Re: Pros and Cons to using oil as coolant

Posted by chadd on May 11, 2007 at 11:32:49 from (155.92.32.46):

In Reply to: Pros and Cons to using oil as coolant posted by JohnG(TX) on May 11, 2007 at 08:44:16:

I think Janicholson hit the nail on the head, the best coolant is the one with a specific heat that is greatest. For oil to be used as an effective coolant, a much larger radiator and cooling passages with more surface area would be required. Technically, diesel engines are partially "oil cooled" as their oil is cooled by running through a heat exchanger called an oil cooler, which then transfers heat to the water engine coolant. There are dozens of tubes in there to get enough surface area to transfer the heat to the water. This system is more a matter of preventing the oil from breaking down and becoming excessively thin due to high temperatures than actually cooling the engine itself.
I don't know about the lubricating of water pumps, either. I wouldn't think it would help most water pumps remain lubricated, because, unless the seals are shot, the liquid they pump should not be in the bearings in the first place. The higher viscosity of oil will probably severely stress the shaft bearings from the extra bending load until the oil gets hot, though.
The heat transfer would be significantly slower, probably the equivalent of a thick layer of scale throughout the cooling passages.
As I understand it, water should not boil to remove heat from an engine. If it does, it is the symptom of something wrong. The water is simply a working fluid that absorbs thermal energy and relocates it to the air moving through the radiator. If it is boiling you are not moving the heat elsewhere, and instead concentrating it in the water causing it to boil. Boiling of cooling water is one of the main causes of cavitation erosion in diesel engines (although it is caused by a local pressure drops around the sleeves, and not by the temperature).
Also, the thermosyphon may not work at all. In order for free convection to occur, the bouyancy force of the liquid must be stronger than the viscous force. Oil has very high viscosity, although it depends almost entirely on temperature. The engine would probably have to get quite hot before you would ever get the oil moving. Not to mention the fact that it isn't going to want to go through the radiator fins either. There are oil cooled motors out there, but they usually rely on air cooling to supplement the oil. Good topic, I never would have thought about using oil in the cooling system. . .

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