Question about heat transfer and motor oil.

[CptanPanic]

Hello,
I am doing an article for an aircooled vw magazine in order to find the best motor oil is in regards to cooling. You see these engines are cooled primarily by running the oil through the oil cooler. Also there has been a urban myth in the industry since the 70's that synthetic oils reject heat, thereby showing a lower temp on the oil-temp gauge, but the engine was actually running hotter. What I want to to do is to scientifically determine quantitatively which oil cools best. I was originally going to find the heat capacity per volume, but now I have doubts to whether this is the right property to test. My problem is I think a higher heat capacity would mean more heat would be carried out of the engine, but does this have any correlation to the ability for the heat to be transferred into the oil? Any help and/or direction would be appreciated. Also keep in mind I do not have access to a lab, so I will have to use kitchen-type tools, ie coffee-cup calorimeter, etc. I already have everything to measure the heat capacity per volume.
Thanks,
Brian Wagener

 

[sirmick]

The property you are looking for is the thermal conductivity typically abbreviated k. Units should be Power/(Length*Absolute Temperature), e.g. W/(m*K). This will help.

Secondly, it would be useful to know the viscosity, density, and specific heat of the different oils. These help determine how fast the oil is flowing through the cooling system and the temperature distribution. If the viscosity and density are similar between oils and the specific heats are different, the oil with the lower specific heat will carry away less heat.

However, if all properties are equal, excluding thermal conductivity, the oil with the higher thermal conductivity will cool the system better.

Mickey

 

[CptanPanic]

Is there a simple experiment for thermal conductivity? Would the oil companies disclose this data? Also in this circumstance, which would be more important, specific heat or thermal conductivity?
Brian

 

[sirmick]

In convective heat transfer there are too many variables involved to make it possible to say which variable is the most important in general. So much depends upon the geometry of the system.

I would like a high specific heat, high heat conductivity oil. I couldn't say which one to sacrifice in general. If possible, I would pick the oil that had high heat conductivity and try to increase the fluid flow rate. The low specific heat can be a problem since the fluid closer to the outlet of the cooling system will be hotter than one with a higher specific heat thus reducing the heat transfer in that region. This can of course nullify the effects of having a higher heat conductivity.

The best test I can think of, is to test the system as a whole.

Mickey