Blankets keep people warm at night -- how about engines?

Dear Car Talk

Dear Car Talk | Nov 01, 2005

Dear Tom and Ray:

In the 1940s and early '50s, we had a big old buffalo robe, and I remember my "old man" putting this blanket on the hood of the car or on the engine to keep it warm in the winter. I'm wondering now if this had any effect in cold temperatures as far as keeping the engine warm for cold Minnesota morning starts. -- Ken

RAY: Well, let's start by discussing blanket technology, Ken. A blanket works by slowing heat loss. So, if you wrap yourself in a blanket at dinnertime and then sit outside in zero-degree weather overnight, you'll lose less heat and be warmer the next morning than you would have been without the blanket.

TOM: But is the same true for a car? Well, the big difference between you and a car engine is that you keep generating heat all night. That's a function of being alive. The engine just starts losing its heat as soon as you turn it off.

RAY: So the question is, Does a blanket slow heat loss enough to keep some heat in the engine 12 hours later? The answer is, probably not.

TOM: In cold weather, an engine normally loses most of its heat in about three hours. The only exception -- but it's an important one -- is the oil, which can take as much as eight hours to reach complete equilibrium with the outside air, depending on how hot the oil is and the outside temperature.

RAY: Can a blanket extend the time it takes for oil to get stone cold by an additional four to six hours? I don't think it's likely, when the temperature is well below freezing. But if the car were driven at 11 p.m. and then started again at 6 a.m., the blanket might make a difference.

TOM: But while a horse blanket might have been the only option for your old man, there are better alternatives these days. There are block heaters that you can plug into an electrical outlet. They actually generate heat and make an enormous difference in brutally cold weather. And then, of course, there's the greatest advance ever in cold-weather car-starting: The heated garage.

RAY: And if you build one, Ken, put the horse in there, too. He'll appreciate that a lot more than a stinkin' blanket.

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