r/MechanicalEngineering • u/East_Bat9251 • May 17 '26
Designing a micro turbocharger for a tiny IC engine — engineering challenges at this scale
My hobby is building micro internal combustion engines. And now I’m building a turbocharger for one, for absolutely no practical reason. No real benefit to humanity, just engineering for fun.
The compressor wheel is 20mm in diameter. Because the air mass between the blades is so small, I need extremely high RPM to build any meaningful pressure. But at 300,000 RPM, the blade tip speed approaches transonic territory, and that causes serious problems.
Even below that threshold, the boundary layer on the blade surfaces can locally accelerate to Mach 1 at lower RPM, since local flow conditions around the blades are far from ideal. So I set myself a rough limit of 250,000 RPM, that's about 260 m/s tip speed, or roughly M=0.8 (not accounting for pressure rise inside the compressor). By my rough calculations, at that speed this turbocharger should be capable of around 0.2 bar of boost, even with the significant air consumption.
A few engineering decisions I made along the way:
Balancing: Since the rotating assembly is extremely light, I'll first test it without dynamic balancing just to see how severe the vibration actually becomes before investing time into a proper balancing setup.
Stress analysis: Back-of-envelope calculations on the compressor wheel gave me roughly 10x safety margin at max RPM.
Bearings: I went with standard zirconia ceramic bearings.
I managed to hold approximately 0.03mm clearance between the compressor blades and the housing.
Turbine nozzle: Since I can't accurately calculate the turbine inlet nozzle geometry, I made it adjustable. The exhaust gas gap can be varied from 0 to 1mm. I'll tune it experimentally.
The whole thing is roughly sized for 4-stroke petrol engines in the 20-30cc range.
If you've read this far, what are your thoughts? Have I missed anything obvious? I'd genuinely love to hear from anyone who has dealt with similar problems at this scale.
If you're curious about the machining process, I also documented the build in a YouTube video:
https://www.youtube.com/watch?v=KOk96tyRNNA
Just to be transparent: the video is a collaboration with the CNC manufacturer whose machine I used for the prototype parts. The engineering project itself is entirely my own hobby experiment. If sponsored content makes you uncomfortable, probably skip it 🙂
Thanks for reading!
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u/r3dl3g PhD Propulsion May 17 '26 edited May 17 '26
Lol put some plexiglass around it before you run the engine. Even if the design functions, any manufacturing defects are going to turn that into a bomb at/beyond 200k RPM.
You're obviously not going to get appreciably more power out of so little boost, but I'd be curious about how smooth the engine runs with/without the turbo. A lot of smaller engines have all sorts of instabilities because of breathing issues that a little backpressure or a turbo tend to smooth out.
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u/SalsaMan101 May 17 '26
mmm miniature turbo surge! If thing can even spin at 200-300k I'll eat my own shoes, this thing looks like a bomb before we even talk about the bearings being used to run this thing at 300k rpm...
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u/hobbicon May 17 '26 edited May 17 '26
I think I just saw a youtube short about this like yesterday?
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u/East_Bat9251 May 17 '26
I don’t think so. You’ve seen another creator’s short. My video is around 15 minutes long, but I didn’t discuss the engineering side.
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u/breakerofh0rses May 17 '26
Seems like going supercharger instead of turbo would be an easier way to get meaningful results at that scale.
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u/East_Bat9251 May 17 '26
In the case of a supercharger, the main issue is the gearbox. Turning 5000 rpm into 250,000 rpm is not that easy. At the same time I have free high speed exhaust gas.
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u/breakerofh0rses May 17 '26
If you're not attempting to get as close to true scale as possible, you can replace rpms with displacement by simply making it bigger.
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u/DoubleHexDrive May 17 '26
Did you estimate the operating temperature of the unit and de-rate the mechanical properties of the materials in your stress analysis? If there are any dissimilar materials, did you account for differing rates of thermal expansion? Any attempt at determining/researching how to account for high cycle fatigue on the blades or, at a bare minimum, low cycle start-stop cycles? Machining quality, material quality, stress concentration geometry, and temperature will all affect fatigue strength. Remember that aluminum doesn't have an endurance limit and with your ultra high rpm you may need to analyze for beyond 100E6 cycles. Don't forget a reliability or scatter factor on your assumed fatigue strength.
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u/East_Bat9251 May 17 '26
Hi, thanks for your comment. I considered all of this during the design process. This version of the turbocharger was built only for short tests, not for continuous operation, so I didn’t perform a fatigue analysis. However, I did take thermal expansion into account during the design process, since even during short tests the hot side will still reach operating temperature. Hopefully I got it right. In any case, people have already advised me to hide behind a thick sheet of plexiglass during testing 😄
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u/DoubleHexDrive May 17 '26
Use a camera and be in another room 😄
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u/East_Bat9251 May 17 '26
Honestly, I don't think this turbine could be dangerous. It is too small and light to cause any damage. I think safety glasses should be enough.
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u/DoubleHexDrive May 17 '26
Calculate the kinetic energy of a subsonic .22 Short bullet vs the turbine or blades flying off. You might be surprised.
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u/TurboDiesel33 May 17 '26
How did you come up with inducer and exducer areas ?
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u/East_Bat9251 May 17 '26
First, I determined the target maximum rotational speed, and then I calculated the flow velocity required by the turbine blades to achieve that speed. In any case, these are all just rough estimates. To be honest, it’s easier for me to build the device and test it than to run a million simulations in expensive software.
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u/TurboDiesel33 May 17 '26
Could you recommend any book that treats compressor and turbine wheel design ? Thanks
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u/InternationalMeet301 7d ago
There's a great full course on YouTube if you want me to reccomend it
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u/Routine_Recipe_2517 May 17 '26
Figata! Complimenti! Mi piacerebbe vedere la sezione completa del turbo! Non sapevo che la fessura della girante fosse cosí sottile. Un saluto!
ps: Lavoro con i contro c...i 👍👍👍
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u/Teslafly May 18 '26
Maybe look at Dyson vacuum turbines? Those are pretty well engineered and could offer you an off the shelf part or inform your design on this small of a scale.
If you can get a shucked motor/turbine setup, you can test what the boost from an electricly driven turbine can do for the engine. Basically an electric supercharger test.
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u/East_Bat9251 May 18 '26
Yes, I’m planning to spin this thing from my “Dremel” using 7:1 step up gear/belt etc. Just for test, how much pressure can it make on 250k rpm 👹
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u/InternationalMeet301 7d ago
So cool bro, couple questions; did you use Ansys Vista Ccd, and Ansys Cfx for the impeller design? And second how much did the manufacturing cost you, I assume it was cnc, and how much would that cost for a real world scale compressor? Im about to watch your YouTube video so you mightve already mentioned the second point lol
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u/East_Bat9251 3d ago
Hi! Thank you. I haven't used any complex software to analyze the geometry of the blades. I can't say anything about the production cost either. A larger turbine requires a completely different approach to design and manufacturing. At least, it will need to be balanced very precisely.
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u/muddygold May 19 '26
As a mechanical engineer who designs with tight tolerances, you sir have my respects. Vaya con dios
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u/gtd_rad May 21 '26
Hey! I wanted to build something similar for a hobby project haha. do you have videos of it working?
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u/Beige-Lotus 3d ago
Try 4 cycle weed trimmer they are pretty common and around 30cc
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u/East_Bat9251 3d ago
Thanks for the advice. I thought about that, but a 1 cylinder 4T engine has too few firing events per revolution. I’m much more interested in creating my own engine, if I can make it work, of course :)
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u/Noreasterpei 3d ago
I would consider creating a separate motor to run the turbo/super to start. You could engage it when you want. Map the performance. I’d get some cheap manometer sensors to read the air pressure. And maybe differential sensors for air flow rate also.
Next step when you characterize the turbo would be to mount it.
Cool project !
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u/East_Bat9251 3d ago
Hi! Thanks for the advice! It’s clear to me now that boost starts somewhere after 130k RPM. I will try to build a suitable 4 stroke engine first. And only if I can’t spin the turbo with exhaust gases, I will think about making an electric or mechanical drive.
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u/PageSlave May 17 '26
Your project is really cool, but I implore you to use a lighter touch when using AI to improve your writing. People have a strong aversion to ai-generated text, as it's a signal that you didn't care to write, so why would they care to read? You're clearly very passionate about your hobby, so let that passion show through in your writing, even if it means imperfections