Internal combustion engines for use in vehicles usually have an engine cooling system which includes the engine that cooperates with a radiator and a pump for circulating cooling fluid to and from the engine. A fan is used to force air through the radiator to cool fluid heated by the engine and passing through the radiator during operation of the vehicle.
Generally, there are two classes of cooling fan systems employed conventionally to force air through a radiator: (1) engine driven type cooling fans which derive the required power from the crankshaft of the engine, and (2) motor driven cooling fans utilizing an electric motor as a driving source. In the engine driven type cooling systems, a fluid-friction or viscous-drive fan arrangement is provided to utilize the rotation of the engine to power the fan. A disadvantage of this design is a degree of slippage between the input and output parts of the viscous coupling. In addition, there is generally no regulation of the speed of the fan. Fan speed depends on the temperature of the air stream surrounding the bimetallic elements of the viscous coupling. Since fan speed is unregulated, when the vehicle is at idle, the number of revolutions of the engine is low and thus the number of revolutions of the cooling fan is also low. Thus, the engine may tend to over heat due to the lack of capacity of the cooling fan to supply required air to the radiator.
In the case of electric motor driven fans, the fan operates generally at a constant speed independent of the driving conditions of the vehicle. Thus, if the number of revolutions of the cooling fan is preset to suit the cooling requirements for conditions during idling, the cooling capacity tends to be inefficient when the vehicle is operating at high speeds. On the other hand, if the number of revolutions of the electric motor driven cooling fan is preset based on cooling needed when the vehicle is running a high-speed, a large capacity motor must be used which may be too cumbersome for use in many vehicles.
In an attempt to solve these problems, hybrid mechanical and electrical drive engine cooling fan systems have been proposed using an electric motor to drive the cooling fan during idling conditions and using the engine output shaft to drive the cooling fan during high speed vehicle operating conditions. A one way clutch is provided to control whether the electric motor or the engine output shaft drives the fan.
Although the hybrid systems work well for their intended purpose, these systems generally require complicated mechanical connections between the electric motor and the separate clutch which makes the system bulky and thus consumes valuable engine compartment space.
Accordingly, a need exists to provide a simple and compact vehicle cooling system which is efficient and provides effective cooling when the engine is operating at both high speed and at idle conditions.