Why does car smoke hang in the air when cold?

[GearheadTool]

I am talking about "tailpipe condensation." And not just the whispy stuff that isn't all that noticeable.. I mean the "gets forcefully THROWN out of the exhaust, then leaves a trail and hangs in the morning, all three cars in a line from a stop light on a cold-ish morning." Why does that happen?

I have not yet been fully able to explain why some cars go down the highway with condensing smoke out of the tailpipe, when some do not. Then, some start smoking again when coming to a stop.
Weymouth, Quincy, Boston.. This happens all over, last few days.

Why does it hang like that?
Why does it sometimes, and sometimes not?
And does anyone else think there is something wrong with their car when they see this?

 

[NateDN10]

Any kind of "smoke" is a bunch of very tiny particles suspended in air. In the case of normal water vapor out of a tail pipe, the particles are tiny water droplets. They disappear as they drop out of the air or evaporate. In cooler weather, they evaporate more slowly, and therefore hang in the air for longer.

Actual smoke, like from burning oil, hangs around longer because it's composed of solid particles, not liquid. So the only way for them to disappear is to fall to the ground or be dissipated enough that you don't see them.

 

[Clevy]

It's moisture. My mustang will actually drip water from the pipes in the summer

 

[eljefino]

It's because the Rapture actually happened on 12/21/12. If you can't see your breath or your car's exhaust you've lost your soul.

It's the coldness of the exhaust pipe. A hot rod with twin pipes or just a large diameter pipe has more time for gasses to cool off and condense the water vapor so you can see it.

Want fun, watch a prius drive around in the winter. Their smoke is random as the gas engine comes and goes.

 

[Phishin]

Water is a by-product of combustion. Anytime anything burns (adding oxygen to the substrate), water is formed.

In the engine, the water is a vapor because the combustion temperature is about the boiling point of water, causing all the water to be gas.

When the vaporized water hits colder air, the water vapor cools below the boiling point, and water droplets form. It's the same principle as to why your beer or Pepsi cans sweat in the summer....the cold can caused water vapor in the ambient air to condense on the cold can, back into liquid water.

Cold air is more dense than warm air, and if the density of the air with water droplets is similar to that of the ambient cold air, then the "smoke" will hang as you call it. If there is any wind or going down the road, you'll notice there isn't a "hang" because the droplets quickly disperse enough into the atmosphere that a sufficient amount isn't there to create the "smoke" effect. The smoke effect requires a certain concentration of water droplets.

 

[SatinSilver]

It's where the big puffy clouds in the sky come from.

 

[440Magnum]

What youre seeing isn't smoke at all, its water vapor condensing. It takes the right combination of temperature of the exhaust gas, temperature and humidity of the ambient air around the car, and minimum amount of "stirring" of the air for the vapor to hang around for any length of time. Different cars are in different states of being warmed up, have different tailpipe lengths, and spit their exhaust out at different velocities, changing the 'stirring' factor. Same reasoning applies to why you sometimes see jet contrails, and other days you dont. Different humidity/temp conditions at altitude.

Ever notice that diesels don't tend to produce vapor like gasoline cars at all? Their exhaust is too dilute for condensation under most conditions (since they don't have a throttle, diesel exhaust at idle is mostly just air.) But if the ambient temps get REALLY cold, they'll steam also.

 

[BearZDefect]

And especially with cold weather and short trips where the distant parts of the exhaust do not get hot, significant amounts of that water vapor condense inside the cold exhaust pipe and muffler. When you see water suddenly spew out of a tail pipe it is condensation that collected in the muffler, some of which was pushed out when the throttle opened and exhaust gas flow surged.

This water that accumulates in the muffler can cause internal rust problems in older cars (like mine) that do not have a stainless steel exhaust.

 

[RF Overlord]

Originally Posted By: eljefino
It's because the Rapture actually happened on 12/21/12. If you can't see your breath or your car's exhaust you've lost your soul.
I think this answer is closest to the truth.