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I noticed that in winter, the smell of cars (to be precise the smell of the exhaust gases) is much stronger in winter. I just can't figure out why. Has this got something to do with the air temperature or with the humidity?

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While car engines might look simple there is a vast amount of technology installed to ensure as complete a combustion as possible. Complete combustion according to the equation

$$\ce{C_{$m$}H_{$n$} + \frac{2$m$ + $n$}{2} O2 -> $m$~CO2 + \frac{$n$}{2} H2O}$$

(not accounting for oxygen, nitrogen or other elements present in the fuel) requires a precise amount of oxygen added. Too little oxygen will result in uncombusted hydrocarbons while too much oxygen will result in $\ce{NO_{$x$}}$. A small oxygen sensor is thus installed to improve the oxygen regulation. (It is a very interesing compound because it functions as an oxide ion conductor.)

The thing about this oxygen sensor is that its function is temperature dependent: It needs to be heated to $\approx 500~\mathrm{^\circ C}$ to function properly. This is achieved quickly due to the heat of the exhaust, but does take a few minutes from starting the car. So throughout the first few minutes, the oxygen sensor does not function and incomplete combustion results in $\ce{NO_{$x$}}$ and uncombusted hydrocarbons that cause the odour. Once working temperature has been reached, the smell disappears mostly.

In cold weather it will take longer for the oxygen sensor to function properly and thus more incomplete combustion is observed. This becomes more pronunced the colder it gets.


†: Its German name is Lambdasonde from the Greek letter $\lambda$ which is used as a variable to determine if the correct amount is present or not. At the correct oxygen concentration, $\lambda = 1$.

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  • $\begingroup$ A couple of points- oxygen sensors are usually heated in modern cars (if the sensor has 4 wires running to it, it's heated) so the warmup time is much shorter. However, engine management systems will not enter closed loop control (where the oxygen sensor reading is used to control the fuel mixture) until the engine has reached a certain temperature anyway. $\endgroup$ – squigbobble Mar 1 '16 at 13:16
  • $\begingroup$ @squigbobble I didn’t know that. (But all of what I do know is from a bachelor’s introductory lecture to solid state chemistry which was long ago, so no surprises there ;).) $\endgroup$ – Jan Mar 2 '16 at 21:59
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It's the air temperature. Petrol engines need a richer (a larger proportion of fuel to air) mixture in order to run properly when the cylinder head and intakes are cold due to the fuel condensing onto the inside of the intake and ports before reaching the cylinder. The richness is roughly proportional to the temperature of the engine so the colder the engine is, the richer the mixture will be. The excess fuel does not burn completely and ends up as a variety of smelly chemicals in the exhaust.

For modern vehicles with a catalytic converter, there's also a secondary effect where the cat takes longer to reach its operating temperature so more uncatalysed emissions escape out of the exhaust, they smell bad too.

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