Sorry for the delay as well. Thanks for the responses, and it gives me a better idea of where you're coming from when focusing on the subject of fan control. Grab a cup of coffee, I'll start basic, and then I'll throw a lot of different examples out there, and after a few (or several) maybe one will click. Some folks respond to basics, and others to lots of examples, so I hope you understand I'm not trying to overwhelm or bulldoze.
So the questions and simplest answers would be:
PSIG wrote:[1] How could only outlet CLT sensor data be used to determine if the inlet coolant temperature or the engine load were excessive without fan operation?
is, as simply as possible, “When it's too hot.”
and
PSIG wrote:[2] How could only inlet coolant temperature data be used to determine if the outlet coolant temperature or the engine load were excessive without fan operation?
is, as simply as possible, “Not without more info.”
Here I will take a wild guess and assume you will not accept those responses, so onward we go, but 2nd things 1st:
cx500tc wrote:The most reliable means is, as most OEM's have done for decades: relying on the temperature of the coolant leaving the radiator to control the fan.
This may be your largest single point of comfort and primary defense. Unfortunately, we do not agree with this, as it is not true for the vast majority of automotive (and most other types of) cooling systems. So, if you are basing your position on this point, and feel you are simply reinforcing proof; this is where the entire topic will fall in the hole. Be aware, that although you may feel you have a 'better idea' to control the engine temperature with fan operation based on radiator outlet/engine inlet temp (and I would like to hear the control logic you propose), it is not the primary control method the majority of the world uses or has used. Just to be clear, I am all for better ideas, but you are in the minority position and spitting into the wind at this point. This is why Speeduino is yet another of hundreds of systems that use engine temperature as the fan control basis—it works—because it is founded on solid principles of operation. The following is explanation of how it's generally been done everywhere for 50 years to now.
Although there are exceptions to every rule, and I'm sure you can find at least one, the fact is that the vast majority of fan control systems (both OEM and aftermarket) are based on limiting the measured engine temperature. Again, in the vast majority of cooling systems, engine's mass temperature is mirrored by the coolant temperature near the engine outlet. This is true with any circulating coolant commonly used, such as water, glycol, oil, etc. While some systems may use additional sensors near the coolant inlet for additional information, the primary source of info to determine when to run the fan is based on the engine temperature, and that is usually based on coolant temperature near the outlet.
This method rests on the primary goal of the system—engine temperature control within specified limits—requiring the engine temperature to be the basis for the goal. The goal is not coolant temperature, or oil temperature, or air temperature, but specifically engine temperature with a specific range or target. All of the factors than can affect engine temperature, that the fan can influence, are accounted-for with this method. Your primary answers were:
cx500tc wrote:[1] Engine outlet CLT sensor data cannot alone be used to determine if the inlet coolant temperature or engine load are excessive. All said sensor can tell you is the temperature of the coolant leaving the engine.
And what does that coolant temperature tell us?
If you are driving down the road, and you change nothing but increase the coolant temp to the engine, will the engine temp increase? Yes. And, if you are driving down the road, and you change nothing but increase the engine load, will the engine temp increase? Yes. So, the engine temp, being reflected in the outlet coolant temp, does indicate those conditions. It also reflects the effects of any and all other conditions that affect engine temperature, and as the fan is a component directly effecting the engine temp, the engine temp is used to control it.
cx500tc wrote:[2] Engine coolant inlet temperature cannot be used to determine the engine coolant outlet temperature, at all, regardless of engine loading or cooling fan operation.
That is correct, it cannot sense outlet temp and therefore engine temp. It can't tell if the engine is freezing or melting. So, why would you want to use it as your primary control, when it can't see the information it's ultimately supposed to control?
The examples you gave in your first answer are indeed factors in engine temperature:
cx500tc wrote:[a] generally normal operation at above idle engine speeds and relatively low vehicle speeds;
faulty thermostat- stuck partially opened and thus allowing some, but otherwise restricting coolant flow which could allow excessive heat to build in the coolant within the engine;
[c] high engine loading such as full-throttle application for an extended period while travelling uphill or pulling a heavy load;
[d] excessive coolant inlet temp caused by faulty radiator- partially plugged or damaged internally or externally, faulty or inefficient cooling fan or fan switch, insufficient coolant volume, damaged or inoperative coolant pump or pump drive ancillaries, or faulty plumbing / hoses;
[e] faulty automatic transmission using an in-radiator transmission fluid cooling loop transferring excessive heat into the coolant.
Interestingly enough, so are all the examples you gave for [2]:
cx500tc wrote:[a] thermostat operation;
temperature of coolant entering the radiator;
[c] efficiency of the radiator including losses due to damage, clogging, et cetera, of the radiator;
[d] air flow and air temperature across the radiator;
[e] operation of the cooling fan;
[f] coolant volume / level;
[g] coolant pump operation;
[h] condition of all devices attached to the cooling system including plumbing / hoses, and to some extent the cabin heater;
air conditioner, if equipped, increasing the temperature of the air across the radiator due to the condenser operation.
As we can see, as causes for affecting engine temperature, all of these factors are accounted-for in the resulting engine temperature. All of them. Indeed, test the theory, and pick any example you gave from either list. Any of them would affect the temperature of the engine. The cumulative effects result in the actual engine temperature. Therefore, the answer to question [1] is, and as simply as possible, “When it's too hot.”
If the engine temperature is too hot, it is too hot. That is not silly. That is a true statement. Once the thermostat has reached maximum flow, there is one primary function the cooling system can use to avoid excessive engine temperature, and that is to turn the fan ON. If there is a failure (which is irrelevant to fan function in normal operations), then there is no recovery function for the system, and the engine overheats. The cooling fan has no other way to avoid engine temperature overheating than to turn ON. What is the purpose of the cooling system and fan? Engine temperature control. How is engine temperature measured? Outlet coolant temperature.
Conversely, if we only sense the coolant temperature at the engine inlet; we have information, but only relative to the coolant. Not the engine. If our engine temperature limits are 180°F to 200°F, what should the coolant temperature be? 199°F? 176°F? How do we know? And indeed, without knowing which of the factors you listed earlier are currently involved and to what level, we cannot control the engine temperature accurately. We cannot know if the engine is working hard (and needs cooler coolant to stay below 200°) or is decelerating down a mountain (and needs hotter coolant to help stay above 180°) or if there are too many bugs in the fins, or any other factor. The fan control does not know what temperature the coolant leaving the radiator should be to cool the engine properly under the conditions of the moment. Indeed, to ensure that the engine does not become too hot, the fan should probably run too much, just in-case the engine is working hard, or the air is hot, or whatever. But that means excessive fan operation, just in-case, because we don't have enough info for accurate operation to control the engine temperature (the goal). So, if you want minimal fan operation while still effectively controlling engine temperature, control it from the engine temperature.
Well, that's about the best I can do to outline the situation from my position form as many angles as reasonable, except that I can address your AC statements. As to the AC situation:
cx500tc wrote:Even the fan on the A/C condenser outside my house doesn't turn on unless the temp of the Freon leaving the condenser is too high... as in it's not controlled by the temp of the Freon leaving the evaporator, but rather the condenser. If it's too hot outside, which means a less efficient transfer of heat from the condenser to atmosphere, the fan runs, but if the outside temp is low enough and there's some ambient air flow, the fan runs less often.
... well it's not exactly that simple since the fan is controlled by the low side pressure, but the effect is the same....
But it is that simple. That's a perfect example of using the control target as the measuring basis. The target specification is low-side pressure control. Therefore the basis for the cooling must be the pressure data, and not on the high side, but on the low side. The system only has one method to maintain the pressure below the specified limit—the fan. If the measured pressure is too high, turn the fan ON. If it's not too high, turn the fan OFF. Sound familiar? It does not matter (or know or care) if the air is hot, or the bugs are thick, or the high-side pressure is wacky. All of those influence the low-side pressure measurement. Only the target measurement is of primary importance, includes the effects of those other factors, and is therefore the basis to control the fan.
Yes, I am a temperature control fanatic. You could probably call me anal on the subject. As a point of control, the greater the engine specific output/power, the more critical temperature control is. In many racing applications, a total variation of only 10°F (5.5°C) may be permitted under any conditions after warmup, else effects from dimensional changes including power loss or damage may occur. Temperatures outside of the narrow specified range is considered a system failure. Period. The ultimate unreachable target is zero temperature deviation, with useful increases in power and reliability, because there would not have to be any concessions due to changes in temperature. The more your temperature changes the "safer" your tune has to be to compensate.
I sure hope some of this helps us get on the same page, and you can fill me in on how your system can improve the fan control situation.
David