Patent ID: 7047911

Claim:
A hydraulically controlled fan drive system for an engine comprising: a housing assembly containing a hydraulic fluid within a fluid reservoir; an input circuit capable of rotating said housing assembly at a given rotational speed; a clutch plate assembly contained within said housing assembly; a fan shaft coupled to said clutch plate assembly; a fan coupled to said fan shaft; a pitot tube coupled within said housing assembly and receiving at least a portion of said hydraulic fluid from said fluid reservoir at said given rotational speed, wherein said portion of hydraulic fluid defines a static pressure within said pitot tube at said given rotational speed; a piston fluidically coupled with said pitot tube, said piston capable of moving towards and engaging said clutch plate assembly as a function of said static pressure, wherein the engagement of said clutch assembly causes said fan shaft and said fan to rotate in response; and a fluid controller fluidically coupled between said pitot tube and said fluid reservoir, said fluid controller using a binary control strategy to control an amount of an engagement of said clutch plate assembly between a fully engaged clutch position, at least two partially engaged clutch positions, and a disengaged clutch position; wherein said amount of said engagement controls the rotational speed of said fan used to maintain the engine at a desired operating temperature as a function of said given rotational speed.