## not-Eric and the half-a-Bcvp

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### not-Eric and the half-a-Bcvp

Mike, how about a threat on the half-BCVP?

your comment on halving the engine.
I had the same idea a few years back.
I even made a first prototype that didn't work.
Which doesn't mean much because its the only
VPJ prototype I have ever made, and my failure rate is 100%
But the theory sounds good.
;-)

another way to look at it is:-

take any old basically working VPJ (ha)
Add a closed end pipe sticking out of the combustion chamber.
The pipe's lengh, allowing for temperature, etc. should transmit the pressure pulse and reflect it back just in time for the next combusion.
In all practicality, better make that pipe slide adjustable.
Will probably need different lengths for starting and running temperatures.

Now, is that a BCVP, a Half BCVP, or something else?
If the pipe is narrow, the amount of energy running up the tube and back will be small, and a greater proportion will go out the exhaust. Efficiency increase small. Thrust higher.

If the pipe is wide, as wide as the combustion chamber, then most of the energy will go up the tube and back, with little available for exhaust/thrust. However, the large returning pulse should improve the efficiency.

If we look at limiting factors (ah, some maths!)
Then with an infinitely small tube, its just the standard engine.
Nominal thrust, Nominal efficiency.
With an large diameter tube (one that captures and reflects all of the combusiton pressure wave, we have increased efficiency, but zero direct thrust.
(Direct thrust is the momentum change from the energy that goes out the exhaust or other orifice, rather than up the resonance tube).
Even if we modify the direct thrust by its efficiency factor to get an actual thrust, its still zero.

Presumably with a positive pressure gain on each pulse, the combustion pressure will rise with each pulse until it reaches some steady-state value limited by the "Q" of the engine.

But, in the limiting case, thrust is stil eficinecy times zero, = zero.

So, we can plot tube diameter against efficiency and effective thrust.
At zero tube, we have nominal thrust and efficiency.
At large tube, we have a highly efficient noise machine whose only real output could best be described as leakage.

There may be an optimum point somewhere in between, or not.
This optimum point would produce greater efficiency (I'm talking about fuel efficiency), enough thrust to make the machine viable.
Unfortunately I can't tell without lots of maths or simulation or building one or ten.

For simulation purposes, this engine would probably map out as a T shape,
and would need to simulate multiple pulses to see where the pressure builds to. Well beyond anything we have available.

As for maths, a spreadsheet should be possible for some simple stuff.
Just black box stuff - energy in resonance vs energy through exhaust, (Q)etc.
Might be enough to indicate if there curve shape will have a useful maximum point or not.

On the construction side, does someone have a nice healthy reliable pulseject that they could cut a hole in the combustion chamber?
Might be able to screw in a variety of tubes, of different lengths, or adjustable. Smaller tubes shouldn't destroy the engine operation (much),
so hopefully it would still start. Then the tube could be successively crimped to shorter and shorter lengths, measuring thrust for each length.
Measuring sound levels should be a good indicator of optimal length also.

I'd expect wider tubes to be more of a problem.

Ideally, the end of the tube should be at the "front", so that when the pressure pulse is reflected back, we get momentum transferred from it in the right direction.
Smoothly curved pipes would be ok.

in the extreme case, the engine might consist of
A long cylinder, closed at the front end.
Combustion occurs at the rear end.
There are enough funny shaped pipes and thingies around teh combustion area to ensure good breathing. i.e. get rid of burnt gass thoroughly, and suck in a good healthy charge for the next pulse.
The "breathing apparatus" can be curved to point forwards or backwards as appropriate, to optimise thrust.
The bulk of the thrust comes from the main pulse hitting the front of the cylinder and reflecting back.

Phew. Hows that for a brain dump.

Don

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### re: not-Eric and the half-a-Bcvp

OK, Ok, brain in positive feedback loop - overheat imminent.....warning

If the point of combustion is considered to be at the rear open end of the tube, then breathing is not a problem. Lots of fresh air right there.
So what we have now is a tube, closed one end, open the other.
Fuel is introduced at the open end where it burns, producing available energy.
The tube constitutes a resonator of calculable frequence and Q.
Ergo, it resonates. just like blowing across the top of a tube, or bottle, etc.
And we are basically back to a Jam-Jar design, but with the fuel supply near the opening.

However, the tube is capable of resonating to sinusoidal or non-sinusoidal wave forms. Basically, you can shove any wave form in the end and it will be returned a bit later, a bit distorted (spread).
The energy source is capable of working at a range of frequencies and waveforms.
So, will it resonate sinusoidally, or with a nice sharp high-pressure, high efficiency spike? Murphy's law, and entropy, imply that any nice sharp starting spike will decay to sinusoidal fairly quickly. Bummer.

This tells me something very interesting:
We have lots of theories about why some engines work and some don't, why sume are powerful and some weak, why some are easy to start and others not. In particular, many people are looking at the resonance of the engine.
I have now followed a thought experiment attempting to improve the efficiency of an engine by adding a resonant tube.
The conclusion is that, from the point of view of efficiency, we need engines that not only resonante, but do so in a spiky, non-sinusoidal manner. (Bruno used to discuss this in the BCVP by having a conical reduction at the end of the blast tube, to "pile up" the blast to give higher pressure. I am now considering the same concept from a different point of view. )
We need techniques like this to keep the pulse sharp and clean, and not let it spread out.

Prediction for experimentation:-
VPJs that "crackle" will be more efficient than those that "roar",
all other things being rougly equal.
Could anyone will wide experience with the little beasties confirm or deny this prediction?

Don

Eric
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### re: not-Eric and the half-a-Bcvp

Huh, what? Did I miss something?

Where do you want the hole in the combustion chamber? My old 12 lb chinese engne can run without the spark plug in place, roughly a 5/8 ish hole, what size tube are you thinking of.

I could probably make some mods to that.

One thing I was working with for a while is having an intake on the front of the engine and the rear of the combustion chamber, so the intake streams would collide, instead of the closed end tube on the front.

Eric

Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

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### re: not-Eric and the half-a-Bcvp

Hi Eric,

I guess a 5/8 hole would be just about perfect, seeing as you have one already, and I have no other guidance.
What diameter is the chamber that the hole goes into? The ratio of areas is probably the important thing.

or a resonance tube that is open ended instead of closed.

Might well work, but it would be a different concept to what I am proposing.
The main difference would be that an open ended tube reflects back a low pressure pulse, instead of a high pressure one, so it would reduce combustion efficiency instead of boosting it.

(Once the low pressure reflects again, it turns back into a positive pressure pulse, but by that time it is very weak - too weak to be usable.
This is partly because the low pressure pulse itself is relatively weak compared to a closed end positive pressure pulse.
This, I think, is the reason why the VPJs are so difficult. It is difficult to get a good strong "suck" to bring in a fresh charge.)

If you run multiple intakes, then I would guess it would be about the same as having a single larger intake, of the same total cross sectional area.

I can see your point about using the intake air momentum to produce a pressure pulse when they collide, but its probably about the same effect as a a single intake flow hitting the front wall of the chamber.
(A head-on collision between two cars is the same as a collision between a car and an immovable wall).

It would be great if you could try the resonator tube, and measure any change in thrust and efficiency.

I'll review yesterdays ramblings later and try to explain better. Some diagrams might help.
(I have this tendency to occasionally blurt out stuff that even I don't understand the next day. Frustrating.)

Don

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### re: not-Eric and the half-a-Bcvp

The chamber is about 3.375". The 12 lb thrust represents a conservative thrust value that you can get by just sticking the fuel injector where ever, with finer tuning I have actually gotten more than 16 lbs out of it.

The closed end tube has to be the perfect place between too small of a diameter and too large so it doesnt act as a poorly designed combustion chamber, too far in either direction would be useless. My biggest worries is that even if you had a considerable diameter tube, that the wave reflection would be relatively insignificant when applied to the whole combustion chamber, unless the engine was specially shaped for this effect. Having a small tube instantly open into a much larger tube might cause it to act as an open end reflection instead of continuing to propogate.

As far as the opposed intake design, its all about timing, not so much as the wave reflections. The force may be the same if it smashes into the front wall or the two colide, but the collision between the two puts the air mass at a different point in the engine, and creates a more unified mass which upon impact will expand outward to the CC walls so it will be a nice "pancake".

With a simple chinese type setup the incoming air usually smashes into the bottom of the combustion chamber, into the front wall, and then up towards the top of the CC before getting ignition. The larger the engine gets the more chaotic the intake charge will be. The pancake fuel charge is much more desireable than the crazy pocket ridden vortex from a normal chinese style intake, especially since you get better pressure peaks from the combustion etc etc.

The ideal situation is to have a number of intakes situated so that the collision vectors are ballanced to create perfect outward expansion.

If you remember the escopette I made back a while ago, that I turned into a chinese engine, what I didnt post was that before turning it into a chinese engine I left the front intake on and tried the one of the first opposed intake engine tests, it was a crude test but that is where much of my current research is going, among other things.

Perhaps those who know about the escopette recouperator know what I might be doing with an opposed intake engine with all intakes having either a recouperator or an augmenter on it :)

Since I can fab something up quick that just screws in the spark plug hole I will give it a try, but im afraid with a tube that size the results would be rather inconclusive.

Eric

Talking like a pirate does not qualify as experience, this should be common sense, as pirates have little real life experience in anything other than smelling bad, and contracting venereal diseases

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### re: not-Eric and the half-a-Bcvp

Hi Eric,

I see your point about the "shaped-charge" effect. could be very interesting. I'll need to let it sink in for a while.
(Maybe you could offset them a little and get a swirl effect to improve mixing, or somesuch.)

Thats definitely a large combuster and a small tube.
On the plus side it shouldn't interfere with basic operation much.
On the down side, its unlikely its effect on the engine will be measurable.
But if it is measurable, it may prove valuable. Thanks.

Don