cx500tc wrote:I like the idea of CF control, but using the coolant temp leaving the engine as opposed to the temp entering the engine doesn't seem proper to me since the coolant temp leaving the engine will be thermostat rated temp or higher and not necessarily indicative of the coolant temp and radiator performance and ability to remove heat from the coolant.
Yeah, gotta go with Rich and Moysie on this one. The entire exercise is to keep the engine within a very narrow temperature range. This is generally within 15°F to 20°F (8°-11°C) of the thermostat rating, as the thermostat functions from closed to fully open in that range, and is all the capability it has to maintain temperature. The mass of the engine mirrors that temperature and is why most engines have their temperature sensors at or near the thermostat and coolant outlet. The cooling system must maintain the proper temperature range, whether it's powering a boat on a winter lake of cooling water, or in a ratty rusted truck crossing Death Valley in summer. Same temperature goals, based on coolant outlet temperature, regardless of the inlet temperature.
cx500tc wrote:If the temp of the coolant going in isn't significantly lower than the temp of the coolant leaving, there's no real cooling going on.
Correct, but that condition is reflected in the exiting temperature, whether the hot condition is caused by hot coolant entering, or if a large load is on the engine. Whatever the cause, it doesn't care, and only knows it's not enough. So to control temperatures that are escalating, the fan turns ON, increasing the heat shedding (cooling capacity) to lower the exit temperature of the coolant to within the proper range. Using your example in that process; if it was hot coolant going in as the problem, then the problem was solved right there.
cx500tc wrote:Which leads to: if the temp of the coolant leaving the engine is 240F, which is entirely possible with an engine being highly loaded, and the radiator itself can lower the temperature adequately without the fan running, for instance when moving down the road at a good clip, there's no need for the fan to run... is there?
Yes, there is, as it's too high
if 240°F is out-of-range. "Too hot" is a relative term, based on the proper temperature range for that engine, and if it's within the range or above it. The system does not know if this was a short burst of power, or if you are still on the throttle and it's going to melt. Either way it's too high.
If the cooling system was fully adequate, then the exit temperature of the coolant would not exceed the range. But it did. So the fan turns ON to bring it back into range as quickly as possible. Actually, the fans(s) should turn ON
before it exceeds the range, in an effort to keep temperature in-control. A cooling system of sufficient capacity and proper operating condition will quickly warm to range and not exceed the range.
cx500tc wrote:Case in point: my motorcycle. When travelling down the road the radiator fan rarely turns on simply because there's enough airflow across the radiator to keep things cool even when under a heavy load.
Bingo - the exiting coolant (and therefore engine) are cool enough, and not threatening to leave the proper range, so the fan is OFF.
cx500tc wrote:The fan itself presents a significant load to the electrical system, and on a bike with an already highly loaded system, having the fan not run when not needed is necessary.
While that is not the fault of the cooling system; it is pointless to have the fan run when not necessary, and so it only runs when the exiting coolant (engine temperature) is heading out-of-range. Fortunately, that's good for your electrical system, and exactly what you're after.
cx500tc wrote:And I won't go too far in mentioning the fan itself only covers about 1/3 of the radiator, at most, so it really wouldn't do much were it to run when moving.
Irrelevant, except to note that your fan being so small, it should probably turn ON sooner to allow more time to counter rising temperatures that are threatening to go out-of-range. With the point being to keep it within range, most fans will turn ON aat about half range and up, to attempt to keep it under control. If the temperatures rise above range before the fan runs, then the thermostat is out of capacity, the system is already at max flow, and the cooling air is only just starting. Too late, and a greater overheat is likely, especially if the conditions (usually high load) is still present and still pushing temps up. While unnecessary fan operation is useless, engine temperature control is critical to performance, efficiency, and engine life. If in-doubt I would run the fan too much as the safer alternative.
David