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Comment by thechao | Hacker News Reader

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.

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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

It's an interesting discussion, because if you assume access to atmospheric oxygen, gasoline is MUCH more energy dense.

    TNT.....:  4 kJ/gram and  7 MJ/liter
 
    Gasoline: 43 kJ/gram and 33 MJ/liter

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...

In the context of the conversation, it doesn't actually refute the point though, which is that explosive danger is contingent on a much broader set of relations than simple energy density. Gasoline just demonstrates the disconnect. Tacking on additional conditions to make high-density batteries dangerous is exactly what the engineering goal should be.

As a stupid example, if we have a high density battery technology that never explodes unless, say, under magnetic forces only seen being produced by magnetars, that's a pretty safe battery.

Gasoline + LOX beats TNT on energy density per unit mass and per unit volume, no? There's a reason that kerosene+h2o2 is a classic rocket fuel, and TNT is not.

It's usually kerosene + O2, not H2O2. But H2 + O2 is another classic liquid rocket fuel that has higher specific energy (energy per mass), but lower energy density (energy per volume) than kerosene, and is used mostly as a 2nd stage fuel (where mass is most important). Its lower density and the need to keep it very cool mean heavier tanks, and it's also hard to work with due to being such a small molecule that likes to leak, which causes development costs to be higher.

> 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.

eeeh that still doesn't matter really. Gasoline is safe(r) because it needs conditions to explode (spreading into area then ignition), instead of just detonator. Diesel even more so

A damaged gasoline tank that's near empty might explode but more likely it will just leak and start burning, and probably at that moment be still extinguishable. Explosives, well, you won't have time to blink.

Batteries ? Zero chance to extinguish, gonna let it burn thru. THAT is the problem

To stoichiometrically burn 1 gram of gasoline requires 3.5 grams of oxygen, so an energy density of 9.5 MJ/g, still more than double TNT. The state of the oxygen doesn't impact its mass.

Air is usually quite available though.

Many batteries are sealed specifically for this reason.

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.

Yes, that's what makes it interesting.

Interesting discussions about energetics are best had from a distance.

I can remember several situations where “a little bit of gasoline” caused a lot of problems. It’s a relative term. You need a lot of air to make it really shine, but “a little bit of gas” goes a long way lol.

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...>.

"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.

I'm still surprised we haven't figured out a bioreactor you can feed your scrap food and turn it into energy, methane combustion notwithstanding. Like imagine feeding a roomba some sugar and it's set for a few days lol.

You could probably even power a time machine without needing plutonium.

But only if said Roomba is unable to find enough sugar on the ground for a few days. Otherwise, it's completely self-sufficient, you just have to leave enough crumbs on the floor.

Lots of experiments along those lines, actually.

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.

Requires a good mix though. The parent is still correct. Mythbusters spent a long time trying to get cars to blow up in a dramatic hollywood fashion in various episodes and determined that under most conditions it wouldn't happen.

They did get it to happen, they just had to try really hard.

In most accidents the gas tank will not blow up.

High velocity accidents where the gas is quickly dispersed into the air is pretty much the only way that happens.

Even then it's just a slow-burning fireball, not detonation, no shockwave.

Still more than enough to burn your windpipe, skin to 2/3rd degree and permanently disfigure you

All of which are symptoms of burning, not explosion.

And was still not really possible without some really unlikely conditions. Not that car fires aren't dangerous, regardless of whether the vehicle is ICE or otherwise, but the fireball thing doesn't really happen.

"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.

We used to take a half-full can of gas, then get a ping pong ball, drill a hole in it, fill it with bleach, then put tape over the hole. Drop the ping pong ball in the gas can, screw the lid on really tight, then wait. It used to take about 10 minutes for the gas to eat its way into the ping pong ball, but when it did, and the bleach mixed with the gas... boom! The scary part is only worked, maybe, half the time. So there would be cans of gas/bleach just sitting in a field, waiting to explode or release really harmful chems into the air. We were kids, young and stupid.

WTF. Seriously? Just plain old Clorox? Which is ~95% water?

BRB, stopping by the laundry room on my way to the garage...

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?

That is definitely one of those “it worked the first 50 times no problem” tricks.

...it was probably diesel not gasoline

Oh, it was most definitely gasoline. As long as there is no vapour hanging above the container, it works just fine... as the neighbour post says, the first 50 times. Then one day the old timer blows up himself and the shop

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.

I remember something about someone trying to do a photo shoot with candles in abandoned cars in a junkyard

Edit: https://petapixel.com/2015/06/11/explosion-at-junk-yard-phot...

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