pulse detonation engines

[flyboy]

i was surfing the net yesterday and found somthing about these engines that they use less fuel and give out more power

i wanted to look at a diagram of the PDE but never found one and would like to find out more about ho these work since i am unfamiliar with them

[Mokele]

I've actually worked alongside someone working on them, and seen one working. Indeed they *are* quite efficient, but suffer from two problems. First, the timing needs to be very finely tuned and precise, or it just stops dead. Secondly, it makes a LOT of noise. And I mean that a 3 foot long, 3 inch wide tube of metal was producing noise that was nearly ear-splitting even *through* the high-quality noise-insulating headphones I was wearing. These things are *LOUD*.

 

The diagram is pretty simple. Get a metal tube, cap one end, inject a fuel-air mixture, and ingite it. Repeat. If the mixture is right for the diameter of the tube, it'll transition from a sub-sonic deflagration wave into a super-sonic detonation wave, which, when it exits the end, imparts thrust.

 

Mokele

[flyboy]

so there isnt an intake valve or anything like that?

 

also...how hard would it be to make one of these?

[d22k]

ive seen a version of one of these engines on scrap heap challence, it was just a u shaped pipe, both ends were open.

[flyboy]

that was probobly a pulsejet

 

how hard would it be to make one?

and would they be effective at low altitudes?

[d22k]

that was probobly a pulsejet

 

how hard would it be to make one?

and would they be effective at low altitudes?

 

sounds about right.

 

well, they made one out of scrap in 10 hours, it was running on a racetrack in england at no more than 100M ASL.

 

It propelled a ~250Kg 3 whelled craft at over 60MPH, it was VERY loud though

[Mokele]

so there isnt an intake valve or anything like that?

 

Not as such. You have to mix the fuel and air before injecting the mixture into the main tube.

 

how hard would it be to make one?

and would they be effective at low altitudes?

 

They are effective at low alts, and the calculations behind them are pretty simple (or you can just tinker until it works right).

 

But for the love of your eardrums, wear a *lot* of hearing protection.

 

Mokele

[insane_alien]

hearing protection: 1 pair of noise resistant headphones with 4 large duvets tightly wound round your head.

[calbiterol]

What? I'm sorry, did you say something?

 

[Kingpin1989]

Yea, I saw an article on this a while back...

http://www.popsci.com/popsci/aviation/article/0,20967,473272-1,00.html

good stuff...just wear earplugs

[vrus]

If you want to make one yourself, you're either going to make a model to look at, or you're gonna blow yourself, your work-place and the model itself to bits. If you still wanna try it call the paramedics first.

 

This is not something you can just make yourself. If it was that easy, why aren't they being utilised ?

 

Normal gas-turbines and other combustion engines burn the fuel at sub-sonic speeds, called Deflagaration. This is 'Detonation' and way more violent. The flame travels at supersonic speeds, creating a shockwave! For it to work, you need pressure, and hell loads of it ! And they haven't got a proper valve system for intake, exhaust & compression, as they disintegrate or break up due to the immense power of detonation. This thing is way more powerful than your average jet engine. It might be able to cause speeds of upto Mach 7 with ease !

It has an effieciency of 50-55 % compared to the gas-turbine's 30%.

 

I repeat

IT IS NOT POSSIBLE TO MAKE THIS AT HOME !

 

Anything similar to this is probably a simple pulsejet with an augmentor.

[insane_alien]

it is actually possible to build one at home. i have seen it done and heard it fired up(although i was 1 1/2 miles away)

[vrus]

it is actually possible to build one at home. i have seen it done and heard it fired up(although i was 1 1/2 miles away)

 

Pulsejets can be that loud, or even louder!

Its just not possible; yet.

[calbiterol]

Before doing my research like a good little student, I was about to post this: "You would be highly surprised. It can be done at home, and not just a pulsejet."

 

Then I did my research.

 

A pulse detonation engine, or PDE, is a type of propulsion system that is designed primarily to be used in high-speed, high-altitude regimes. To date no practical PDE engine has been put into production,[/b'] but several testbed engines have been built that have proven the basic concept. In theory the design can produce an engine with the efficiency far surpassing gas turbine with almost no moving parts.

(Emphasis added)

 

I have seen pictures of those prototype testbed engines, and they are quite fascinating. My curiosity with these is also thanks to PopSci. Anyways, long story short, PDE's are a LOT more complex than pulsejets.

 

Wikipedia article on PDE's

[vrus]

 

PDE's are a LOT more complex than pulsejets.

 

 

That's my point.

And where on wikipedia does it say that you can make one at home ?

 

Go to an external link and see what it says :-

 

"Amateur PDE Development

Unfortunately pulse detonation engines are probably not the sort of thing that is well suited to amateur development.

 

The risks of containment failure (ie: a catastrophic explosion that turns your engine into a grenade) and the difficulties involved in obtaining reliable detonation of readily available fuels make both dangerous and likely to be quite disappointing.

 

The sad truth is that you're unlikely to be able to coerce a mixture of gasoline and air or propane and air into true detonation using something you've built in your garage -- and if you do, the paramedics will likely have to take you away in a plastic garbage bag (that's if they can find all the pieces ;-). "

 

The link is :-

 

http://www.aardvark.co.nz/pjet/pde.shtml

 

Major Areonautic Firms might have built test-beds, but make one at home ? I don't think so ...

[calbiterol]

My point was the same as yours, vrus. Wikipedia never says that making one at home would be anywhere near possible.

[vrus]

So you agree with me ?

[calbiterol]

Yep, that was the idea.

[RICHARDBATTY]

You realy need to look at making a valveless pulse jet. They are quite simple and require no external fuel mixing. The reasons these devices are not used commercialy are noise and inefficiency. A quick search found this site http://www.aardvark.co.nz/pjet/valveless.htm but there is lots of info and some with online calcs. Have funn .

[vrus]

Yep, that was the idea.

 

I'm cool with that !

 

PDEs require detonation to work. That's what seperates them from other types of jets (and ofcourse the pulsing). And detonation is supposed to be hard to create too, right ?

 

I'm not sure if you all knew this, but I didn't. The knocking or pinging ina car engine is also Detonation ! The unburnt fuel air mixture explodes before the flame front reaches it causing detonation. I read it somewhere on wiki, can't remember where though....

 

Also I have a few doubts about detonation. How can you actually cause it rather than Deflagaration. I'm not sure my understanding on this is right.

What factors will affect whether it takes place ? Will largely increasing the compression/pressure create it ? If so, then I have a few ideas that might work !

[calbiterol]

I'm not sure if you all knew this, but I didn't. The knocking or pinging ina car engine is also Detonation ! The unburnt fuel air mixture explodes before the flame front reaches it causing detonation. I read it somewhere on wiki, can't remember where though....

 

Yeah, i seem to remember hearing about that somewhere, but the term "detonation" is often used very loosely, so I never took it that way before. That's actually rather interesting - it makes sense, too.

 

Also I have a few doubts about detonation. How can you actually cause it rather than Deflagaration. I'm not sure my understanding on this is right. What factors will affect whether it takes place ? Will largely increasing the compression/pressure create it ? If so, then I have a few ideas that might work !

 

From my understanding (which is not very good either), it's all about helping the shockwave to propagate. I think the wikipedia article (or a link from it) explained the process in more detail. The shaping of the detonation chamber takes a big role, too.

[vrus]

I am unable to understand the DDT (Deflagaration - Detonation - Transition) concept properly. Also, what makes a detonation a detonation and how is it sustained ? In other words, why doesn't it slow down into Deflagaration ? What force keeps it a detonation ?

The typical solution is to use a Deflagration-to-Detonation Transition (DDT) - that is, start a high-energy deflagration, and have it accelerate down a tube to the point where it becomes fast enough to become a detonation.

 

The typical solution is to use a Deflagration-to-Detonation Transition (DDT) - that is' date=' start a high-energy deflagration, and have it accelerate down a tube to the point where it becomes fast enough to become a detonation.

[/quote']

 

When it says have the high-energy deflagaration accelerate down a tube they mean the exhaust right ? If not, WHAT ?

 

How can the 'deflagaration' become faster ? And how will that make it a detonation ?

[YT2095]

Ooooookay!

 

this REALLY does NOT belong here in this thead, DDT does NOT apply to pulse engines in fact if it were to apply to ANY type of engine it would be a Deisel sort (and they don`t fly).

 

DDT in The context mentioned above is about Explosives only, in effect a charge in a critical diameter (enough to be resonant hydrodynamicaly) will at at 1`st Burn (deflagrate) and do this at such a rate as to build up a pressure wave (or shock front) that shock front then actualy breaks down the molecular bonds in the explosive and you no longer get the products of burning, but often just pure gasses, when this DDT takes place the shock wave propogages through the material (often several kilometres per second) and "sets off" the rest.

it burns.

builds up pressure and speed while burning.

this builds up a frontal shock wave.

That then propogates Down to the rest of the explosive using the explosive material as the medium to propogate this shock.

the material undergoes a transition where the shock is powerfull enough to break molecular bonds.

 

*BOOM*

 

the reason tubes are mentioned btw, is simply that given a tube of a certain diameter the DDT can be measured by where the tube starts to split, it`s sort of a Linear witness plate

[calbiterol]

this REALLY does NOT belong here in this thead, DDT does NOT apply to pulse engines in fact if it were to apply to ANY type of engine it would be a Deisel sort (and they don`t fly).

 

YT, I have to disagree. DDT does not have anything to do with pulsejets, no, BUT the thread was started as an inquiry about pulse detonation engines. Pulse detonation engines use DDT as a method to produce detonation in the engine. In fact, the quote that vrus mentioned was from the wikipedia article on pulse detonation engines. Given all of the above, I would consider it a perfectly valid discussion in this thread.

 

vrus, as far as sustaining it, they don't, really - hence the pulse part. The repeat the process (up to) thousands of times per second.

[Mokele]

calbiterol's right, the deflagration/detonation/transition scheme not only works for engines, that's where I learned it (though I've forgotten most of it). Back when I was in Aerospace engineering, I had to take two combustion courses, both of which included this. Detonation is simply rarely used except in pulse-det engines because it's such a royal pain in the ass to work with, in terms of forces, design constraints, noise, etc. So far as I recall, DDT applies to everything that burns in an atmosphere under any circumstances.

 

Mokele