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Pulsejet thrust output

Posted: Mon Apr 05, 2010 8:09 pm
by HPSCL
When a valveless pulsejet design is advertised for sale on ebay and it states that the engine will put out a given amount of thrust; is that thrust achieved, using propane vapor? Or will it only put out that amount of thrust, with a liquid fuel?

I ask, because I vaguely remember reading somewhere, that the thrust output can be doubled, when switched over from propane to a liquid fuel...

Hypothetical situation: an engine was just advertised on ebay, claiming that it put out 12 pounds of thrust... it came with a fuel pump, 4 sets of injectors... blah,blah,blah. From the looks of it (size / quality) it appeared to be nothing more than a piece of junk... and I thought: "No way, will that thing put out 12 pounds of thrust???" - The engine had some sort of cover over the engine, that the fuel inlet seemed to be routed through and this cover was beat to $#%^.

I look at Eric's engine (on Beck Technologies) and a given engine is advertised for a certain amount of thrust... From the looks of them (again: size / quality) I have no doubt that they will put out, as advertised.

However, even if some professional builder like Eric, advertises a given engine design to put out a certain amount of thrust and that thrust measure is on propane vapor alone... then wouldn't it be fair (legal) to advertises them, at twice that figure?... and then (of course) to note that the engine will put out this amount, if run on a liquid fuel?

Re: Please put your "I am new and need help" question here.

Posted: Mon Apr 05, 2010 8:30 pm
by Mike Everman
Hydrocarbon fuels are all in the same ballpark in terms of mass per time per thrust. There would be no 2x factor one to another.

Re: Pulsejet thrust output

Posted: Mon Apr 05, 2010 10:07 pm
by Eric
HPSCL wrote:When a valveless pulsejet design is advertised for sale on ebay and it states that the engine will put out a given amount of thrust; is that thrust achieved, using propane vapor? Or will it only put out that amount of thrust, with a liquid fuel?

I ask, because I vaguely remember reading somewhere, that the thrust output can be doubled, when switched over from propane to a liquid fuel...

Hypothetical situation: an engine was just advertised on ebay, claiming that it put out 12 pounds of thrust... it came with a fuel pump, 4 sets of injectors... blah,blah,blah. From the looks of it (size / quality) it appeared to be nothing more than a piece of junk... and I thought: "No way, will that thing put out 12 pounds of thrust???" - The engine had some sort of cover over the engine, that the fuel inlet seemed to be routed through and this cover was beat to $#%^.

I look at Eric's engine (on Beck Technologies) and a given engine is advertised for a certain amount of thrust... From the looks of them (again: size / quality) I have no doubt that they will put out, as advertised.

However, even if some professional builder like Eric, advertises a given engine design to put out a certain amount of thrust and that thrust measure is on propane vapor alone... then wouldn't it be fair (legal) to advertises them, at twice that figure?... and then (of course) to note that the engine will put out this amount, if run on a liquid fuel?

Yes well you are right, almost everyone selling engines on ebay are exagerating, or flat out lying about performance. Aerosplice especially, they are trying to say that the mishapen junk engine not much larger than our 4 lber, is putting out 12, when in their own test videos they show that its probably not even putting out 2 lbs thrust.

Some engines will run slightly better on liquid fuel, but you arent going to get a 2x increase. Our 10lb thermo puts out about 14.5 on a methanol mix (about 12 max on prop), but that is because methanol considerably cools the fuel air mixture causing it to have very dense charges, meaning more fuel and air in the chamber per cycle.

Re: Advertised Thrust

Posted: Tue Apr 06, 2010 6:27 am
by HPSCL
Mike & Eric,
Thank you, for your replies. I tried to find the info that I claimed to have seen, but could not find. - Only thing I did find was a statement or two, that properly designed augmenters may double the thrust output of an engine?

Eric, your statement about the dense charge makes sense, as a denser air charge makes more horsepower in a car's engine.
Is this why some videos show a pulsejet engine running at a higher throttle, when liquid propane is introduced, or is that method just a why of forcing an extra amount if fuel, into the engine? (Rhetorical question)

I've tried, several times, to slowly tip my BBQ-sized bottle, to make the engine scream - but because the liquid doesn't come out steadily, it'll scream for a few seconds, before flaming out. I'm happy, that my poorly designed engines run at all, because I only build them for fun. My only goal is to make them as loud as possible, with the limited resources available... and for the money I'm willing to put into something.

After seeing that "12lb thrust" engine listed on ebay, I was tempted to list my current engine- simply for "spite", for lack of a better description of my thoughts.- I was going to state that MY engine, (though large): 1.) Starts easily 2.) Has a wide throttle range and 3.) Makes a lot of noise, while belching fire almost a foot out of the intake! - But I didn't, because you (Eric) had some of your engines listed as well and I didn't want people to think I was including yours, along with the dented one, when I would go on to state that the claims of over 10 pounds of thrust, were greatly exaggerated.

Funny, with that particular engine: I watched the countdown, until there was only 15 minutes left on the auction... Then it supposedly sold for a little over $150 and then the listing disappeared. The next day, it was back - showing the exact same pictures!? (dents and all). :? :roll:

I really can't think of a use, for a mid-sized thermojet, anyway- other than having something that's cool, because it glows red hot and make a $#%^load of noise? Their shape, is by no means, "aerodynamic" and they eat too much fuel, to be hooked to a compact fuel source. The larger ones, like the lockwood-hiller, or your 70lb. 180* design - can (at least) be used to power a go kart or something similar.

Thanks again, for the quick replies and sorry, for the boring rant.

Re: Please put your "I am new and need help" question here.

Posted: Tue Apr 06, 2010 12:27 pm
by Eric
From the looks of his feedback rating, I think Aerosplice is using multiple accounts to boost their feedback.

Our thermojet's are pretty efficient, the only thing is that they can tolerate the richest fuel burn conditions possible, it will increase thrust some, but the fuel consumption goes up dramatically above its rated thrust, so a lot of home made ones are probably seeming a lot more fuel hungry than they are.

We have 1 and 2 lb tank setups that they run well off of.

My never-ending quest to understand injector designs

Posted: Mon Apr 12, 2010 10:29 pm
by HPSCL
Sitting in the garage and wondering why Llama with Hats is so damn funny, I had an epiphany:

Does the speed at which the propane vapor flows out of the nozzle, have to be "in balance" with the
cyclic-rate of the pulsejet engine?

To start the engine resonating, you may require a certain stoichometric mix, as well as requiring how fast that mix is delivered.

I'm going to randomly pull a figure out of my butt and lets say (hypothetically) that an engine is resonating at a certain frequency and this frequency is the hot gas moving at 2100fps, through the
engine.

Depending on the overall length of the engine, the inlet gas may need to be moving at a cetain multiple of the speed of the hot exiting gas, or even simply matching it.

The inlet gas is at 20psig but the injector is a perfectly round 3/16" I.D. hole. - The speed at which the gas exits the hole is a mere ??70fps??

Crushing this 3/16" diameter hole in a vise, the hole now has the same surface area, but the gas has to move through a choked area (venturi). The 20psig inlet pressure now forces the gas to come out of the hole at ??210fps??

The same applies to water flowing out of a garden hose - pinch the end together and the water comes out with greater force and at a much higher speed.

I am also guessing that the length and angle of a flare on the intake and the tailpipe will also greatly affect how the engine cycles?

If the "x" amount of gas is flowing in at too high of a speed, the engine will blow itself out, as you attempt to meter more in.

If the same "x" amount of gas is deleivered at too slow of a speed, the engine will not even begin to rumble/gurgle at low (starting) inlet pressure.

I've also been able to tune my engines injector size, be noticing if the end of the tailpipe is turning blue... which I believed to be an indication that the engine was running way too lean.

Too rich and the engine will simply sit there and belch a large flame out of the exhaust.

I've been trying to take Eric's advice and consider that a turbulent flow is much better for mixing the fuel/air. But the more turbulent the flow becomes, the more difficult it is to get the engine to run, in the first place. The better the laminar flow is out of the injector, the easier my engines seem to start...
and this may be entirely due to the fact that my engine's are poorly designed and quite different than his designs.

This all came about after having the most difficult time ever, in getting a severely shortened engine to run again. It runs now and is twice as loud as before, due to the shorter tailpipe length. The combustion chamber seems WAY too big for the tailpipe's volume, or perhaps it's still too small...

I really have no clue, as I receive conflicting opinions on design ratios. These thoughts are nothing more than my own theories and I was only wondering if ANY of this made sense to anyone else?

Re: Aerosplice 12 lb valveless EBay pulsejet motor

Posted: Wed May 19, 2010 5:16 pm
by croatiablu
I happened across this site while searching for options to power a 60” WS F18 that I am building and joined this forum in order to reply to this post. Like many others, I am not in the position to sink thousands on a turbine unit. In the course of my search, I found Aerosplice's 12 pound valveless motor. After speaking to the owner of Aerosplice, I went ahead and purchased one of the 12 pound motors referenced here. Based on having run one of these motors, it sounds like the comments here are based solely on viewing the item on EBay. I've had the opportunity to run this motor for myself and achieved 11 pounds thrust on liquid propane. I have no doubt that the motor can provide more thrust than this but 10 pounds is more than sufficient for my needs here. The unit itself is all stainless steel from stem to stern with continuous welds that are not spot welded. While the consensus here seems to suggest that Aerosplice is selling snake oil, here, I very much have to disagree. I am more than happy with the unit and currently moving forward with this project.

As for my customer dealings with Aerosplice, there was a delay in shipping which the owner resolved for me by picking up the tab of overnight shipping. This and taking my calls at all hours are more than enough reason that I highly recommend this vendor. He has also been a huge assistance in ascertaining pumps for me.

Anyone quesitioning the validitiy of my statement can view the Ebay transaction under handle Croatiablu.

Thanks

Croatiablu

Re: Please put your "I am new and need help" question here.

Posted: Wed May 19, 2010 5:27 pm
by Mike Everman
Thanks for the review!

Re: Please put your "I am new and need help" question here.

Posted: Thu Jun 24, 2010 8:51 pm
by mitchell struwig
Hello everyone, im new two the forum.

I have just completed a new valveless pulsejet, just short of a meter. Im working on a drawing with dimensions, but maybe you'lle recognize my problem without the drawing.
It doesnt want to start. I try to start it by turning on the gas and lighting it at the tailpipe, Then I put a blow dryer to the intake and the small flame at the end escalates to a fireball then a blue blowtorch like flame, then the flame almost dissapears into the cc and makes a low bellow of a pop, the flame then returns to a blue flame and after about 2 seconds it makes another pop and repeats itself. But sometimes the flame dies and I have to relight and when I take the blow dryer away it dies, I havent achieved the constant rumbling of a true pulsejet yet.
Does anyone know what could be wrong, maybe too short or too long tailpipe or intake (we cut the intake shorter before and it helped slightly), not enough gas (I think im using butane, not sure, maybe butane propane mix), or maybe starving it of air because intake is much smaller than exhaust?

Thanks.
Mitch

Re: Please put your "I am new and need help" question here.

Posted: Thu Jun 24, 2010 9:23 pm
by metiz
Hello Mitch

It's hard to tell if there are any fundamental problems but regardless of that, there are a few things you should look at:

1) starting air. A blow dryer will not work on 99% of all engines. You need either a shopvac, a leafblower or best of all, an aircompressor.

2) you CAN start engines on butane. The only reason propane is advised is that butane tanks ice up realy fast reducing pressure. That said, reading your description it looks like you are not feading it nearly enough gas wich bring me to point:...

3) Loose the friggin' restrictor!! Unless you have a small engine, wich you have not, a restrictor like you'd use for your barbecue will NOT allow your engine to even start. A small camping like propane/ butane bottle will also not work. get the big bbq variety

4) Although lighting at the tailpipe does somewhat work, you want the flame to be inside the combustion chamber. the best way of getting it there is to ignite at the combustion chamer. use either a sparkplug or simply toss a sparkler through the intake/ tailpipe into the cc.

5) your injector (position) may be at fault but we'll need some data to diagnose that.

To help you further we'll need to see some plans.

Re: Please put your "I am new and need help" question here.

Posted: Fri Jun 25, 2010 8:51 pm
by mitchell struwig
Thanks for the reply!

Your advice makes sense, when I did the first test I couldnt turn the gas up to full because this would make a big red flame (meaning too much fuel to air), but only when I hit a sweet spot in the middle I achieved a blue flame, so it only makes sense that to turn the gas up to full I would need more air. So for my next test I will use a air compresser and hopefully I will be able to turn the gas up to full. I will also try to get sparklers, but for now Im sticking to lighting at the end.

As for the injector I think that it is not far enough into the cc, but I couldnt get it in all the way. Attached are two pictures of the pulsejet (sorry about the quality, had to take the pics with my cellphone) and one drawing of the injector.

Re: Please put your "I am new and need help" question here.

Posted: Fri Jun 25, 2010 9:04 pm
by metiz
Ooookay...

Bad news: that engine will not work. The combustion chamber is waaay to small, the intake is waaay to long and fat. Not like it would make a difference with this engine but you injector is also waaaay to far inside your intake. Chinese style engines (your engine is technically a chinese) like injectors at about 25/30% inside the intake. You may also want to try a simple open end.

From the photo's at least it looks like you've got some epic welding skils. Use an existing design like the 2.25 pound fwe engine to get you started with pulse-jets before you try designs of your own. There are tons of other plans here ("looking for free plans, look here") and scatered throughout the valveless forum that you can also try.

You COULD rebuild the combustion chamber and intake but that would be a lot of trial and error to get something that may only barely run.

Re: Please put your "I am new and need help" question here.

Posted: Sat Jun 26, 2010 1:55 am
by idunno
Hello,
I made a pulsejet with these dimensions, and I cannot seem to get it to work. It will "idle" at a low speed if I feed it compressed air, but I cannot get it to run by itself, nor can I get the firings to occur quicker. If I try to feed it more fuel, it will stop the pulsing, and shoot a flame out of the exhaust. I tried air intakes that are 1 3/16" in diameter, and the same length, but it had the same problem. I have also cut down the 1 3/16" air intakes down 1/2" at a time until they were about 1" without success.
any suggestions?

thanks
- Ryan

Re: Please put your "I am new and need help" question here.

Posted: Sat Jun 26, 2010 10:54 am
by metiz
Hello

Try to increase your intakes by about 35mm. also remove your fuel injector from the end of your combustion chamber and replace it for two intakes with an inside diameter of about 4mm positioned about 50% inside both intakes. Don't forget to flare both intakes.

I tried your injector idea once on a tried and true engine and i got the same result as you'it only ran on forced air

Of yeah, if you have a restrictor on your peopane tank remove it. your thermojet is way to big to run of off that

Re: Please put your "I am new and need help" question here.

Posted: Sat Jun 26, 2010 5:24 pm
by mitchell struwig
Hooray!

I found hope. After making changes such as cutting the injector and positioning it on the lip of the intake, also using a stronger blowdryer(which I melted), I was able to make it idle for 2 minutes and the front half of the cc went red hot, I took a video of it just before the red hot 2 minute run. The only problem is that it will only run with the blow dryer and the flame often dies.

What am I missing?

Video of short idle, with blowdryer:
http://www.youtube.com/watch?v=WeS3-ESJan8