I can see problems with either approach if an on/off fan control is used:
1) if the sensor is at the radiator inlet then the fan could be running when
it is not needed as airflow without the fan might be enough to cool the
coolant suffiently and the fan running would "over cool" it. There would
also be a large time delay in responding to the cooling also as the cooled
water would have to make it around the engine and back to the radiator.
2) If the sensor is at the radiator outlet then the fan does not come on
until the coolant exceeds the desired temperature. The coolant will then
cool down below the desired temp and the fan will constantly be cycling on
and off.
With an on/off control I think situation 2 (at the radiator outlet) would be
preferable as this would respond more quickly to cooling needs. This type
of control is also very cheap, if not real precise.
Something a little more esoteric:
(the controls engineer in me just has to present this ;-)
If the electric fan has a variable speed control (other than just on/off)
then a sensor just on the outlet of the radiator could be used vary the fan
speed proportional to the temperature of the coolant. The controller would
turn the fan on at minimum speed at some temperature (maybe right at the
desired engine inlet temp) and run faster as the coolant temp increased --
reaching full speed well below maximum allowed engine inlet temp.
The components to build a variable controller are actually pretty cheap.
The problem is determining the parameters of the control problem and the
degree of variation in the parameters under different operation conditions.
Seems like it might be a good problem for a "fuzzy logic" type controller
and use of a sensor on both inlet and outlet. Then the controller could be
fairly independent of the exact parameters and self adjust to changes in
operating conditions.
This more advance controller would give a much more consistent temperature
control to the engine but is probably overkill. Option 2) above is the best
bet.
Bob
