Fan temperature sensor

[baganz.sven@web.de]

Dear forum colleagues,

taking the cooling system apart I found that the housing for the Otter switch is completely corroded.
But only from the outside
I even have holes which go completely through the material. I don't know how this could happen.
Now I want to install a coolcat temp sensor.

Can you give me a recommendation which sensor I should use?
coolcat included a 75° and a 85° one.
Is it better to use the lower temp to switch the fans on?

Best regards

Sven

[jagwit]

Definitely the 75C one!

The cooling system will then have a much deeper reserve to deal with "high heat generation" situations.

[MarekH]

On my car, the temperature at the Otter switch is about 15'c lower than that at the back of the engine during normal running once the thermostats open. Also, for an 82'c thermostat, the temperature cycles between 82'c and 88'c normally when the car is stationary at idle.

That means you want the fan to kick in when the temperature at the bottom of the radiator exceeds ~73'c.

I'd chose the lower temperature Otter switch.

All that will happen is that if the radiator now cools too much, the thermostats will close up and choke off the water supply to the radiator. Better that than allowing the temperature to run away upwards and pull the fans in much later. A fan failure under those circumstances gives you less time to retake control before it is too late.

kind regards
Marek

[baganz.sven@web.de]

Thank you for your quick replies and recommendations!

Sven

[myJagV12]

Hi !
I decided to PWM control my fans but the soldering of the device was a bit challenging.

BR, Tom

[jagwit]

Very good work Tom!

Are you intending to sell the controller?

[myJagV12]

Selling or making profit was not the idea and the unit was not tested on the road. If it works satisfying i could provide PCB boards with or without parts and the firmware required for one to assemble. Having them soldered pays off only with 50+ pieces and I think not so many will be interested ? The BOM is only around 15USD but soldering is quite time consuming

[jagwit]

I have 74ºC thermostats in my car with an ECHLIN FS155 68-72ºC fan switch.

What I would like to do with your "intelligent fan controller" is replace the otter switch (fan switch) in the bottom hose with a temperature sensor and then program the controller to start the fans at say 68ºC (at a lowish speed) and linearly increase the fan speed up to max at 72ºC.

This would have the effect of pre-emptively starting the cooling effort by the fan rather than waiting for the exit temp of the radiator to reach 72ºC and only than reactively start the fans.

[myJagV12]

Well that is what it does and the input from the otter/AC overrides and forces full speed. The sensor shown in the video is stainless steel (DS18B20 chip) but there exist various housings for this. My V12 has a copper pipe close to the otter switch which would be a good mounting point.
I programmed some "maps" that map the fan speed depending on outside temperature and engine temperature. It was the idea to already start the fan at reduced power when it is hot outside even if the engine temperature is not critical yet. Another approach would be the "climb-rate" of the coolant temperature but I did not try this yet. The program also detects ignition off (battery voltage) and can run with ignition off for a predefined time (again variable depending on the ambient temp) but this does not make too much sense if the coolant is not being pumped (electrically). To be able to control such electric pumps (some uprated their jag like that) a switched output is missing in the design.
The fan speed is always ramped from the actual to the wanted speed reducing noise and power surges.
I have read someone had problems with his AC/switch driving the AC-fan - this could be another case where such PWM switching (again together with a second output) would make sense and could be used instead of the AC main-relay.

// 7 rows for ambient temps 0, 5, 10, 15, 20, 25, 30
// and 10 coolant temps from 50, 55, 60, 65, 70, 75, 80, 85, 90, 95
static const unsigned char pwmLUT[PWMLUT_ROWS][PWMLUT_COLS] = {
{ 0, 0, 0, 0, 50, 65, 80, 90, 100, 100 },
{ 0, 0, 0, 10, 50, 65, 80, 90, 100, 100 },
{ 0, 0, 15, 20, 60, 70, 80, 90, 100, 100 },
{ 0, 10, 20, 40, 62, 74, 82, 92, 100, 100 },
{ 0, 15, 30, 60, 65, 75, 85, 95, 100, 100 },
{ 0, 20, 45, 65, 75, 80, 90, 97, 100, 100 },
{ 0, 25, 60, 75, 85, 87, 90, 100, 100, 100 }

The table holds the fan speed - e.g: winter = 1 line, summer = 7th line
So in summer the fan would run 25% if the coolant is > 55°C, 60% at >60°C .....
The actual map may not make sense as it is now but the idea should be clear.
Currently this map is "hardcoded" in the firmware - might be nicer to be able to change it by bluetooth or infrared :-)

[MarekH]

Philip, you may be slightly overthinking this. If the temperature is or may become too high, why do you want to half run the fans? You just want to run them full-on, and when you do, the thermostats will soon shut off the flow to the radiator anyway, so any further attempts at cooling will be thwarted.

This sort of setup only really brings anything to the table when it is used to control electric water pumps in place of the mechanical pump. We know that the cooling is at its most marginal when airflow and waterflow are at their minima (a stationary or slow moving car at low rpm) and you can always just have an Otter override switch on the dash for this scenario. Other than that, the thermostat will design away all of your excess cooling capacity efforts.

I did try something like this with electric water pumps as part of my lpg setup, but abandoned it as it really didn't make any useful difference. See http://www.jag-lovers.org/snaps/snap_vi ... 1287409769 for a similar experiment I trialled back in 2010. The water pumps were brushless Bosch types from a Merc heater circuit.

Having a massively overspec'd radiator will work better I would have thought. Your thermostat and Otter temp thresholds want to work together.

kind regards
Marek

[myJagV12]

"the thermostats will soon shut off the flow to the radiator anyway"

How long this takes should depend on the temperature of the water in the cooler ?
You are also assuming that maximum fan speed is required which can not be true imo.

BR, Tom

[jagwit]

Philip, you may be slightly overthinking this. If the temperature is or may become too high, why do you want to half run the fans?

I don't think so Marek. And to the second question, no.

The principle of fitting a fan switch with low temp ratings is to get the fans to activate sooner, creating cooler coolant with which to cool the engine. but the fans will only activate when , in my case, the temp reaches 72DegC. With Toms device, one can get them to start up even sooner, albeit at lower speed, but as the temp (at the fan switch location) rises, the fans will run faster, until, when the temp reaches 72degC, they run at full speed anyway.

The difference is that with Tom's device, the cooling system has less "catching up" to do because the fans started even earlier than 72degC and the whole system will be better "prepared" for a sudden increased heat load. I can also see that with Tom's device one might have a situation where the Thermostats will have less need to go to fully open resulting in an even more stable cooling system.

But all this is still rather academic. Its just the engineer in me wanting to always see if one can't optimise things even more. Yesterday, I went for a 100km drive and on the back got stuck in severe traffic in about 30degC heat. The gauge never went above N, so one can argue, why bother ? In my case, its in my head.

[myJagV12]

The gauge never went above N, so one can argue, why bother ? In my case, its in my head.

Same here :D And I did not like the fans to suddenly run at full speed together with loud noise while standing at a traffic light - switching back off again soon.

[MarekH]

That's right - running a fan (pre-emptively or not) just does the cooling quicker, not "more".

Consider a thought experiment. You have three radiators:- a rubbishy small blocked radiator, the stock radiator, and an infinitely powerful fan assisted radiator.

At 73'c, it makes no difference - the radiator isn't receiving any water flow.

At 74'c, each radiator starts to receive flow. What happens to the water temperature? Well, this is just the equilibrium point between heat production and heat dissipation. When there is more heat production than heat dissipation, the water temperature rises. When heat dissipation is greater than production, the temperature drops. It is subject to a floor though, as the thermostat will choke off the supply of water to the radiator if the returning water drop the temperature lower than 74'c.

All that is being achieved with fans is that the rate of dissipation becomes greater. I would argue that the health of the cooling system can be judged by the rate at which we can dissipate heat, so the first radiator will always struggle to bring the temperature down and the equilibrium point will meander upwards and hopefully still settle without a runaway reaction. The third radiator, being infinitely powerful, stamps on the temperature straight away and will almost instantly force the thermostat shut again. Effectively, the thermostat will only be allowed to burp small amounts of hot water to the radiator.

Somewhere in the middle is the real world. It takes time for the water to go around the long loop to and from the radiator and it takes time for the thermostat to change position. The actual water temperature is always "wrong" but so long as the radiator is more powerful than the heat source, you don't care, as you can't actually control that.

Knowing the coolant temperature on its own isn't going to measure what you want to measure or make the system "pre-emptive".

What you are actually asking for is this:-

We want two water temperature sensors on the engine - one on the input and one on the exit.
We also want two temperature sensors on the radiator - one on the top hose and one on the bottom hose.
We now measure the rate of change of temperature of the engine water and subtract the rate of change of radiator water from that. As soon as the number is positive (and above our target temperature), we implement "Project Big Cooling Fan" and whenever it is below we don't.

Thanks to the delay and hysteresis of the thermostat, the result is always going to be a bit out of synch anyway. (Simply correlating the output to coolant temperature does not achieve the goal of pre-emptive design or take into account how well the radiator did.)

All you are asking for is to make the radiator more powerful more quickly, on demand, much like a combi-boiler does. The thermostat will still choke off the flow once the temperature drops.

Empirical work also tells us when our cooling is most marginal. It is when coolant flow and airflow are at a minimum, so the focus of any useful modification ought to be on upping the flow (of air and water) when revs are low. We don't much care about the radiator outside of the minimum operating condition, as it is always partially turned off by the thermostat. So this is a nice idea, but it doesn't put the focus on where the focus needs to go, which is low rpm running.

I could show you some graphs of heat generation and heat dissipation if you like - I also have a sensor in the Otter housing and flow meters in all of the top hoses, so monitor and log these on every drive.

kind regards
Marek

[myJagV12]

That's right - running a fan (pre-emptively or not) just does the cooling quicker, not "more".

Thank you for the long expertise given. I do know that I can't increase the cooling capacity by PWM controlling two fans and a better radiator or stronger fans would do this job. But i prefer fans being switched on longer with less power than switching them on and off with full power. Anything is better than a otter bypass switch in the cockpit (i'd rather switch on AC in that case).

[MarekH]

You are switching them on and off with full power every time. Whether that consumes more or less power than their steady state always-on running condition is a function of the on:off pwm ratio. Take a look at the graph of inductive load current versus time - it's not a square tooth profile.

kind regards
Marek

[myJagV12]

"You are switching them on and off with full power every time."

@11kHz yes - I hope you don't want to teach me electronics as when I look at your prototype ....

[jagwit]

running a fan (pre-emptively or not) just does the cooling quicker,

At 73'c, it makes no difference - the radiator isn't receiving any water flow.

At 74'c, each radiator starts to receive flow.

I would rather phrase it as follows: "running a fan (pre-emptively or not) just starts the cooling earlier".

You make it sound as if the thermostat is closed one moment and then 100% open the next?

To me, the thermostat should NEVER be either fully closed or fully open once the engine is at operating temperature. It should be "somewhat" open and merely go "more open - but not fully open" (sending more coolant to the radiator and less to the bypass circuit) or "less open - but not fully closed" (sending more coolant to the bypass circuit and less to the radiator) around some operating point, in the region of 74ºC. This can only be achieved if the coolant entering the engine is sufficiently cool to achieve the desired engine cooling without the thermostat needing to go fully open.

Starting the fans earlier at low speed, ramping up as the radiator exit temp increases) with Tom's device would contribute greatly towards achieving the aim of having a thermostat that never goes either fully open or fully closed. In this scenario, change of the the coolant temp entering the radiator should be limited to the reaction time of the thermostat only. It should not be visible on the temp gauge due to the slow reaction time of the bimetallic operation of the gauge and hence should present a perfectly constant temp on the gauge.

[MarekH]

Whether or not you are "cooling earlier" is down to the thermostat - it controls the flow to the radiator, not the temperature at the Otter switch. I think you may have meant "cooling more rapidly", rather than earlier, as running the fan earlier is obviously putting more airflow into the equation. Whether you need to do this is moot - the XJS runs 88'c thermostats and they don't automatically overheat. In principle, I don't see a problem with wanting to cool rapidly once you have decided to cool. My original point was to question why you'd want to half cool (moderated via PWM) once you'd decided intervention via a fan was needed. If you need to drop the temperature, do it as rapidly as possible I would have thought.

"Earlier" is when you have relatively little flow diverted to the radiator, so I'd question whether operating the fan then is optimal use of energy. You want the fan intervention when it is probable that the radiator alone cannot bring the temperature down in a timely manner. If however, it is as you envisage, that the thermostat is neither fully open or fully closed, then you probably don't yet need the fan. Logic says you need the fan when the bypass has been shut off and 100% flow to the radiator isn't bringing the temperature down. That is why I defined the problem as one of rate of heat production minus rate of heat dissipation. I see the temperature as the result of those two processes, not the driver.

If you pop a thermostat into a water bath, it isn't digital, but there is a temperature when it suddenly seems to move a lot - neither opening nor closing is a linear function. Mike Frank's website will fill in the details I'm sure.

Running lower temperature 74'c thermostats in lieu of 82'c ones does nothing for you other than to buy you a bit of time should the radiator fail. You have the time it takes to raise the coolant temperature 8'c to pull over, assuming you notice the gauge has gone up to 8'c higher than it usually does.

kind regards
Marek

[jagwit]

I'm going to try another angle to get my thoughts across:

Let's pretend we have a magic radiator to somehow keep THE SAME VOLUME of coolant that would be in the radiator at a constant 3degC. This device has some magic way of "instantly" cooling coolant coming from the engine down to 3degC.

Right, now we go for a drive. I still have 74degC thermostats in my engine. Clearly in this case, the fan switch and the fans and air flow will play no role whatsoever. Surely you have to agree with me that the thermostats will remain ALMOST closed, that MOST of the coolant being pumped into the block by the water pump will be coming via the bypass route and that a very small amount of 3degC coolant from the magic radiator will be combined with the bypass coolant, but the engine will be running at 74degC, correct?

I can now increase the temperature of the magic radiator and as I do, so the thermostats will gradually open more reducing the ratio of bypass coolant and increasing the ratio of magic radiator coolant to still maintain the engine at 74degC.

As I increase the magic radiator temp even more, there will come a point where the thermostats are wide open, the bypass route fully closed and from this point onward the temp of the coolant of the magic radiator will determine the engine temp.

My point is this: The cooler the volume of coolant in the radiator, the LESS of the coolant in the radiator is needed to maintain the engine temp at the thermostat temp. The less I need to use, the greater the buffer (the more robust the cooling sytem) I have, before we get to the point where the radiator + fans can no longer keep keep the engine within "thermostat regulation".

The whole point of installing the 68-72degC fan switch was to start cooling the radiator "stash" earlier/sooner/at a lower temp. With Tom's device I can now start the fans at even lower temps, thus "staying ahead" even more than what is currently the case, even with my 68-72degC fan switch.

But, in line with the above one could argue to simply start the fans at full speed at 50degC and switch them off at 45degC, hence no need for PWM. And if there was such a fan switch, I would have fitted it. Having the fans come on slowly at 68 and ramping up to 72, is just a more elegant (refined) approach than hard on/off.