r.b pelteire.question

[Mike Everman]

OK, good enough for me! Seems the difference from a normal PJ cycle then, is that the pulse from CC1 is forced to do more work than mere ejection, hitting a pressure head in CC2, and is not fully utilizing it's inertia to create the low pressure zone in CC1 that draws a new air charge in. So if we can name the demon, can we not banish it?

[Graham C. Williams]

Dear Mike.
A few things to think about.
1) The interface is not planar; this is only a one-dimensional approximation.
2) The combustion generated compression wave travels ahead of the combustion-generated interface. The interface travels at about half to a third of the speed of the compression wave.
3) When the interface (generated by combustion in chamber A) starts to compress the other charge (in chamber B) it will generate compression waves that travel back towards chamber A. This will increase the pressure in chamber A.
4) If the pipe is long enough, the combustion compression wave, from A, that has reflected in B and partly compressed the charge, and is heading back down the pipe towards A will oppose the flow of the interface. In addition the compression waves generated by the compression of charge in B will add to the strength of the original compression wave.
The net effect (I'm speculating now) will probably be a general bogging down of the motor as a result of the reduced ability to breath. The augmented compression waves will slow the induction mixture to below flame speed causing premature ignition before the interface has much chance to act upon the mixture.
Graham.

Graham.

[Bruce]

All this talk of waves might be helped by some analogies.

A wave as such is not always a transfer of mass -- sometimes it is only a transfer of energy.

For example -- waves roll onto beaches all around the world -- but the water level doesn't keep rising does it?

Of course not -- because the wave is merely energy that displaces the water temporarily.

However, sometimes a wave *is* accompanied by a mass-flow and a good example of this is if a dam breaks in a river.

A huge wave is generated -- that wave being the leading edge of a mass-flow consisting of all the water that was behind the dam.

If you measure the level of water ahead of and behind that wave you can see that there is a difference - unlike ocean (energy) waves which leave the seal-level unchanged.

These distinctions are important when you're trying to model a pulsejet in your head (or otherwise).

Engines such as the Lockwood (and indeed, most pulsejets) operate on mass-flows, which of course, have accompanying pressure-waves.

In the case of the Lockwood (and many other pulsejets) it's better to think of the pressure wave produced by combustion to be a side -effec of the massflow rather than the other way around.

The *power* within a pulsejet engine is contained almost entirely within the mass-flows, not the pressure waves. Sure, there is some power in those pressure waves but compare the energy levels of a 4-foot high ocean wave with those of a 4-foot high mass-flow generated by a breaking dam and you'll see the difference.

If you're floating in the ocean when a 4-foot wave goes by you'll bob up and down but won't be in any danger. If you're floating in a river when a dam breaks and a 4-foot wave comes roaring down -- then you're in big trouble.

The difference?

Both waves are 4-feet high but the ocean wave has only a tiny fraction of the energy that the massflow in the dam-break scenario has.

I have come to the conclusion that Trying to compress gases with pressure waves in a pulsejet is like trying to nail jelly to a tree. It can be done to a small degree but the results are seldom worth the effort.

It is much better to look at the reasons *why* you might want to compress gases in a pulsejet.

This is the tack I ultimately took with the X-Jet. If compressing the gases is so very difficult, why not try to achieve the same result that compression gives by using an alternative mechanism.

Think laterally folks :-)

[Viv]

Bruno from memory 10-30 metres a second for deflagration 500 metres a second for front mode combustion (fast deflagration) and detonation from about 2000-3000 metres a second, yes 2 to 3Ks.

Viv

[Bruno Ogorelec]

Bruce,

I agree with you almost all the way, but at the very end, we diverge. They way you describe the energy of the mass flow and the energy of the wave is the reason why I have become disenchanted with wave engines, elegant as they look to some people.

Yes, I agree, we should think laterally, but I prefer to think laterally on how to achieve compression, rather than on how to find other means to get to the same result you get with compression.

That is a part of the reason why I am no longer particularly active in the effort to develop the BCVP engine. (Another reason is that I no longer understand what the Albion team in the UK is really doing, ha-ha-ha... They have gone off on a tangent my tired old brain is not able to follow.)

Instead, I have concentrated on other ways to utilize the mass flow to raise the mean effective pressure in the pulsejet cycle. The secret -- I think -- is in abandoning high compression. In pulsejets, the onset of diminishing results in raising the compression ratio is quick. The secret is in boosting the pressure a little. you want it to be higher than in a 'normal' pulsejet but lower than in a piston engine. That is the spread in which most of the available boost is hidden. After that, further efforts are self defeating. You just generate additional problems.

As for the wave energy, well, it's there for the taking but it's a tricky thing, as Graham's example shows. Nailing jelly to the tree is not a bad simile. But, it should not be completely dismissed. You can't just let the energy go without trying to harness it if at all possible.

Bruno

[Mike Everman]

Graham and Bruce,
I am trying only to think in terms of mass flow, and I take it as a given that the pressure wavefront does little but make noise and travels at the speed of sound (dictated by temp and density of the gas in the CC). The mass transfer, while slower, must compress the opposite charge as a result of the air piston's deceleration and we must,
a: give it time to do this (the critical dimension on the sketch).
b: allow its trailing edge to continue, creating the all important low pressure zone behind it.

If the air piston (mass flow, now) stops and rebounds suddenly, this device will not breathe correctly. The E-P diagram seems to pay little heed to this, as well as constricting the main exhaust for higher velocity going into the turbine; a detrimental back-pressure that will inhibit the bypass we're depending on to charge chamber 1.

I beleive all of this has very little to do with whether the elements are 1/4 or 1/2 wave all by themselves; and I suspect it would only work synchronously with no Blast Compression.

And to qualify all of this, my family holds that I am often wrong, but never unsure! ;-D (turns out to be true...)

[Bruno Ogorelec]

Mike,

Placing the 'diverter' where you did simply blocks the exhaust where the pressure fluctuations are the least and shifts it to the places where fluctuations are higher, meaning that instead of a constant-pressure effluent you will get fluctuating-pressure effluent. As constant-pressure only exists in theory enyway, your change will just make the small fluctuations big. If you want to drive a turbine, this will notably lower its efficiency.

[Mike Everman]

Placing the 'diverter' where you did simply blocks the exhaust where the pressure fluctuations are the least and shifts it to the places where fluctuations are higher, meaning that instead of a constant-pressure effluent you will get fluctuating-pressure effluent. As constant-pressure only exists in theory enyway, your change will just make the small fluctuations big. If you want to drive a turbine, this will notably lower its efficiency.

but if it's not there, I see big, big turbulence!! I don't think in any case that this is a high effeciency thing.

[Bruno Ogorelec]

but if it's not there, I see big, big turbulence!! I don't think in any case that this is a high effeciency thing.

What turbulence? Do you get great turbulence in a diverging exhaust pipe? Not if it's in the right place. You must have seen exhaust pipes joined at right angles on high-efficiency machines.

I am not certain what you mean by saying you don't see it as a high efficiency machine. If we forget about the dispute on whether teh E-P works or not and assume that it does, its mean effective pressure is way higher than in a pulsejet. If that does not spell efficiency in an internal combustion engine, I don't know what does.

Pressure waves stay inside the engine and do useful work instead of getting dispersed into atmosphere. No power is wasted on reverse exhaust flow. Finally, the gas exhausts at close to constant speed, meaning that you can power a turbine with it efficiently, unlike any other pulsejet.

Where do you see inefficiency coming from?

[Andrew Parker]

I think I understand the mass flow stuff, but doesn't the E-P release the "mass flow" at the velocity antinode halfway down the tube? Bodine shows a back pressure valve at the exhaust port of his e-p design. Could the effect of the mass flow be tuned in this way?

Also, check out http://www.macrosonix.com/other_technologies.htm to see what pressures can be acheived acoustically.


Andrew Parker

[Viv]

Thanks Andrew I had forgotten all about that web site from the old Acoustics group days.

Attached is a nice little picture of a mass flow leaving the end of a pipe and generating a vortex and also you can see the shock front of the compression wave preceding it.

Imagine this at the boundery of the two exhausts in the EP.

Viv

[Mike Everman]

What turbulence? Do you get great turbulence in a diverging exhaust pipe? Not if it's in the right place. You must have seen exhaust pipes joined at right angles on high-efficiency machines.

I am not certain what you mean by saying you don't see it as a high efficiency machine. If we forget about the dispute on whether teh E-P works or not and assume that it does, its mean effective pressure is way higher than in a pulsejet. If that does not spell efficiency in an internal combustion engine, I don't know what does.

Pressure waves stay inside the engine and do useful work instead of getting dispersed into atmosphere. No power is wasted on reverse exhaust flow. Finally, the gas exhausts at close to constant speed, meaning that you can power a turbine with it efficiently, unlike any other pulsejet.

Where do you see inefficiency coming from?

Easy, killer. I am persuing this because I do understand that more compression is more efficient. I meant that I feel there's more turbulence without the diverter. The diverter segments the air piston off and uses it repeatedly. Allowing the other flows a more direct path out, in my opinion. I'd try to improve on the paper-clip, if you asked me to.

With what I don't know, we could fill a book. I think I'll just watch for a while, I'm in over my head...

[Bruce]

> Also, check out
>http://www.macrosonix.com/other_te
> chnologies.htm to see what
> pressures can be acheived
> acoustically.

Ah, but we're talking a whole different kettle of fish with those acoustic resonators.

The accumulate the energy of many hundreds (or thousands) of individual pressure waves into a single pressure differential.

With a pulsejet we only get *ONE* pressure-wave per operating cycle.

There's no accumulation of energy -- what you get from a single combustion cycle is all you've got to compress the next charge.

[Graham C. Williams]

Dear Mike and Bruce.
How does pressure, low or high, propagate through a gas?
Please Bruce, in your terms.
Dear Mike.
Your drawing shows the gas flowing down the exit tube; it comes off the back of the hot plug. But surely, using the analogy of the cork in a bottle the flow into the exit pipe must be off the front of the plug, because the pressure is higher. Later, as the back of the plug passes the hole a reverse flow, to fill the low-pressure region behind the plug, must flow in the opposite direction to your arrow. Unless we are to assume that the induction has brought the pressure in the combustion zone back to ambient by this time.
The act of compressing the gas in the right hand chamber must slow the hot plug of gas. After all, you are extracting energy from this plug to compress the new charge.
How is the compression of the gas to be propagated through the new charge? Is it in one direction only or as Newton says equal and opposite? Consider the gas at some distance ahead of the hot plug (Hot interface), it cannot have any knowledge of the hot plugs existence until that information has been passed to it. How is this information propagated?
Graham.

[Mike Everman]

Later, as the back of the plug passes the hole a reverse flow, to fill the low-pressure region behind the plug, must flow in the opposite direction to your arrow.

I didn't consider that much in my diagram; for that matter, I guess I had assumed that the bypass would be roughly the same pressure as that region behind the plug.
surely you don't mean there's something wrong with my CFD algorythm???