# Soak back temperature

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1. Oct 7, 2014

### stinsonbr

Could someone explain what an object's soak back temperature would be? My idea of what it is relates to an object's temperature peaking before cooling down due to the absorption of leftover energy (i.e. a car engine is being cooled by a fan. The fan and engine both shut off, and the engine temperature increases slightly before cooling down). Can anyone explain this in another way, or point me in the right direction if I'm off track?

2. Oct 12, 2014

### Greg Bernhardt

Thanks for the post! Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post?

3. Oct 14, 2014

### Staff: Mentor

4. Oct 15, 2014

### stinsonbr

5. Oct 15, 2014

### NTW

Your explanation is right, I believe... If one considers, separately, the engine block plus cylinders, etc. and the coolant, what probably happens is that the mean temperature of the block, with the engine running, is always higher than that of the coolant. This temperature is kept stable by the coolant continually absorbing heat from the block, and continually delivering it to the outside air through the radiator. The temperature of the coolant is always lower than that of the block, because of the 'heat cascade'. Both temperatures, that of the block and of the coolant remain stable since the all the rejected heat of the engine is delivered to the coolant, and all that heat is delivered, in turn, to the outside air. All the heat energy rejected by the engine ends up in the surrounding air, while both the engine block and the coolant temperature, although different, remain constant.

But then you switch off the engine, the coolant flow stops, the radiator fan stops, and the coolant does not deliver any more heat to the air. But the engine block is still in contact with the coolant, and as its temperature is higher, it continues delivering heat to it. And that heat is no longer evacuated to the air. Hence, the engine block cools down, and the coolant heats up till an equilibrium is reached. The temperature of final equilibrium will depend, among other things, on the initial temperature of the block and the coolant, on the respective masses, and on the global specific heat of the block and of the coolant. But, for a given time after switching the engine off, the coolant temperature will rise...