thermo fan switch, install in inlet or outlet of radiator?

[kewp]

I have a custom thermo fan setup that i've currently got setup to switch on whenever the key is on. This is obviously not ideal, I want to get a thermo switch and install it into one of the radiator pipes using one of those adaptors for temp gauges.
Is it better to put the switch into the inlet or outlet of the radiator?
cheers

[river]

Hi,

One would think the inlet (ie water coming into the radiator from the engine) as that is the temp of the water as it exits the engine.

seeyuzz
river

[allencr]

The radiator cools the engine and the fan cools the radiator, so the fan should cool the radiator when the coolant Exiting the radiator, going into the engine, is too hot, like it is from lack of air flow.
There's no use having a fan running while driving at 150kph & the outside air temp. is almost 0, just because the engine with a 90 thermostat is being thrashed and very hot coolant is coming out of it. The air flow will be doing a much better job then a fan ever could.

[kewp]

hmm, thats sort of what I was thinking, that the fan only needs to come on when the water exiting the radiator is too hot...?

[codez]

It will work either way but it will be more efficient if u have the switch in the outlet.

Because if u measure the temp. from the the inlet, that temperature is different than the water temp. entering back into the engine and thats the water temperature you want to keep under control.

[kaido]

yeah river is right you want it on the engine outlet / radiator inlet, reason being is that the if you look at the temperature of the water coming out of the engine, you know thats the maximum temperature your engine sees, if its operating off the water into the engine, it can and will be hotter inside.

[river]

Hi,

Yep, that's why the thermostat is at the outlet of the engine (inlet to the radiator) and that is also the area where the temp gauge is placed. You need to see what temp the water is coming from the engine, not what the temp of the water is going into it.

seeyuzz
river

[codez]

oh yeah, makes sense..

/admits defeat lol

[river]

Hi,

No.. not defeat, but victory as you learnt something, and that's a good thing.

seeyuzz
river

[twentyEight]

Yep, that's why the thermostat is at the outlet of the engine (inlet to the radiator)

Not on all engines...

3S-GTE (and possibly other modern Toyota engines) has the thermostat on the engine water inlet.

[river]

Hi,

Indeed my friend, but you know my baseline is the 18R-G/2T-G engines.

I didn't know Toyota made other engines!

seeyuzz
river

[hemi twofifteen turbo]

Actually most factory setups switch on thermo's based on the radiator outlet temp. The 4age for example has the thermostat on the lower radiator hose. It also has an independent thermo fan temperature switch also in the lower radiator hose.

If the engine is running too hot, the thermostat opens to allow more water through, cooling the engine and solving the problem. The thermostat regulates the temperature of the engine.

The radiators job is to cool coolant. If it's not doing it's job due to lack of airflow (sitting in stationary traffic for example) that becomes apparent by looking at it's output -- hot water -- that needs some extra airflow to cool it -- thermofan.

In a nutshell...

The thermostats job is to regulate the temperature of the engine
The thermofans job is to limit the temperate of the water exiting the radiator in a low or no air flow situation

[river]

Hi,

There's no use having a fan running while driving at 150kph & the outside air temp. is almost 0, just because the engine with a 90 thermostat is being thrashed and very hot coolant is coming out of it. The air flow will be doing a much better job then a fan ever could.

If the engine is overheating, regardless of the outside temperature or how fast or slow you are going, a thermo fan will kick in - unless you got some sort of speed sensor to disable the thermo fan above a certain kph rating. You set the termo fan to a temp that is within the safe limits of the engine temp. If the fan is coming on while driving fast, then you've either set the temp for fan activation too low, or your engine is overheating and no amount of air is going to help you.

If you are going to switch on the fan from the inlet to the engine, it would have to be set at a lower temp than if you had it connected to the outlet of the engine.

You want the fan to come on only after the thermostat is fully open. I can't see why you'd want the thermo fan kicking in if the thermostat is not fully open.

Using arbitary numbers... if the thermostat opens at 82deg you would want the fan to kick in at 90deg.

If you are using the outlet of the engine (going into the radiator) then you'd have a thermo switch that turns the fan on when the water is 90deg.

If you are using the inlet of the engine (coming from the radiator) then you'd want a switch that turns the fan on at (say) 70deg - assuming there is an approx 20deg rise in temp when the water exits the engine.

If you use a switch that turns on the fan at 90deg, and it is placed on the engine inlet side, then you can be pretty sure the water exiting the engine is a lot hotter than 90deg.

seeyuzz
river

[Hiro]

I'm going to join the radiator-outlet bandwagon, looking at it from a control-theory point of view.
Think of the the radiator and engine as a dual system, and the output of the second system is also the input of the first system and vice versa(thus creating complete closed loop). The radiator cools the coolant as a relative function of the temperature differential between the outside air temperature (assumed constant in this case), and coolant temperature (ignore efficiency and convection variations as well) and the engine heats it up as a relative function of engine block temperature and coolant temperature. But the system is not perfect - the heat absorbed by the engine may be greater than that given off by the radiator, which causes thermal runaway and overheating of the engine/boiling of the coolant.

Thus, a control needs to be added (we're up to operating temps so the thermostat is assumed to stay open). The control in this example is the thermofan - when not enough heat is being given off by the radiator the fans kick in to boost the convection through the radiator and improve heat exchange. Since it is a dual system though (radiator = 1, engine = 2), the control is only acting across the first system - it has no effect on the engine system apart from modulating the input temperature. For the controller to have the best effect on the output of the first system (which is the input of the second system) the sensor must be placed between the two systems - otherwise you are introducing lag into the system by needing to wait for the coolant to travel through the radiator AND the engine before the feedback is registered, which can create a hunting scenario. If the sensor is after the radiator, then any improvement in the efficiency of the radiator is detected immediately, and will provide a better control of the temperature of the coolant entering the engine.

[river]

Hi,

I see what you mean and fully understand where you are coming from.

But, in a closed system, the feedback can be placed either at the input to the radiator or the input to the engine. In your argument you placed it to the input of the engine. While this can see the immediate performance of the radiator, it also has an induced lag. By the time you see the temp coming into the engine is too high, then the coolant coming out of the engine is much higher. This too can create a hunting scenario.

So, based upon the assumption of a closed loop with feedback, both systems are similar, except the temp which the fan would be activated.

seeyuzz
river