Senior design pulse jet barely runs...

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armada7-ar5
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Senior design pulse jet barely runs...

Post by armada7-ar5 » Wed Apr 25, 2012 8:03 pm

Hi everyone,

I've built a valved pulse jet for my senior design project at Columbia University (testing pulse jets in NYC is interesting). The pulse jet will run, but only with help from compressed air to atomize the fuel at the top of the fuel line and send it into the combustion chamber with some fresh air. The valves are circular (10 holes) and currently represent 75% of the cross-sectional area of the tailpipe. The combustion chamber is 3" diameter and the tailpipe is 2" diameter. The tailpipe is 25" long and the chamber is around 4". Video:

http://vimeo.com/41032618

I've included the video link of the pulse jet running with significant help from an air tank. Obviously, I'm wondering if you guys have any idea what the issue may be. I'm wondering if I need more than just a simple fuel tube perpendicular to the airflow at the venturi intake to effectively atomize the fuel? Doesn't seem like there's much suck from the pulse jet itself once its running. Other thoughts were that the petal valve isn't being allowed to flex enough or the valve area is still too small. Any ideas?

Hopefully I've provided enough information. Thanks!

--Jeff

Mark
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Re: Senior design pulse jet barely runs...

Post by Mark » Wed Apr 25, 2012 8:53 pm

Maybe a photo of the valves would help. You could try methanol which is more forgiving. Sometimes when injecting air, the fuel can get pushed back down the fuel line instead of what you want. Other factors are the play of the reeds, too much or too little and like you mentioned valve area. I've built a few simple valved pulsejets out of plumbing pipe and spring steel for reeds. Sounds like you are almost there. Just keep thinking of some other variable to try. Maybe the metering of the fuel/air ratio is off. Here's about as primitive as it gets, it didn't take that much skill, just simple trial and error. You don't have to be smart to build a pulsejet, you just need to become familiar with it's characteristics.
This is kind of a chugging start but I have had many that go full grease from the moment of the spark. That's the neatest thing how sudden they can start - instant on.
http://www.youtube.com/watch?v=NU3cQ8_upWk
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armada7-ar5
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Re: Senior design pulse jet barely runs...

Post by armada7-ar5 » Wed Apr 25, 2012 11:35 pm

Thanks Mark. AND by the way, we often would have trouble getting any pops or bangs out of the pulse jet from a cold start with methanol or methanol/ether mix, so we have been using 100% starter fluid as a fuel source (which is almost entirely diethyl ether). Even when we preheat the combustion chamber with a torch, 50% methanol 50% ether usually does not ignite for us.

I've attached photos of the various parts on the pulse jet so hopefully that helps. Right now I'm thinking that maybe bending the petal valves so that they are always slightly open may help. Again, any help is greatly appreciated. Thanks!

--Jeff
Attachments
topview.JPG
A top view of the pulse jet. Disregard the tailpipe extension added on, it was used today to rule out the length as being the issue.
venturi.JPG
A photo of the diffuser and the fuel line at the intake.
valveplate.JPG
valve.JPG
petalvalve.JPG
intake.JPG

Mark
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Re: Senior design pulse jet barely runs...

Post by Mark » Thu Apr 26, 2012 2:19 am

I think you should go with straight methanol and just heat the jet up a bit. That gets it to evaporate and going if it is cold out. Here's something illustrative to try. Put some methanol in a jam jar and run it. Then try a mix of ether and methanol. And then try just ether. I bet you will find the plain methanol a winner hands down.
What brand of starter fluid are you using? Usually there are a few other ingredients. I have played with pure ether and it's very flammable but it becomes too rich or sluggish very easily. You can run a pulsejet on just about anything, but ether is expensive and not really one that stands out. Is your ether dispensed from an aerosol can or are you using a typical fuel tank? Believe me I have tried a lot of fuels and methanol is the most forgiving liquid fuel I know of.
You might try cupping your hand over the intake to see if you are getting a strong enough vacuum effect to draw in/aspirate enough fuel. When I was building mine I would prime it with methanol and then spark it, holding my hand partially over the intake to feel the draw and it's sometimes kind of startling. Maybe you need a bit more enclosure/constriction around the fuel metering jet but in the video the venturi appear fine. Also you could try short bursts of air at different angles to see if that helps. Sometimes you can wet your spark plug with fuel spray and then you lose your spark. Your reeds and jet look like they have plenty of intake area, and the play of the reeds shouldn't be/allow more than 1/4 inch. If they don't shut fast enough, sustained feedback won't develop. The combustion chamber should be somewhere around twice the diameter of the exhaust pipe for the small pulsejets typically. Is there enough gap for fuel and air to skirt around your reed stop/retainer, it looks like it's very close to the diameter of the reeds but maybe it's just the angle in the photo? What thickness did you make your reeds and what diameter are the 10 ports?
It all looks good enough to run, the reeds and valves look neat and even. Just keep tweaking it, you're almost there I think. A closer look at your fuel metering jet in the venturi might help to see how it is putting out fuel. Seems it's most likely just a fueling problem, getting enough fuel flow through the tubing and getting the metering right should do it. I like to have the fuel level about a 1/3 of an inch from overflowing in the line so the fuel is there if it catches. Again, I would use plain pure methanol that hasn't had a chance to absorb water from the air, such as a bottle of HEET if you haven't already.
I can't tell how the fuel is spraying out but it should spray from the center of the venturi otherwise the fuel isn't going to get mixed optimally. Now that I look at your needle valve it might be a little small for the jet but I can't really say. Also methanol will require much more fuel flow than ether and that may be why you are perceiving better results with pure ether.
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armada7-ar5
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Re: Senior design pulse jet barely runs...

Post by armada7-ar5 » Thu Apr 26, 2012 4:06 am

Thanks again for the quick response, Mark. The starter fluid is called Thrust, and yes it does have other additives. We could only find ether in aerosol form so we actually just spray it into a small rc fuel tank with a straw (the limits of being in NYC).

I'd like to clarify: what do you mean by fuel metering jet? Because right now we simply have a plastic fuel tube protruding from the front of the venturi with an inner diameter of around 1/8". The fuel tank is at a level that gets the fuel in the plastic tube about an inch or so below the opening. Is this a mistake that we are just using a tube rather than some other mode of aspiration? In the video of the pulse jet you posted what's going on at the inlet/fuel line interface that I can't see? When you say the fuel should "spray" form the center of the venturi I'm assuming you mean spray as a result of airflow over the tube. Just trying to clarify cause we're newbies here.

I'm a little worried that our tailpipe diameter is too large for the pulse jet to run on it's own (it's 2/3 the diameter of the CC). I think you're right about the reed stop, I will likely reduce its diameter tomorrow to provide for more airflow. The reeds are .006" thickness shim steel and each of the 10 ports is 0.5" in diameter.

Our needle valve is very small, perhaps we'll try and grab a bigger one with a larger tube to go along with it soon. Our methanol is pure lab stock but it may be older and contain some water at this point. I'll post a video update tomorrow or Friday. Thanks again for the help!

--Jeff

Mark
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Re: Senior design pulse jet barely runs...

Post by Mark » Thu Apr 26, 2012 4:32 am

http://www.brasslite.com/SiteImages/Misc/HEET.html

This is how my fuel metered out on the jet in the video. And the other side of that valve, (the center petal valve), the one on the right a Dynajet and the bigger one on the left another homemade of mine. The other v-valves/pyramid valves for outboard motors. I think more fuel flow might be the answer, maybe a larger needle valve or just omit the needle valve and crudely try the 1/8 inch straight tube "metering jet orifice" in the center of the venturi if you can hold it in there. You can really slop in the methanol, most engines aren't very fussy with pure methanol. Thanks to Larry for drawing the sketch over my simple valve. It's a nice illustration.
Attachments
Pyramid valves and petal valves.JPG
Mark_valveplate_&_venturi_crop1.jpg
Last edited by Mark on Thu Apr 26, 2012 5:19 am, edited 1 time in total.
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Mark
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Re: Senior design pulse jet barely runs...

Post by Mark » Thu Apr 26, 2012 5:03 am

At the top of the jets are the metering jets. Notice the blowpipe at an angle to inject air over the ports. The little Tigerjet and redhead Dynajet have two little holes and the blue Bailey jet 10 little holes, if you can blow the image up to 400% you can see them. One time I ran that Tigerjet on methanol just by attaching a fuel line sleeved over the blowpipe and it ran fine! I used an air compressor to aspirate the fuel whereas usually you might use a tire pump out in the field. I should say the metering jets are technically the little fittings that screw inside the top part of the devices, each metering jet size can adjust for different fuels or altitudes. I have 3 sizes for my Bailey jet. The metering jet accepts the fuel line and the other end screws into the part leading to the fuel ports/holes.
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Mark
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Re: Senior design pulse jet barely runs...

Post by Mark » Sun Apr 29, 2012 1:59 pm

One time with a smaller Dynajet-sized pulsejet made of plumbing pipe I just mounted it on a board with a few nails to hold it in place and elbowed a piece of copper tubing into the front of the venturi so that it rested inside the throat a bit and ran it that way. I think the inside diameter of the tubing was around 1/8 inch and maybe pinched down a bit from being cut with a pipe cutter or I might have also crimped it just a slight amount fiddling around. I don't know why but I always started it from a cold start, priming the engine and then sparking it. It either started on the first spark or not. I had to air it out if I didn't catch and try again. That was my first pulsejet.
Another tidbit that may be illustrative is the time I tried to unscrew the flowjector from my Dynajet when it was still kind of hot and I broke the blowpipe off. When aluminum is hot it expands quite a bit and the metering jet/air injector device locked into the aluminum head. Anyway, I had to improvise by using my air compressor with a nozzle fitting and puff in starting air that way because of the missing blowpipe. If the injected air was not angled properly over the fuel holes/ports the jet was very fussy in starting. If your air injection angle is off even a slight bit, it can really make starting difficult. I think it might even have driven the fuel back down the line on some occasions. It's always a good idea to use clear fuel line so that you can observe the fuel level as you puff in air. Sometimes you can see the fuel go in the opposite direction it's supposed to. Instead of creating a low pressure you create a high pressure. The fuel has to be right up there near the exit so that if the engine catches or revs it will get fed in time.
The Dynajet instructions say try short bursts of starting air rather than a constant stream. Maybe longer bursts sweep from an initial low pressure to a high pressure on some homemade designs or if you vary the angle when injecting air. Or maybe the fueling could become too rich or high air flow interfering with the feedback, you could try adjusting the air pressure too. Each pulsejet often has it's own proclivities. Hope this helps, I'm no expert, just a dabbler.
http://www.youtube.com/watch?v=68MRkGxhvRE
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armada7-ar5
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Re: Senior design pulse jet barely runs...

Post by armada7-ar5 » Tue May 01, 2012 1:27 am

Success! This thing finally runs on its own:

http://vimeo.com/41325228

Made the reed stop smaller in diameter, using only a fuel line now instead of a needle valve, increased the valve area, and put a nozzle on the front of the venturi. Thanks for the help!

Rocket Man
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Re: Senior design pulse jet barely runs...

Post by Rocket Man » Sat Jun 30, 2012 12:06 pm

trust = doing this from memory I think it is = exhaust pipe cross sectional area x 3.4 = thrust in lbs for gasoline.

exhaust pipe length = diameter x 11 = length

combustion chamber diameter = cross sectional area of tail pipe x 2

combustion chamber volume = 25% of total engine volume

cone shape = 25% of total engine volume

Air intake = 50% of the cross section area of the exhaust pipe.

Reed valves should not open more than about .200". You can adjust the frequency of the valves my limiting how far they can open. If they open too far they beat them self to death. .010" thick reed valves work best.

Reed valve hole diameter should be limited to about .400" diameter if possible. When the valves slam shut they tend to bow down into the holes this causes the valve metal to bend and crack.

I have more information about how to keep valves from getting hot and how to make adjustable reed valves but no time to list all that now. I can successfully run my engine at 100% throttle static thrust for 40 hours with very little signs of the valves getting hot and no damage. If you switch the engine over from gasoline to alcohol thrust increases about 140%. Ram air will increase thrust too. Mixtures of gas/kerosene, gas/fuel oil, alcohol/water will increase thrust.

VIDEO http://www.youtube.com/watch?v=1VNyTsUT2Xg

luc
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Re: Senior design pulse jet barely runs...

Post by luc » Mon Jul 02, 2012 1:11 pm

Although your engine seems to run now, one thing you need to check is your valve material thickness.

If the valve material is too thin, the valve won't last and a good sign of this is that you can see the petal starting to bulk and you can see the holes imprints on the petals.

If the valve is too thick, you will end-up having to maintain compress air to assist the engine bending the valve, or if your engine is able to bend the valve alone, it will still end-up with power lost.

I remember messing with mine trying to find out what was going on. I made the mistake of probably changing everything on my engine before I decided to investigate the valve to later find out the problem was just the valve's thickness.

My engine ran best with best power with .018" valve ... If this can help ... :wink:
Luc
Designer & Inventor

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