4 ms·

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/minut

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

steve_adams_86 3y ago
I’m always blown away by how efficient animals are. No wonder we get fat.
itishappy 3y ago
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.)
- m3047 3y ago
  > 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.)
  - itishappy 3y ago
    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.

steve_adams_86 3y ago

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

itishappy 3y ago

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.)

m3047 3y ago
> 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.)
- itishappy 3y ago
  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.