In thermal-fluids systems, the focus is on energy: its use, conversion, or transmission in one form or another. For example, consider a few of the energy flows in a car. Gasoline is stored in a tank until its energy is needed to move the vehicle from one place to another, and then the gasoline is pumped from the tank to the engine. In the engine the fuel is burned, and some of the released chemical energy is converted to useful mechanical power to propel the car. Mechanical power is also extracted to drive: the water pump used in the engine-cooling system; the alternator to provide electrical power for the CD player, lights, cooling fan motor, and fuel pump; and the air-conditioning system.