Chapter 6 Lubrication and Cooling Systems Principles of Engine Lubrication The primary purpose of a lubricant is to reduce friction  between moving parts. Because liquid lubricants or oils  can be circulated readily, they are used universally in  aircraft engines. In theory, fluid lubrication is based on the  actual separation of the surfaces so that no metal-to-metal  contact occurs. As long as the oil film remains unbroken,  metallic friction is replaced by the internal fluid friction of  the lubricant. Under ideal conditions, friction and wear are  held to a minimum. Oil is generally pumped throughout the  engine to all areas that require lubrication. Overcoming the  friction of the moving parts of the engine consumes energy  and creates unwanted heat. The reduction of friction during  engine operation increases the overall potential power output.  Engines are subjected to several types of friction. 