Automotive/car radiator fan
QUESTION: Hii ,, i own a ford figo tdci 2015 model car , i want to ask that how much is the normal time taken for a radiator cooling fan to shut off automatically when the car is kept stationary after a journey(with AC). I have noticed that my car fan takes too much time (20-25 minutes) to shut off automayically. I have driven other diesel cars too , i think its too much time.Is there any problem in the radiator fan??
ANSWER: Sorry for the delay! As it happened, I needed to produce some course notes on Temperature control, and decided to work out my draft notes via my reply to you!
First however: As indicated in my AllExpert notes, I do have shop information ONLY for those vehicles I myself own, and thus can not easily refer to detailed operating specifications for those which I do not, such as your car. Hence my reply is general, to pass along some thoughts and concepts ... I do hope that you find them useful! Incidently, I ALWAYS STRONGLY RECOMMEND that anyone who is serious about maintaining or modifying their vehicles GET the OEM SHOP MANUAL first, along with any other resources on the electrical and mechanical technologies used in cars that one may find helpful.
Now, to answer your very last question first, that being: "Is there any problem in the radiator fan?", I can only say that, given that the fan appears to operate (whether only because the engine had been running and its timer has not timee out yet, OR because the Temperature setpoint ultimately was reached while the engine was operating, the radiator fan DOES appear to be doing what it is intended to do: run to remove excess heat from the system.
Having established that, we shall note that the "control device" does appear to do what it is intended to do as well, that is: turn ON the fan once its high-Temperature setpoint is reached (IF applicable - that is, UNLESS the fan DOES turn ON ANYWAY once the engine is running) and then WILL shutdown the fan at some point after the engine has been shut down and has cooled off sufficiently. Note that I choose my words for the foregoing so as to allow for either of two scenarios: (i) the control device is a Thermostat, or (ii) the control device is a Timer.
Now, I would expect that the control device for an engine cooling fan probably IS a Thermostat, as that pretty much is a standard practice for temperature regulation or, in this case, mitigation. However, I do speak from a North American - in fact, Canadian - viewpoint, and it may well be that the use of a Timer to assure fan shut-down at a reasonable period after the engine goes off-line may well be a preferred approach in very hot climates. If it ALWAYS takes 20 to 25 minutes for your fan to shut down, regardless of ambient temperature, then your vehicle may well use a Timer for this purpose.
Still, I would think it more likely that a Thermostat is being used, as it is in most such applications, and so I shall proceed onward from this point with the assumption that Thermostatic control IS applied, as it most commonly is ...
The most common thermostats for straight-forward ON<->off control usually have a band gap (hysteresis) between their contact-closed (ON) points and their contact-open (off) points. Specifically, a thermostat intended for the use of, say, room or space heating appliances would be configured so that, for a given Temperature selected on its range, say a setpoint of 20C on a room thermostat, the heat will NOT be turned on just as the room Temperature drops to 20C, but would hold off until the T has dropped to, say, 19 or even 18C. Then, when the heat does come on, the temperature at the thermostat will then be allowed to rise, NOT to 20C, but to 21 or even 22C, say, before the heat is shut off again.
Similarly, a cooling thermostat will close only once its ambient temperature has risen ABOVE the setpoint by two or three degrees (per the specification for the system) and thereafter will reopen again only after its ambient temperature has dropped BELOW that setpoint by the prescribed margin.
This band-gap action is provided intentionally, so as to avoid excessively-frequent cycling ON-off-ON-off of the controlled equipment; while it might appear that frequent action would provide a tighter control on absolute Temperature, such fine-tuned control rarely is necessary. Further, the short-cycle operation that such control would cause is UNdesirable from the perspective of service reliability and operating lifetime for most controlled equipment. Motors especially are adversely affected by unnecessarily-frequent starts, so it is always much better for a given system to run for a while, once on, and to be idle again for a while, once off.
Your automotive cooling system differs from the ON<->off control point-of-view in that automotive thermostats generally are proportional, effectively working as a variable valve so as to modulate the flow according to the temperature schedule implicite in the materials from which the expanding and contracting mechanisms of the thermostat are made - parts which act to move that valve portion of the stat as needed to throttle the flow of coolant, and thus heat.
It is important to know that the purpose of an automotive - or any - cooling system is to move heat in a controlled manner, away from where it is being generated and to where it may be otherwise harnessed, or else dissipated.
Know that it takes time for heat to be removed, because it takes time for heat to flow. Per the Second Law of Thermodynamics, heat will naturally flow from a higher Temperature (ie: "hotter" ) body or region to a lower Temperature ("cooler" ) body or region. Your fan aids in cooling the region to which it is applied. By extention of the second law, that fan then also aids the rest of the system to cool as well, as long as the fan is running, since heat will continue to flow towards any regions where the temperature is being reduced, as long as any temperature differential remains present. Should that fan shut off while there are still temperature differentials in the system, then the paths that heat will take to attempt to ultimately equalise that temperature differential could well cause heat to flow back through an area that previously was being cooled, and cause that region to RISE in temperature yet again ... if there is a LOT of heat still in the system, then that temperature rise could even be damaging!
This can be illustrated via the example (JPG) attachment; while this example differs, in that it relates to the removal of heat from an alternator rather than from an engine cooling system, the fundamental principles of thermal flow are the same, as are the circumstances for the removal of heat or, in this case, NOT, due to the elimination of the flow of the air to remove that heat BEFORE the bulk of the heat has been removed. Looking to the upper right corner of the curve of alternator Temperature, one can see that, while the Temp increased as it approached the equilibrium point (the point where ultimately the heat being shed will be equal to the heat being generated), it dropped significantly once its electrical load was removed, as long as the alternator itself continued to run - the running is important, since the alternator's own cooling fan is driven by the alternator shaft. However, the alternator is stopped (when the engine is shut down) shortly thereafter. At that point, the Temperature in the region where the alternator Temp is being measured starts to RISE again! This is because, in the absence of the fan's action, the heat now is being passively REdistributed wherever the various paths of thermal conductivity in the alternator will take it, and some of it builds up again in the region where the fan was previously removing it! Subsequent to that, as new paths are established, the alternator begins to cool again, but now at a MUCH LESSER rate than it did while the fan - along with the unloaded alternator - was running.
The point made by this example then is simple: You do NOT want to shut off the vehicle for the removal of heat - the air flow - before enough heat has been removed so as to NOT cause unexpected and possibly damaging temperature rises that could happen if that shut off was premature. Therefore, if the fan is properly controlled to shut off once an appropriate Temperature for shutdown is reached, then let it run until that condition is met. (The same edict could apply for a correctly-engineered Timer-based system, the operative condition in this case being "appropriate Time" until that condition is met.)
So, what are YOU going to do now? There are several things you can do, if you are serious about pursuing finding answers and working out remedies on your own. The first is to acquire the shop literature, as already indicated at the top. The second would be to get some useful instrumentation. In this case, a good-quality automotive service digital multimeter (DMM) would be a core tool, and most of these DO include a temperature range and a thermocouple (aka Resistance Temperature Detector, or RTD) that can be used to provide the input for that range. (Now, many such DMMs offer "logging" capability, using an RS-232C or a USB or even an InfraRed (IR) means by which to couple the DMM to your PC, and then use either the logging software provided with the DMM, or else just copy the comma-delimited values into Excel or Quattro or whatever and create the useful graph or graphs therefrom ... this is all sorts of fun for those of us so inclined! However, you can just as easily record temperatures onto paper, using a watch to track time, and plot a graph manually, if you wish.
Whatever you choose to do - or not - the salient point is to find out how long it takes for the system temperature to drop to any level below the bottom end of normal operating temperature, 180F or 82C in many cases. If your data collection shows that the fan continues to run even after this range of temperature is reached, then perhaps an adjustment (if available) of the Fan thermostat contact-opening setpoint is warranted. (Similarly, IF a timer is used rather than a stat, then a shorter time setting might be in order.) If, however, the ability to adjust settings is NOT available, than thermostat (or timer) replacement might be necessary. In either case, our concern really is only to limit or mitigate the excess temperatures that arise right after engine shutdown, so as to limit pressure rise in the cooling system, and possible loss of coolant.
In general. a shut-off temperature about equal to the normal reclosing point of the coolant thermostat (ie: ~180F or 82C, for the common 180F stat example) would be a good target. Note that automotive fan thermostats are intended to start only when or if the engine coolant Temperature becomes excessive, a few degrees above the point where the coolant thermostat would be wide open. For gasoline engines, this would be about 200F (93C), whereas for Diesel engines (cast-iron ones), this can be above the atmospheric pressure boiling point, as high as 110C - which is below the 120C "redline" on my OM617 engine. The idea is that the primary control for engine cooling is the modulation of the temperature of the liquid coolant, with back-up "high-limit" protection provided by the fan which boosts the air flow through the radiator, and which also limits Temperature rise post engine shutdown by continuing to remove surplus (and also excess) residual heat from the stopped engine by running until the engine temperature is no longer excessive.
Anyway, I share these thoughts with you - if they are helpful, that is great!
If not, then I thank you anyway for getting some source notes for an upcoming writing task out of the way.
Best regards and cheers ... E G Kenward
---------- FOLLOW-UP ----------
QUESTION: Thanks .... you explained so well!
Now since i have just purchased this car in november 2015 and it has done only 3800 km. I am depressed of this problem. I had a service check up of my car , they said everything is fine. They even said that you can turn off the engine when the radiator fan is rotating. Please please .... Can you tell that will that harm my car engine or not?
Having a more clear understanding of your concern now, I would say that you need not worry about engine damage, or seal or cooling system damage for that matter, arising from the continued temporary running of a cooling fan. The opposite is true, in fact.
This matter of the radiator fan running on after the engine is shut off happens to be pretty much a standard operating mode by which much of residual heat is dispatched from the engine in the period after shutdown. This is found on most cars of the past two or three decades, and was made possible by the changeover from fans driven by a drivebelt on the engine to electrically-powered fans. The electric fan simply will run until the contact on the electric thermostat controlling the fan opens, which it will do once the engine temperature has dropped to the predetermined point beyond which the fan is considered to be no longer needed.
Obviously, the fan does draw power from the battery alone when it is running after engine shutdown, however, the system - including the sizing of the battery - is engineered to accommodate this. Ultimately, a battery will deteriorate and have to be replaced, of course, but that is the case in any system relying on a starting battery. Periodic checks can verify battery condition, to avoid unpleasant surprises on the road, and should be done with all automotive (and other) systems anyway. Preventative maintenance accompanying these periodic checks will also catch issues before they become problems.
Leaving this to trusted mechanics is just fine. However, there is no reason why you cannot get involved in the preventative-maintanance process yourself, to a limited extent, or to an increasing extent, however far your interest takes you. The manufacturer's literature and manuals are always a good starting point, and may be all that you would need. There are other more general-application books, by Audel, and Chilton, for example.
This could open a whole new interest.
Best regards ... EGK