3 ms·

Excellent analysis, thank you, but this line is a little strange: > The reason fuels are so good is that this release needs a second component (oxygen) that is

Excellent analysis, thank you, but this line is a little strange: > The reason fuels are so good is that this release needs a second component (oxygen) that is kept separated. This makes high energy densities safe. Don't fuel tanks (for internal combustion engines, for example) typically also contain oxygen? You make it sound like a full tank is safer than a half-empty tank. I assume you're actually saying that for safety we want the reactants to be stable at common atmospheric temperatures and pressures, and we don't want reactants that spontaneously combust on contact with air or common materials (including each other).

BenjiWiebe 5y ago
They don't contain nearly enough oxygen to liberate all the stored energy. Also, especially since gas tanks are no longer directly open to the atmosphere, I wouldn't be surprised if the oxygen concentration was too low most of the time for combustion to be possible.
- dane-pgp 5y ago
  In principle we could calculate the weight of oxygen in half a fuel tank's worth of air, and work out how much energy that would release if it was used in combustion with the fuel there. In practice, though, you're right, that we'd have to think about the oxygen concentration, which would immediately start dropping as the fuel burned. Also, it's not easy to calculate what temperature the tank would reach, and for how long, and whether that would cause it to rupture. For reference, though, the testing process for fuel tanks can include: "the tank being exposed to a pan of raging petroleum coming through specifically designed firebricks to intensify the effect of the naked flame. The tank has to be capable of securely containing diesel during the test for 90 seconds." https://corillaplastics.co.uk/rotationally-moulded-diesel-fuel-tanks/ https://corillaplastics.co.uk/rotationally-moulded-diesel-fu...
dotancohen 5y ago
> You make it sound like a full tank is safer than a half-empty tank. Back when I was working as a Ford tech, I was told that a full tank is a fire hazard. An empty tank is an explosion hazard. You can run away from a fire. If the fuel pump can be changed without dropping the tank (e.g. by going under the rear seat or truck bed), then it is preferable to work on a full tank.
l33tman 5y ago
A full tank is safer than an almost empty tank.. Also the tank is designed to keep the vapor part of the gasoline too saturated to explode, this is why there is a latch on the fill tube. I found this Quora post: https://www.quora.com/How-come-the-gasoline-in-gas-tanks-never-explodes-What-makes-a-spark-so-impossible https://www.quora.com/How-come-the-gasoline-in-gas-tanks-nev... And even if it can explode in the situation where there's almost no fuel left, in that case it's not as bad as liberating the full energy of a full tank, which is what you'd do if you have a flywheel spinning with the same total energy as a full tank of gasoline. That would be madness. Heh I came to think of the recent demo of the company that wants to spin up satellites on earth and THROW them into orbit. If something goes wrong in that spin-up, they would destroy the entire launch facility and whatever is in the way. https://youtu.be/JAczd3mt3X0 https://youtu.be/JAczd3mt3X0 I once had a CDROM open while the CD was spinning at like 40x or so, it ejected and went into the plaster wall. And that was just a CD..
wruza 5y ago
You may think of it as one key for all energy boxes (a broken valve or a hole in a gas tank) vs multiple keys for every energy box (oxygen molecules). To burn a liter gasoline you have to mix it with the ~same amount of oxygen. It never explodes all at once except in that rare situation. An empty tank is exactly that, because gasoline is volatile. Energy boxes that can be opened with a single key are more dangerous, cause more prone to accidents. Pressure, kinetic storages, self-sufficient explosives are all much more dangerous than stable chemical bonds. Also, gravitational potential can be accumulated in a single-key way too. That’s why cliffs are dangerous, but staircases are less so, because energy release is dosed with each step and flight. You have to actively err on stairs to take fatal damage.

BenjiWiebe 5y ago

They don't contain nearly enough oxygen to liberate all the stored energy. Also, especially since gas tanks are no longer directly open to the atmosphere, I wouldn't be surprised if the oxygen concentration was too low most of the time for combustion to be possible.

dane-pgp 5y ago
In principle we could calculate the weight of oxygen in half a fuel tank's worth of air, and work out how much energy that would release if it was used in combustion with the fuel there. In practice, though, you're right, that we'd have to think about the oxygen concentration, which would immediately start dropping as the fuel burned. Also, it's not easy to calculate what temperature the tank would reach, and for how long, and whether that would cause it to rupture. For reference, though, the testing process for fuel tanks can include: "the tank being exposed to a pan of raging petroleum coming through specifically designed firebricks to intensify the effect of the naked flame. The tank has to be capable of securely containing diesel during the test for 90 seconds." https://corillaplastics.co.uk/rotationally-moulded-diesel-fuel-tanks/ https://corillaplastics.co.uk/rotationally-moulded-diesel-fu...

dotancohen 5y ago

> You make it sound like a full tank is safer than a half-empty tank. Back when I was working as a Ford tech, I was told that a full tank is a fire hazard. An empty tank is an explosion hazard. You can run away from a fire. If the fuel pump can be changed without dropping the tank (e.g. by going under the rear seat or truck bed), then it is preferable to work on a full tank.

l33tman 5y ago

A full tank is safer than an almost empty tank.. Also the tank is designed to keep the vapor part of the gasoline too saturated to explode, this is why there is a latch on the fill tube. I found this Quora post: https://www.quora.com/How-come-the-gasoline-in-gas-tanks-never-explodes-What-makes-a-spark-so-impossible https://www.quora.com/How-come-the-gasoline-in-gas-tanks-nev... And even if it can explode in the situation where there's almost no fuel left, in that case it's not as bad as liberating the full energy of a full tank, which is what you'd do if you have a flywheel spinning with the same total energy as a full tank of gasoline. That would be madness. Heh I came to think of the recent demo of the company that wants to spin up satellites on earth and THROW them into orbit. If something goes wrong in that spin-up, they would destroy the entire launch facility and whatever is in the way. https://youtu.be/JAczd3mt3X0 https://youtu.be/JAczd3mt3X0 I once had a CDROM open while the CD was spinning at like 40x or so, it ejected and went into the plaster wall. And that was just a CD..

wruza 5y ago

You may think of it as one key for all energy boxes (a broken valve or a hole in a gas tank) vs multiple keys for every energy box (oxygen molecules). To burn a liter gasoline you have to mix it with the ~same amount of oxygen. It never explodes all at once except in that rare situation. An empty tank is exactly that, because gasoline is volatile. Energy boxes that can be opened with a single key are more dangerous, cause more prone to accidents. Pressure, kinetic storages, self-sufficient explosives are all much more dangerous than stable chemical bonds. Also, gravitational potential can be accumulated in a single-key way too. That’s why cliffs are dangerous, but staircases are less so, because energy release is dosed with each step and flight. You have to actively err on stairs to take fatal damage.