temperature sensor IN radiator => electric fan

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

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Bob

Your forgetting the engine thermostat, you do not have to worry about overcooling ( unless you freeze the rad. core) as the engine thermostat determines the operating temp regardless of the rad. temp.

Harold

conditions.

Variable speed controller: