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  #1  
Old 01 March 2008, 02:42 AM
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Driver Put a blanket over your car's engine at night

Comment: The use of a blanket to keep your car motor "warm" during those
cold bitter nights.
Many people here in Wisconsin beleive this!
AAA recomends using a blanket over your hood to keep the cold from
preventing rough starts on your car.
I think this is bunk.
The temperature of the vehicle will drop with the weather.
A blanket keeps warmth in as an insulator. Once the motor cools to the
same temperature as the outside the blanket serves no purpose.
I even argued with a "scientist" about this.
He kept telling me as the windchill increased the blanket kept the motor
warmer. The temperature stayed the same just the wind increased.
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  #2  
Old 01 March 2008, 02:34 PM
bjohn13
 
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We tend to go rounds about this locally, and the validity of it comes from the way you interpret what is being said.

A blanket over a car's hood, in theory, should keep the heat in the engine longer, but the actual temperature the engine will reach once it compeltely cools down will be the same. Here in North Dakota, where the actual temp tends to drop below -40 F from time to time, we get used to going out every 3-4 hours to start our vehicles to make sure that we can get to work. Throwing a blanket over a car's hood could feasibly increase the time between starts to 4-5 hours, but to me, that extra hour just isn't enough incentive to go deal with a blanket in the all too often windy cold (and it would have to be a pretty big blanket to do any good). Now, if you throw in a block heater, and it's pretty stupid to not have a block heater if you are parking your vehicle outside for any amount of time in this neck of the woods during the winter, a blanket can make all of the difference in the world. However, with a block heater, heat is actually being generated from underneath the blanket.

Of course, the argument around here has more to do with windchill factors. Many people claim that parking a car with the engine facing the wind will cause the engine to get colder than if the engine is facing away from the wind. The fact is, the engine will cool down to the same temperature regardless of which way it is facing. The difference is, if the engine is facing the wind, it will cool down sooner than if the engine is facing away from the wind. When the actual temp is -30 and the windchill is -100, this can make a significant difference.
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  #3  
Old 01 March 2008, 03:39 PM
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I agree with bjohn13. In Montana where we have similar temperatures block heaters make a huge difference but a blanket is warm because it traps heat, not creates it.

Wind chill is an approximate feeling of temperature when the boundary layer of air around our bodies is moved. If it is above freezing but wind chill goes below freezing water will still not freeze. I guess what I'm saying is that your car doesn't care if it is 30 degrees or 30 degrees with a wind chill of -100. It's all the same true temperature.
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  #4  
Old 02 March 2008, 12:46 AM
RichardM RichardM is offline
 
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cageboy is correct in that wind chill has no affect on inanimate objects. A strong wind may carry the heat away quicker, but the end temperature is the same regardless. A blanket or other insulation will keep the wind from chilling the engine as quickly but it will not limit the ultimate low temperature. A blanket and a block heater will work in tandem.

No a guy from Texas is not an expert on cold weather but I did use the block heater on my truck for the very first time this past weekend in Salem Missouri. The truck started very easily and we had heat inside the cab very quickly. The windshield however, cracked from a small stone chip.
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  #5  
Old 02 March 2008, 06:22 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by RichardM View Post
cageboy is correct in that wind chill has no affect on inanimate objects. A strong wind may carry the heat away quicker, but the end temperature is the same regardless.
Not true. Windchill affects any wet (or damp) object. A wet hunk of steel will cool below ambient temperature if there is a wind.

A wet blanket will do the same thing.
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  #6  
Old 02 March 2008, 06:44 PM
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Quote:
Originally Posted by jimmy101_again View Post
Not true. Windchill affects any wet (or damp) object. A wet hunk of steel will cool below ambient temperature if there is a wind.

A wet blanket will do the same thing.
Yeah, but they always spoil the party.
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  #7  
Old 02 March 2008, 07:12 PM
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Quote:
Originally Posted by jimmy101_again View Post
Not true. Windchill affects any wet (or damp) object. A wet hunk of steel will cool below ambient temperature if there is a wind.

A wet blanket will do the same thing.
That's what I thought. I have a shelter on our deck that I put up during the winter. It's just a frame covered with plastic tarping. It has no R value, and no heat source, it is not close enough to the house. Yet the temperature in the shelter is always a few degrees warmer than the temperature outside, and I've always assumed this was because the tarp, while doing nothing for heat, did minimize the effect of windchill. If that is correct, then a blanket over the hood would accomplish the same thing - to keep the cold wind off the engine, and thus prevent it from reaching a lower temperature.

If that's not true, however, you would still have the issue that a blanket would increase the effectiveness of a block heater, since as other have noted, a blanket simply preserves the heat that is there. The block heater would produce heat, the blanket would help preserve it.
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  #8  
Old 03 March 2008, 12:52 AM
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Quote:
Originally Posted by jimmy101_again View Post
Not true. Windchill affects any wet (or damp) object. A wet hunk of steel will cool below ambient temperature if there is a wind.

A wet blanket will do the same thing.

I'm trying to figure out how evaporate cooling would have any effect on an object that is below freezing.
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  #9  
Old 03 March 2008, 12:53 AM
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Quote:
Originally Posted by callee View Post
That's what I thought. I have a shelter on our deck that I put up during the winter. It's just a frame covered with plastic tarping. It has no R value, and no heat source, it is not close enough to the house. Yet the temperature in the shelter is always a few degrees warmer than the temperature outside, and I've always assumed this was because the tarp, while doing nothing for heat, did minimize the effect of windchill. If that is correct, then a blanket over the hood would accomplish the same thing - to keep the cold wind off the engine, and thus prevent it from reaching a lower temperature.
I have a theory about this, but I'd like to wait for someone else's input before I make a fool out of myself.
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  #10  
Old 03 March 2008, 02:47 AM
cageboy
 
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Quote:
Originally Posted by callee View Post
That's what I thought. I have a shelter on our deck that I put up during the winter. It's just a frame covered with plastic tarping. It has no R value, and no heat source, it is not close enough to the house. Yet the temperature in the shelter is always a few degrees warmer than the temperature outside, and I've always assumed this was because the tarp, while doing nothing for heat, did minimize the effect of windchill. If that is correct, then a blanket over the hood would accomplish the same thing - to keep the cold wind off the engine, and thus prevent it from reaching a lower temperature.

If that's not true, however, you would still have the issue that a blanket would increase the effectiveness of a block heater, since as other have noted, a blanket simply preserves the heat that is there. The block heater would produce heat, the blanket would help preserve it.
Does your shelter consist of wood? Wood fibers tend to hold onto heat, enough so that a log cabin will hold heat well through the night.

Simply speaking, wind chill is an effect on our skin, not metal. Otherwise a plane flying past the speed of sound would have its rocket fuel frozen (occurs approx. -150 F.)
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  #11  
Old 03 March 2008, 05:37 AM
Singing in the Drizzle Singing in the Drizzle is online now
 
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Quote:
Quote:
Originally Posted by RichardM
cageboy is correct in that wind chill has no affect on inanimate objects. A strong wind may carry the heat away quicker, but the end temperature is the same regardless.

Not true. Windchill affects any wet (or damp) object. A wet hunk of steel will cool below ambient temperature if there is a wind.

A wet blanket will do the same thing.
Wind chill has a effect on inanimate object as well animate. Air movement affects how fast non heat producing things will cool down to the ambient temperature or if heat producing how extra cooling it receives.

Wet thing become cooler that the ambient temperature because evaporation always produces a heat lose. The affect of wind just helps it along and therefor makes it even more cooler. It a separate affect from wind chill, but helped by it.
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  #12  
Old 03 March 2008, 03:23 PM
RichardM RichardM is offline
 
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But the blanket is dry because it is wrapped in plastic. Hence, wind chill has no affect.
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  #13  
Old 03 March 2008, 09:46 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by bjohn13 View Post
I'm trying to figure out how evaporate cooling would have any effect on an object that is below freezing.
Look up the definition of "sublimation".
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  #14  
Old 03 March 2008, 09:49 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by callee View Post
I have a shelter on our deck that I put up during the winter. It's just a frame covered with plastic tarping. It has no R value, and no heat source, it is not close enough to the house. Yet the temperature in the shelter is always a few degrees warmer than the temperature outside, and I've always assumed this was because the tarp, while doing nothing for heat, did minimize the effect of windchill.
What you are describing is a "hot house". The essentially non-insulating plastic does block the wind. The plastic also traps solar heat. The two together will cause the air inside to be above ambient temperature.
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  #15  
Old 03 March 2008, 10:03 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by cageboy View Post
Simply speaking, wind chill is an effect on our skin, not metal. Otherwise a plane flying past the speed of sound would have its rocket fuel frozen (occurs approx. -150 F.)
"Wind chill" as used by a weatherman is based on the affect on a human body. "Wind chill" also has a more general meaning and can be used to describe the increased rate of heat loss caused by wind. It is also used to describe the increased heat lost caused by wind on a wet object. Evaporative cooling can be a very significant affect.

A plane flying faster than the speed of sound will be heated pretty significantly by friction. Even at just Mach 1 the heat generated is enough to need to be accounted for in the aircraft's design. At higher Mach numbers the heat generated is very significant. Even if someone could come up with an aircraft engine that worked at speeds of say Mach 10, the engineering challenges of coming up with an airframe that'll withstand the temperatures would be substantial.

The shuttle glows cherry hot during its decent into the atmosphere. Even with a hyper-mach speed.

So, it doesn't matter how fast a supersonic aircraft is moving, the heat caused by friction is much greater than the increased rate of heat loss caused by the higher air speed.

At speeds more typical of wind, a hunk of metal will cool to ambient quicker at higher wind speeds. A hunk of wet metal will cool to below ambient in a wind. This is the basis of a "wet bulb hygrometer".
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  #16  
Old 04 March 2008, 12:18 AM
bjohn13
 
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Quote:
Originally Posted by jimmy101_again View Post
Look up the definition of "sublimation".
So, even an object that is covered in frost will cool to below the ambient temperature when subjected to a steady wind?

I'm also guessing that this is dependent on the humidity as well. If the ambient temperature is cold enough for frost to form, does sublimation still occur?
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  #17  
Old 04 March 2008, 02:54 PM
jimmy101_again jimmy101_again is offline
 
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Quote:
Originally Posted by bjohn13 View Post
So, even an object that is covered in frost will cool to below the ambient temperature when subjected to a steady wind?

I'm also guessing that this is dependent on the humidity as well. If the ambient temperature is cold enough for frost to form, does sublimation still occur?
You are correct. "Wind chill" is a function of moisture on the object and the relative humidity of the air. At 100% relative humidity there is no evaporative (or sublimation) cooling. That is how a wet bulb hygrometer works, it measure the relative humidity (RH) based on the degree of evaporative cooling. No cooling means the air is at 100% RH.

If frost is forming (or it is raining or snowing) then evaporative cooling won't occur since the RH is very near 100%. If conditions change and the RH decreases then evaporative cooling comes into affect again. Note that this has little to do with the actual air temperature. Air at 5F will still cause evaporative or subliminal (?) cooling as long as the RH is less than 100% for the particular temperature.

People in cold climates can still dry wet clothes by hanging them outdoors, even when the air temperature is well below freezing. The clothes dry fairly slowly but they will eventually dry.
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  #18  
Old 04 March 2008, 06:25 PM
bjohn13
 
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Thank you for the responses. Now, a co-worker with a degree in meteorology has promised me he is going to go dig out some old textbooks to prove me wrong.
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  #19  
Old 04 March 2008, 08:00 PM
RichardM RichardM is offline
 
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Is your co-worker named Greg D. ?
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  #20  
Old 05 March 2008, 01:19 AM
bjohn13
 
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Quote:
Originally Posted by RichardM View Post
Is your co-worker named Greg D. ?
lol....no
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