Small Engines(scottlocklin.wordpress.com)
scottlocklin.wordpress.com
Small Engines
https://scottlocklin.wordpress.com/2026/03/25/very-small-engines/
20 comments
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...
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...
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.
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!
> could we make a 2 inch diameter turbine engine reliably?
https://en.wikipedia.org/wiki/Capstone_Green_Energy
https://en.wikipedia.org/wiki/Capstone_Green_Energy
A group at MIT tried, unfortunately it doesn't seem to have gone anywhere
https://news.mit.edu/2006/microengines
>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.
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.
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.
This article scratched an itch.
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.
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?
Approximately all gas appliance pilot lights.
https://www.acservicetech.com/post/how-the-gas-pilot-light-f...
https://www.acservicetech.com/post/how-the-gas-pilot-light-f...
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.
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.
The only commercial kitchen stove I interact with on a somewhat regular basis is a miserable piece of crap. It has multiple pilot lights, some of which go out frequently and stink up half the building with unburnt fuel. I suspect that just the pilot lights consume $50-$100/month of natural gas.
Stoves seem to be somehow special in safety regulations. I guess regulators assume that they are never operated unattended, so they tend to have no real built in safety features. Both commercial and residential stoves will happily operate unlit at full power.
Stoves seem to be somehow special in safety regulations. I guess regulators assume that they are never operated unattended, so they tend to have no real built in safety features. Both commercial and residential stoves will happily operate unlit at full power.
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.
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.
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.
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)
https://calteches.library.caltech.edu/1976/1/1960Bottom.pdf (8 pages, PDF)
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.
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.
The entire section on large and small flying insects has been debunked:
https://arstechnica.com/science/2026/03/leading-explanation-...
https://arstechnica.com/science/2026/03/leading-explanation-...
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