Abstract:
A hydraulic power steering system for a diesel-fueled truck utilizes vehicle fuel as a working fluid. The steering system includes a power steering gear connected to the steerable wheels of the truck. A first pump flows fuel from the truck&#39;s fuel tank to both the steering gear and to the engine. A second pump increases the pressure of the fluid to the steering gear to provide power assist. The flow to the steering gear returns to the fuel tank for recirculation.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to a hydraulic power steering system for turning the steerable wheels of a vehicle. 
   Heavy-duty trucks use a hydraulic power steering system to provide power assist in turning the steerable wheels of the truck. 
   A conventional hydraulic power steering system includes a pump that flows high-pressure working fluid to a fluid motor. Power steering fluid is utilized as the working fluid. Power steering fluid is a specialized hydraulic oil supplied from a reservoir located within the engine compartment. The fluid is continuously recirculated through the system and may be replaced at regularly-scheduled maintenance intervals. 
   A major concern with hydraulic power steering systems is heat. The power steering reservoir is heated by its close proximity to the engine. If overheated power steering fluid is circulated through the steering system, pressure seals in the system may be damaged. Replacing damaged seals increases maintenance expenses and warranty costs. 
   As air pollution requirements for trucks become increasingly restrictive, engine temperatures are increasing. It is becoming more difficult to place the power steering reservoir in a location that provides adequate cooling. Furthermore, as the engine compartment becomes more crowded, finding a suitable location for the reservoir becomes even more difficult. 
   Therefore there is a need for an improved hydraulic power steering system that provides sufficient cooling of the working fluid utilized in the power steering system. 
   SUMMARY OF THE INVENTION 
   The present invention is an improved hydraulic power steering system that utilizes vehicle fuel as the working fluid. The fuel is preferably stored in a conventional fuel tank outside of the engine compartment. The fuel tank effectively cools the fuel and eliminates overheating. Oil coolers, water coolers, and extra hosing often used to provide additional cooling in conventional power steering systems are eliminated. 
   A hydraulic power steering system in accordance with the present invention includes a power steering gear with a fluid motor to turn the steerable wheels of the vehicle. A control valve actuates the fluid motor in response to steering wheel position. A supply line connects the power steering gear with the fuel tank and flows fuel from the fuel tank to the power steering gear. 
   A return line returns the fuel from the steering gear to the fuel tank. A truck fuel tank is much larger than a conventional power steering fluid reservoir. The tank is located outside of the engine compartment, exposed to the wind stream generated by truck movement. The size and location of the fuel tank enables the tank to function as an effective heat sink or radiator that cools fuel stored in the tank. 
   In a preferred embodiment of the invention the hydraulic power steering system is integrated with the fuel system. A fuel pump flows fuel from the fuel tank to both the engine and the power steering gear. The flow from the fuel pump is divided or bifurcated into a first part flowing to the engine for combustion and a second part flowing to the power steering gear. A power steering pump further pressurizes the second part of the flow to the operating pressure of the fluid motor. 
   In addition to eliminating overheating and the need to locate a steering fluid reservoir within the engine compartment, power steering systems in accordance with the present invention have a number of additional advantages. The cost of the power steering supply reservoir is eliminated. The cost of power steering fluid is also eliminated, as is the need to change power steering fluid and dispose of the used fluid in an environmentally responsible manner. Each time the fuel tank is filled, fresh working fluid for the power steering system is also provided. 
   Additionally, the power steering pump can be designed to operate more efficiently than conventional power steering pumps. Conventional power steering pumps must be capable of operating with negative head pressures. By flowing pressurized fluid to the power steering pump, the power steering pump can be optimized to operate with only positive head pressures. 
   Other objects and features of the invention will become apparent as the description proceeds, especially when taken in conjunction with the accompanying 3 drawing sheets illustrating an embodiment of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is representational view of a hydraulic power steering system in accordance with the present invention; 
       FIG. 2  is a representational view of a truck equipped with the hydraulic power steering system shown in  FIG. 1 ; and 
       FIG. 3  is a representational sectional view of the rotary control valve shown in  FIG. 1 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate a hydraulic power steering system  10  in accordance with the present invention. Power steering system  10  turns the steerable wheels  12  of a heavy duty, over-the-road truck  14 . Truck  14  is equipped with a diesel engine  16  located within engine compartment  18 . A fuel tank  20  outside of the engine compartment contains a reservoir of diesel fuel  22 . The capacity of fuel tank  20  is 300 gallons, a conventional size for a large truck. 
   Fuel system  24  flows fuel from fuel tank  20  to engine  16 . In the illustrated embodiment fuel system  24  includes a conventional common rail fuel injection system  26  that injects fuel into the engine for combustion. An accumulator  28  serves as a common rail, receiving fuel and forming a common supply for a number of fuel injectors  30 . The injectors are actuated by fluid from a supply reservoir  32  (flow to only one injector is shown in  FIG. 1 ) pressurized by a high-pressure injection pump  34 . The actuator fluid can be diesel fuel  22 , in which case reservoir  32  can be fuel tank  20 . Alternatively the actuator fluid can be engine oil, in which case reservoir  32  is an oil reservoir. A drain line  36  returns unburned fuel to fuel tank  20 . 
   Hydraulic power steering system  10  includes a fluid supply system  38  that flows diesel fuel from fuel tank  20  to a power steering gear  40  operatively connected to wheels  12 . The fuel is used as a working fluid in steering gear  40  as will be described in greater detail below. 
   Fluid supply system  38  includes a fuel pump  42  connected between a pump inlet line  44  and a pump discharge line  46 . Inlet line  44  fluidly connects the pump  42  and fuel tank  20 . Fuel filter  48  is located in the inlet line. Additional fuel filters can be provided in series or in parallel with fuel filter  48 . 
   Fuel pump  42 , lines  44  and  46 , and fuel filter  48  are shared in common with fuel supply  24 . Pump flow bifurcates at a flow junction  50  at the end of discharge line  46 , the flow dividing into a fuel flow to the engine through fuel supply line  52  and a power steering flow to the power steering gear through power steering line  54 . Fuel pump  42  is sized to meet the combined flow requirements of power steering gear  40  and fuel injection system  26 . 
   Power steering system  10  includes a power steering pump  56  that receives pressurized fuel from fuel pump  42  via power steering line  54 . Power steering pump  56  increases the pressure of the fuel discharged from fuel pump  42  to the working pressure required for actuating the steering gear. Power steering pump  56  is preferably a vane pump, but other types of pumps can be used. 
   A discharge line  58  flows high-pressure fuel from the power steering pump  56  to a control valve  60 . Control valve  60  regulates flow to a fluid motor  62  in response to steering wheel input. Fluid motor  62  drives a movable output member  64  in a conventional manner to turn the tires. Output member  64  can be a Pitman arm, a rack, or other conventional output member. Motor lines  66 ,  68  flow fuel between the control valve  60  and the left and right motor chambers of fluid motor  62 . A return line  70  returns the exhaust flow from the control valve  60  to the fuel tank  20 . 
   Power steering pump  56  is preferably modified from a conventional power steering pump that pumps power steering fluid. The modifications optimize performance of pump  56  with diesel fuel. 
   Power steering fluid has a viscosity of about 60 centiStokes at 15 degrees Centigrade, and diesel fuel has a viscosity of about 5 centiStokes at 15 degrees Centigrade. Pump clearances are preferably modified to accommodate the reduced viscosity of diesel fuel. 
   Conventional power steering pumps have seals formed from a nitrile base formulated for use with power steering fluid. Diesel fuel pumps also typically have seals formed from a nitrile base, but the base is formulated for use with diesel fuel. Pump  56  has nitrile-based seals similar to those in diesel fuel pumps. 
   Control valve  60  is also preferably optimized for use with diesel fuel.  FIG. 3  illustrates control valve  60 . Valve  60  is a rotary control valve that includes an outer valve sleeve  72  and a valve core  74  rotatably mounted within sleeve  72 . Rotation of valve core  74  from a neutral position to the position shown in  FIG. 3  closes orifices  76  and flows high-pressure diesel fuel to the appropriate fluid motor chamber to turn the wheels left or right. Operation of control valve  60  is conventional and so will not be described in further detail. 
   Orifices  76  are modified from a conventional control valve that flows power steering fluid. Orifices  76  are sized to accommodate the reduced viscosity of diesel fuel. Orifice size can be determined using the same engineering methods used for conventional control valves that flow power steering fluid, but using the fluid properties of diesel fuel instead of power steering fluid. 
   Other rotary control valves used in power steering systems are known and can be readily adapted for use in the present invention. Axial control valves are also known whose orifice sizes can be modified in a similar manner for use in the present invention. 
   Power steering system  10  is an open-center system in which working fluid flows through the control valve  60  when the control valve is in the neutral position for straight-ahead steering. Closed-center power steering systems are also known in which the working fluid does not flow through the control valve during straight-ahead steering. A closed-center system can be readily adapted in accordance with the present invention to use fuel as working fluid. 
   Engine  16  drives fuel pump  42  and power steering pump  56  through drive shafts  78  and  80  (see  FIG. 1 ). Fuel pump  42  discharges fuel at a discharge pressure of about 75 pounds per square inch. The pump flows approximately 4 to 8 gallons per minute flows to power steering pump  56  and the remainder flows to engine  16 . Power steering pump  56  further pressurizes the fuel to up to about 3000 pounds per square inch to drive fluid motor  62 . 
   Operation of the power steering gear  40  is conventional. Control valve  60  is mechanically connected to the truck&#39;s steering column (not shown) to open and close the valve. Other types of control valve arrangements are known and can be adapted for use in the present invention, including those in which the control valve is opened and closed by a motor in response to a data signal representing desired wheel orientation. 
   Return line  70  returns diesel fuel from control valve  60  to the fuel tank  20 . Fuel tank  20  is exposed to the air stream generated by the truck&#39;s motion. The size and location of fuel tank  20  enables the tank to function as an effective heat sink or radiator, cooling diesel fuel returned to the tank. Any heating of the fuel caused by flow through the pumps and steering gear is effectively removed before the fuel recirculates through the power steering system, thereby avoiding overheating of working fluid. 
   Engine  16  operates until the fuel tank  20  runs dry. When the engine stops from lack of fuel, pumps  42  and  56  also stop. Power assist is lost in the same manner that power assist is lost in a conventional hydraulic power steering system employing an engine-driven power-steering pump. 
   Fuel tank  20  is periodically refilled with diesel fuel, providing fresh working fluid for the power steering system without the necessity of disposing old power steering fluid. 
   While the illustrated embodiment illustrates use of the invention with a diesel-powered truck, it is understood that a power steering system in accordance with the present invention can be adapted for use with other types of vehicles and other types of fuels. 
   While I have illustrated and described a preferred embodiment of my invention, it is understood that this is capable of modification, and I therefore do not wish to be limited to the precise details set forth, but desire to avail myself of such changes and alterations as fall within the purview of the following claims.