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Comment by hn_throwaway_99 | Hacker News Reader

Thanks for posting this - I wasn't previously aware of the delineation between a shock wave and a pressure wave that the author explained clearly.

I'm curious, is there something specific about "high explosives" vs "low explosives", beyond just the magnitude of energy released, that explains why some generate shock waves and others pressure waves?

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In a nutshell, high explosives detonate, low explosives combust. The two processes are distinctly different in how they operate at the molecular level. Basically, detonation is when a shock wave goes through the material, causing it to "blow up" on a molecular level as it passes (the molecules split as the shock passes through). Combustion happens much slower and is usually a chemical reaction involving some sort of fuel and oxidizer that is mixed and burned. I'm not an expert, but you can see the difference in a gasoline engine. As you crank up the compression ratio the air/fuel mix in the cylinder will eventually undergo a change from combusting to detonating, producing engine "knock" and rapidly destroying your engine if you let it continue. On a gas motor, around 13:1 is a good spot to see this happen. Someone will have more for you, but I think this is the basic concept seperating the two.

Am I right in my understanding then that diesel engines work entirely on the principle of detonation, and that's why they're heavier as everything is designed to cope with what would make a petrol engine ping?

Does that make any sense?

Or is diesel injected in to the cylinder in a way that more or less produces a controlled combustion?

Detonation in an engine means something different. Detonation is when the fuel starts burning from a source other than the spark plug and the two flame fronts collide.

No, Diesel engines are not based on detonation but also on a controlled combustion. The combustion is triggered differently that is all.

Diesel engines have a very high compression ratio, like 20:1 or more, but diesel fuel has a very high octane rating in comparison with gasoline, so it still combusts, not detonates, at these high ratios. What makes a diesel different than a gas engine is that the heat generated from the compression of the air/fuel mix is what actually causes the mix to start burning in the cylinder. So, for this reason you don't see any ignition system on a diesel usually, it doesn't need it (though maybe glow plugs or a block heater in cold areas, but not an actual ignition system). But the mixture still combusts in a diesel. Detonation is incredibly destructive to any engine, even a big beefy diesel. It can destroy any engine very quickly.

Thanks all for clearing that up.

>but diesel fuel has a very high octane rating in comparison with gasoline,

"octane" is the ability of a fuel to "resist combustion" from compression. Diesel fuel is not rated in "octane" but the opposite: the ability for it to combust from compression. For some reason in the "old days", it was commonly thought by some that by adding some diesel to gasoline that it would help "lubricate" the motor (as diesel does have an oily feel). However mixing diesel with gasoline only lowers its octane rating.

Yeah, I am not sure if octane is the right word here, you are probably right in what you say. I just know that diesel fuel is much more resistant to detonation at high compression ratios, given how a diesel works. In a gas motor, octane gives a measure of this, so I used the word there, but as you say it is probably not the right word.

Diesel has a low octane number. Though octane isn't used for diesel fuel, it can be determined and is around 15-25.

Diesel engines avoid pre-ignition knocking by compressing only air. There is no compression of air/fuel mix. The fuel is injected into the hot, compressed air at the top of the compression stroke (at something like 3000+ PSI, IIRC).

The fuel does not have to be particularly volatile because of the high compression and the injector's atomization.

In theory, diesel engines can run on "anything". Diesel engines have been run on crude oil taken directly from the ground (of good purity), and on vegetable oil.

Modern diesel injectors:

30,000 to 40,000 PSI

https://www.buyautoparts.com/blog/4-common-diesel-fuel-injec...

You can do that with old diesel engines but new ones are too sensitive and destroy themselves if you try to run them on anything but diesel.

It mostly because the high pressure pump use the Diesel for lubrication so if you use something else the pump destroy itself sending tiny metal shards down the pipe and into the injectors.

That's why accidentally running a diesel car on gas turn expensive fast. The contrary is much cheaper as just have to flush the fuel lines and you're good to go.

So gasoline mixture is both a high and low explosive? Your example seems to contradict your definition. This is also what seems confusing in the article too. Ammonium nitrate might be a low explosive but in this case it seemed to detonate. At that point is there any qualitative difference in the reaction between low and high explosives?

I am not an expert and do not understand it fully, but I know that fuel/air mix does make a phase transition of sorts and becomes a high explosive when the compression ratio is high enough. At low ratios it burns, at high enough ones it really does detonate with a shock wave running through it and causing the whole charge to blow up almost at once. Ammonium nitrate does the same if there is a big enough kick to get a strong detonation wave going through it, though I think there is much greater chance of this if there is some sort of solvent (gas, diesel, etc) mixed in with it.

> I think there is much greater chance of this if there is some sort of solvent (gas, diesel, etc) mixed in with it.

The fuel is what burns turning into combustion byproducts. Larger volume -> high pressure -> physics magic I don't feel like explaining -> shock wave. Burning pure AN is a waste, you're basically burning super lean. Add fuel and you get closer to stoic. More efficient reaction -> more output volume per input volume -> more bang.

It's like the difference between making a bunch of gas fuel (like propane or something) go up as a woosh vs a bang but with solid inputs instead of gasses.

I'm also no expert, but I thought that "detonation" in a gas engine just meant that there was too much fuel being injected, when the engine is at too low revs, so the piston can't move quickly enough to absorb the expansion and convert it to spinning the crankshaft - very similar to if the timing of the spark was too early.

Nothing to do with a transition to high explosive, although it will destroy an engine just the same.

And if you're "inside" an explosion that's too slow to generate a shock wave, you may not even be seriously injured.[0] At least, if the pressure spike is too low to rupture your body. And as long as objects don't get accelerated toward you.

As a child, a friend and I were electrolyzing water, in an enclosed space. And so the hydrogen flashed. But the concentration was so low that the air just turned faint blue. There was no damage, to us or the building.

0) https://www.stuff.co.nz/national/114372936/how-did-anyone-su...

The technical term for a subsonic combustion wave is deflagration.[0] A normal internal combustion engine deflagrates. If it starts pinging, that is detonation, and damages the pistons, etc.

[0] https://en.wikipedia.org/wiki/Deflagration

I've been led to believe that 'high' explosives are explosives that explode/combust faster than the speed of sound. Others (like black powder) burn slower.

More than just burn, 'high' explosives detonate [0], where the pressure wave itself provides the activation energy required to sustain the reaction.

[0] https://en.wikipedia.org/wiki/Detonation

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