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Post by ericthegreatish on May 9, 2024 19:39:55 GMT -5
Good evening everyone, I've been lurking this forum for about 6 months now reading up on the fundamentals of these DIY engines as well as professional papers (comparing with NASA general equations) on the subject. I'm a mechanical engineering student and have recently become fascinated by these DIY jet engines, even though I come from the high performance automotive world. Anyways, I think I have a reasonable understanding of how different variables affect each other as far as calculations and in the real world go, however I am a little stumped at the moment regarding my Jet Nozzle...
Here are my specs and calculations:
T3 Variant eBay turbo 2" compressor inducer - I'm aware not the best choice, but trying to keep the project at $1,000 or less
Diesel fuel being used
5"x12" flame tube
0.02cuft inlet area
0.7lb/s mass flow rate
6gph fuel required for 60:1 AFR
20psi out = 34.7P2
2.36PR
116C rise @ 70% eff.
0.278kcal/Kg/C Cp diesel fuel
108C @ 78% eff. Tdrop thru turbine
953K TOT
1.29 Jet nozzle PR
52C Tdrop thru nozzle (628C)
1,142ft/s Nozzle exit velocity
40.86cuft/lb nozzle exit density
2.25in dia jet nozzle
~25-26lb thrust
My issue is that I think the calculated jet nozzle diameter is bigger than my actual turbine outlet... unless I'm measuring wrong somehow. Should be the exit diameter of the turbine, correct? Or do I need to take the turbine housing off and measure the larger diameter of the turbine wheel? If the calculated diameter is indeed larger, would I need to make a CD shaped nozzle?
I know there are a lot of smart people on this forum so I'm hoping someone can show me my duhhhhh moment. I have already began collecting materials and made my flame tube, so I'm hoping I won't have to completely redesign. I will upload pictures later. Thank you in advance!
-Eric
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Post by racket on May 9, 2024 20:12:45 GMT -5
Hi Eric
If your turb wheel exducer diameter is less than 2.25" and your comp inducer is 2" you might not be able to flow your 0.7 lbs/sec as it'll depend on your turb scroll A/R.
Could you please provide a Link to the turbo specs
Cheers
John
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Post by ericthegreatish on May 9, 2024 21:04:47 GMT -5
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Post by racket on May 9, 2024 23:57:08 GMT -5
Hi Eric If you checkout the closet Garrett turbo www.garrettmotion.com/wp-content/uploads/2022/06/Turbine-Flow-Maps-GT30-scaled.jpg you'll notice the 0.63 turb scroll only flows ~19 lbs/min Corrected Flow at a 2:1 PR and at only 69% effic , even with full expansion across the turb stage using your 2.36 PR it only goes up to ~20 lbs/min Corrected Flow . Assuming a TIT of 1061K and a 2.36 Pr that equates to a Actual Flow of ~25 lbs/min or 0.4 lbs/sec which on the small GT28 comp map www.garrettmotion.com/wp-content/uploads/2018/05/Comp-Map-GT-2871R.jpg is OK Now with a lot less mass flow due to the tight turb scroll , your potential jet nozzle sizing will be smaller ............I'd suggest NO jet nozzle to begin with as the very low efficiency of the turb stage will require a larger pressure drop so less energy left over for making thrust The engine should run :-) Cheers John |
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Post by ericthegreatish on May 10, 2024 5:29:16 GMT -5
Hi Eric If you checkout the closet Garrett turbo www.garrettmotion.com/wp-content/uploads/2022/06/Turbine-Flow-Maps-GT30-scaled.jpg you'll notice the 0.63 turb scroll only flows ~19 lbs/min Corrected Flow at a 2:1 PR and at only 69% effic , even with full expansion across the turb stage using your 2.36 PR it only goes up to ~20 lbs/min Corrected Flow . Assuming a TIT of 1061K and a 2.36 Pr that equates to a Actual Flow of ~25 lbs/min or 0.4 lbs/sec which on the small GT28 comp map www.garrettmotion.com/wp-content/uploads/2018/05/Comp-Map-GT-2871R.jpg is OK Now with a lot less mass flow due to the tight turb scroll , your potential jet nozzle sizing will be smaller ............I'd suggest NO jet nozzle to begin with as the very low efficiency of the turb stage will require a larger pressure drop so less energy left over for making thrust The engine should run :-) Cheers John Thank you for the insight. I will do the math out as well based on those specs. I will start without a jet nozzle, but rather than the hot exhaust gasses just coming out the back of the turbo, should I add a short pipe of equal diameter to it just to help direct the flow and heat? I will be back this weekend with more #’s. Thanks, Eric |
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richardm
Senior Member
Joined: June 2022
Posts: 413
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Post by richardm on May 10, 2024 9:23:14 GMT -5
You might add a short pipe or just operate the engine in a good headwind..
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Post by racket on May 10, 2024 23:46:50 GMT -5
Hi Eric
A short pipe is handy to fit a thermocouple into for measuring your turb gas temps ...............and firing her up pointing into the wind is a good idea whether a little DIY gas turbine or a huge aircraft jet engine as Richard said ;-)
Cheers
John
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Post by ericthegreatish on May 12, 2024 8:15:51 GMT -5
Hi Eric A short pipe is handy to fit a thermocouple into for measuring your turb gas temps ...............and firing her up pointing into the wind is a good idea whether a little DIY gas turbine or a huge aircraft jet engine as Richard said ;-) Cheers John Thanks guys, I’ll try to orient it to dissipate heat better. I’ll make a short “nozzle” at first that doesn’t decrease in diameter. My professor wants a small “enclosure” around the turbo and pressure vessel so I just won’t make it too large. I’m thinking a probe type thermocouple to measure the T2 and TOT? Thanks, Eric |
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Post by ericthegreatish on Aug 4, 2024 11:14:55 GMT -5
Well, it’s been a few long months of work getting this thing to run but the day finally came. Between school and work and my cars it has been 24/7 nonstop. The engine runs, but it has some issues. 1 - it won’t run without leaving the air compressor nozzle going to help feed the turbo, I’m guessing the turbo just isn’t spooling fast enough? 2 - I have to leave the fuel control knob at 2% or else the engine runs rich and TIT just starts climbing. This is with a 3.5GPH nozzle. I have an even smaller fuel nozzle coming (1GPH), so maybe that will help. Pics on the way. 7 weeks til final presentation!!
-Eric
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Post by racket on Aug 4, 2024 17:36:22 GMT -5
Hi Eric
Certainly sounds like you haven't got the RPM up to self sustain levels , this will also account for the high temps , we often need to use very high temps momentarily to get "over the hump" of spoolup , 5 seconds of high temps ( 1000 deg C ) during a 20 second spoolup is generally OK .
Some video of a spoolup will help :-)
Cheers
John
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Post by ericthegreatish on Aug 7, 2024 20:41:00 GMT -5
Hi Eric Certainly sounds like you haven't got the RPM up to self sustain levels , this will also account for the high temps , we often need to use very high temps momentarily to get "over the hump" of spoolup , 5 seconds of high temps ( 1000 deg C ) during a 20 second spoolup is generally OK . Some video of a spoolup will help :-) Cheers John Hi John, Yeah, I have been using an intake that I made for it and a piece of copper brake line angled into the compressor wheel to spool it up (100psi). I can get it spinning pretty fast but once I get it "running" where TIT is lower than TOT (and it has a very distinct sound), any extra fuel just enriches the mixture too much and creates smoke... I can get the temps pretty high, TIT at 1000 deg C, but it doesn't seem like the turbo is actually spooling too too fast from it. It seems like it hits a wall at some point and the temp just rises and makes a ton of smoke. I had the engine run away on me once from too much fuel and that was the only time it actually spooled at the speed I think it needs to, but I haven't been able to replicate that with the proper temps and fuel settings. My biggest concern is the fact that I have to be able to control this engine from 15ft due to a waiver my school made me sign to be able to do this project, so using a leaf blower is out of the question unless I can somehow use that from a distance... that's what has caused most of my headaches so far. As far as videos, do I have to upload to YouTube first then post a link? I'll attach a few pics... Thanks, Eric |
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Post by racket on Aug 8, 2024 2:22:24 GMT -5
Hi Eric
TIT should always be HIGHER than TOT due to the energy extracted by the turbine stage .
Would your school allow you to do a "dry" spoolup at close range so that you can measure the RPM produced by your stating method , without any fuel its quite safe .
It certainly sounds like you are reaching "the hump" and don't have enough starter power to get it over the hump , common problem with our engines ............and all turbine engines :-(
You could try making a nozzle for the end of your copper brake line to increase the velocity of the air coming out, its positioning is also critical for extracting as much energy from it as well .
Yep , vid to youtube then post a link , would be much appreciated , a pic is worth a thousand words :-)
Cheers
John
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Post by ericthegreatish on Aug 8, 2024 13:02:12 GMT -5
Hi Eric TIT should always be HIGHER than TOT due to the energy extracted by the turbine stage . Would your school allow you to do a "dry" spoolup at close range so that you can measure the RPM produced by your stating method , without any fuel its quite safe . It certainly sounds like you are reaching "the hump" and don't have enough starter power to get it over the hump , common problem with our engines ............and all turbine engines :-( You could try making a nozzle for the end of your copper brake line to increase the velocity of the air coming out, its positioning is also critical for extracting as much energy from it as well . Yep , vid to youtube then post a link , would be much appreciated , a pic is worth a thousand words :-) Cheers John Alright, I guess I was a little backwards on that… whoops… Yes, the school only sees what I send pictures of, so I could measure RPM with a tool on Amazon I found. As far as the copper brake line, I tried squishing the end a bit and it helped, but obviously I didn’t decrease the area very much. Any ideas on a smaller nozzle? I’ll have to upload a few videos. Thanks once again for the tips! Thanks, Eric |
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Post by ericthegreatish on Aug 8, 2024 14:21:03 GMT -5
Also, I am controlling the fuel pressure using PWM and am getting larger spikes than anticipated. Before, the needle would bounce around within a few psi range, now right before I get over “the hump” the pressure is bouncing drastically. Would a “water hammer arrestor” solve this?
Thanks for any input,
Eric
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Post by ericthegreatish on Aug 8, 2024 15:02:14 GMT -5
Update!!!!!!
Got it to self sustain with the air nozzle off! It definitely spooled way up compared to before. Only issue is heat. I jumped back up a size for the fuel nozzle and it definitely liked it, but I’m still running at TIT 1000-1050 deg C. I couldn’t get it any lower without the fuel cutting out. And yes, Racket, I got the TOT much lower than the TIT… not sure how I mixed it up.
I should also mention I damaged the turbo the first time I tried running it, but I have a replacement to put in once I work out some kinks. I’m thinking the bearings got cooked and the extra resistance is possibly preventing it from spooling up higher. I also think the nozzle cone diameter might be causing too much back pressure, but wouldn’t the TOT be what is affected since it’s after the turbine?
I am beyond excited and my brother ran outside to catch a video since he heard it even while playing Xbox XD. Huge milestone on this project for sure.
Thank you to everyone who has input so far,
Eric
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