Why don't we power our mobile devices with petrol engines?

[Ranger_Jeff]

Comedian George Gobel had a routine about his playing a steam powered guitar.

[Qwertol]

@iljitsch

You changed units in the wrong direction, it’s centimeters. Wikipedia:

“Of all the common nuclear fuels, Pu-239 has the smallest critical mass. A spherical untampered critical mass is about 11 kg (24.2 lbs),[1] 10.2 cm (4”) in diameter."

Of course you may want to reserve some extra space for a boiler, steam turbine, generator and, last but not least, shielding.

If you want to go small and don’t mind some radiation, antimatter would probably be the way to go.

You DON’T want your nuclear reactor to reach critical mass (by the way size given by you does not include use of neutron reflectors).

As Derleth mentioned generators using radioactive materials can be small, but the problem is that it wouldn’t be a good idea for people to carry them inside their pockets…

[iljitsch]

I’m not sure how much fissible material you need to run a reactor, but I’m assuming the minimum would be close to the critical mass; my understanding of the whole process is that a reactor runs at just blow critical to get nice and hot and generate all that steam. In practice it’s probably more because you need space to insert control rods and for heat exchange.

About RTGs: it looks like that’s actually possible: a gram of plutonium -238 generates about half a watt and the shielding would be 2.5 mm of lead or even less. Apparently, Pu238-RTGs have been used to power pace makers.

[Hail_Ants]

@Fear_Itself

Here is a butane-powered fuel cell charger for laptops and phones. I think fuel cells are more efficient than internal combustion engines and produce less waste heat.

Miniaturized butane fuel cell system enables new USB battery charger

I remember about 20 years ago (right when laptops were reaching critical mass) reading an article in a computer magazine which stated that the above devices were going to be the next big thing and replace chemical batteries en masse! Needles to say it didn’t happen.

[Dag_Otto]

I think it would be OK for Apple devices, but you know those assholes with Androids would be changing the pipes on their phones.

[upend]

there’s a few fuel cell cars being launched in the US (and Europe) this year - Toyota, Honda and VW are 3 manufacturers that spring to mind.

as the byproduct is water - I’m not sure they’d make great power sources for mobile phones though. “My suit was ruined when I made a phone call”!

more practically, not sure if I’d want to be jogging with a pressurised hydrogen tank strapped to me. You’d also have prohibitive cost at present.

[Quercus]

@iljitsch

I wonder though if there are any possibilities for building an internal combustion engine that’s optimized for this kind of use. Maybe burn a small amount of fuel in a chamber slowly, and then use the expanding gasses to put pressure on piezo crystals to generate electricity?

The problem is that any engine which depends on generating heat is going to get more and more inefficient as it gets smaller. If it runs on heat, you need to keep the heat in one place, and as things get smaller, the surface area (which lets heat out) gets bigger in relation to the volume (which is where heat is). So smaller engines leak (waste) far more heat that bigger ones.

That’s leaving aside the issue of getting rid of the waste heat – not exactly an easy thing to do if the engine is supposed to be in someone’s pocket.
Fuel cells do not depend on heat, so conceivably, engineering improvements could make [fuel cell + fuel storage] competitive with batteries even for small applications. [But of course a new battery technology could put batteries way ahead, too]