Howling mode

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evildrome
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Howling mode

Post by evildrome » Fri Jan 30, 2004 12:52 am

Hi All,

I'm normally in the triangle Lockwood thread but I thought I'd start a new thread for this.

Today I continued to try to get my square jet working. I got some popping & banging as usual & had to give up in the end as the hairdryer (my wifes!!) ingested some blue exhaust flames which re-arranged the plastic fan blades to their detriment.

Now... before that happened I managed to get a mode of combustion I have not seen (or heard of) before. The noise produced was like a high pitched screaming, nothing like Lockwoods I've heard in videos. At a guess I'd say the pitch was maybe 4 times that of Chris' triangle Lockwood. It was bloody loud too but not as loud as the popping & banging.

There was a very focused blue flame coming from the exhaust & I could see dust from my firebricks dancing on the test rig, obviously being vibrated very fast at a short amplitude.

I actually think it was serendipitous. The hairdryer was set on low speed initially. As it ingested the hot exhaust it softened the fan causing it to flatten & speed up. Then I got this howling/screaming mode. It probably wouldn't have happened without the dryer 'modification' :)

I could keep this mode running for as long as I used the dryer but it was not self sustaining. I felt that it 'wanted' to though when I took the dryer away.

Has anyone else had a valveless run this way?


Cheers,

Wilson.

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Post by resosys » Fri Jan 30, 2004 1:11 am

It sounds to me like it was starting to resonate.

How long is your tailpipe?

In my experience, with the air flowing, the engines tend to pop and growl a lot. When you hear a distinct change in pitch, chances are it's resonating at some level.

The triangle video clips are not a good representation of the sounds that happen since the engine was already very hot and was almost resonating with the air wand blowing. Once the spark plug mount tore open on the combustion chamber, it was getting more air and stayed running without the starting air. I was also tweaking the fuel level at the tank to get it to run.

I'd say you are very close. It's a small engine and it'll probably be harder to get running.

Chris

evildrome
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Post by evildrome » Fri Jan 30, 2004 9:51 am

Chris,

I'll try & get some video of this happening. A picture of my jet is in the last message of the triangle Lockwood thread.

Cheers,

Wilson.

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Post by mk » Fri Jan 30, 2004 8:52 pm

evildrome wrote:Has anyone else had a valveless run this way?
I'm having exactly the same problem.

Take a look at "small lockwood - propane injection problems?".

I'm using a hot-air gun (it delivers hoter air than a hair-dryer) for delivering air. As long as air is forced to flow trough the pj it produces a this constant high singing sound (after some bangs at the beginning of the starting procedure) and the conical blue flame at the exhaust. I found out that it is a small, high frequenzy pulsating combustion, because an unconstant airflow from the combustion chamber is getting back through the intake (against forced airflow[!], thats why your hair-dryer melted). If I take this starting-air source away, my engine doesn't want to run (just a constant burning inside cc at injector outlet). Actually I have a needle valve assembled between the propane bottle and the engine, but tomorrow I'll try without this valve. I just hope that temperatures around 0°C aren't to cold for good propane flow.

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Post by evildrome » Fri Jan 30, 2004 10:13 pm

Hi MK,

My propane supply is in the same position as yours (III).

I am beginning to think that my CC is not large enough. I have built a square section PJ with flat ends. The volume of the square is less than that had I made it circular. Flat ends also lessen the volume.

There is a picture of my PJ in page 6 of the triangle Lockwood thread.

Do you have a picture of yours? We really should try & get video with sound of this singing mode of combustion.

Cheers,

Wilson.

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Post by resosys » Fri Jan 30, 2004 10:45 pm

evildrome wrote:We really should try & get video with sound of this singing mode of combustion.
Certainly get video and audio of all of it. I think I better understand the "singing mode" reference now. In the image below, the engine wasn't running yet and the flames were shooting out of the tailpipe in a conical shape. Is this what you are experiencing?

If so, how are you applying air to the intake?

I've found that when it's in this mode, there is too much air being forced into the intake. I've had good luck backing the air supply away from the mouth. We've also had good luck "popping" the starting air on and off.

Our normal starting procedure is to flood the engine with propane, then hit the spark and pop the starting air on. The rush of fresh air creates the correct mixture at the spark plug and there is a large bang, due to the massive amount of propane in the engine. The bang seems to help get things going, causing pressure waves to go to the right places.

The square engine will probably be tougher to start than a round one. There are a lot of reflected waves going on that may cause issues. The smoother the engine, the easier it seems to run, in my experience.

The triangle engine that I built was hard to start. It took us many minutes to get it to run. That's many minutes of constant propane, spark and air in different combinations. One of my little round lockwoods is also tough to start. The propane flow rate has to be pretty close to right on, or it flames out. Interestingly, once it is hot, it will auto start by simply turning on the propane. I'll have to set it up and take some video.


Chris
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evildrome
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Post by evildrome » Fri Jan 30, 2004 11:04 pm

Hi Chris,

> I think I better understand the "singing mode" reference now. In the image below, the engine wasn't running yet and the flames were shooting out of the tailpipe in a conical shape. Is this what you are experiencing?

No, not really. I have seen my jet do that, but this is something else. The combustion is definitely happening inside the jet. What you see in your picture is a diamond shaped flame where the flame initially spreads out & then cones back in. What I'm seeing is like the tip of a longer blue cone protruding from the end of the exhaust. I think there is resonant combustion in the horn rather than the CC and what I'm seeing is the tail end of it. The combustion looks nearly complete, no yellow at all.

Cheers,

Wilson.






If so, how are you applying air to the intake?

I've found that when it's in this mode, there is too much air being forced into the intake. I've had good luck backing the air supply away from the mouth. We've also had good luck "popping" the starting air on and off.

Our normal starting procedure is to flood the engine with propane, then hit the spark and pop the starting air on. The rush of fresh air creates the correct mixture at the spark plug and there is a large bang, due to the massive amount of propane in the engine. The bang seems to help get things going, causing pressure waves to go to the right places.

The square engine will probably be tougher to start than a round one. There are a lot of reflected waves going on that may cause issues. The smoother the engine, the easier it seems to run, in my experience.

The triangle engine that I built was hard to start. It took us many minutes to get it to run. That's many minutes of constant propane, spark and air in different combinations. One of my little round lockwoods is also tough to start. The propane flow rate has to be pretty close to right on, or it flames out. Interestingly, once it is hot, it will auto start by simply turning on the propane. I'll have to set it up and take some video.


Chris

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Post by resosys » Fri Jan 30, 2004 11:26 pm

evildrome wrote: No, not really. I have seen my jet do that, but this is something else. The combustion is definitely happening inside the jet. What you see in your picture is a diamond shaped flame where the flame initially spreads out & then cones back in. What I'm seeing is like the tip of a longer blue cone protruding from the end of the exhaust. I think there is resonant combustion in the horn rather than the CC and what I'm seeing is the tail end of it. The combustion looks nearly complete, no yellow at all.
Right. My image is a bit misleading since the propane was WAY up and the image exposure time was relatively long.

You are probably correct about the resonant combustion in the horn. Pull the starting air back. Take it away completely and wave it in front of the intake, increasing the amount of time it's there . Move it closer and further. It's almost always a bang that makes my engines run. The gargling, growling, howling, and singing are all indicative of the starting air not doing the right thing.

Anyone else have any thoughts?

Chris

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Post by Bruno Ogorelec » Sat Jan 31, 2004 2:22 pm

The photo showing the big flame shooting out of the exhaust is a sign that the combustion is taking place in the exhaust tube, not in the chamber. You should properly have clearly visible flame only from the intake. When the engine is tuned correctly, the exhaust should only emit hot air or perhaps barely visible flame. See the picture below. It was built by Paul Sherman.
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Post by vhautaka » Sat Jan 31, 2004 2:46 pm

Boy, that one has a big chamber! At least it looks so.. I want to build one as nice and fat as that :)

Anyway,

I had the same thing, gas burning apparently in the exhaust pipe.

I already asked this in my Lockwood thread, but nobody answered yet...

Could this flame-in-the-back be caused just by too little propane pressure / too little mixing? The engine fell within Lockwood's ratios for the different part sizes but the -10C weather with no warming for the gas cylinder could make the pressure lower than one tenth of what it would be at, say, +15C.

We got it to brap & bang occasionally, but it only pulsated with the flame in the tailpipe. Also we didn't run out of propane in acouple of minutes as we should have. :)

- ville

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Post by mk » Sun Feb 01, 2004 1:26 am

I noticed that singing/howling mode just appears at really small engines, otherwise you'll have to use an air source delivering a huge volume of air at a high speed, but you'll just get one of it at all.
I think at small engines the air HAS a relatively big volume (in relation) and HAS a high speed (like hair-dryers deliver the air). Though the air blows the flame out of the cc into the tail pipe. At my small engine was (I had to cut it in pieces for a new one) a rugh welding at ~1/2 of the tail pipe length. The wirls around this place built up a kind of flame holder, though the resonance length was about the half of the tail pipe length, too, what rises the frequency. The engine gor red hot at the combustion zone. But burning in the tail pipe means that there isn't enough space for getting a powerful combustion, what rises the frequenzy as well, though you get a kind of a little, at a high frequency (because of the screaming), pulsating engine, even more compareable to a ramjet. The conical, blue exhaust flame indicates good lean combustion. I had been able to tune the flame color and the screaming sound with the fuel valve a bit, what shows that it is indeed a kind of a true pj engine.

I'm sorry not having a camera or microphone or something.

mk

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Post by evildrome » Sun Feb 01, 2004 2:01 am

MK,

Thats very interesting. I didn't see this effect until I added about a third of the length to my existing exhaust. The piece I welded on wasn't perfectly aligned & there will be a lip. The size & shape of the blue cone I'm seeing is consistent with combustion at the joining of the two pieces.

I'll try to get video tomorrow.

Cheers,

Wilson.

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Post by mk » Sun Feb 01, 2004 2:21 am

I also had a little widening at this place. Perhaps this had an influence, too. Rough places and relatively big diameter changes at a short length are playing the most important roles. (Does your engines tail pipe constantly widen?)

A large tail pipe supports creating the "howling mode", too. I tried to get my small engine run with a shorter tail pipe. In this case was no "howling mode" occuring.
I think that the mass of air/fuel flowing through the engine (or rather tail pipe), that has a ground frequency, plays a roll, too. If the mass and the speed of the air/fuel mixture stands in a specific relation to the tail pipe dimensions, "howling" might be supported. But I don't know anything special about it. I'm just thinking about...it might be wrong, anyway.

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Post by mk » Mon Feb 02, 2004 10:21 pm

Today I got my engine running 4 times, but only for about 10sec. Earlier I wrote, I wasn't able to get the howling mode at this engine, but I got it today!
I tried to lenghthen the tail pipe (outlet: 2.0inches) with a 2.9inches diameter piece of pipe of an old exhaust. It lenghtened the tail pipe for about 4,7inches (it was approx. 9.5inches long). Then I tried starting again, but finally the battery of my ignition source lost its charge after the testings of the last days. So I decided to light the outflowing propane at the exhaust with a lighter. Of course a kind of fireball occured. The engine burned something else than lean at the exhaust end, by blowing air into the intake, too. I turned the main valve of the propane bottle to "focus" the fat yellow flame to a conical blue flame. Once the mixture got lean enough, the conical blue flame within the "howling mode" occured. I got the burning in about a third of the tail pipe length or less, especially in the end, near the added pipe. Keeping air forced into the intake keeps the "howling mode" till the fuel gets raw. I could even put the air source away for max. a second, by putting it back the "howling mode" occured immediately again (I think, mixture lit up at hot walls; you can get them red hot glowing, when diameter isn't too big in relation to the delivered air volume and its speed).
So my assumptions towards rapidly widening diameters (no complete outflow of frontiers, another pipe is needed!) and(/or) wirls at rough or rather dented areas (effect is the same: whirls are effectively decreasing inner diameters -> building up a gaseous venturi) seem to be correct. These things are building a small combustion chamber in augmenter style or rather "a half" lockwood combustion chamber, if you want, a (low velocity?) ramjet. I also noticed flames comming back (against direction of forced airflow) between the tail pipe and the added pipe. This shows that "howling mode is a pulsating combustion at a high frequency.
May be the the augmenter style of the added pipe has also positive influences on the "howling mode".

I think that's all I noticed...
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Flame colors at "howling". (green=unlit fuel/air mixture; my assembly)
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Post by evildrome » Mon Feb 02, 2004 10:35 pm

I managed to get video of my square VPJ. It never really runs but what is interesting to note is that, towards the end of the video, my assistant turns down the gas. As he does so the combustion moves from the exhaust up to the CC. As it transitions from one to the other, you can hear it just starting to howl before the flame front moves all the way round to the CC.

http://www.pgoffline.com/movies/PJ-run.mpg

Cheers,

Wilson.

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