Et al...I have been following the thread about the issue of water
speed, etc. and find it interesting that perhaps a return to some
basic principles are in order. Cooling is a result of a proper balance
between water flow, radiator efficiency, temp of the air passing
through the radiator and the mass, weight of the air that and that
includes the moisture content. A radiator efficiency can best be
described by the delta temp across it at a given flow rate. Normally
a 25-30 degree drop is satisfactory to return water to the engine,
cool enough for it to absorb BTU's from the block. Remember, cooling
is the result of taking heat out, not putting cold in. The greater the
delta temp is, the greater the heat transfer.
OK, now the problem becomes one of determining the flow at the delta
temp required to transfer heat from the block to the water. That can
be derived through the math involved, or by empirical testing. Water
can be passed through the system as fast, or as slow as necessary to
provide the heat transfer that cools the engine. Turn to the
radiator. A radiator does nothing more than transfer heat to a cooler
source. The secret of thermal transfer of a radiator is found in the
material of the radiator, steel, copper, alum, the total number of
inches that are in contact between the tubes carrying the coolant and
the radiating area of the fins. Air passing through a radiator should
be turbulent in order that the maximum area of the fins are in contact
with the air mass passing through the radiator. One of the main
reasons that radiators do not work, is due to the fact that the air
mass passing through the radiator is laminar.
Consider water at a given temperature passing through a pipe, say 6"
that is surrounded by a coolant at a given temperature. If the water
is passed through the pipe at a given flow, the temperature at the
output of the pipe can be measured and compared with the inlet
temperature. Provided the tube is clean and smooth, the water in the
middle of the pipe is not mixed as it flows and the result is that the
water flows through the pipe in a laminar condition, The delta
temperature is then the efficiency of the system. Now place something
in the pipe that causes the water traveling through the pipe to become
turbulent, allowing the water to be mixed as it passes through the
pipe. The more water that comes in contact with the inner surface of
the pipe, the greater the transfer. Yes, the head pressure goes up and
that requires a larger force at the inlet to get the same flow, re a
larger pump. Though water and air are different media, the principle
is the same.
Consider the thermostat, there only to keep the engine at a
temperature that is best for the thermal efficiency of the
engine. Modern engines run at much higher temps than older engines as
the tailpipe requirements and fuel efficiency dictate. That is also a
function of the fuel being burned. The thermostat should never be in
the system to control flow, other than to maintain a block
temperature. Modern engines are pressurized to 20 psi, or more, to
keep boiling temps to above 230-250 degrees, or more. That keeps the
water in contact with the inner surfaces of the block and prevents
bubbles from forming. When a gas bubble forms, generally as a result
of boiling, the system avalanche, resulting in loss of contact between
the coolant and the inner surfaces of the block. No water contact, no
heat transfer.The thermostat should not be in the equation for thermal
transfer, certainly never to provide pressure within the block.
If one looks at the engine designs 30 years, or more ago, the emphasis
was not on thermal efficiency. First, because of emissions issues, and
second because fuel was formulated differently. The engine designers
today go the great lengths through computer modeling to design engines
that have coolant flow characteristics that provide for constant flow
through all parts of the block and heads, thereby providing a
constant, known, heat transfer. Modern engine systems are designed
knowing the total requirements for keeping an engine at a constant
temperature. Look at the temp gauge of a modern car, it never varies
from the center mark, regardless of the outside air temperature. 30
years ago, whenever you took a trip, you always saw cars on the side
of the road with steam coming from the radiators during hot weather. I
can't remember the last time I saw that. That's because designers now
design the total system to contend with the extremes.
So, what does this all mean? First, race car engines that are produced
by engineers in the field know that the total system makes the car
win. Second, older engines, modified to produce more horsepower, re
more heat, must have the rest of the support systems adjusted for the
thermal transfer. That means understanding the increase in thermal
units that must be accounted for and providing the system to transfer
them to the atmosphere. Is it easy, no. Most do so by trail and error,
as there is really no other way. Trial and error leads to some success
and that knowledge is transferred to others by way of things like this
net and publications, but it does not change the basic principles of
the law of thermodynamics which always apply.
Cheers....
Bill Bollendonk, 1976 MGBGT factory 3.5L Rover, 1937 MGTA,
!967Morgan DHC
and many cars going back almost 60 years.
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