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Comment by sandworm101 | Hacker News Reader

The hood is exactly the wrong place. Blocks, radiators, even oil pans would be far better.

But imho the real market for these will be in low-power environments. Or defense: submarines have near-unlimited heatsink potential and lots of <100 heat sources.

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That made me wonder if anyone had tried tracking submarines based on the heat they leave behind - turns out the Soviets had a technique that integrated various sources including heat :

I don't really think submarines are going to stay stealth from major powers for much longer. Underwater drones are too easy and once they lock on a sub they can follow it constantly.

The ocean is a really, really, really big place. You don't even have to submerge in order to stay relatively hidden. We don't publish the exact coordinates of carrier groups because they're really hard to find when underway. You can fly a plane around for days looking for one and not find squat.

No number of drones that we can realistically produce and operate can adequately blanket the ocean. The Pacific ocean is bigger than all of the Earth's landmasses combined. Imagine trying to find a ship-sized vehicle moving through an area the size of Wyoming by plane, that's about how hard it is to find a whole carrier group just moving on the surface of the water, with publicly available information.

Without it, you're looking for a needle in a haystack the size of Wyoming.

Really?

Surface area of planet Earth: 500M km2.

Horizon radius at a height of mount everest: 300km.

Surface area visible: 282K km2.

Number of drones needed to cover the area: 1773.

Multiply it by 5 to 10 to be sure.

Incredibly realistic.

Why not make them fly higher so you can increase the horizon even farther? In fact, why not put them in orbit?

Yeah, but orbit is predictable and traceable, while drones could also fly under the clouds if required. I just don't understand the "Earth is so huge" argument.

Is it, really? I've thought the only reason we can do drone strikes in Syria is exactly because it's really hard to hide if you're always being tracked.

Carrier groups aren't hidden from powers with reasonable satellite constellations or radar / sonar groups. They're hidden from Iraq or Saudi Arabia or the journalists at the New York Times.

And my first thought was like your's, but I came to think that between subullites, refined SIGINT analysis for surfaced subs, and the few locations on earth we know manufacture these things I think it's going to be more possible to follow submarines than you're making it out to be.

Humans have to disembark these things, at least if we want humans on them. Even in a future where all subs are autonomous they'll still need to be manufactured somewhere and will still require supply (re)provisioning.

But I'm open to changing my mind on this if you have a special insight.

We already follow subs using our subs using this kind of information. Drones don't meaningfully affect this state of affairs.

The Swedes sunk the USS Ronald Regan in war games using a sterling engine powered sub. It makes it hard to track since it doesn't have the traditional explosion noises of a diesel and it lacks the heat sink noise of water cooling a nuclear core.

https://www.youtube.com/watch?v=saCdvAp5cow&t=354s

YT link without a time point: https://www.youtube.com/watch?v=saCdvAp5cow

Diesel subs don’t operate their engines while submerged unless they are recharging in safe waters, so there aren’t any ‘explosion noises’. They have batteries that are used for propulsion, and the Diesel engines are technically generators, as their propulsion system is electric - like a train.

I know I was just pitching for a customer level add-on and not a factory level one. There's also the exhaust pipe which has high temp gas in. I can't find it again sadly but I found a japanese lab who made thermoelectric pipes with not too poor efficiency (aimed at building water pipes but probably workable for vehicles).

The problem with extracting heat from exhaust pipes has never been conversion efficiency but weight. To harvest a useful amount of heat from a combustion engine requires lots of pipe, (surface area) which impacts engine performance. It might work in a fixed power station where weight/size isn't an issue, but you aren't going to see them on a motorcycle exhaust.

having a heavier material around the pipe would impact performance ? Im surprised but Im not an engineer so Im learning.

No, having a long pipe impacts performance as it puts back pressure on the engine.

Some resistance like concept for fluids ?

A manifestation of drag: https://en.wikipedia.org/wiki/Drag_(physics) Stuff doesn't move through pipes for free. Given that we'd probably have to put tons of turns into this pipe, too, those are even more expensive.

Contrary to probably-locally-popular opinion, cars have been engineered to within an inch of their lives, and the quest for better fuel economy has been a huge focus of the industry for decades, pretty much countered only by "customer safety" and "what will the customers pay for". If there was an efficient way to recover the heat, it would probably already be getting used, as evidenced by all the other mechanisms already in a car if you look.

I do get all this, but you know very well that "what customers would pay for" is very much contextual and thus "energy economy" is not only a fixed law.

I'm sure zero percent of the market cared about pollution or high efficiency in the 90s and today it's a sizeable chunk (Im guessing but I'd bet a dollar it's at least a single digit %).

Today having something in that scavenge heat at some cost might be a plausible marketing argument.

Back to physics, so pipe are like wires for electrons, some energy will be wasted. But aren't turbo equipped engines using part of the flow to improve the engine fuel quality ? I should search if people try to expand on that. I know Turbos had some issues but I never read in details and this was long ago.

> But aren't turbo equipped engines using part of the flow to improve the engine fuel quality

No, turbo is used to force more air into the intake side of the engine. It uses exhaust gas pressure to spin a turbine which is used to compress more air into the cylinder between cycles. That, combined with more fuel to maintain proper balance, gives you increased combustion, thus more power.

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