On cold days when an engine is first started the...

Dear Car Talk

Dear Car Talk | Jan 01, 1994

Dear Tom and Ray:

On cold days, when an engine is first started, the exhaust is visible as a white smoke. Then, when the engine warms up, the exhaust becomes colorless. Why is this?

TOM: Great question, John. My brother happens to be an expert on this issue. After years of sniffing around tailpipes, he has concluded that it has to do with the temperature of the exhaust system.

RAY: When the engine runs, it combusts gasoline and air. And one of the by-products of that combustion is water vapor. The water vapor that comes out of the engine is very hot, and comes out as steam. And if you remember your high school chemistry, you know that steam is invisible.

TOM: But then that steam has to travel from the engine all the way through the exhaust system--to the back end of the car--where it's released into the atmosphere. And when the exhaust system is cold, some of that steam gets condensed back into water vapor. And when it comes out as water vapor (as opposed to steam), you can see it.

RAY: Eventually, the exhaust system reaches operating temperature (about 300 degrees Fahrenheit) And by then, it's hot enough so that the steam is STILL steam when it comes out the tailpipe, and you can't see it anymore.

TOM: Of course, on really cold days, the end of the exhaust system may never reach operating temperature, so on those days, you may see some water vapor all day long (especially on those real long cars)! 1449

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