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Comment by dosman33 | Hacker News Reader

Amazing, I actually saw one of these today at a flea market. I noticed it was hydraulic powered but didn't suspect it was for use under water.

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> I noticed it was hydraulic powered but didn't suspect it was for use under water

I suspect a gasoline-powered chainsaw wouldn't work underwater... which made me wonder if maybe that does exist because you could feed it O2 from a supply tank just like the fuel, maybe even from the diver's own air tank?

Now I want to know how bouyant battery-packs are and if an impractically-heavy battery-powered chainsaw work well underwater?

The exhaust bubbles surely would steer up debris and clog your sight

Not if you route it through a flexible hose a couple feet long. Not as bad, at least.

For the first question, according to [1] gasoline uses 9000L air per liter of gas and a liter of gas runs a chainsaw for an hour ish, something like 150 L/minute. This is an order of magnitude higher than humans or so, so a scuba tank that lets you dive for an hour would only let you chainsaw for maybe 5-10 minutes?

[1] https://news.ycombinator.com/item?id=9504215#9505263

I’m always blown away by how efficient animals are. No wonder we get fat.

And that's at atmospheric pressure! 100ft would be 4x the pressure at the output, requiring 4x the flow rate to do the same work. (Assuming you don't have some sort of magic low-pressure high-volume chainsaw "rebreather" apparatus.)

> requiring 4x the flow rate to do the same work.

I strongly suspect that this is incorrect. A given amount of oxygen and fuel releases the same amount of energy at any (reasonable?) pressure. 4x oxy (and fuel) --> 4x energy... this is the principle of turbochargers and superchargers (although the limiting factor is compression).

Interestingly, and paradoxically, I suspect that at an appreciable depth you mitigate the "air pump" effect and you don't need the compression stroke (or as much of one) to achieve a particular fuel mixture density prior to ignition so you'd lose "compression braking" and gain some efficiency.

(I also suspect that what happens if your vessel implodes and you're suddenly subject to 6,000psi is that the now superheated oxygen reacts with any oxidizable material at hand in an explosive exothermic reaction.)

I think we're both right and describing different things.

The air in the engine will be 4x as dense, but the flow rate will be the same.

The air in the tank cannot change density, and will therefore have a 4x higher flow rate.

The result is that storage is worth less strokes/breaths at depth.

I wonder if you’d get issues running an ICE under water if you went very deep, due to pressure changes on the exhaust system. Assuming you could seal up the system well enough to work at depth, and provide O2 as you mentioned.

Even the pressure going into the combustion chamber could be a complicated thing, right? Say you’re at 20 feet, that’s a fair amount of pressure. But what if you’re at 80 feet? Would it still be able to function the same way? I assume no, but I also know nothing about running ICEs under water.

If you can push the fuel and air in at ambient pressure, it shouldn't matter much right? Because the 'neutral' point of the cilinder pressure would be just as high as the pressure of the exhaust.

You'd probably lose some efficiency because the difference between peak pressure and neutral pressure in thr cylinder feels like it would be smaller. Perhaps not, because you could fit much more fuel and air in the cylinder for a bigger bang?

Most of your engine's power will be spent pushing the exhaust into the water.

I believe he is insinuating a "two hose" approach

All of your densities would be higher. Input, chamber, and exhaust would all have proportionally higher pressures and use more air. (And gas? I don't know if it's aerosolized or vaporized.) In theory this should all cancel out, although the absolute pressures and variable reaction rates and friction probably cause issues.

If nothing else, you'd have a hell of a time insulating the high voltage parts of the ignition system from all of the rest of the metal bits.

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