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Comment by bserge | Hacker News Reader

I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection.

That said, this reminded me of a lake in Chelyabinsk that is so polluted you can set it on fire according to some locals (I don't remember the video, it was a few Russians filming a short tour of the city)

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The river running through Cleveland used to be so polluted that it caught fire at least a dozen times.[1]

[1] https://www.smithsonianmag.com/history/cuyahoga-river-caught...

And provided the spark that caused the Clean Water Act to pass. Impossible to imagine something similar today.

Unless you live in Flint. Then nothing is impossible to imagine.

https://en.wikipedia.org/wiki/Flint_water_crisis

Sorry that was ambiguously worded. My point was that nothing like the CWA was even floated afterwards, since everyone knows the politics of today wouldn't allow it.

Maybe in the US, but I've seen some horribly dirty waterways in other countries.

I wonder if this is how Terry Pratchett was inspired, as the Ankh sometimes caught fire.

https://discworld.fandom.com/wiki/Great_Fire_of_Ankh-Morpork

Fires are less likely to ignite in very cold weather as the environment is taking energy out of the reaction. Once it gets going cold doesn't matter much.

To a running fire, everything is cold.

Poetic. I love it.

https://americanenglish.state.gov/files/ae/resource_files/to...

Wow, this brings memories, I remember reading this story when I was around 9-10 y/o. Is it from the Tales of the North?

>I mean, cold itself (within Earthly limits) doesn't put out fires. In fact, it might make them burn better due to convection.

Colder air will mean denser air and with that is more oxygen, so yes that is plausible.

Also less humid air

Does water already in vapor form really impact fires?

It appears humidity doesn't really impact an already burning wildfire, but lower humidity means drier fuel which makes the fire more likely to spread. Conversely, higher humidity can make a fire less likely to spread to moister fuel[1]

[1]http://www.auburn.edu/academic/forestry_wildlife/fire/relati...

Peat itself is often fairly damp.

> I mean, cold itself (within Earthly limits) doesn't put out fires.

However, heat is very much a part of the fire triangle.

But any fire is magnitudes hotter than -50. That cold is a blip on the fire's radar. Once started, and as long as there's fuel and oxygen, it won't stop because of negative Celsius temperatures.

It can significantly slow down wild fires which need to heat a constant supply of fuel to ignition temperatures. Wood burns at 180 degrees Celsius, so the difference between 20c and -50c is +160C vs +230C or over 40% more heat.

This is especially impactful when looking at how easily sparks propagate fires across gaps.

A typical wildfire is not putting much of its overall energy output into bringing fuel up to combustion temperature, though. The combustion energy in a kilo of wood is measured in megajoules; the amount of energy needed to elevate the temperature of a kilo of wood by a degree is measured in kilojoules. So while you're right that the energy budget needed to combust new fuel goes up by 40% or so if the fuel is at -50°C, the energy budget going into that is only a percent or so fo the total energy the wildfire is putting out. The rest of the energy beyond what's needed to bring fuel up to combustion point is going into making massive walls of glowing flames, mile-high convection columns of air, sending smoke thousands of miles, etc., etc...

That is, in many ways, precisely what makes it a wildfire, rather than a smouldering ember.

In the limit - when a fire is dying out - then I guess, for sure, changing the fuel temperature can sap a larger part of the energy budget.

The point is propagation speed. A burning tree surrounded by acres of burning trees is largely irrelevant at that point. It might happen to produce spark that travels miles, but that’s relatively rare. Which is why firebreaks can be useful.

My uncle used to joke that his car ran better in the mountains because the air was cleaner and thus had more oxygen. He was not serious, but in case of cold air there would be more oxygen per unit volume due to density so that might improve the combustion.

Ignoring, of course, the fact that "in the mountains" means an increase in altitude and thus a decrease in air density. My carbureted vehicles that were jetted for sea level always ran like shit in the mountains. And fuel injected vehicles run fine (because they can adjust for the altitude on the fly), but with noticeably less power.

Apologies for pedantically deconstructing what your uncle meant as a smart-assed joke.

Lower air density means old carburetted engines run leaner - which since most such engines are tuned to run slightly rich for reliability in favour of slightly lean, gives you more power up until the point they overheat and if they're a 2 stroke motor relying on the oil mixture in for lubrication the fuel, seize.

One of the old traditional racetracks here in Australia is Mt Panorama at Bathurst. It's got about a 180m elevation change between the start/finish/pit straight and the top of the mountain. Back when they were allowed to race motorcycles there, old 2 stroke racebikes were notorious for seizing at the top of the hill if they'd been jetted down at the bottom and hadn't been set "rich enough" to allow for the altitude change. Many many riders crashed and said "It was running really strong, til it locked up on me and spat me down the road..."

(No apologies at all for my off topic response to pedantry...)

Mmm, pretty sure sea level carbs run rich at altitude. Less air means a need for less fuel for the same ratio. But the fuel side didn’t change because we didn’t rejet, so now too much gets pumped in. If I’m guessing the bikes might have seized at the bottom of the hill instead? Or it just took until the top before the damage was done.

Ahhh, yeah - you're right. I've got it back to front there.

In my defence it's been a long time singe they've been allowed to race motorcycles there. At least 20 years. And now you mention it I do recall bikes seizing at the end of pit straight, right down the bottom of the hill.

In my defence..

In your defense, anyone old enough to remember carbs how work, even if only slightly, is forgiven the occasional lapse of memory. ;-)

Uhhh, thanks. I think.

(And get off my lawn!)

Could the lower air pressure at altitude still end up pushing less fuel in through the jets? It's not obvious to me how this would behave when deviating from normal conditions.

Carbs are half black magic and half voodoo.

If you find someone who _really_ knows how they work, pay them what they ask, and keep going back to them...

Some Rotax two-stroke engines running pre-mix have been used in ultralight aircraft, so I think appropriately sized altitude-compensating carburetors designed to run a petrol-lubricant mixture may be commercially available. However, I imagine it's pretty expensive to put aircraft parts (even ultralight parts) on a racing motorcycle.

These carbs use an aneroid to move a needle valve at the base of the jet as the atmospheric pressure changes, effectively dynamically re-jetting the carb on the fly.

Neat!

Got any keywords that'd lead me to something like that on Google quicker than me randomly searching? (Or even a link to such a carb?)

Edit: Found it. "High Altitude Compensator". Thanks anyway.

> And fuel injected vehicles run fine (because they can adjust for the altitude on the fly)

A bit of pedantry: modern automotive electronic fuel injection with exhaust oxygen sensors adjust much better to altitude, but the first aircraft and automotive fuel injection systems used the Venturi effect acting on a spring-loaded diaphragm to estimate mass flow rate, and simple mechanical computer to adjust fuel flow needle valves. Early aircraft mechanical fuel injection systems contained an altitude-compensating aneroid, but I doubt early automotive models did.

Many small recreational aircraft still use altitude-compensating carburetors that contain aneroids that act on needle valves to compensate for altitude.

Even today, the oxygen sensor is a relatively expensive part, and it's possible that early electronic automotive fuel injectors didn't measure and compensate for oxygen concentration in the exhaust.

So, an old automobile retrofitted with an aircraft altitude-compensating carburetor may very well compensate for altitude better than an early fuel injected automobile. Though, I assume only a handful of cars have ever been retrofitted with altitude-compensating carburetors.

Heat engines generate power from a difference in temperature. The larger the temperature difference, the more power. You can increase the temperature difference by making the heat source hotter or by making the heat sink colder.

The air pressure is also higher at colder temperatures so if the fire is underground it'd have more forces working in its favor to cycle fresh air to it and the relative temperature difference between the freshly heated and exhausted air and the fresh air would be wider causing there to be more turbulence cycling the air around.

Hence the invention of the intercooler diesel engine.

The turbocharger is what helps at high altitudes, not the intercooler which just cools the air. The turbo packs more air in to the engine, which helps at sea level but makes a big difference at high altitude. Superchargers do the same.

It's actually one way of putting out otherwise very persistent fires, especially the ones that like to only smoulder (really nasty chemicals, metal compounds, etc.) Removing heat from the reaction lowers the ignition temperate of the surrounding material to a point that fire dies out on its own.

Side note: it doesn't work that well on burning plutonium.

I think heat is meant to represent the ignition source. The reaction creates heat which then disapates into cold, right?

If this were to happen in absolute 0, would it be possible to sustain a fire? From learning more (just now) about absolute 0 seems it wouldn't but for other reason.

Can someone answer this, I find it a really fascinating question. How does cold effect fire if at all?

Cold affects the speed at which molecules are moving around. Fire is a phenomenon produced generally by exothermic chemical reactions between molecules. The part of the fire you see is, if I understand correctly, the photons emitted by excited electrons moving to a lower energy state after the reaction.

All other things being equal, the colder it is, the less the candidate molecules are moving, and thus the less likely they are to encounter each other and react. IIUC almost all chemical reactions are less vigorous at lower temperatures, disallowing convection of course.

At absolute zero, ambient oxygen would solidify. Not sure what happens if you drop a match on a bunch of solid oxygen, but I imagine it's not good.

It would go out. In a room cold enough for Oxygen to turn solid, there would be no gaseous oxygen to sustain the match’s combustion as it falls. The wood would have some latent heat, but not enough to re-ignite itself in a room that cold.

Pure oxygen won't burn anyways. What would it oxidize? If you had frozen a mix of oxygen and hydrogen (without the two separating) it would be possible to ignite and burn it.

It would oxidize the match that’s being dropped in.

Truly pure oxygen won't burn, it needs fuel. So, what would probably happen is it would burn super bright for a split second as the match itself was consumed, and then extinguish itself as it has no more fuel.

> I mean, cold itself (within Earthly limits) doesn't put out fires

This is not a meaningful phrase when cold is the absence of heat. You need to describe the motion of the cold air against the warmer system to get a meaningful phrase.

Being able to set a river on fire is not terribly unusual for a polluted river. I remember about ten years ago I posted up at the schuylkill, waited for an oily patch, and was able to set it on fire for about 5 seconds. These oily patches are uncommon. I have no clue where the oil came from but with the schuylkill it's anyone's guess. The patch itself is necessary for lighting the river on fire; you can't just set fire to an arbitrary section of river.

Things that are deemed to be fuels emit way more energy than energy needed to heat air from ANY temperature. The fire would continue even if the air was liquid.

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