Tuned intake and not tuned

[Maxime Chamberland]

Hello

I want to know the effect of a short intake and a long intake?
and which is the difference between a tuded intake and one not tuned on the performace of the engine and valve life?

[Bruno Ogorelec]

Hello

I want to know the effect of a short intake and a long intake?
and which is the difference between a tuded intake and one not tuded on the performace of the engine and valve life?

Maxime, you are asking a wrong question. Better to say, an imprecise question.

To put it very simply, the engine behavior is largely determined by the the combustion chamber-tailpipe combination. Once you have settled on that, you design the intake to suit this combination the best.

The intake must fit the rest of the engine in diameter, shape and length. The values of all three factors must be within a certain narrow range that will suit your combustion chamber-tailpipe combination.

If you have an engine that is very well tuned and you change just the length of the intake, performance will deteriorate. Either thrust or fuel efficiency or both will get worse.

However, in many cases you will be able to make up for at least a part of the lost performance by adjusting the rest of the engine (the combustion chamber-tailpipe combination) to suit the new intake length. So, the 'too short' pipe can become 'good' again.

In other words, we should not be talking of the intake in isolation, only in the context of the rest of the engine. Also, we are not talking just of the length but also the diameter, the slope (if any) the presence or absence of flares, pinches etc. All those factors will affect its behavior.

[pezman]

I think that it would work for improving static thrust. An L/2 intake pipe would be needed (e.g. for a 250Hz engine, a 65cm pipe - basically tripling the size of the engine).

A cool variant might be a "canard" pj, with intake and outlet check valves, no tail and a tuned intake. Without a tail, the pj could probably run a bit faster, and that would shorten the length of the intake needed. Another advantage is that it would put the jet right in the back of any vehicle, and the intake tube could function as a fueslage (like an old F-86 Sabre)

For a traditional pj, like a Dynajet or a Bailey, I think that such a tube would "compete" with other elements of the design and detract from overall performance if it had any effect at all.

[Bruno Ogorelec]

Pezman, would you care to sketch the thing you are talking about? I can't quite visualise it. Are you saying that you would use the intake, rather than the exhaust, to determine the working frequency? Or you would not use acoustics at all?

[Maxime Chamberland]

it not what I mean!

your engine have it own frequency this is dertermine by it length
but the intake have some effect on the valve frequency it what I mean.

try to run one of your engine without the intake and place it when the engine run.

[Bruno Ogorelec]

Maxime,

Of course it has some impact on the frequency but what's the point of that oservation?

The frequency is mostly determined by the clamber/tailpipe combination. The intake is also driven by the impulse set by the clamber/tailpipe. The better you tune it to resonate with clamber/tailpipe, the better the engine will function.

If you change the intake so that it no longer resonates with this primary generator of signal, you will not just get a smooth change of frequency, because we are not talking of an empty pipe that you strike with a hammer and listen to the sound. You will change the working cycle.

The resonance of the pulsejet also governs aspiration -- pumping of gases. A change that moves the resonance regime too far from the narrow design range disturbs the aspiration flows and the process does not take place as it should. Performance suffers.

In the 1950s, the Turbocraft company produced a valved pulsejet whose valve head incorporated a brass sleeve on which the entire valve mechanism could slide in and out by about half an inch or so. It affected the frequency and allowed very fine tuning to the conditions (air temperature, humidity etc.). Please note that they went to the trouble of moving the valve head, not stretching or shortening the intake, which would have been much easier.

[dynajetjerry]

Bruno,
You have piqued my interest by referring to a Turbocraft pulsejet. Do you have a model number and other specific information on the model that had a valve, inside a brass sleeve, that moved 1/2 inch in opening and closing?
I own one design that appears to be a pressure jet. It is an early version of the P-82. I've not run it because the needle valve leaks very badly against the propane pressure of 150 psi.
Tim Dannels is including information on all Turbocraft engine we can locate, in his new listing of model engines. I supplied some of it but have not seen examples of many that were advertised. Don Laird made drawings of a few.
I must add that I am unawareof anyone who is familiar with any Turbo-craft product and can confirm any of the performance figures that were listed.
Jerry

[Bruno Ogorelec]

Jerry, the model is P-80. It used a modified Kavan carburetor and a glow plug. I'd like to see the drawing of photos of the head to see how the frequency change was effected -- if indeed it was. As you note yourself, Turbocraft did not pursue truth in advertising very zealously when doing its sales literature.

Unfortunately, I only have an old note on it, no photos or drawings. I also have a drawing of the outside of a Kavan carb, but you must be familiar enough with that one.

[dynajetjerry]

Bruno,

I suspect your P-80 is among those drawn by Don Laird. He borrowed one from a friend and I made all the precision measurements for his use.

It is a 1 in. dia. straight pipe design with a round reed valve entrapped between the 1 ft. long pipe and the bottom of the machined valve head. I am pretty sure the valve was not fabricated by Mr. Britt (the owner, I'm told, of Turbocraft,) because it is such a complex and small piece of sheet st. steel. The 1/2 in.port is in the center of the head and the center of the valve is a 9/16 in. disc. What makes it unique is the suspension of the center disc. There are 3 zig-zag fingers that extend from the disc to the outer ring, similar to but much more sophisticated than, the spiders that support some hi-fi loud speaker cones and voice coils. The Kavan carb. is held in the head with a set screw. The movement of the center disc valve, axially, would probably be less then 1/16 in., if it ran at all.

If you are interested, I can copy Don's drawing and "snail mail" it to you as soon as you E-mail your address to me. As yet, I am unable to include any attachments to my E-mail messages.

I must point out that, regardless of Britt's claims of performance of his products, NO ONE, to my knowledge, ever succeeded in getting a P-80 to run. Some of his other jet engines may have run but with much less thrust than he claimed. Throttling is anybody's guess.

Jerry

[Bruno Ogorelec]

Jerry, yes, I have heard that caveat about Turbocraft products from other people, too. I've heard of people offering good money as wager to anyone who can make a U-22 work, for instance.

Thank you for the offer of a drawing copy. Frankly I am more interested in valveless engines and, yes, I would very much like to see a drawing of a valveless Turbocraft, even if it was mostly hype.