Just need a few pointers...

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thecheat
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Re: Just need a few pointers...

Post by thecheat » Wed Oct 01, 2008 1:15 pm

well, let's go ahead and calculate the pressure on the bolts. the can of the combustor is 5.8 inches, that's 26 sq inches. At 15lb pressure (that's normal boost right?) there's 396 lbs of pressure on it. HOWEVER, there's actually minimal pressure on the flametube itself, it's "submersed" all around by the pressure, so it has nearly no net force on it. The can however, has all the force on it's rear wall, and so it has all of that force on it. If you divide the force by the 8 bolts, that's 49 lbs of pressure on each. BUT, there's one more, and that's the sparkplug, I threaded that through the can, and the flametube, so it's able to bear a bit more of the pressure. (it's also significantly larger then the other bolts, so we'll give it the value of 3 bolts in circomfrence to bear the strain. that would mean there's effectively 121 bolts, each with a pressure of 36lbs on each bolt.


That's all if I did my calculations right... still, 49-36lbs is a good bit of force. I'm fairly confident it would hold, but, I do think I see the need to add some bolts the the rear of the engine too. :wink: those bolts in the front are air cooled though, (since the inlet is right there) and I usually can touch the can where they are without getting burned. They aren't losing any integrity from heat. I also haven't noticed any signs of strain on the bolts... but like I said, I'll get any form of doubt out of the equation on this one. 8)

but, due to your valid concerns, I may be investing in some more stainless bolts... and a tap.


OK, onto the actual design of the combustor. No, I didn't use the jet spec program... actually, I've never heard of it... :| But, I did follow Sal's guide to the flame tube.
-take the area of the compressor wheel inlet, 30% for primary, 20% for secondary, and 50% for tertiary (if I remember correctly)

The reason I chose the small size was to up the amount of holes. I was looking for a nice even distribution of air around the flametube for even combustion. seems I defeated myself. :wink: Ok, so some larger holes in a few spots it is! Then also make the new holes on the injector aim for the enlarged flametube holes...

Would it just be better if I replaced the nozzle and drilled more (smaller?) holes with a larger total area that aim for the abundance of small holes?

Or, could I even mix the propane inside the can at the primary holes? Maybe the flame speed is too slow to be able to go through those holes, and not allow for any back tracking of the gas?

Or, should I leave the nozzle like it is, but drill 6 larger holes in the flametube more towards the rear of the tube where the holes already point?


I hope this stuff isn't bothering you, because you're helping me on heck of a lot! :D


PS: I think I'll go check that calculator you mentioned out...
Lasers, jets, and helicopters HURAH!

thecheat
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Re: Just need a few pointers...

Post by thecheat » Sat Oct 04, 2008 3:11 am

ok, here's the deal. I removed the flame tube and decided "heck with modifying the nozzle" mainly because it's fairly well stuck in place, and, I have no idea how I'll get it back in once I get it out. :lol: Definitely a spot of poor engineering... :roll: I'll figure out something I guess.

So I removed the flame tube and approximated where the jets of propane from the injector are hitting, and drilled two holes. on big and one small. Why? Er... I have no idea. for some reason I just wanted to do that. maybe for asthetics of something you'll never see. :roll: Well anyways, I lit it up tonight and had some improvement, then started adjusting the nozzle to see if I could hit a mixing "sweet spot" with those holes I drilled.

To be honest, I don't think it's quite tuned yet, but I did hit a "much better" spot and it spooled up very nicely. I was able to get it really spinning, and, PULL OFF THE AIR! it's officially now self sustaining!!! Still produces a heck of a lot of smoke, but whatever, it clears up once it actually gets self sustaining. I definitely need to make sure to top that oil resivoir off...

So, it's running! now I'm putting the jet pipe together... and hope to do some thrust tests in a few days. Not expecting too much, but shooting for 25lbs! :D
Here's a link to the video! Forgive the cinematography, running a jet and a camera is nearly impossible. :wink: I'll get something better together once I get the testing rig up, I promise. And some of those elusive daytime shots.

http://www.mediafire.com/?sharekey=6bd5 ... 9ed398672e


PS: so, about those afterburner plans, where might I find such things, or does anyone have some pointers? I'm looking for effective, and efficent, but I still want a bit of flames coming out the rear... :twisted:
Lasers, jets, and helicopters HURAH!

racketmotorman
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Re: Just need a few pointers...

Post by racketmotorman » Sat Oct 04, 2008 7:10 am

Hi Cheat

Your P2 (boost) of only 15psi is very modest , I ran a P2 of 45 psi on my jetbike .... http://www.racketmotorman.turbinebuilde ... hp?album=3 , a "safer" 40psi on my kart .... http://www.racketmotorman.turbinebuilde ... hp?album=4 , and about the same with Fat Mumma http://www.racketmotorman.turbinebuilde ... hp?album=4 , and if for any reason you experience a runaway , you might find "instantaniously" theres >30psi inside your combustor , so please play safe and build for the exceptional case , the last thing you need is the combustor exploding along with the turbo during any "unplanned" runaway .

The 30-20-50 division is the same as in Jetspecs which was developed from my Rule of Thumbs (RoTs) , but if the holes are only plain drilled holes , not bellmouthed or plunged , then the actual flow is only ~60% of what is possible if bellmouthed ( vena contracta problems) , so always err on the generous side with the FT hole area if only plain holes , and very small holes (<2mm dia) flow almost nothing because of "boundary layer" reasons , thats why all those small holes in your primary zone aren't working properly .

I'd strongly suggest you get the engine well sorted with a plain jetnozzle BEFORE starting any afterburner construction , if you put an afterburner on an unsorted engine you're only going to compound problems when you try to tune her, you won't know whether its the base engine or the afterburner causing the problems .

Having just got your engine to selfsustain , theres still a lot of work to do before its safe to start pumping huge quantities of fuel into the exhaust , you'll save time and money if you go thru the steps methodically :-)

Once you have running parameters for your "sorted" engine , its fairly simple to calculate the afterburner requirements , much easier than trial and error .

What sort of instrumentation do you have on the engine ?? .................at a minimum you need an oil pressure g/g , P2 g/g to give you a rough idea of the rpm/tip speeds and a turbine outlet temp (TOT) g/g so you don't overtemp her .

Cheers
John

thecheat
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Re: Just need a few pointers...

Post by thecheat » Sat Oct 04, 2008 12:26 pm

Currently, the only instrumentation is an oil pressure gauge. I do have a pyrometer for the TOT and a pressure gauge for P2's but haven't rigged them up yet. (I have to get a jet pipe on before the pyrometer is used, but I could start installing the p2 Gauge any time now...) I know that's generally not a good idea, but I was running it "easy" last night. I got it selfsustaining, and then turned it up a touch, and left it there. I absolutely do NOT want to burn it out before I get some use out of it, so I'm playing it easy. :wink:

I hope to have a tachometer on, but, it only goes to 99,999 RPM, my engine is rated to 120,000rpm. :roll:

But, there are enough leaks in the CC setup currently, that I know I'm not getting very impressive figures for pressure, and I know it's running a bit inefficiently. I'll be using some muffler repair putty to plug those up. :D

Ok, so if I'm only getting ~60% of the air volume going through, that means I should start opening some of the holes... I still think it's burning a bit inefficiently, so...

1- open holes
2- reinforce CC
3- get jet pipe and gauges mounted
4- Test
5-Tune
6-Afterburner!
7-Mount!

All right, so I'll get to work on that. Once again, thank you very much! I'll get to work on it.
Lasers, jets, and helicopters HURAH!

racketmotorman
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Re: Just need a few pointers...

Post by racketmotorman » Sat Oct 04, 2008 8:14 pm

Hi Cheat

Please get those gauges installed as quick as possible , you're flying blind at present :-( ...........its very easy to overtemp her at marginal rpm near selfsustain where comp/turb efficiencies are very low , a "safe" idling P2 is ~5-7 psi where the comp is working nicely and your TOTs should be at their lowest, its very difficult to judge rpm just by the sounds she's making , combustion should also improve as airpressures rise .

Do you have a compressor map for your engine ??

Cheers
John

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Re: Just need a few pointers...

Post by thecheat » Sat Oct 04, 2008 8:33 pm

you mean the cacluations? or some 3D sim... I think I have some the the calculations, but nothing but that.

But yeah, I'll get the gauges on before the next test. I've got a bit of re-fitting to do anyways. Now that I know the thing WILL work, it's time to get it working right. :D
Lasers, jets, and helicopters HURAH!

racketmotorman
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Re: Just need a few pointers...

Post by racketmotorman » Sat Oct 04, 2008 8:45 pm

Hi Cheat

Compressor maps like .......... http://www.turbofast.com.au/FlowT3.html ... so you've got an idea of what your compressor should/could be providing .

Yeh, now you've got her fired up , thats the biggest hurdle over with, the rest can be more enjoyable :-)

Cheers
John

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Re: Just need a few pointers...

Post by thecheat » Sat Oct 04, 2008 11:15 pm

hmm... I'll have to check that serial number and see what I actually have....

but, let's just go with the top chart so I can understand this.

Image

Ok, I see this, but what exactly am I looking at?

the pressure ratio is pressure out to ambient (or pressure in?)
the corrected air flow is... lbs of air per minute, which would describe how much air is being blown, right?
surge limit is what?
Then the lines are showing how efficient to blade speed the compressor is?


Then finally, where do I want to be on this chart? it would seem that I would want to be as far to the right as possible without being too high up on the chart...

hmm...
Lasers, jets, and helicopters HURAH!

racketmotorman
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Re: Just need a few pointers...

Post by racketmotorman » Sun Oct 05, 2008 12:24 am

Hi Cheat

Where you would want to end up if using this comp is at ~2.5 pressure ratio ( PR) , with 16 lbs/min flow on the ~145,000 rpm line , right at the top of the 70% efficiency "island" , efficiency starts to drop off quickly once the rpm increase above this point and we enter the 65% island where the compressors power requirement increases rapidly for very little "pressure energy" gain, we need to stay above 70% if we can .

Our mass flows average ~12-13 lbs/min /square inch of comp wheel inducer area , so this map would be for a comp inducer with ~1.33 sq ins of area , or ~1.3-1.5 inch dia , a very small turbo with pretty poor performance as a GT .

Pressure ratio is absolute pressure out divided by ambiant absolute pressure in , so 2.5 PR is 36.75psia out (~22psi P2 gauge pressure ), ambiant at 14.7 psi

The surge line/limit is the point where because the flow has been reduced too much the comp flow starts to break down with air flowing backout of the compressor intermitantly , the compressor "growls" , on full sized engines it must be avoided at all cost otherwise the engine can be destroyed , even with our small DIY engines it can lead to bent shafting ,........... from bitter and expensive experience I can vouch for this :-((

Cheers
John

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Re: Just need a few pointers...

Post by thecheat » Mon Oct 06, 2008 2:05 am

ah, ok, that helps a TON!

ok, so we want 2.5 for our pressure ratio, at (since we're using this example) 145,000 RPM...

so, if every thing is working right, I need to design for 25lbs of pressure. (I believe in over building if possible!) 8)

so back to the origninal calculations, 26 sq inches times 25PSI would be 650 lbs of pressure. Needless to say, that's a ton, or 1/4 of a ton. so those 8 screws just ain't going to hold. (or may, but why risk it?)

Fortunately I KNOW there's no where near that on it right now. if it was getting above 10psi I'd be fairly amazed. the CC is a tad leaky. But back to the math:

so, I'd say a "safe" pressure on the bolts would be 30lbs or less force on each bolt. that would result in 21 bolts minimum. I have 9 right now, so we need 11 more. No use in being un-safe! :D

so 11 SS bolts are on the list. Gee, I'm going to have to find a way to attach those without tapping. :? I have no idea how I'll get those nuts to stay, but, that's my problem not yours. :wink:

so, more holes until I move the heating pattern back into the flame tube and away from that turbine wheel for a T.I.T. temperature of ~1400 degrees F, and something like 1200 TOT. So, if that's right, and the info here: http://www.cowtown.net/mikefirth/techspec.htm is right then I'm about at the right temperature! (cherry red heat is 1375 F) :D

I'll make sure to monitor it with my pyrometer, but, the good news is that I probably wasn't damaging anything like I thought I might have been. the heating pattern is still off a bit in my mind... but I'll work with the compressor flows chart and shoot for that island! :D


VERY good then! so mount the pressure gauge and see how that's doing, then check the speed of the machine... and see what where I'm at. Check the temps, make sure I'm not doing anything bad to the turbo and change the holes accordingly. More holes if the pressure is too high, and towards the back if it's running hot, towards the front if it's running cold (that won't be a problem I'm afraid)

Then we mount the jet nozzle and see how that affects everything THEN start adjusting the cone accordingly. Then think about an afterburner.

It's good I got that all figured out. That'll help a whole lot!
Lasers, jets, and helicopters HURAH!

racketmotorman
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Re: Just need a few pointers...

Post by racketmotorman » Mon Oct 06, 2008 6:36 am

Hi Cheat

You might need to checkout the comp maps here ......... http://www.turbobygarrett.com/ .... to see why I recommended a stronger construction , some of the maps go to a 5:1 PR , thats ~60psi gauge pressure :-)) ..................lotsa other handy info as well .

With regards your temps , simply changing hole sizes and distribution whilst trying to "cool" down or "heat " up the engine can be a bad thing to do , the overall distribution at 30-20-50 is the correct ratio to have annd its generally fuel problems and/or an unsuitable turbo configuration causing the temperature problems .

Without a jet nozzle on her she should run with TOTs of ~7-900 F at modest P2s of <15psi , but once the correctly sized jet nozzle is installed and the pressure ratio across the turbine is reduced , the temps have to climb to compensate , the reduction in pressure ratio is required so that there is a pressure ratio across the jet nozzle to accelerate the gases for thrust production .

Cheers
John

thecheat
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Re: Just need a few pointers...

Post by thecheat » Mon Oct 06, 2008 12:03 pm

well, I've got to make sure what model exactly my turbo is, but at least I have the correct idea, and how to read this exactly.

Ok, so doing that with the flametube is a no-no... well, I still need to widen some of the holes. Thanks for claryifying that! :D

I may do some more testing today, and hopefully I'll get that jet nozzle installed.
Lasers, jets, and helicopters HURAH!

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