Recent comments in /f/askscience

ECatPlay t1_ja4e72v wrote

Reply to If the fuel that goes in car engines is extracted from hydrocarbons, which consist of only Hydrogen and Carbon, and those hydrocarbons react with Oxygen in the air (combustion reaction), to produce CO2 and H2O, why do we get a bad smell from car exhaust fumes if both gases are odorless? by Protoflare

As you note, a car's gasoline engine converts hydrocarbons and air into CO2 and H2O, both odorless, providing the energy that makes the car go. But there is also nitrogen in the air in the combustion chamber. And although we think of N2 as inert, a small amount does get incorporated into the oxidation chain reaction, to form a mixture of nitrogen oxides, NOx. This, along with unburnt hydrocarbons, lead to smog, so catalytic converters were developed to combat this.

Modern Three-way Catalytic Converters not only convert CO to the less toxic, CO2, and oxidize any remaining hydrocarbons; they also reduce most of the NOx back to N2. But a small amount of NOx gets reduced to ammonia, NH3, as a side product. And NH3 has a noticeable, pungent smell down to 5 parts per million in air.

And although gasoline is primarily composed of hydrocarbons, there are traces of other elements: inhibitors and detergent additives, along with residual sulfur compounds. And whatever sulfur there is, winds up as hydrogen sulfide, H2S, after the catalytic converter. The sulfur level in gasoline should be pretty low now days, below 10 ppm, so this may not sound like much of a problem, but H2S becomes noticeable as a rotten egg smell below 0.1 ppm.

These traces of ammonia and hydrogen sulfide are primarily responsible for the bad smell you notice.

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gwplayer1 OP t1_ja4a4we wrote

Reply to comment by mdogm in How old is the ISS REALLY? by gwplayer1

That's a valid point. Supposedly the "Big Bang" was an almost instantaneous expansion but instantaneous relative to what? It's kind of opposite the effect at the event horizon of a black hole where, from the outside perspective (earth), something near the event horizon is slowing down but for the particles perspective, everything appears normal.

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