6 replies to this topic

[schuy]

Which of these substances take longer to cool down from their optimal work temperature, and which take longer to heat up to the optimal work temperature?

Liran.

[J. Edlund]

It usually takes longer for the oil to get up to temperature. About the cool down I don't know.

[acey]

It depends on which oil and for what? If we talk about roadcars the oil takes about 15 minutes more to get to worktemp than water. I would think that water also cools down faster since oil usually operates at a higher temp.

[clSD139]

Just don't thrust the central gauges of a Clio 16v. It can be very expensive...

[schuy]

I should have been more specific- I meant road-bikes.

Thanks for the information so far guys!

[david_martin]

Originally posted by schuy
Which of these substances take longer to cool down from their optimal work temperature, and which take longer to heat up to the optimal work temperature?

Liran.

The heat capacity of typical low viscosity synthetic oils is roughly 1.6 kJ/Kg/K at STP, typical radiator water is probably close to 4 kJ/Kg/K - ie radiator water requires the gain or loss of approximately 2.5 times as much heat to raise or lower its temperature. The thermal conductivities of the same oil and water complicate things a bit further too: oil is approximately 0.15 W/m/K at STP, whereas water is around 0.6 W/m/K. So while water needs a larger change in net heat to change its temperature, it is a significantly better conductor of heat than oil is.

So purely on the basis of the thermal-physical properties of engine oil and radiator water, it would seem logical to conclude that bulk water is easier to heat up and cool down, but requires more heat exchange to do so (which makes it a good cooling medium - it can absorb a lot of heat and it is a good conductor, so it is relatively easy to transfer heat to or from it), whereas bulk oil is more difficult to heat up or cool down but heating or cooling it down requires less heat exchange.

The real answer is a hell of a lot more complex than that - it lies in the relative heat balances of the oil and water circuits as much as the properties of the oil and water. The engine oil operates at a much higher temperature than the coolant does, but the total heat capacity of the oil circuit is a lot lower than the coolant circuit (a quick comparison of the relative capacities of the oil and coolant circuits and surface area of a typical radiator and oil cooler is quite illuminating in this case).

I would guess that the oil circuit temperatures probably lag behind the coolant circuit temperature during the warming up and cooling down cycles of a typical otto cycle engine, but that is purely gut feeling, I don't have any measured data to back it up.

My, that was a rather long winded way of saying, "I'm not sure"

[schuy]

Originally posted by david_martin

I would guess that the oil circuit temperatures probably lag behind the coolant circuit temperature during the warming up and cooling down cycles of a typical otto cycle engine, but that is purely gut feeling, I don't have any measured data to back it up.

My, that was a rather long winded way of saying, "I'm not sure"

Thanks mate, you helped aplenty