My new Engine

[ganuganu]

The "combined" intake to tail diameter ratio is approximately 0.558 This typically results in the CC is starved for air, and is not completely driving the tail pipe. Adding more trapped mass behind the tail pipe only increases the problem.
Some injector setup can entrain more air and help produce better results, but 0.558 represents a large quantity of entrainment to balance.

Looks like the minimum intake length is very short also, adding to the intake leakage.

how did you calculate that ratio "0.558"? k do you say there is pressure leakage for the word "intake leakage", if so i designed an augmenter at intake side with diameter of 28mm and length 15mm.

[PyroJoe]

Below is the calculation. Augmentation will probably have a mild effect until the intake area is more balanced with the tailpipe.

[ganuganu]

with the same calculation i calculated for eric's(beck tech) 3lbs engine the value was 0.566.
And what should be the ratio to get my engine stable.

[PyroJoe]

What you have built is considerably different than Erics. What would you propose the ratio should be?

[ganuganu]

i made the design in following manner initially took 7.62 as cc dia and doubled it to get 15.24cm in which removed 25% which gave 3.81 and 11.4cm. Made the 11.4as cc length and 3.81 as choke length. And usually i wont take the tailpipe dia as half of the CC dia but this time i thought to make a try and made the tailpipe dia as 3.8cm, and the intake dia was 80% of the tailpipe diameter so i got 1.5cm and 1.5cm each intake. And while i design thermojets i usually keep the intake length as the CC diameter. But after you showed the difference between mine and erics i admit that my intake pipe length is small. And i also think that intake diameter should be more. Please correct if iam wrong.

[PyroJoe]

Ah I see, you took 80% of 3.8cm tailpipe and probably had 3.0cm as the intake diameter. And that would have been good, except to split the intake into two intakes you divide the diameter by 2 and the results were 2 intakes at 1.5 cm.

The intake diameter cannot be simply divided by two. Typically it is the intake area that is divided by 2. when the area is divided the intakes show to be 2.12 cm in diameter.

3.8cm times 2.66 would give about 10.1cm minimum intake length.

[ganuganu]

oh thats really a silly mistake i have made, but i will say you the reason how i thought to divide the intake dia as two. At the very beginning i dont know how to design the engine so i referred many engine(thermojet) designs for eg.eric 3lbs, metiz's 5lbs engine designs. when i tried to analyse the design eric used 3.17as tailpipe dia over which 80% gives 1inch after dividing we get two half inch pipes. In the same way metiz engine was designed tailpipe dia was 5.2cm 80% gives two 2.1cm dia intake pipe. I need to skill myself more to get a steady design procedure. There are so many confusions about this engine it really challanges me.

[PyroJoe]

No worries, Erics thermo is a special design, and I have looked at it many times. It is one approach to reduce the intake leak, but the rest of the engine is designed to allow that type of setup.

[ganuganu]

Now in my design there is intake(pressure) leak and engine is starving for air. I was totally upset about the engine's performance because it was hard to start the engine and once started i cannot open the fuel valve more that two turns. so many problems were there. Here does the thrust depends upon the noise that engine produces? because my engine now produces irritating noise at the beginning itself thats a strange thing i have ever seen in my built.

[PyroJoe]

Noise and thrust are typically not related. Noise can reveal data important to how the engine is cycling. Small engines can be more irritating than the big engines due to their high frequency.

[ganuganu]

I will change the intake diameter as 2.1cm each and will give it a try. i will post the video soon.

[ganuganu]

joe i have a good news for you, successfully started the engine and checked the thrust without augmentation. Dead weight with engine was 610grams and at full throttle it pushed out till 2.5kg so the difference gives 1.89kg of thrust approximately 4.16pounds. Next iam going to add some augmenter to both intake and exhaust side at intake aug dia is 3.3cm and length is 21cm. For exhaust dia is 5.2cm and length is 15.2cm. Thanks joe. i will post the video of the engine after adding the augmenter.
One more question: does the formula to find frequency(valveless pulsejet) is sound speed/4*L or sound speed/2*L.
Do we have to add the length of full engine or just multiply only exhaust pipe length.

[PyroJoe]

Good work
I would test with only the intake augmentors and spend time adjusting the gap, that tail has plenty of mass and it would be better balanced without the tail augmentor.

I don't use any specific formula to find frequency, I have tried several techniques and none of them are accurate. Engine temperature, and aerodynamic valving, and chamber shape appears to have a factor in shifting the frequency. Typically I will record the sound and run it through software. Mostly the sound will show 2 seperate frequency. It is mentioned in the forum about a fundamental frequency and a secondary also open pipe, closed pipe.

Sometimes if the engine is throttled down, one frequency will become clear.
I have a small engine that produces a very specific frequency at any throttle setting but the majority of engines have 2 or 3 different ones.

[ganuganu]

k then i would add the augs only to intake pipes. Actually i got that formula from this link

http://www.pulse-jets.com/download/docu ... lsejet.pdf i dont know how they calculated the frequency as 46. I planned to place the intake augs at a distance of 4.2cm.

[PyroJoe]

Make sure you can adjust the augmentors without much work.

It appears they found 46 from running a real engine.

The result was 50.1 from equation (12.14)
the way the formula works is to take the speed of sound divided by the result of 4 multiplied by the total length of the pipe.

frequency = (speed of sound)/ (4 x total pipe length)

The 4 comes from the assumption the pipe is acting as a quarter wave resonator. Keep in mind this was a valved engine.