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Homework Help: Car Thermostat Temperature Problem

  1. Apr 6, 2012 #1
    Solution Required :

    Engine Temperature should not exceed 96 °C

    Specify Thermostat starting temperature for controlling engine temperature within limits with tolerance of +/- 2 deg

    Constraint :

    Radiator Fan starts at 90°C & stops at 84°C

    Coolant from engine must be circulated through radiator when engine temperatures cross its specified limit

    No other Data has been provided.

    Solution Guess:

    Avg is of 84 & 90 , ie 87°C ; assuming Fan speed is based on keeping the engine at optimum temperature.

    Please help me guys
  2. jcsd
  3. Apr 8, 2012 #2


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    Staff: Mentor

    I've pondered this, but can't get a grip on it. When you find out the answer would you come back and share it? I think the question must rely on a more intimate understanding of an engine's cooling than I have -- which isn't saying much! ☹
  4. Apr 8, 2012 #3
    Still trying to get a correct answer
    Appreciate your interest though

    Thank You
  5. Apr 11, 2012 #4
    Answer is 88 to 90 degrees, ie thermostat opens partially at 88 and completely upto 90

    Radiator fan starts only when Natural convection ( or forced convection due to motion of the vehicle ) is not sufficient to cool the coolant in the radiator. The fan provides extra air circulation to cool the radiator n the coolant in it.
    This means the thermostat has to be open ( it should be operational , allowing coolant to absorb heat from the engine ) before the the fan starts working.
  6. Apr 11, 2012 #5


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    Staff: Mentor

    Okay, that wasn't difficult. :smile:

    What I've been wondering is: If the fan can reduce the temp down to 84° before it cuts out, does this mean the thermostat's wax can refreeze as the engine temp cycles down whenever the fan cuts in? I'm finding it difficult to picture the wax refreezing in a running engine, but maybe it does https://www.physicsforums.com/images/icons/icon5.gif [Broken]
    Last edited by a moderator: May 5, 2017
  7. Apr 14, 2012 #6
    Well in Full working Condition , don't think a Fan can cool an Engine upto 84 C ,
    Also since it has not been mentioned , I am assuming 84 would be coolant temp n not the engine temp

    About the Wax refreezing , that is plausible ; infact the way I understand it , a thermostat keeps opening & closing( which in turn means wax melting n freezing ) in a normal ( optimum ) condition
    (Check How stuff works )

    The working of a radiator is not so easy to explain ; the heat transfer , it depends on Temperature difference between Coolant & Engine ; between the coolant & surrounding air , coefficient of convective heat transfer in the radiator ( which depends on whether the fan is On or Off) ; mass flow rate of air in the radiator ( again depends on the fan )
    Coolant Pressure & mass flow rate ( Depends on Coolant Pump Capacity & Thermostat position respectively )
    but most importantly on how much Heat the engine is producing ( which again depends on throttle position , engine load etc

    Hope this helps and not confuses you :)
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