[Diy_efi] RE: Timing and dyno pulls

M. Claywell clay0052 at umn.edu
Wed Dec 25 05:00:38 GMT 2002



On 23 Dec 2002, Adam Wade wrote:
> --- Grant Beaty <gbeaty at ufl.edu> wrote:
> 
> > So higher cooling system temp helps heat transfer
> > from coolant -> radiator as well as engine ->
> > coolant?
> 
> Yep.  That's physics for ya.  ;)  The larger the
> differential between two temperatures, the faster the
> heat transfer.

True, however the heat transfer rate (Q dot) for the engine-> coolant must
go down as coolant temperature goes up. However, if you look at increasing
coolant temperature lets say from average of 90C to 120C, and the air temp
is 20C, and the average head temp at the water jacket surface is
250-350C(guessing here) then there is a much larger percent difference in
the the temp difference for the coolant to air than comapared to the jacket
to coolant. See what I am saying? Hotter coolant will decrease the heat
transfer from head to coolant but higher coolant temp should more than make
up more this loss due to the higher heat transfer to the air surrounding
the radiator (assuming the radiator is properly designed and not limited
due to some other factor).

Q_dot (heat transfer rate) = h * A * (Temp difference) 

h is a heat transfer coefficient and takes into account every imagineable
enviromental factor (really fudge factor, as I think of it). A is surface
area.

> Fourier's Law: The heat flux resulting from thermal
> conduction is proportional to the magnitude of the
> temperature gradient and opposite to it in sign.
> 
> Since the thermal conductivity between a radiator's
> metal and the air is the lowest thermal conductivity
> of anything in the circuit, increasing coolant temps
> by pumping more heat into the system from the cylinder
> head WILL result in an increase in coolant temps,
> because getting the heat out to the air is the
> "hardest" part of the circuit.  But higher coolant
> temps will shed more heat to the air than lower
> coolant temps, and the opposite is also true for air
> temps; it's the differential.  IT's also one of the
> reasons vehicles are more likely to boil over in hot
> weather, and why some people put cardboard in front of
> part of their radiator in cold weather (although if
> their thermostat is working properly, that's not
> really necessary).

Convective heat transfer is going be the limiting factor for heat transfer
from the coolant to the air, or the cyl. head, etc to the coolant. Did you
mean convection instead of conduction??

Mark


> 
> =====
> | Adam Wade                       1990 Kwak Zephyr 550 (Daphne) |
> |   http://y42.photos.yahoo.com/bc/espresso_doppio/lst?.dir=/   |
> | "It was like an emergency ward after a great catastrophe; it  |
> |   didn't matter what race or class the victims belonged to.   |
> |  They were all given the same miracle drug, which was coffee. |
> |   The catastrophe in this case, of course, was that the sun   |
> |     had come up again."                    -Kurt Vonnegut     |
> 
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