www.pulse-jets.com

Here it is, all the discussion about Chinese engines being quarter wave engines, and LH engines being half wave engines, and I can put it to rest right now. Thanks to Graham for pushing me toward this.

The short of it is, they are the same. If you take the intake off of the front of an LH and put it on the back of the CC as shown, both engines have the same frequency. Faaascinating. I don't know about you guys, but I expected the freqency to drop a for a lot for a closed end of the same length.

The Chinese style has the advantage that pressure waves cross the CC twice per cycle, so it allows for higher pressure peaks and therefore frequency and therefore shorter engines. They're about 1/7 shorter for the same frequency just moving the intake.

At the end of my test program, I'll know if you can attain the same thrust as an LH for a given CC dia with the chinese layout. I'm going to predict not. I think the forgiving nature of the Chinese layout comes at the price of max thrust, but as a little bird kept a chirping, it has more thermal efficiency.

I asserted the other day that we should look at the S/LH was two quarter wave tubes back to back, with the pressure antinode (call it Pantinode) at about .25L, and that the Chinese was two quarter wave tubes with pantinodes that are the closed end. I believe this is an appropriate way to look at them.


I'll be gathering thrust and sfc curves at every step. The current setup has the cc entry at L/8 and the intake length L/6. I wanted L/6 for the entry, but the cc wasn't long enough, and FWE is good with it, so press on. Overall length is 42".

My plans, in order:
1. blend the intake flare better, I was in a hurry, gather data as is.
2. pinch the intake a bit, maybe pinch the fuel jet a bit, see if I can get a larger spread between low and max thrust.
3. take a section of intake out to make it 1/6 of the tail only, allowing re-installing of this section with band clamps.
4. Lengthen the CC until the inlet to cc is at L/6 and add a bit to the exhaust to get me to L/3 ass-to-intake (like a classic chinese).

Mike Everman wrote:At the end of my test program, I'll know if you can attain the same thrust as an LH for a given CC dia with the chinese layout. I'm going to predict not. I think the forgiving nature of the Chinese layout comes at the price of max thrust, but as a little bird kept a chirping, it has more thermal efficiency.

Great stuff! I'm sure gled you went to the trouble of making a comparison. There's no other way we could have known. Mike, this is a major contribution, I'd say.

As for thermal efficiency, it's better than thrust and more difficult to achieve. You want thrust, you boost the volume a little bit. As I often say, volume is cheap.

Well done, Mike!

Bruno already pointed the things out.

So I'll conclude that the comb. chamber (with regard to the internal conditions) is the important thing of a pulsejet, acting as a Helmholtz resonator in any case...Hmmm...What are we going to draw out of this fact?

Please take and publish the SFC, SLS and maybe other figures here or write the results in an email to me. Sooo interesting...

Thanks, Marten. The work is just beginning. I want to find out if the intake will be acoustically matched to the tail or the tail+CC. Also, I'd have to shorten the CC if I want the closed end to be in the same place with respect to the inlet as the LH, or lengthen the CC if I want to preserve the L/3 for inlet to closed end as in the Chinese. I suspect this last one is not the metric to be preserved, but we shall see.

Got to hit the road, I'll check in tonight!

looking forward to the next installment Mike.

Cheers

Nick

Sweet Jesus, I step away for a few weeks and Mike is off doing crazy pulsejet research!

Good stuff!

I'll send that damn ignition circuit out tomorrow, I promise!


Chris

So mike, you are saying the 1/7th shorter for the same frequency is essentially the lenght of the intake not sticking out the front?

I really really really would love to see you build a kazoo jet with 2 or 3 smaller intakes. One lockwood style, and one chinese style, on the same engine, just to see what would happen. I have been toying with that idea for quite some time, and your 2 kazoo tests showed that it could work very well. I will probably actually try that tomorrow if its nice out, now that I am really intrigued. I think that if the engine was designed right you could boost compression and suction a lot.

Eric

Eric wrote:So mike, you are saying the 1/7th shorter for the same frequency is essentially the lenght of the intake not sticking out the front?
Eric

Yes, CC head to exit. This thermo-acoustic length is what sets the frequency of the motor, and the intake must match. I expect the chinese type layout will need a little correction because acoustic waves are crossing the CC hot zone twice. Perhaps L/6 is a bit long for the intake to be in tune on this layout. We'll see.

Speaking of tests, I took the first Kazooenstein that I'd cut down the tail on, since it has the lengthwise relations, within reason, as the problematic rev10 engine. It was one of the early attempts to use L/3 from inlet to exhaust throat, just by shortening the tail. If you recall, it's the one I could barely start, only hot, and get running on "high" only, and the hotspot is "blown" down the exhaust port. This engine is 42" long, and the intake was 7". When it was 49" long, it ran beautifully, that is, the intake was L/6. (L=all but intake)

I shortened the intake on this one to 5.8", and it runs wonderfully! Starts easily and the entire CC is involved (red). It really cranks! I'll run again this weekend and post photos. I'll cut the intake on rev 10 today, which I'm most excited about running!

Not the least of which is re-examining the uflow files in light of this, because models with the intake being L/5 or L/6 both seem to run in UFLOW fine, so I'm missing something in uflow interperetation.

Yea I dont think valveless pulsejets are as picky as everyone thinks, as long as you arent building something microscopic. I was running one of the fwe prototypes the other day and the spark plug wasnt screwed in tight and actually vibrated out, the engine still ran, fire was coming out the spark plug nut, it wasnt running well but it was running.

I also tried the large chinese by injecting propane through the open spark plug nut, that way it mixed with some air as it was going into the cc. I didnt get enough fuel flow, but i got it to pulsate nicely without any forced air.

Well I got a freebie gas leaf blower that I am rebuilding the engine on... now im off to build a motorjet/ram jet contraption.

Eric