On March 2nd, 2017 we learned about

The causes of your car’s visible exhaust clouds on a cold morning

On a recent cold morning, my kids became rather excited about the visible exhaust coming from a car in front of us. The two worldly second graders in the back seat declared that there was clearly something wrong with that car, and that the thin, white exhaust puffing out of the tailpipe was evidence of terrible pollution. Being the more experienced, supposedly knowledgeable adult in the car, I countered that it really wasn’t any more exhaust than any other car puts out, but that the cold air was causing it to become visible thanks to condensing moisture. All cars have exhaust like that, but like our breath, we just don’t see it most of the time. The two girls seemed to accept this answer before moving on to other important topics, like fart jokes, which is a shame, because I was wrong.

Exhalation vs. exhaust

My logic about cold air would make more sense if cars were biological systems, and if their exhaust was more like ours. When we breathe out, we expel mostly nitrogen we never needed, unused oxygen, carbon dioxide and various other gases inhaled from the atmosphere. We also expel a bit of water vapor from our moist lungs, and on a cold, dry morning, that water vapor will quickly cool as it exits our warm body, condensing into little clouds in front of our mouths. We breathe the same things out in other conditions, but the right mix of cold, dry air is what’s needed to form visible breathe.

At this point, you are probably remembering that cars don’t have any warm, moist lungs to speak of, so their visible exhaust is coming from a different source. What comes out of the exhaust pipe is a mix of various components created, or left over, after burning gasoline to move the engine’s pistons. As with burned wood or other fuels, if less fuel is burned, more will of that material will be left over to go into smoke. On a cold morning, especially when a car has been turned on for the first time in a few hours, catalytic converters won’t be at their optimal temperatures, and fuel won’t generally vaporize as completely, which means that more waste products like hydrocarbons, carbon monoxide and nitrogen oxides will be left over as smoke. So what my passengers were seeing was indeed excess exhaust coming from the car ahead of us. However, as long as that smoke cleared up as the car’s engine warmed up to normal operating temperatures, you wouldn’t need to assume that the care was broken. It was just a bit less efficient than usual.

More serious smoke

If smoke continued to be visible from the exhaust pipe, or if smoke of a different color was coming out, there would be more to be concerned about. Continuous white smoke may indicate a cracked cylinder-head, or burning coolant somewhere in the engine. Gray exhaust is likely a sign of burning transmission fluid. Bluish smoke indicates that oil is burning, and black smoke is probably tied to an air filter problem, or maybe even gasoline leaking and inefficiently burning on the outside of another component. Even under optimal conditions, a combustion engine exhausts hydrocarbons that greatly contribute to the greenhouse effect, and in worse cases damage the atmosphere’s ozone layer. But continued visible exhaust also means that your car is in need of repair if you want it to run smoothly the next time you have to take the kids to school.

Source: Types of Smoke From Your Car Tailpipe & What It Indicates, Pro Car Mechanics

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