Comment by thechao
9 hours ago
Gasoline has 10x the energy (MJ/Kg) than TNT, but it's not a "bomb". Being a "bomb" is about energy release rate. A battery is still controlled by its chemistry.
9 hours ago
Gasoline has 10x the energy (MJ/Kg) than TNT, but it's not a "bomb". Being a "bomb" is about energy release rate. A battery is still controlled by its chemistry.
Liquid gasoline does not have anywhere near the energy of TNT, or even a battery for that matter. It contains basically zero releasable energy. It needs oxygen or another oxidizer to react with to actually release any energy. TNT and batteries' energy density calculations include the oxidizer and the oxidizer is in close proximity to the fuel (molecularly so in the case of TNT). If you 10x the "energy content" of gasoline it's still rate limited by access to oxygen. If you 10x the energy density of a battery (the type with the oxidizer contained within the battery, not a fuel cell or metal air battery) you've got 10x the energy ready to be released quickly if oxidizer and fuel mix in unfortunate ways
Energy capacity vs. energy release rate, or energy vs. power, tends to follow an inverse relationship, at least within similar domains.
The fuels which afford the highest rates of storage tend to release it more slowly. Chemical explosives typically have about 1/10 the energy storage capacity (by mass) of fuels, as several people have pointed out.
Similar relationships exist for other forms of energy storage. For electricity, fuel cells, chemical cells ("batteries"), flywheel / reaction storage, and capacitors tend to offer a trade-off between total storage capacity and reaction time. Capacitors can react at intra-phase rates (that is, within a single 60Hz power cycle), whilst batteries and flywheels can take longer to come online.
See for example: "Battery Parameters" <https://www.monolithicpower.com/en/learning/mpscholar/batter...>.
It's an interesting discussion, because if you assume access to atmospheric oxygen, gasoline is MUCH more energy dense.
The previous point is fair though. What is the energy density of gasoline in a stoichiometric mixture with its oxidizer and not already detonated? That's where TNT beats it by a long shot. The mixture with any chance of being stored would be some relatively low pressure gas.
Edit: Or, a fair closed-system comparison to a battery would need to include a liquid oxygen tank or similar so you have the two components stored but can control the delivery...
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That's a huge assumption that won't always be the case though. A gasoline tank with a small hole in it is a lot safer than a liquid-electrolyte battery with a hole in it.
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"Liquid gasoline does not have anywhere near the energy of TNT,"
Energy density of gasoline is approximately 44 to 45 megajoules per kilogram.
Energy density of TNT is approximately 4.184 megajoules per kilogram.
Gasoline, when in liquid form, is not mixed with oxygen and burns, releases energy slowly. In comparison with TNT which is both the fuel and oxidizing agent releases energy very fast, it has detonation velocity of 6,900 m/s.
https://en.wikipedia.org/wiki/Table_of_explosive_detonation_...
There is a way how to use the energy density of gasoline for explosive purposes, Thermobaric weapons.
"A fuel–air explosive (FAE) device consists of a container of fuel and two separate explosive charges. After the munition is dropped or fired, the first explosive charge bursts open the container at a predetermined height and disperses the fuel in a cloud that mixes with atmospheric oxygen (the size of the cloud varies with the size of the munition). The cloud of fuel flows around objects and into structures. The second charge then detonates the cloud and creates a massive blast wave."
https://en.wikipedia.org/wiki/Thermobaric_weapon
A snickers bar has nearly 5× TNT’s energy per kilogram—TNT merely has the habit of being extremely impatient about releasing it.
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One of the problems with gasoline is its explosive nature, which the fuel storage and delivery system of a car attempts to mitigate. It has a better profile for burning off than a battery though since it will spill out and burn away from the vehicle.
Gasoline isn't that explosive. You can shoot up a gas tank and it will just burn. What you see on TV is movie magic. Gasoline vapor can explode, but you need the right air to fuel ratio and a spark.
Uh… gasoline has a super high vapor pressure. If you start out at the wrong ratio, it does what it can to make that ratio right.
I’ve seen a few gasoline fires that start by burning, and then much more rapidly expressing potential.
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"Fast X: why cars don’t really explode when they crash"
"Petrol and diesel can only explode when under pressure and mixed with air and in the case of petrol, have a small amount of energy added in the form of a spark or a flame. Engines pressurise the fuel/air mixture in the cylinder and so produce small, confined explosions which turn a crankshaft and drive the wheels."
"When cars are involved in collisions, fuel lines are often torn and petrol leaks out onto a hot engine. Liquid petrol can catch fire in the presence of air. But it can’t explode because it’s not under pressure and is in the liquid phase rather than a vapour."
https://theconversation.com/fast-x-why-cars-dont-really-expl...
> Gasoline ... it's not a "bomb"
My teen years would beg to differ, when some neighborhood friends and I decided to experiment with gas and bleach. The neighborhood residents, fire department and local police also had an opinion, too. (We all got into BIG trouble. Definitely one of the dumber things I did as a teen.)
Bleach? What does that do?
My high-school crowd stopped just short of building a multistage FAE device. I left early for college and everybody else was short of cash, and that was the end of our neighborhood R&D program. All pre-9/11, needless to say. It's a lot less fun when the possible punishment goes from detention at lunch to waterboarding in Gitmo.
Poorly stored gasoline can spontaneously combust, though. So more like a firebomb.
No it can't. To combust it needs ~ 500F. If you left it out uncovered in the open and allowed the it to vaporize with a spark nearby, it might explode, but thats not spontaneous combustion. Gasoline sitting in a sealed container wont spontaneously combust.
Didn't everyone have one of those old mechanic friends who liked to put the cigarette out in the can of gas to scare the tourists?
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Gasoline doesn't do that. Perhaps you're thinking of the following?
What it does do is slowly polymerize, becoming useless as an ICE fuel in time, typically in a year or two. This is why backup generators should run on propane, which has no degradation mechanism.
No, I'm thinking of improperly stored gasoline evaporating and combusting.
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