Radiator Cap

[Michael Skolones]

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     Newton tells us that the rate of heat flow from one region into 
     another is proportional to the difference in temperature between the 
     two regions (in this case, the block and the water).  Therefore, given 
     an engine block of a certain temperature, cooler water will remove 
     more thermal energy per unit time than warmer water.  A 
     lower-temperature thermostat will ensure a cooler supply of water to 
     remove the thermal energy from the block, and so cooler water is in 
     this sense more efficient.
     
     Each volume-unit of water is in contact with the block for less time 
     if the water is flowing quickly, as compared to water that is flowing 
     slowly.  Therefore each volume-unit of water carries away less thermal 
     energy if the water is moving quickly.  But in a quickly-flowing 
     cooling system, more volume-units of water are flowing through the 
     cooling passages per unit time, so the amount of thermal energy 
     carried away per unit time will be the approximately the same 
     regardless of flow rate, the only difference being the change in the 
     rate of thermal exchange with temperature difference as described 
     above.
     
     On the other hand, the question is somewhat irrelevant to automotive 
     cooling systems.  To my knowledge a higher-temp thermostat flows at 
     least as much as a lower-temp thermostat for a given engine, once both 
     have reached the fully-open position.  The only difference is the 
     temperature at which the valve fully opens, not in how much it opens.  
     
     mike skolones


______________________________ Reply Separator _________________________________
Subject: Re: Radiator Cap
Author:  diy_efi at coulomb.eng.ohio-state.edu at Internet
Date:    10/13/97 4:57 PM


> ....................................................... As for the
>higher temp thermostats, I believe the reasoning is the higher opening temp 
>makes the water stay in the engine longer in order to absorb more heat from 
>the engine. Use too cool a thermostat and the water doesn't have enough 
>time to leech off enough heat, totally remove the thermostat and you risk 
>actually overheating, not necessarily boiling over but overheating the 
>engine. Water temp doesn't mean a damn thing, it's the block and head temp 
>that counts.
     
now, here's where I have a problem -- probably showing just how cirrusly 
uninformed I really am !!  ..... I used to make water-cooled heatsinks. 
Doesn't mean I knew anything.  As long as the water contacts the
metal, what difference could it possibly make how fast it moves. 
IMHO, this is the most ridiculous line of reasoning I've heard in a
long time (no offense to you, Chris, ... I assume you're just repeating 
what you've heard or read -- seen it in some radiator ads myself).
     
I mean, it's a law of physics that delta-T is required to remove 
heat .... and if you leave the water there, it warms up and then 
doesn't remove as much heat !!!
     
I'd like to know .... not really losing sleep over this, but surely there's 
some guru summers that can ease my tortured mind ???  (I mean, I've
run lots of engines with no thermostat -- always assumed the only problems 
were due to water not getting up into some high places that collect
air pockets and the engine oil not getting hot quick enough to lube 
at first start.)
     
What difference does the rate of flow have to do with heat transfer ?? 
Like I said, seems to me that it's totally irrelevant as long as the 
liquid stays in contact with the metal -- in fact, seems t'me that the 
faster the better, since that keeps the delta-T at it's highest !!
     
At this point, I pronounce the ads by the rad makers "HOKUM".  Now, 
someone prove me the idiot that most have long suspected  ;-)
     
Tom Cloud
     
   Warning, dates in calendar are closer than they appear !
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From: Tom Cloud <cloud at peaches.ph.utexas.edu>
Subject: Re: Radiator Cap
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