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?

Submitted by Protoflare t3_11ck5es in askscience

Obviously there would be a catalytic converter, which does try to convert carbon monoxide into carbon dioxide, but both gases are still odorless, so I was wondering why that was.

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ECatPlay t1_ja4e72v wrote

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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supreme_harmony t1_ja4ot1d wrote

This is a fantastic answer. The only thing I would add to the above is that no burn is ever perfect. Exhaust fumes from a car therefore also contain trace amounts of unburnt fuel and even partially burned hydrocarbons of various complexities. These are tiny amounts, but enough for our noses to pick it up.

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Drink15 t1_ja4zrsq wrote

There is also oil that get past the seals and burns with everything else. It’s a small amount in new cars but increases as motor wears

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smolgaybigcity t1_ja62ydy wrote

N2O2 is another byproduct, albeit produced in the lowest amounts, but still happens. It has an acrid odor.

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ECatPlay t1_ja8gcvv wrote

I think you mean N2O4, which does indeed have an acrid odor. This is the dimer of NO2, and is in equilibrium with it, such that both species are present:

2 NO2 ⇌ N2O4

This is generally understood to be part of the mixture generically referred to as NOx. But point taken.

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KingSpork t1_ja6m5c2 wrote

So does this mean that car exhaust smelled different pre-catalytic converter?

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ECatPlay t1_ja6pex2 wrote

Oh, yes. In particular, some refineries blended extra aromatics in with the gasoline to increase the octane: benzene, xylenes, and alkylates. And before catalytic converters and emission standards limited unburnt hydrocarbons in the exhaust, these could give it an almost pleasant odor, especially when it was running rich. (After all, benzene compounds get the name “aromatic” because of the type of odor they have.)

But only if the engine was burning clean and not also burning oil. That made the exhaust smoky. Now days engines are manufactured to closer tolerances and you seldom have to add oil. But back in the day it was routine to check your oil every time you stopped for gas, because there was always a little bit of oil making it past the rings and getting burned in the cylinders.

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trapperjohn3400 t1_ja7n8o7 wrote

Yes it could vary from a light, sweet smell, to a horrible acrid smell. But they were all quite smelly. It made driving in stop and go traffic in a city very, very unpleasant.

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MisterSquidInc t1_ja6docu wrote

I've noticed when running E85 (in this case race fuel, so guaranteed 85% ethanol) the exhaust has a sweet, almost fruity smell to it.

Do you know what causes that?

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murderhalfchub t1_ja7mqjt wrote

My guess would be incomplete oxidation of the ethanol to aldehydes. Aldehydes are responsible for fruity smells like grape, banana, apple, etc. But this is a guess.

Source: studied chemical engineering in college and took organic chem

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Suicicoo t1_ja7056u wrote

would that be better with "clean" manufactured fuel (called E-Fuel in Germany) and a separate O2-supply?

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Pun-pucking-tastic t1_ja75b05 wrote

E-fuels are still carbohydrates. You get the same issues of aromatic compounds that are left unburnt, or incomplete combustion leading to CO emissions.

If you would run the engine with a supply of pure oxygen instead of air you could avoid the NOx problems but that would be ridiculously expensive to do.

Really, the only issue e-fuels don't have is the sulphur content, but that is already pretty low in modern car fuels.

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oheffendi t1_ja7cw7c wrote

Many diesel cars, who traditionally have been the stinkiest of the bunch now use a special additive known by the trade name AdBlue (urea solution ) to help reduce NOx emissions. My car uses it and the reduction of the smell is quite noticeable.

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theRose90 t1_ja7cwiw wrote

Cars also burn a tiny bit of lubricant oil every cycle, which can have other additives that may cause a smell.

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RCrl t1_ja56lgb wrote

Combustion is incomplete and you have other trace elements (and not trace - like fuel additives) in the fuel / atmosphere.

There's also a system on your car that pulls gas out of the crankcase and through the engine. It's full of oil vapor and singed fuel that only sorta burns on its way through the combustion chamber / cat.

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Bsoton_MA t1_ja50hf0 wrote

Gasoline has a little bit of sulfur from petroleum. When sulfur combusts it forms sulfur dioxide which smells. A catalytic converter should prevent this, which is why it does not smell as bad as with one.

Edit: without —> with

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Origin_of_Mind t1_ja6hsnb wrote

When you start the car, the engine is cold. In this situation, there is much more unburned fuel which passes into exhaust. The catalytic converter is also still too cold to do anything. This is called cold-start emissions.

What you smell particularly strongly at that time is aromatic compounds from the gasoline. In chemistry, they were named "aromatic" precisely because they smell. A representative aromatic compound that is responsible for the exhaust smell during the cold start is benzene, although many of its derivatives are also present.

Incidentally, in many countries gasoline is called benzin/benzine.

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WulfRanulfson t1_ja6ur42 wrote

Petrol fuel is not just hydrogen and carbon, it comes from crude oil which is decomposed organic material. This material contains all sorts of compounds including Sulphur, benzene, hydrogen sulfide, various metals.

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Of-Lily t1_ja7yvtq wrote

Combustion of hydrocarbon is a fairly fractious process. (pun intended!) The parent hydrocarbons have various structures and each one of those multiple pathways to break down.

The noxious odors come from primarily smaller hydrocarbons with various structures and and nitrogen oxides. carbon monoxide is odorless but is also produced and incompletely converted. It is a particularly harmful byproduct.

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