Technical help required

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farhad_sft
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Technical help required

Post by farhad_sft » Tue May 31, 2011 4:48 pm

hi guys, i know i am giving you headache but i really need your help. this is what i have designed for my aerospace B.Eng final semester project :
A valved pulse-jet with following parameters and specification:
combustion chamber length: 12 cm
combustion chamber diameter: 8 cm
the cone (between CC and exhaust) length: 12.5 cm
exhaust pipe length: almost 40 cm
exhaust pipe diameter: 4 cm
valved area(air inlet area): 10 holes with 1.5 cm diameter each one
* i did not put diffuser in the front! because i thought it is not that much important!! u think i am stupid?
i have used petal valve which i made out of Scraper because i couldn't find spring steel in the market. i did grinding on one surface and it bends easily and works good now.
for fuel i am using cooking gas (cylinder) without regulator. and for fuel nozzle i just made 10-15 holes with less than 1 mm( say .7) diameter on a rounded 6.5 mm diameter copper pipe.
valve plate is made out of aluminum with a cone in front.
the spark plug is almost at the end of CC( not on the cone) and i am using small motorbike 12 volts spark plug-
now! when i am trying to run the engine, using pneumatic/blower, there is only boom sound in the beginning and when i open the gas more, there is no combustion at all, and some times the gas burns outside of the engine, at the end of exhaust! when i decease the gas flow almost till close, the boom sound happens again, i donno what the hell is wrong with this? the design or the valve or ....
plz note that the valve is closed in normal case( engine off), should i keep some clearance between the valve and the valve plate?
and note that, when the engine gets hot, i get more booms but still not running, i just keep playing with the gas flow.
plz go through my design and tell me where my problem is, otherwise i will fail this project :( if u have any question plz inform me i will tell u every thing
hope to see ur replies buddies
all the best :wink:

Viv
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Re: Technical help required

Post by Viv » Tue May 31, 2011 5:19 pm

Hi Farhad

Just a thought but if you have any pictures of your engine you could upload to the site that would help people greatly in diagnosing your problems, a drawing too of your dimensions would be nice as well.

While your doing that a quick question,
Are you sure there are no air leaks in your welds? even a small pinhole can some times stop an engine starting properly?
Have you made sure your valves seal when closed by blowing back along the tailpipe?
If you blow down the inlet do your valves open? as you ground your own valves from a scraper they may be too stiff to open properly

Welcome to the forum

Viv
"Sometimes the lies you tell are less frightening than the loneliness you might feel if you stopped telling them" Brock Clarke

Viv's blog

Monsieur le commentaire

Mike Everman
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Re: Technical help required

Post by Mike Everman » Wed Jun 01, 2011 2:12 pm

Yes, you need the diffuser at the front.
Mike
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PyroJoe
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Re: Technical help required

Post by PyroJoe » Wed Jun 01, 2011 2:40 pm

This one is close except for the combustion chamber length:
(aerojet plans)
Attachments
aerojet.JPG

larry cottrill
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Re: Technical help required

Post by larry cottrill » Wed Jun 01, 2011 4:03 pm

farhad_sft,

I can't judge your design or construction (I DO agree that the front end "diffuser" shell is necessary, though). Here is an idea that I usually offer to valveless engine builders, but it applies to valved engines too - just in a little different way. It has to do with starting air technique.

My observation has been that the hardest thing for beginners to get right (at first-time startup) is the right amount of starting air. The builder almost always believes that it takes a real blast of air for starting. On small engines like this, that is most assuredly NOT TRUE. Starting air MUST be limited - if too much is used, you will never get your engine to run.

My old Dynajet is a good example. Compressed air is used, and it is delivered through a fairly tiny needle aimed directly at a small part of the valve area. What matters is that (a) you get just enough air to mix with fuel and blow a couple of the valves open; and (b) that you "pulse" the air (a push-button valve works well for this).

On an engine like yours (I assume you do NOT have a permanent starting air pipe aimed at the valve), the best solution would be a small compressor with a hand-held "blow gun" valve (like you'd use to blow dust out of equipment). With a small compressor and hose, you'd probably need 35 or 40 PSIG in the hose. You would aim it into the diffuser at an angle that would let it blow past the side of the cone onto the valves on one side of your engine. While you pulse the air (one to two short bursts per second), you would gradually open up the propane until you get a really good bang with EVERY burst of air and then NOT open the fuel valve much further. In the next few bursts, you should start hearing the pulsejet "roar" briefly, and then a longer burst of air should get it going on its own.

A blower is MUCH trickier (in my personal opinion), and it doesn't help that so many sizes and speeds are available. You can find out how close you need to be by testing without fuel. Listen at the tail of the engine as you gradually approach the front end with the blower, starting about a metre out in front. Come closer and closer until you hear the valves open and the wind just starting to blow through. You will probably be amazed at how far out in front of the engine the blower is at this point ! Now, for actual starting, you can be a LITTLE closer - but now what you do is "scan" the blower in front of the intake by waving it slightly from side to side. (This is roughly similar to "pulsing" the air flow.) As you do this, again gradually open the fuel valve until you start getting good bangs and then brief "roars". Now, SLOWLY swing the blower back toward the intake centerline as you open the fuel just SLIGHTLY more and you should have it running.

It usually takes several good "bangs" before a pulsejet will run, because it generally is necessary to purge the engine body of cool, dense air. Once the whole engine is full of heated, low density air, it should be ready to run once the air/fuel mixture is tuned properly.

Get that front end diffuser in place and go for it. Good luck!

L Cottrill

larry cottrill
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Re: Technical help required

Post by larry cottrill » Fri Jun 03, 2011 4:56 pm

farhad,

A couple other things:

In my opinion, if your valve and valve plate are properly designed and constructed, there is no need to have the valves slightly open when they are at rest. I know this has been often debated. However, there are lots of valved engines that start and run just fine with the valves resting flat against the valve head - why change it?

You have said nothing about providing the big domed retainer behind the valve. This is critical - you MUST provide a smoothly curved surface for the valves to "roll out" against as they open. If you don't have this, your valves will fatigue and break very quickly after you get your engine running.

L Cottrill

luc
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Re: Technical help required

Post by luc » Wed Jun 08, 2011 3:27 pm

Hi Farhad,

Saw your posting taking my morning coffee and said to my self "Hoo well ... Why not ... Have been away from this site to long".

So, I took the liberty of doing a 3D model of your engine according to the dimensions you posted (See the attached pdf file), which is your engine's exact replica (Took me 3 minutes).

Looking at your engine and the data I have, most of it seems fine next to few details and here are my comments ...

The tail pipe

I find your tail pipe a bit short and your problem looks like mine years ago. Go excessively longer on your tail pipe and cutoff later. The goal here is to have the engine running and then, cut the pipe down to tune and improve the engine. An engine with a to short pipe will not run in opposite to an engine with longer then necessary pipe. Especial if you used the "Pulsejet calculator", for this program has a tendency to produce engine to close to run/no run break point and if you missed that point ... Well ... No run.

The valve system

Here too, you seem to respect the valve entrance area required for this engine BUT;

1- Remember that a holes with sharp edges only performs at 60% of their actual surface area. Right now your total hole area is 17.67 sq./cm which I find bit marginal but Ok still, but IF your valve plate holes have sharp edges, then you're down to 10.60 sq./cm which is totally not appropriate. If this is the case, your engine is "Air starving" and why you get the initial BOOM and nothing after. If this is the case, take a Dremel grinder and round-up the valve hole's edges on both sides, so you get around .8 or .9 efficiency and you will save your valve too.

2- The other thing is your spring valve thickness. Go to thin and they will get destroyed in no time. Go to thick and the engine won't run. When I did mine, I found the .018" would work best and especially … Last the longest.

In conclusion and after looking at your engine compared to mine and the data I have, I believe your engine problem runs around 3 causes :

a) Tail pipe length (To short to my opinion)
b) Valve holes total area (IF you're using sharp edges).
c) And your valve thickness.

Next to the above, I will go with Mike (i.e: You need an intake) and Larry (i.e: No need to have the valves slightly open and round these holes edges).

P.S : Front round edges for air flow "efficiency" and back round edges for valves long life time … :wink:

Good luck … :wink:
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Farhad engine.PDF
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Luc
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luc
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Re: Technical help required

Post by luc » Wed Jun 08, 2011 3:44 pm

I forgot ... :(
Remember that your valve too act as a restriction even when open. They don't literly "Get out of the way" when they open and this too has to be taken in account when calculating and selecting the valve system total area.

For mine (See the attached), I had to use a specific hole geometry design to maintain the desired "Total valve hole area" according to effeciency and valves restriction.

This is why I say your valve entrance area is marginal (On the down side).

Good luck ... :D
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Luc's Valve Plate.PDF
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Luc
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Jonny69
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Re: Technical help required

Post by Jonny69 » Thu Jun 09, 2011 2:10 pm

luc wrote:Saw your posting taking my morning coffee and said to my self "Hoo well ... Why not ... Have been away from this site to long".
Well well well, great minds think alike. Good to see all the old faces in here :D

I agree that the pipe is too short. For a 4cm diameter you need a total length of about 90cm.

I disagree with luc about the valve area though. What he said is completely correct but you should have sufficient area there to get the engine running. You won't get top power but it should run. My 50mm exhaust valved motor has 10 13mm holes and it runs well. It could run stronger with more valve area but it runs.

My second concern is the fuelling. They use a lot more fuel than you think they will. A little stove bottle won't be enough. On my 50mm exhaust valved motor I use a 4kg bottle of propane with a 4 bar variable regulator on it. To give you an idea how much fuel it eats it'll ice that bottle up in about 5 minutes...

Finally, what kind of valve retainer have you got on the back? Just from experimenting, it needs to be curved, diameter needs to be to about the middle of the valve holes, the valves want to open no more than 4-6mm max and it needs to hold the valves closed when it's not running.

farhad_sft
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Re: Technical help required

Post by farhad_sft » Fri Jun 10, 2011 3:47 pm

Hi real friends! 1st i really appreciate ur supports specially Jonny. Luc, Pyro, Viv, Larry and Mike. sorry for delay in replying u people.
even i was thinking about that this engine needs more air and my air to fuel ratio in not correct. u know why, because last night while testing, the time i was increasing the fuel, there was no flame at all, specially when i was switching off the blower! or the gas was just burning out of the engine. now i will do the following and let u know the resul
1- add length to the exhaust pipe;
2- decrease valved area and smooth the holes from out side( from inside i can not make any change because the valves won't be able to close the hole properly and there will be a leak :( )
3- put a cone as a diffuser in front

dear all, plz note my valve is not too stiff since i already grind the valve from one side to make it thinner. and there is another problem which i came to know! my valve plate is too thick, almost 1cm ! is this ok?
thanx for ur replies guys

luc
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Re: Technical help required

Post by luc » Fri Jun 10, 2011 4:29 pm

farhad_sft wrote:my valve plate is too thick, almost 1cm ! is this ok?
Ouffff ... That's alot of metal, but in fact, it's probably making the air flow smoother and more linear compared to a thin plate, which is better. Next to the unnecessary weight added to the engine, I don't see any problem with a thick plate and who cares since this engine is not to fly.

I am also a bit worried when you mention "Grinded my valve", for we all know gringers or grinding leave scratch and scratches mean "Leaking" and breaking eventually. Maybe go to a local 2 cycle engine repair shop and try to get a bunch of used "Reed valve" that you can after, cut and rivet to your plate or maybe find another way to mount them. 2 cycle engine's reed valve make great pulsejets valve you know.

Hope this helps,

Cya,
Luc
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farhad_sft
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Re: Technical help required

Post by farhad_sft » Fri Jun 10, 2011 5:24 pm

luc wrote:
farhad_sft wrote:my valve plate is too thick, almost 1cm ! is this ok?
Ouffff ... That's alot of metal, but in fact, it's probably making the air flow smoother and more linear compared to a thin plate, which is better. Next to the unnecessary weight added to the engine, I don't see any problem with a thick plate and who cares since this engine is not to fly.
dear Luc, i know what u mean about the valve grinding results, but i grinded from 1 side only. the side which is not facing the valve plate. now how much should i increase to my exhaust pipe ?

luc
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Re: Technical help required

Post by luc » Fri Jun 10, 2011 5:46 pm

farhad_sft wrote:now how much should i increase to my exhaust pipe?
Baaaa ... Any lenght you want budy. Personaly, I would double it just to make shure you have an engine that start and run. You can after cut it off to tune your engine.

As I said before, a too short pipe will compromize the engine running where a longer pipe will just make run at a lower frequency ... But it will run at least ... :D ... Which is the goal here ... :wink:

Cya,
Luc
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farhad_sft
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Re: Technical help required

Post by farhad_sft » Sat Jun 11, 2011 9:15 pm

luc wrote:
farhad_sft wrote:now how much should i increase to my exhaust pipe?
Baaaa ... Any lenght you want budy. Personaly, I would
thanx buddy :D :wink:

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