Hacker News Reader

Comment by zhengyi13 | Hacker News Reader

In '99, while working at a solar hydrogen startup as a recent grad, I was exposed to a fellow who proposed to transport H in the form of H2O, and his solution for cracking it was to make purple ping pong balls filled w/ Na. He'd deliver a hopper of Na ping pong balls, and a machine filled with water. Feed balls into the machine, which would then use a ram to split the balls underwater, and siphon off the resulting H2 gas to feed to your fuel cell.

The racket it made, with a loud kerchunk splitting the balls, and the hissing immediately following, was just slightly terrifying.

This paste seems a good deal saner in comparison.

View on HN

Not that internal combustion engines are the model to aspire to... but they're just a metal contraption full of explosions. Being terrifying isn't necessarily a disqualifier, haha.

Full of combustions, not explosions.

“The speed of the reaction is what distinguishes an explosive reaction from an ordinary combustion reaction.” - Wikipedia on explosion.

“An internal combustion engine nominally operates on a controlled rapid burn.” - WP on combustion.

Internal combustion engines do in fact attempt to burn fuel and oxidizer at maximum stoichiometric efficiency, and they also are trying to burn it as quickly as possible to capture all of the heat rather than exhausting a partially burnt mixture and wasting fuel.

Effective explosives have the exact same chemical goals and nearly the same thermodynamic goals.

If that isn’t a carefully controlled explosion I don’t know what is. I mean, come on, a piston moving up and down thousands of times per minute isn’t explosive enough for ya?

Engines combustion is designed to occur as a propagating flame front in the cylinder initiated by the spark plug. Great efforts are made to ensure that this is precisely what happens.

The alternative is a detonation, i.e. engine knocking. This causes a spike in cylinder pressure and will quickly destroy an engine. It is for this reason that engines are absolutely not trying to burn fuel as quickly as possible.

This is a difference of detonation vs. deflagration. Whether you consider both of these to be forms of explosions is a semantic issue.

> ...trying to burn it as quickly as possible...

No, the speed of burn in an internal combustion engine is not explosive:

“Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.” - WP on engine knocking, which is undesirable and may damage the engine.

Sounds to me that the problem is not the speed of the explosion that happens when engine knocking, but the timing of it.

There are HCCI engine prototypes, but they are finicky, because the mixture (mostly regarding initial temperature and pressure) has to be in a narrow flammability band, so that it ignites precisely at Top Dead Centre from adiabatic compression heat.

If it ignites earlier, the pressures are far higher than normal. If it doesn't trigger, it won't for the complete cycle, as the piston expands and the temperature drops.

Free-piston engines have an advantage there, as they don't force the compression ratio with a crankshaft. They would decelerate the piston somewhat earlier and harder than necessary, but there are no bearings that have to handle this load.

Their issue has more to do with extracting energy from the oscillation, and keeping a stable operating point that's neither prone to stalling nor prone to runaway.

> Full of combustions, not explosions.

It's both, no? Aren't all chemical-based explosions considered combustion?

If "explosion" means "a violent release of energy", that seems a reasonable word to describe what happens in a cylinder in about ~5 ms.

Explosion is a faster speed of burn than what an internal combustion engine is designed for.

Yes. And for another example: gunpowder is not considered an explosive.

In fact, only half of the hydrogen originates from the POWERPASTE; the rest comes from the added water.

Indeed, it seems it's an hybrid of your fellow's tech, using magnesium both as a substrate for H2 and as a reducing agent for water: Mg + 2H2O -> Mg(OH)2(aq) + H2(g)

Less explosive than Na, hopefully :)

Reminds me of my undergraduate days in the 1980's, when the chemistry majors' idea of fun was stealing chunks of sodium from a lab and throwing them off a bridge into a nearby river.

Boom.

I had a high school science teacher who was demonstrating controlled burning of sodium and potassium one day by slicing of pieces from a large sample. Being covered in oil, the main chunk got away from him and dropped into the lab sink which was unfortunately wet from previous cleaning. A vigorous reaction ensued :)

There was another group during that era doing something similar with sodium hydroxide, which is a little less... violent. Not quite sure I recall how that chemistry worked, but I think the idea is 'find something where the hydrogen bonds are easier to crack than H2O'.

Now there are other groups trying to figure out if they can increase the power capacity of batteries by replacing the electrolyte. Which sounds like a fuel cell/battery hybrid to me.

Aluminum oxidation.

It would be lot easier to just use sodium directly and have hydrogen as a by-product. (See my other comment.)

If you're going to use metallic sodium as energy storage you might as well use it directly in a battery.

Reply on news.ycombinator.com