ordonezs
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https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 2, 2020 22:37:16 GMT -5
Hi all! Background:5 years ago, I build a jet engine with a friend for my high school senior project. It did not run for many, many reasons. To name a few: -Using standard 0.4psi propane grill regulator -Gravity fed oil system -Untested ignition system -We had no idea what we were doing Recently, we built a proper test stand for the jet engine and redid the ignition system (now using a glow plug that goes nicely with all the other 12V automotive systems I'm using). Most importantly, I took time to do many tests on the combustor. Yesterday, the jet engine self sustained in several tests! It lives! Startup pictures: Test video here: youtu.be/uHVQpqusdMQProblem: Despite the jet engine self-sustaining, it would not run very well. It seemed to be very hot (begins smoking and shoots sparks out the back) and would struggle to run for very long. My longest test run (linked video) went for about 30 seconds before the turbo slowed down and the turbine housing popped clean off. It seemed like the friction on the shaft had increased, stopping the engine. Not sure what caused the turbine housing to come off. I have many questions, but I am also putting this out here for any feedback from wiser eyes. A few of my own questions are: -Is the smoke normal? From my understanding of turbos, oil tends to leak out through the bearings, so my best guess is that the smoke was just leaked oil burning off and not cause for great concern. Correct me if I'm wrong. -How is the turbine housing held on? Is it easy to fix? -What's causing my engine to run hot and not for very long? I want to be able to run it for several minutes. Engine Specs:Inducer Diameter | 32mm | Flametube Diameter | 68mm | Flametube Length | 230mm | Chamber Diameter | 96mm | Primary Holes (#, D) | 19 holes @ 1/4"D | Secondary Holes | 4 holes @ 3/8"D | Tertiary Holes | 4 holes @ 1/2"D | Oil Pressure | 40psi
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Combustor: Exploded Combustor: Fuel nozzle: View from exhaust of combustor: Test stand pictures:
Electrical system:
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Post by racket on Nov 2, 2020 23:18:47 GMT -5
Hi
A couple of things that will need fixing .
Firstly your flametube holes , theres too much Primary Zone hole area with your 19 X 0.25" holes
A 32 mm inducer has an area of 804 sq mms , your Primary holes need only amount to ~25-30% of that 804 sq mms , so ~200 - 250 sq mms , you currently have 3 times that at ~600 sq mms .
Your Secondary holes , 4 X 0.375" are nearly twice the required area , we only need 20% of your 804 sq mms , so 160 sq mms not your ~280 sq mms .
The Tertiary holes , 4 X 0.5" are also a ~25% too much area , we only need 50% of the 804 sq mms or 402 sq mms , not the ~500 sq mms currently .
Now for the exit from the flametube , we MUST have a smooth transition from the 68 mm FT dia to the turb scroll inlet , some sort of "funnel" arrangement , without it the sharp step at the FT exit will create a vena contracta within the turb scroll inlet reducing flow and potentially putting the engine into surge .
The colouration of your flametube indicates very late combustion , the "purpleing" is down near the Tertiary holes whereas it should be up at the Primary/Secondary holes, this would indicate a propane injector spraying axially down the flametube rather than a radial spread so that the fuel can mix with the Primary air and burn .
A simple propane injector is a short length of 3/8" metal tube with the end blocked/crimped off and 6 to 12 very small holes ( 1mm max) drilled in the tube sidewall to spray out towards the Primary holes.
LOL......that might do for now :-)
Cheers John
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ordonezs
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https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 3, 2020 0:29:02 GMT -5
John, Thanks for your thorough response! I think you're spot on about the nozzle. It's off a pressure washer and looking at it, it definitely is shooting the propane most of the way down the tube before it has time to mix and burn. I was hoping turbulence would help with that but obviously not enough. A new nozzle is the first order of business. Also, got the smooth transition covered. It's not pictured but the other SS turned end-cap is chamfered from the flame tube ID to the turbine inlet ID. It's a 45 degree chamfer though so hopefully that's smooth enough. As for the flame tube, I think I'll try to reduce the primary hole area with a SS hose clamp over one of the rows and maybe cover up a secondary and tertiary hole. What would be the effect of fixing the extremely generous hole areas? I don't doubt your experience, but I'm curious what performance aspects covering some holes will improve (I like to understand reasoning for doing things). Best, Sam PS do you know how to fix a turbo housing? Not sure how it's mounted, wasn't a snap ring or radial bolts...XD
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Post by racket on Nov 3, 2020 1:11:04 GMT -5
Hi Sam Your turb scroll should have small tabs screwed to the scroll in those threaded holes , it looks like whoever owned the turbo previous tried to disassemble it but couldn't get the scroll housing off and simply threw the securing bits away . You'll probably find just bolts and large flat washers will hold it in place , by heating the scroll you've expanded it off its centre housing flange mount, and without any securing devices the gas pressure inside simply blew it off ................LOL, I recon it would have given you guys a scare and a good laugh :-) If you have a tapered entry at the turb scroll end then the FT should be OK . Most of your problem has been the fuel injector , but try to block off some of those Primary holes for a test run after fitting the new injector, because we need to keep our turbine inlet temperatures at a low enough level not to damage the turbine wheel we can only "burn" ~25-30% of the air exiting the compressor, hence 25-30% hole area in the Primary zone , the resulting temperature is up around 2,000 degrees C , this then gets diluted and cooled by the Secondary and Tertiary holes to a temperature replicating the EGT of the piston engine the turbo came off . If we don't have that 30/20/50% division of holes, obtaining the right mixture strength and combustion is difficult, all jet engines have a division of comp air in their flametubes, we find that using the same area for all air entry holes into the flametube as the area of the inducer works nicely at our pressure ratios. Have you looked through this info jetandturbineowners.proboards.com/thread/680/diy-turbines just click on the icons in each section for the info . Cheers John
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Post by sauerkraut on Nov 3, 2020 15:33:49 GMT -5
Thoroughly inspect your turbine wheel. Not only it was certainly making contact with something after the housing separated, but it sounds to me like it was making contact even before then. Most likely the heat shield.
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ordonezs
Member
https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 3, 2020 17:33:22 GMT -5
Thanks John and Sauerkraut, I appreciate the feedback. I should have time in the next day or so to at least inspect the turbo and maybe fix it up with some radial bolts and hose clamps, although I'm not sure I'll be able to run it before I leave home for awhile. Turbine wheel contact might be responsible for the occasional sparks in the exhaust. Either that or the K type thermocouple that pokes out about 1/4" in the connection between the combustion chamber and the turbine inlet. Last question for now: is the smoke cause for concern? I'd wager since it hasn't been brought up so far it's not a giant red flag, and also knowing how turbos sometimes leak oil by design I'm guessing it's just leaked oil burning off. Is that right? Thanks again. Testing video: youtu.be/uHVQpqusdMQ
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Post by racket on Nov 4, 2020 0:41:28 GMT -5
Hi Sam
Please reposition your temp thermocouple to downstream of the turbine wheel to prevent bits going through the wheel if/when it fails .
If you don't already have a P2 pressure gauge installed , fit one as you need to prevent overspeed , keep your comp pressure below 15-20 psi max , small turbos are generally designed for low boost.
As for the oil burning , we'll worry about that if its still there after fixing the combustor .
Cheers John
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ordonezs
Member
https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 5, 2020 21:48:25 GMT -5
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Post by racket on Nov 5, 2020 23:03:29 GMT -5
Hi Sam
I'd use it again :-)
I notice your turbo has variable turbine geometry which I wasn't aware of, at what position did you have the vanes set during the testrun ??
Cheers John
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ordonezs
Member
https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 5, 2020 23:34:34 GMT -5
Hey John,
I think I had them full open (max flow through turbine). I tested the turbine with a blower and found the wastegate-valve-lever position that made it turn (the other position idled it). Gravity would pull it back down to the idle position so I propped it up with a spring and a screw lol
Also, couldn't find a drill bit <1mm diameter. Would a few 1/16" radial holes do for a propane fuel nozzle? I could do 3 or maybe even 2 to minimize total area.
Sam
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Post by racket on Nov 6, 2020 0:05:27 GMT -5
Hi Sam
Fully open would have meant high temps :-( lotsa reasons why , but won't bore you with the details ;-)
Fix it around mid point , you may need to adjust it a tad at a latter date once you have it running reasonably well .
6 X 1/16" injection holes will be fine , just space them at 60 degrees apart to get an even coverage of the FT cross section
Cheers John
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ordonezs
Member
https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 6, 2020 0:28:02 GMT -5
John,
That's interesting. Won't half way mean that some air is bypassing the turbine? Also, I'm an engineer so I love the boring details lol! Feel free to share if you want or point me to some info about it.
Also excited to test with these changes. Expecting to this weekend if everything goes well!
Sam
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Post by racket on Nov 6, 2020 0:50:04 GMT -5
Hi Sam
When you have the vanes fully open , set at a "high" tangential angle , the throat area of the vanes is large , meaning theres little presure drop required, and little velocity increase, as the gases pass through, and minimal gas energy being imparted to the turbine wheel.
The increased area and low power production is a problem that gets "compensated for" by requiring higher temperatures to "fillout" the oversized throats with less dense gas and to increase gas velocities and energy transfer.
Another consequence is it allows the comp to flow in the choke region of its map at low compression efficiencies , this exacerbates the situation further as relatively more power is required than if compression is carried out at higher efficiencies .
We end up with an engine that struggles to keep itself alive.
With the vanes closed to mid point , "lower" tangential angle , more pressure drop required , more energy transfer , less mass flow ( hopefully) and a much cooler running engine , that can then have some backpressure put on it in the form of a jet nozzle or freepower turbine , both of which can then raise temperatures to a design level .
Its a juggling act to get all the components flows to match each other as they perform their various functions .
You may find that you'll need to close the vanes even further and reduce the throat area between vanes for higher power/rpm , the reduced throat area and "low" tangential angle will produce high velocity gases that impart lotsa energy and can keep up with the turbine inducer speed .
Cheers John
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ordonezs
Member
https://youtu.be/hCHDC_XksKY
Joined: November 2020
Posts: 19
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Post by ordonezs on Nov 6, 2020 1:33:12 GMT -5
Thanks for the fast responses and details, John. Think this will all help a lot. Last time I was able to get it to run, but it was clearly being tortured lol. Hoping all these changes will keep it running cool and easy. Made the new propane nozzle with what I had on hand. Only got 4 holes due to the shape of the brass NPT plug I had, but it's got much higher flow rate than the previous nozzle I used. You think this will work? Let me know if there's any glaring issues to correct. Otherwise, looking to run soon!
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Post by racket on Nov 6, 2020 2:53:48 GMT -5
Hi Sam
You'll use less fuel having it injected radially in the Primary Zone rather than squirting down to the Tertiary Zone because you only need to form a combustable mixture with 30% of the air instead of 100%
Cheers John
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