petal valve, then this intake section needs to be the same length from inlet to the average opening point of the valves. This length is critical, acoustically.

[Graham C. Williams]

Hi Grim.
Will your motor run, without forced air, on Propane?
If it will then the pipe and valves are probably OK and you can be sure that it's the pressure drop across the fuel system that's the problem. If the motor runs on Propane, try sleeving down the induction cone. This will increase the air velocity and reduce the pressure over the injector resulting in a better draw of fuel and better atomisation.
I don't know anything about the acoustic length of the induction cone that Mike talks about but have seen these motors running without any cone at all, admittedly I think it was on Propane.
Looking at the design of the valve travel stops. It looks as if you have used the design of the outer shell to perform this function. Do you have too much valve travel would be my next question?

Nice motor.
Graham.

[GRIM]

Two observations.
I think since you are using that curved surface as valve retainer, it should be grooved so you do not briefly make a vacuum between petal and outside wall. I do know that this can screw things up.
Also, the intake looks long to me. If the venturi intake was designed for a motor with a flat petal valve, then this intake section needs to be the same length from inlet to the average opening point of the valves. This length is critical, acoustically.

Hi Mike , thanks for the feedback , I tried shortening the inlet , didnt seem to make much difference , I didnt get to do the grooves yet , I understand the theory and it is a good point , but will take some "imagination" to do the grooves , ha

Hi Grim.
Will your motor run, without forced air, on Propane?
If it will then the pipe and valves are probably OK and you can be sure that it's the pressure drop across the fuel system that's the problem. If the motor runs on Propane, try sleeving down the induction cone. This will increase the air velocity and reduce the pressure over the injector resulting in a better draw of fuel and better atomisation.
I don't know anything about the acoustic length of the induction cone that Mike talks about but have seen these motors running without any cone at all, admittedly I think it was on Propane.
Looking at the design of the valve travel stops. It looks as if you have used the design of the outer shell to perform this function. Do you have too much valve travel would be my next question?

Nice motor.
Graham.

HI Graham ,
I tried propane briefly , but got loads of blowback from the inlet , I guess its one of them "feeling" things , just the right amount of air etc, Will try again, sleeving the inlet is on my list , we shall see

Correct, the wall of the transition is the valve stop , I done it like that in an attempt to provide better cooling for the valves , and you could well be right about the too much travel ,

Will keep experimenting , this engine is making good power, even as it is, I expect some good results once it is sorted , How many times you heard that one

Thanks guys , will keep you informed ,

[Graham C. Williams]

H Grim.
Here is an interesting difference of opinion! I found these some time ago. They are recommended set-ups for valved motors
Nf = Pipe Operating Frequency
Nr = Reed Frequency

Tharratt:
Nf/Nr = 0.8

P. Schmidt:
Nf/Nr = 0.33'
or
Nf/Nr = 0.2

Graham.
P.S. I've lost the source of this information so cannot give it here.

[Viv]

Hi Graham

Looks like an interesting progression to those numbers, personally I went with the Schmidt's number but maybe only because it was the first one I looked at years ago, I wonder if these will plug in to Forrest's excellent thread on modes?

As for the motor it could be the picture but the tailpipe looks ever so short to my eye compared to the bustle and the CC?

Viv

[Graham C. Williams]

OK. Using Forrest's Form we have:

dfr = f/fn
:
dfr = Driving Frequency Ratio
f = Pipe (driving) Frequency
fn = valve frequency

We have:
Tharratt:
dfr = 0.8

P. Schmidt:
dfr = 0.33'
or
dfr = 0.2

Alright, this asks the question "is fn the free reed frequency"? This isn't the place for this discussion. I'll move it over to Forrests excellent post.

Graham.

[GRIM]

Here is a video and some pics of my G59er ,
The fuelling took some sorting out , (learning curve) but figured out the start up routine and got it to sustain reliably,

Running on 97 unleaded gasoline,

It was very bright sunshine , this gets a whole lot hotter than it looks in the vid , also it is made from 1.5 and 2mm steel , so takes a while to warm up ,

In real time it is interesting to look down the inlet and actually “see” the reeds opening and closing whilst running,

In the original vid I managed to capture this reasonably well , In the compressed vid here , it is barely visible ,

Reeds are 0,0085” thick , Driving frequency / valve frequency ratio 0,66 , I was aiming for 0.59, I missed ,
That may explain the extremely eroded valves in the pic , continued testing will define that , and of course full power static runs are the worst case scenario ,

I currently do not have thrust figures , That thrust rig sucks , time for a re think ,

In the table top pic , you may see there are two engines , I ran both today , they currently share the same valve grid , that may change ,

Finally I cannot thank Forrest D Eckstein enough for all his help with the Math ,
I have learned more math and how to work with spreadsheets, in the last month, than in the last 20 years ,

[Mark]

That looks like a clever little fuel tank arrangement. Often, the pulsejet is the easy part and the fueling the tricky part. For example, lot's of things I've made will rev up or run for short durations, but getting a workable/practical fuel feeding system is often the greatest battle or more mental work than the engine itself. Engines and fueling, quite the diversity.

[WebPilot]

Hi G,

Congratulations!

Oh, that's where the G59'er comes from. I get it.

You learned the math because you had a reason and thus, a desire to learn.

Having one of the consequences of my theoretical thread, dynamic modeling of a strip valve experimentally verified ... priceless.
Thank-you.

You're the man!

[larry cottrill]

GRIM -

Congratulations! from me as well -- man, that is just fabulous! Wonderful work, sir, what a nice running beast!

Excellent example of skillful design and fine workmanship.

Darn! Someday I've GOT to make a mild steel one with one of the little valve sets Mark kindly gave me a few years ago.

L Cottrill

[GRIM]

Thanks to all for the comments ,

I will post some photos of this “build”over the next couple of days, I wont post the plans until I have got some accurate data to back them up ,

Here is a photo of the fuel setup, I tried several different styles , this one worked best , it is not unlike the bailey jet setup apart from the mounting *

The flow control fitting, is a SMC pneumatic device , commonly used on pheumatic cylinders ,
Attached is an image , showing some other styles ,
I used style “D” 1/8” npt 6mm tube

The conical thing on the extreme end is what I know as an “olive”, These are commonly used on small bore copper tube with compression fittings ,

This device was constucted and mounted* in this fashion to allow some flexibility for tuning to find the best position etc , and to avoid the need for machining ,

This whole engine was built without any kind of machining , apart from hand drilling and tapping the threaded holes in the valve grid ,

The valve grid itself was fabricated from four pieces of 20x20x2mm square steel tube , left over fom a gate I built for the garden LoL,
I then ground and filed them to shape and then valve seat faces were hand lapped flat, photos to follow,

G

[Mike Everman]

Wonderful, mate! So Cool!

[GRIM]

Thanks Mike
Here is a cross section drawing of the grid,
The green rectangles represent 1mm thick steel ( I call them tuning stubs ) these are rectangular spacers that are sandwiched between the grid and the inlet wall ,
By changing their length one can effectively change the length of the reed valve , thus changing its frequency ,

These also ensure that the shape of the “inlet” inside wall doesnt influence the natural curvature of the valves, thus these valves are indeed free to oscillate as if there were no valve retainer , see the current discussion in, viewtopic.php?f=2&t=4984


However the proximity of the curve would act as an “extreme” travel stop , I could see no evidence of this on the valves though ,

More photos to come,

G.

[Mike Everman]

Neatest valver build yet, man. Nice work!