refrigerators coolants?

In a regrigerator, the coolant evaporates in the freezer. In so doing, it absorbs latent heat of vapourization from the refrigerator chamber, hence cool down the air inside the refrigerator.
The evaporated coolant is turned back into liquid form in the compressor by increasing pressure on it. The liquid coolant is then recycled into the freezer again.
You can find the relevant diagram on the web-page below:
http://home.howstuffworks.com/refrigerator4.htm
The cooling of a car engine is by means water circulating the engine. The water absorbs heat from the engine and becomes hot. This hot water returns to a tank (the radiator) normally placed in front of the engine. The hot water is cooled by forced convection with a fan driven by the engine itself.
Diagram is given on the web-page below:
http://auto.howstuffworks.com/cooling-system.htm