the driving speed of the fan driving motor the regulation of the fan driving motor is such that the valve 104 will provide just enough fluid to drive the motor and thus the fan 82 at a speed commensurate with the need of dissapating heat in the toroidal heat exchanger thus when the engine is cold the fan will not be driven at all this aids in rapidly building heat in the engine to normal operating temperature as the engine heat increases the hydraulic motor will begin to drive the fan at a slow rate just sufficient to cool the engine coolant to the optimum temperature of course when the engine coolant reaches a very hot predetermined temperature the fan will be driven constantly at a high rate of speed the ability to have a coolant temperature control system that can control the temperature of the coolant flowing through an engine is universally desired as all engines operate at optimum temperature efficiency in a specific range historically this has been provided by a thermostat generally located in the engine block proper that would allow fluid to flow from the vehicle heat exchanger or vehicle radiator into the engine block this thermostat is responsive to the engine coolant temperature and when the engine temperature drops below a predetermined point the thermostat will close cutting off flow of fluid from the engine to the radiator.