[HN Gopher] Small Engines
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Small Engines
Author : surprisetalk
Score : 72 points
Date : 2026-04-06 15:11 UTC (3 days ago)
(HTM) web link (scottlocklin.wordpress.com)
(TXT) w3m dump (scottlocklin.wordpress.com)
| davidu wrote:
| This helps define some of the challenges with making very very
| small turbine engines. We have electrical (lithium) powered
| drones but they are heavy and have low energy density compared to
| what a liquid fuel + turbine could provide. But could we make a 2
| inch diameter turbine engine reliably? Maybe!
| intrasight wrote:
| http://lambert-modellturbinen.de/html/english.html
|
| 55mm diameter
| bob1029 wrote:
| > could we make a 2 inch diameter turbine engine reliably?
|
| https://en.wikipedia.org/wiki/Capstone_Green_Energy
| torginus wrote:
| >could we make a 2 inch diameter turbine engine reliably
|
| I mean, technically yes, but in practical terms, no - turbines
| run on the Brayton cycle, where the are under curve efficiency
| is determined by the peak pressures it can withstand. if you
| scale down the turbine proportionally, it gets structurally
| weaker, meaning its efficiency drops. thrust/weight decreases
|
| If you then thickened its walls you would then be able to
| handle higher pressures, but weight would increase -
| thrust/weight decreases again.
|
| So the correct answer is if you really wanted to make a small
| turbine, you could certainly make one, but your design would be
| less optimal than a bigger one, so unless your goal is to go
| small, you would make one as big as you can get away with it.
| harwoodr wrote:
| This reminds me of the explanation of why the US primarily uses
| light water reactors.
|
| Heavy water reactors (CANDU in the case of the explanation) don't
| scale down past a certain point - the water flow ends up with
| weird dead spots and that affects the reaction and cooling of the
| fuel rods. Since they don't scale down well enough, they aren't
| suitable for portable use, such as in submarines.
|
| I'm not sure if it's still the case, but apparently one of the
| largest sources of nuclear engineers/scientists in the US was the
| US navy.
|
| This also lead into the story about Jimmy Carter (nuclear sub
| naval officer at the time) coming to Chalk River in the 50s to
| help with the NRX meltdown...
| buildbot wrote:
| Oh wow, I didn't know he experienced two meltdowns in his life!
| Carter also visited 3 mile island during its meltdown when he
| was president.
| elevation wrote:
| This week I was wondering how long it would take a pilot light to
| deplete a tank of LP fuel (the kind people use for grilling.)
| Several months? A year? For no particular reason, I wondered what
| the limitations would be on shrinking the pilot light. Could a
| small tank keep a flame going for 10 years? 100 years? I sense
| one challenge would be machining a small scale nozzle for laminar
| flow, and carefully filtering both fuel and air inputs to ensure
| the tiny nozzle didn't clog, for instance, with a grain of sand,
| or a piece of pollen. At a small scale, what are the limits of
| flame?
|
| This article scratched an itch.
| amluto wrote:
| A pilot light is tricky: in typical designs, it needs to heat a
| thermocouple enough to produce enough current to drive a
| solenoid to allow the rest of the flame to ignite.
| Thermocouples are outrageously inefficient.
| NetMageSCW wrote:
| The pilot lights I'm familiar with just light the rest of the
| flame directly since they are burning already - turning on
| the fuel is all that is required. What systems uses a
| thermocouple and a solenoid?
| bob1029 wrote:
| I've got a Honeywell digital controller on my hot water
| heater. It's powered by the thermocouple. It can make
| troubleshooting a lot easier because it has flashing lights
| for diagnostics.
| amluto wrote:
| It's extremely common for the mechanism that only allows
| the fuel to be turned on if the pilot is lit to work by
| having a thermocouple in the pilot flame. Some of these
| also power the controls (thermostat, for example) and some
| don't.
| throwway120385 wrote:
| Yeah blowing yourself up with a gas leak is common enough
| when you're working on these systems that it's pretty
| important to have an interlock there.
| compiler-guy wrote:
| Approximately all gas appliance pilot lights.
|
| https://www.acservicetech.com/post/how-the-gas-pilot-
| light-f...
| dghlsakjg wrote:
| Any modern country with safety regulations will require a
| thermocouple in the loop if there is a pilot light on the
| appliance. The last non thermocouple appliance I saw was an
| industrial kitchen stove, but it had been modified for
| propane, and I suspect that the guy who did it ripped out
| the safety stuff.
|
| Every factory appliance will gate the full gas flow behind
| the activation of a the thermocouple.
|
| When you push and hold a dial or button to get a pilot lit,
| what you are actually doing is bypassing the thermocouple
| safety until it gets to temperature. If you release the
| "hold to light" knob too soon your pilot will go out since
| the thermocouple needs ~10 seconds to get to temp.
| NetMageSCW wrote:
| The entire section on large and small flying insects has been
| debunked:
|
| https://arstechnica.com/science/2026/03/leading-explanation-...
| harias wrote:
| Reminds me of Feynman's 'There's Plenty of Room at the Bottom'.
| He discusses miniaturizing ICE engines in page 5 - bottom left
| paragraph.
|
| https://calteches.library.caltech.edu/1976/1/1960Bottom.pdf (8
| pages, PDF)
| torginus wrote:
| Considering the many folk tales of giants and dwarves, featuring
| in all sorts of cartoons, or toy trucks and model trains I played
| with a kid, it's interesting to think scaling in real life works
| very poorly - even going beyond such simple principles as the
| square-cube law, if you think about stuff like a pressure vessel
| with a certain wall thickness that needs to hold 100 bar - the
| thickness needed is the same regardless you have something the
| size of a golfball or a swimming pool.
|
| This is imo why scaling down combustion engines beyond a certain
| point makes little sense - you don't gain anything in terms of
| weight since the wall thicknesses are determined by the pressures
| the engine has to endure which is the same - this is why model
| engines suck - they're not only less powerful than big ones, but
| less powerful per pound.
| pillars wrote:
| Some Model Engineering related resources:
|
| Engineering drawings of small model engines used in model
| aircraft making. These designs are intended for manufacturing and
| practical operation, rather than merely for display, and can be
| used to build fully functional engines.
|
| 1.
| https://outerzone.co.uk/plans.asp?cat=Engines&Xcardsperpage=...
|
| 2. https://modelenginenews.org/midge/index.html
|
| > http://www.model-engine-plans.com/otheritems/JEH_Catalog.pdf
|
| > https://modelengineeringwebsite.com/Midget_gas_engine_1.html
|
| > https://www.adriansmodelaeroengines.com/catalog/product.php?...
|
| > Previous submission related to model engines:
| https://news.ycombinator.com/item?id=46098655
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