User talk:Graham.Fountain/Triumph TR7 Sprint
But where's the fun in that? And if you've already made the [wrong] decision to switch the fan on and off from the temperature in the top hose, why would you use anything more complex than a simple thermal switch anyway – the switching temperatures in the top hose should be entirely determined by the thermostat you’ve got. I would argue that while the thermostat determines the normal range in the top hose, that isn't well linked to where coolant temperature in the bottom hose is out of range and needs the fan to come on. Looking at how long the top hose temperature continues to rise after the fan is turned on, which is some minutes at least at idle, it is clear that the thermostat, which has a time constant in the 10s of seconds, must be doing a lot to mask the changes in temperature in the bottom hose from the switch in the top hose. That, in turn, means either you have to turn the fan on when the temperature is at least close to going out of range and thus, given the delay between turning the fan on and the temperature coming down, allow significant overshoot in engine temperature (I get something like 5 C), or turn the fan on well inside the normal range, and thus will sometimes be on when it's not needed or wanted and wastes power (and, I suppose, might overcool). Whereas, switching as soon as the bottom hose temperature shows a loss of cooling airflow, and while the top hose temperature is still in range, can significantly limit the overshoot and any tendency to turn the fan on when it's not needed. I'm not sure that undershoot is a real problem with the kind of electric fan I'd consider, but turning the fan off as soon as the bottom hose temperature is low enough should minimize that if it really is. I admit that setting the switch temperatures for the bottom hose is a bit more difficult than picking an on temperature near the top end of the thermostat's range. However, it is possible to do it by trial and (hopefully not too much) error. That means a variable switch, e.g. an NTC resistive sender and variable comparator with variable hysterisis — I'm still working on the issue of how much hysterisis is needed, but I think it may always be some. I don't doubt the ready built controllers that are available will have enough range to be used in the top or bottom hose. But they are a bit expensive given the sender, hose adapter, and basic comparator with a 20A relay on board are not much more than £10 the lot (and 2-3 weeks lead time from China) – the cost of putting accessible controls in the cab, as another toy to play with, is separate. It's a shame there don't seem to be any cheap comparator modules with hysterisis control already built in. But that's a nearly trivial mod if you can de-solder and solder components on a PCB and cut the odd track or two – it ain't rocket surgery.
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