Abstract:
An hybrid four wheel drive system characterized by a vehicle having a conventional rear wheel driveline as well as a conventional driving front steer axle powered by an hydraulic motor. The hybrid four wheel drive system is provided in multiple embodiments, particularly with and without an hydraulic pressure accumulator.

Description:
BACKGROUND OF INVENTION 
   This invention relates to a vehicle equipped with an hybrid four wheel drive system that is simpler to install than conventional four wheel drive systems, and may allow partial regenerative braking, depending on the configuration of the invention. Specifically, multiple embodiments are disclosed, all of which are characterized by a vehicle having a conventional rear wheel driveline as well as a conventional driving front axle. In lieu of a transfer case and front propeller shaft, as is often used in four wheel drive vehicles, the front axle is powered by an hydraulic motor. Pressurized hydraulic fluid is provided by an hydraulic pump driven by the vehicle main engine. 
   SUMMARY 
   Mobile vehicles, especially medium and heavy-duty commercial vehicles, are commonly configured with the engine located longitudinally forward of the cab and mounted to a set of frame rails, which form the structure of the vehicle. The engine is coupled to a transmission, which in turn provides power to one or more driving rear axles by means of a propeller shaft. For medium and heavy-duty vehicles having two wheel drive, the vehicle is typically provided with a simple front beam steering axle. For medium and heavy-duty vehicles having four wheel drive, or six wheel drive, as the case may be, the simple front beam axle is replaced by a conventional front drive steering axle. Power is provided to the conventional front drive steering axle by means of an additional propeller shaft, which is connected to a transfer case that is either driven by or integrated into the vehicle transmission. 
   There are several significant drawbacks to the typical four wheel drive configuration for medium and heavy-duty vehicles. The transfer case is heavy and consumes power, even when four wheel drive is not in use. It is prone to wear, and cannot be engaged during normal traction conditions, due to binding caused by rotational differential between front and rear axles. The front propeller shaft, located between the transfer case and the conventional front drive steering axle, passes through an area of the vehicle that is typically congested with hoses, tubes, and various pieces of vehicle hardware. Due to articulation of the vehicle front suspension, there is a large area surrounding the front propeller shaft that must remain clear of obstruction. 
   Currently, there is much prior art involving the use of hydraulic systems in vehicles to transmit power from the vehicle engine to the vehicle drive wheels. Many of these systems utilize an hydraulic pressure accumulator to store energy captured during vehicle braking. The presence of this hydraulic pressure accumulator in the prior art is due to a strong industry interest in increasing vehicle fuel efficiency, especially in conditions where the vehicle is required to start and stop repeatedly. 
   The invention disclosed herein provides an improvement over the prior art, while solving many problems associated with the typical four wheel drive configuration for medium and heavy-duty vehicles. Specifically, the invention retains the conventional front drive steering axle used on medium and heavy-duty four wheel drive vehicles. An hydraulic motor is coupled directly to the conventional front drive steering axle, and an hydraulic pump is mounted to the vehicle engine or Front Engine Accessory Drive (FEAD). Alternately, the hydraulic motor may be mounted to the vehicle frame, and coupled to the conventional front drive steering axle by a short propeller shaft. Also, the hydraulic pump may instead be coupled to a Power Take-Off (PTO) on the vehicle transmission. 
   In either case, the vehicle retains its conventional rear axle or axles, and drivetrain. In this way hydraulic hybrid four wheel drive may be implemented using conventional hardware, requiring very little modification to the design of a given medium or heavy-duty vehicle, while eliminating the bulky transfer case and front propeller shaft of the typical four wheel drive configuration. 
   In addition to the hydraulic motor and hydraulic pump, the invention provides an hydraulic reservoir of sufficient size to meet the requirements of the system, one or more control valves, and, optionally, an hydraulic pressure accumulator. The hydraulic motor, which is coupled to the conventional front drive steering axle, is capable not only of converting hydraulic power provided by the engine driven pump into rotational power that is then transmitted by the conventional front drive steering axle to the vehicle front wheels, but also of converting rotational power transmitted by the conventional front drive steering axle from the front wheels into hydraulic power that is then stored in the hydraulic pressure accumulator. 
   The one or more control valves direct pressurized hydraulic fluid between the engine driven hydraulic pump, the conventional front drive steering axle coupled hydraulic motor, and the hydraulic pressure accumulator depending on the demand conditions under which the vehicle is operating. These conditions will be referred to as “driving”, “braking”, “storing energy”, and “recovering energy”. Potentially, both “driving” and “storing energy” could take place concurrently. The same is true for “driving” and “recovering energy”, as well as “braking” and “storing energy”. 
   During “driving”, hydraulic pressure is directed from the engine driven hydraulic pump to the conventional front drive steering axle coupled hydraulic motor. The hydraulic pressure accumulator is isolated from the system, and little or no makeup volume is taken from the hydraulic reservoir. This condition occurs when the amount of power required by the conventional front drive steering axle is approximately equivalent to the amount of power provided by the engine driven hydraulic pump. 
   During “braking”, hydraulic pressure is directed from the conventional front drive steering axle coupled hydraulic motor to the hydraulic pressure accumulator. The engine driven hydraulic pump is isolated from the system, either by allowing fluid to recirculate to it, or by virtue of its being a variable displacement pump with its displacement set to zero. Makeup volume to replace the fluid pumped into the hydraulic pressure accumulator is provided by the hydraulic reservoir. This condition occurs when the conventional front drive steering axle is being used to help slow the vehicle, and when it is desired to store part of the vehicle kinetic energy as potential energy in the hydraulic pressure accumulator for later use in propelling the vehicle. 
   During “storing energy”, hydraulic pressure is directed from the engine driven hydraulic pump to the hydraulic pressure accumulator. The conventional front drive steering axle coupled hydraulic motor is isolated from the system by allowing fluid to recirculate to it. Makeup volume to replace the fluid pumped into the hydraulic pressure accumulator is provided by the hydraulic reservoir. This condition occurs when there is no demand for power at the front wheels, and when it is desired to store excess rotational energy being created by the vehicle engine as potential energy in the hydraulic pressure accumulator for later use in propelling the vehicle. 
   During “recovering energy”, hydraulic pressure is directed from the hydraulic pressure accumulator to the conventional front drive steering axle coupled hydraulic motor. The engine driven hydraulic pump is isolated from the system in the same way as during “braking”. Excess volume of hydraulic fluid created by the release of pressurized hydraulic fluid from the hydraulic pressure accumulator is returned to the hydraulic reservoir. This condition occurs when there is a demand for power at the front wheels, and when it is desired to return some of the potential energy stored in the hydraulic pressure accumulator to vehicle kinetic energy. 
   As noted previously, both “driving” and “storing energy” may take place concurrently. During this condition, hydraulic pressure is directed from the engine coupled hydraulic pump to both the conventional front drive steering axle coupled hydraulic motor and the hydraulic pressure accumulator. Makeup volume to replace the fluid pumped into the hydraulic pressure accumulator is provided by the hydraulic reservoir. This condition occurs when there is a demand for power at the front wheels, yet the engine is capable of producing more power than is needed to propel the vehicle. Some of this additional energy created by the vehicle engine is then stored as potential energy in the hydraulic pressure accumulator for later use in propelling the vehicle. 
   Both “driving” and “recovering energy” may also take place concurrently. During this condition, hydraulic pressure is directed both from the engine coupled hydraulic pump and from the hydraulic pressure accumulator to the conventional front drive steering axle coupled hydraulic motor. Excess volume of hydraulic fluid created by the release of pressurized hydraulic fluid from the hydraulic pressure accumulator is returned to the hydraulic reservoir. This condition occurs when there is a demand for more power at the front wheels than is being provided by the vehicle engine. In order to supplement the amount of power being provided by the vehicle engine, some of the potential energy stored in the hydraulic pressure accumulator is released into the system. 
   During the condition wherein both “braking” and “storing energy” are occurring, hydraulic pressure is directed both from the engine driven hydraulic pump and from the conventional front drive steering axle coupled hydraulic motor to the hydraulic pressure accumulator. Makeup volume to replace the fluid pumped into the hydraulic pressure accumulator is provided by the hydraulic reservoir. This condition occurs when the conventional front drive steering axle is being used to help slow the vehicle, and when it is desired to store energy being produced by the vehicle engine as potential energy in the hydraulic pressure accumulator for later use in propelling the vehicle. The power consumed by the engine driven hydraulic pump may assist in engine braking through the conventional drivetrain. 
   In the case of a system configured without an hydraulic pressure accumulator, among the aforementioned conditions, only “driving” may be accomplished. Some braking effect may be accomplished by controlling the displacement of the variable displacement engine driven pump, thereby transferring power back to the conventional drivetrain in order to enhance engine braking. 
   In any given configuration, means is provided for controlling the one or more control valves, and for coordinating flow of hydraulic pressure between system components. This means may be electronic or mechanical, or some combination thereof. It may also be automated, so that operation of the system is controlled automatically based on vehicle conditions and conventional vehicle operator inputs such as brake pedal or throttle actuation. Conversely, the control means may directly controllable by the vehicle operator. 
   The figures listed illustrate a vehicle with hydraulic hybrid four wheel drive. Both configurations with and without the hydraulic pressure accumulator are shown. Alternate configurations are shown for the conventional front drive steering axle coupled hydraulic motor, as well as for the engine driven hydraulic pump. Flow diagrams are given showing the various operating conditions, listed supra. 
   The invention as presented is a solution to the problem of providing an economical and simple four wheel drive for medium and heavy-duty commercial vehicles. It allows for installation of four wheel drive onto existing vehicles of this type, without significant revision of the basic vehicle geometry. Additionally, the invention is capable of at least partial regenerative braking, depending on its configuration. 

   
     DRAWINGS 
     FIG.  1 —A top view of a vehicle made in accordance with a first embodiment of the invention 
     FIG.  2 —A top view of a vehicle made in accordance with a second embodiment of the invention. 
     FIG.  3 —A top view of a vehicle made in accordance with a third embodiment of the invention. 
     FIG.  4 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under “driving” conditions. 
     FIG.  5 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under “braking” conditions. 
     FIG.  6 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under “storing energy” conditions. 
     FIG.  7 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under “recovering energy” conditions. 
     FIG.  8 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under a combination of “driving” and “storing energy”. 
     FIG.  9 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under a combination of “driving” and “recovering energy”. 
     FIG.  10 —A flow diagram of a vehicle made in accordance with the first, second, or third embodiments, operating under a combination of “braking” and “storing energy”. 
     FIG.  11 —A top view of a vehicle made in accordance with a fourth embodiment of the invention. 
     FIG.  12 —A flow diagram of a vehicle made in accordance with the fourth embodiment of the invention, operating under “driving” conditions. 
     FIG.  13 —A flow diagram of a vehicle made in accordance with the fourth embodiment of the invention, operating under “engine braking” conditions. 
   

   DETAILED DESCRIPTION 
   The vehicle  101  shown in  FIG. 1  has an engine  102  attached to a chassis  103 . The vehicle  101  also has at least one rear axle  104  and one conventional front drive steering axle  105  attached to chassis  103 . The rear axle  104  is provided with rear wheel and tire assemblies  106 , and the conventional front drive steering axle  105  is provided with front wheel and tire assemblies  107 . The engine  102  provides power to a transmission  108 , which in turn provides power to a propeller shaft  109 . The propeller shaft  109  thereby provides power to rear axle  104  and to rear wheel and tire assemblies  106 . 
   The engine  102  shown in  FIG. 1  is provided with a Front Engine Accessory Drive (FEAD)  110 , which provides power to an hydraulic pump  111  attached to it. The conventional front drive steering axle  105  is provided with an hydraulic motor  112 . The chassis  103  is further provided with an hydraulic pressure accumulator  113 , an hydraulic reservoir  114 , a control valve  115 , and a control means  140 . The hydraulic pump  111  is provided with an hydraulic pump inlet port  116  and an hydraulic pump outlet port  117 . The hydraulic motor  112  is further provided with an hydraulic motor inlet port  118  and an hydraulic motor outlet port  119 . The hydraulic pressure accumulator  113  is also provided with an hydraulic pressure accumulator port  120 , and the hydraulic reservoir  114  is provided with an hydraulic reservoir port  121 . The control valve  115  is provided with an hydraulic pump and reservoir supply port  122 , an hydraulic pump return port  123 , an hydraulic pressure accumulator supply port  124 , an hydraulic motor supply port  125 , and an hydraulic motor return port  126 . 
     FIG. 1  further shows an hydraulic pump and reservoir supply hose assembly  127  connected to the hydraulic pump and reservoir supply port  122 , to the hydraulic reservoir port  121 , and to the hydraulic pump inlet port  116 . An hydraulic pump return hose  128  is connected to the hydraulic pump outlet port  117  and to the hydraulic pump return port  123 . An hydraulic pressure accumulator supply hose  129  is connected to the hydraulic pressure accumulator supply port  124  and to the hydraulic pressure accumulator port  120 . An hydraulic motor supply hose  130  is connected to the hydraulic motor supply port  125  and to the hydraulic motor inlet port  118 . Finally, an hydraulic motor return hose  131  is connected to the hydraulic motor outlet port  119  and to the hydraulic motor return port  126 . Additionally, a line is shown extending from control means  140  to control valve  115 , representing the electronic or mechanical connection therebetween. 
   The vehicle  101  shown in  FIG. 2  has a similar chassis  103 , engine  102 , transmission  108 , propeller shaft  109 , rear axle  104 , rear wheel and tire assemblies  106 , conventional front drive steering axle  105 , front wheel and tire assemblies  107 , as the vehicle  101  shown in  FIG. 1 . An hydraulic motor  112 , having an hydraulic motor inlet port  118  and an hydraulic motor outlet port  119 , is again shown attached to the conventional front drive steering axle  105 . An hydraulic pump  111 , having hydraulic pump inlet port  116  and hydraulic pump outlet port  117 , is mounted to the chassis  103 , and driven by a transmission Power Take-Off (PTO)  132  by means of a Power Take-Off shaft  133 . The chassis  103  is again provided with an hydraulic pressure accumulator  113  having an hydraulic pressure accumulator port  120 , an hydraulic reservoir  114  having an hydraulic reservoir port  121 , a control valve  115 , and a control means  140 . The control valve  115  is provided with an hydraulic pump and reservoir supply port  122 , an hydraulic pump return port  123 , an hydraulic pressure accumulator supply port  124 , an hydraulic motor supply port  125 , and an hydraulic motor return port  126 . 
     FIG. 2  further shows an hydraulic pump and reservoir supply hose assembly  127  connected to the hydraulic pump and reservoir supply port  122 , to the hydraulic reservoir port  121 , and to the hydraulic pump inlet port  116 . An hydraulic pump return hose  128  is connected to the hydraulic pump outlet port  117  and to the hydraulic pump return port  123 . An hydraulic pressure accumulator supply hose  129  is connected to the hydraulic pressure accumulator supply port  124  and to the hydraulic pressure accumulator port  120 . An hydraulic motor supply hose  130  is connected to the hydraulic motor supply port  125  and to the hydraulic motor inlet port  118 . Finally, an hydraulic motor return hose  131  is connected to the hydraulic motor outlet port  119  and to the hydraulic motor return port  126 . Additionally, a line is shown extending from control means  140  to control valve  115 , representing the electronic or mechanical connection therebetween. 
   The vehicle  101  shown in  FIG. 3  has a similar chassis  103 , engine  102 , Front Engine Accessory Drive (FEAD)  110 , transmission  108 , propeller shaft  109 , rear axle  104 , rear wheel and tire assemblies  106 , conventional front drive steering axle  105 , front wheel and tire assemblies  107 , as the vehicle  101  shown in  FIG. 1 . An hydraulic motor  112 , having an hydraulic motor inlet port  118  and an hydraulic motor outlet port  119 , is shown attached to an hydraulic motor mounting bracket  134 , which in turn is attached to the chassis  103 . The hydraulic motor  112  drives the conventional front drive steering axle  105  by means of a front shaft  135 . An hydraulic pump  111 , having hydraulic pump inlet port  116  and hydraulic pump outlet port  117 , is again shown attached to and driven by the Front Engine Accessory Drive (FEAD)  110 . The chassis  103  is provided with an hydraulic pressure accumulator  113  having an hydraulic pressure accumulator port  120 , an hydraulic reservoir  114  having an hydraulic reservoir port  121 , a control valve  115 , and a control means  140 . The control valve  115  is again provided with an hydraulic pump and reservoir supply port  122 , an hydraulic pump return port  123 , an hydraulic pressure accumulator supply port  124 , an hydraulic motor supply port  125 , and an hydraulic motor return port  126 . 
     FIG. 3  further shows an hydraulic pump and reservoir supply hose assembly  127  connected to the hydraulic pump and reservoir supply port  122 , to the hydraulic reservoir port  121 , and to the hydraulic pump inlet port  116 . An hydraulic pump return hose  128  is connected to the hydraulic pump outlet port  117  and to the hydraulic pump return port  123 . An hydraulic pressure accumulator supply hose  129  is connected to the hydraulic pressure accumulator supply port  124  and to the hydraulic pressure accumulator port  120 . An hydraulic motor supply hose  130  is connected to the hydraulic motor supply port  125  and to the hydraulic motor inlet port  118 . Finally, an hydraulic motor return hose  131  is connected to the hydraulic motor outlet port  119  and to the hydraulic motor return port  126 . Additionally, a line is shown extending from control means  140  to control valve  115 , representing the electronic or mechanical connection therebetween. 
     FIG. 4  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are all represented by simple block symbols. The hydraulic pump  111  block symbol is shown with simplified representations of the hydraulic pump inlet port  116  and the hydraulic pump outlet port  117 . In the same way, the hydraulic motor  112  block symbol is shown with simplified representations of the hydraulic motor inlet port  118  and the hydraulic motor outlet port  119 , the hydraulic pressure accumulator  113  block symbol is shown with a simplified representation of the hydraulic pressure accumulator port  120 , the hydraulic reservoir  114  block symbol is shown with a simplified representation of the hydraulic reservoir port  121 , and the control valve  115  block symbol is shown with simplified representations of the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126 . 
   A line representing the hydraulic pump and reservoir supply hose assembly  127  is shown in  FIG. 4  connected to the simplified representations of the hydraulic pump and reservoir supply port  122 , the hydraulic reservoir port  121 , and the hydraulic pump inlet port  116 . Another line representing the hydraulic pump return hose  128  is shown connected to the simplified representations of the hydraulic pump outlet port  117  and the hydraulic pump return port  123 . Another line representing the hydraulic pressure accumulator supply hose  129  is shown connected to the simplified representations of the hydraulic pressure accumulator supply port  124  and the hydraulic pressure accumulator port  120 . Another line representing the hydraulic motor supply hose  130  is shown connected to the simplified representations of the hydraulic motor supply port  125  and the hydraulic motor inlet port  118 . Finally, a line representing the hydraulic motor return hose  131  is shown connected to the simplified representations of the hydraulic motor outlet port  119  and the hydraulic motor return port  126 . 
   In addition, the line in  FIG. 4  representing the hydraulic pump return hose  128  is shown emphasized with double arrows, representing high pressure flow and the direction thereof. In the same way, the line representing the hydraulic motor supply hose  130  is shown emphasized with double arrows. The lines representing the hydraulic motor return hose  131  and the hydraulic pump and reservoir supply hose assembly  127  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “driving” is disclosed. 
     FIG. 5  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure-accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 5  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic motor return hose  131  and the hydraulic pressure accumulator supply hose  129  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic motor supply hose  130  and the portion of the hydraulic pump and reservoir supply hose assembly  127  leading from the hydraulic reservoir port  121  to the hydraulic pump and reservoir supply port  122  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “braking” is disclosed. 
     FIG. 6  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 6  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic pump return hose  128  and the hydraulic pressure accumulator supply hose  129  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The line representing the portion of the hydraulic pump and reservoir supply hose assembly  127  leading from the hydraulic reservoir port  121  to the hydraulic pump inlet port  116  is shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “storing energy” is disclosed. 
     FIG. 7  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 7  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic pressure accumulator supply hose  129  and the hydraulic motor supply hose  130  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic motor return hose  131  and the portion of the hydraulic pump and reservoir supply hose assembly  127  leading from the hydraulic pump and reservoir supply port  122  to the hydraulic reservoir port  121  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “recovering energy” is disclosed. 
     FIG. 8  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 8  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , and the hydraulic motor supply hose  130  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic pump and reservoir supply hose assembly  127  and the hydraulic motor return hose  131  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the combination of operating conditions referred to previously as “driving” and “storing energy” is disclosed. 
     FIG. 9  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 9  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , and the hydraulic motor supply hose  130  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic pump and reservoir supply hose assembly  127  and the hydraulic motor return hose  131  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the combination of operating conditions referred to previously as “driving” and “recovering energy” is disclosed. 
     FIG. 10  is a plumbing diagram of the hydraulic system shown in  FIG. 1  through  FIG. 3 , similar to the plumbing diagram shown in  FIG. 4 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic pressure accumulator  113 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic pressure accumulator port  120 , the hydraulic reservoir port  121 , the hydraulic pump and reservoir supply port  122 , the hydraulic pump return port  123 , the hydraulic pressure accumulator supply port  124 , the hydraulic motor supply port  125 , and the hydraulic motor return port  126  are all shown as simplified representations. 
     FIG. 10  shows lines representing the hydraulic pump and reservoir supply hose assembly  127 , the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , the hydraulic motor supply hose  130 , and the hydraulic motor return hose  131 , in a configuration similar to the plumbing diagram in  FIG. 4 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic pump return hose  128 , the hydraulic pressure accumulator supply hose  129 , and the hydraulic motor return hose  131  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic pump and reservoir supply hose assembly  127  and the hydraulic motor supply hose  130  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the combination of operating conditions referred to previously as “braking” and “storing energy” is disclosed. 
   The vehicle  101  shown in  FIG. 11  has a similar chassis  103 , engine  102 , transmission  108 , propeller shaft  109 , rear axle  104 , rear wheel and tire assemblies  106 , conventional front drive steering axle  105 , front wheel and tire assemblies  107  as the vehicle  101  shown in  FIG. 1 . The engine  102  shown in  FIG. 11  is again provided with a Front Engine Accessory Drive (FEAD)  110 , which provides power to an hydraulic pump  111  attached to it. The conventional front drive steering axle  105  is again provided with an hydraulic motor  112 . The chassis  103  is further provided with an hydraulic reservoir  114 , a control valve  115 , and a control means  140 . The hydraulic pump  111  is provided with an hydraulic pump inlet port  116  and an hydraulic pump outlet port  117 . The hydraulic motor  112  is further provided with an hydraulic motor inlet port  118  and an hydraulic motor outlet port  119 . The hydraulic reservoir  114  is provided with an hydraulic reservoir port  121 . The control valve  115  is provided with an hydraulic pump supply port  137 , an hydraulic pump return port  123 , an hydraulic motor supply port  125 , an hydraulic motor return port  126 , and an hydraulic reservoir supply port  136 . 
     FIG. 11  further shows an hydraulic pump supply hose  138  connected to the hydraulic pump supply port  137  and to the hydraulic pump inlet port  116 . An hydraulic pump return hose  128  is connected to the hydraulic pump outlet port  117  and to the hydraulic pump return port  123 . An hydraulic motor supply hose  130  is connected to the hydraulic motor supply port  125  and to the hydraulic motor inlet port  118 . An hydraulic motor return hose  131  is connected to the hydraulic motor outlet port  119  and to the hydraulic motor return port  126 . Finally, an hydraulic reservoir supply hose  139  is connected to the hydraulic reservoir supply port  136  and to the hydraulic reservoir port  121 . Additionally, a line is shown extending from control means  140  to control valve  115 , representing the electronic or mechanical connection therebetween. 
     FIG. 12  is a plumbing diagram of the hydraulic system shown in  FIG. 11 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic reservoir  114 , and the control valve  115  are all represented by simple block symbols. The hydraulic pump  111  block symbol is shown with simplified representations of the hydraulic pump inlet port  116  and the hydraulic pump outlet port  117 . In the same way, the hydraulic motor  112  block symbol is shown with simplified representations of the hydraulic motor inlet port  118  and the hydraulic motor outlet port  119 , the hydraulic reservoir  114  block symbol is shown with a simplified representation of the hydraulic reservoir port  121 , and the control valve  115  block symbol is shown with simplified representations of the hydraulic pump supply port  137 , the hydraulic pump return port  123 , the hydraulic motor supply port  125 , the hydraulic motor return port  126 , and the hydraulic reservoir supply port  136 . 
   A line representing the hydraulic pump supply hose  138  is shown in  FIG. 12  connected to the simplified representations of the hydraulic pump supply port  137  and the hydraulic pump inlet port  116 . Another line representing the hydraulic pump return hose  128  is shown connected to the simplified representations of the hydraulic pump outlet port  117  and the hydraulic pump return port  123 . Another line representing the hydraulic motor supply hose  130  is shown connected to the simplified representations of the hydraulic motor supply port  125  and the hydraulic motor inlet port  118 . Another line representing the hydraulic motor return hose  131  is shown connected to the simplified representations of the hydraulic motor outlet port  119  and the hydraulic motor return port  126 . Finally, a line representing the hydraulic reservoir supply hose  139  is shown connected to the simplified representations of the hydraulic reservoir supply port  136  and the hydraulic reservoir port  121 . 
   In addition, the lines in  FIG. 12  representing the hydraulic pump return hose  128  and the hydraulic motor supply hose  130  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. In the same way, the lines representing the hydraulic motor return hose  131 , the hydraulic pump supply hose  138 , and the hydraulic reservoir supply hose  139  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “driving” is disclosed. 
     FIG. 13  is a plumbing diagram of the hydraulic system shown in  FIG. 11 , similar to the plumbing diagram shown in  FIG. 12 . The hydraulic pump  111 , the hydraulic motor  112 , the hydraulic reservoir  114 , and the control valve  115  are again represented by simple block symbols. The hydraulic pump inlet port  116 , the hydraulic pump outlet port  117 , the hydraulic motor inlet port  118 , the hydraulic motor outlet port  119 , the hydraulic reservoir port  121 , the hydraulic pump supply port  137 , the hydraulic pump return port  123 , the hydraulic motor supply port  125 , the hydraulic motor return port  126 , and the hydraulic reservoir supply port  137  are all shown as simplified representations. 
     FIG. 13  shows lines representing the hydraulic pump supply hose  138 , the hydraulic pump return hose  128 , the hydraulic motor supply hose  130 , the hydraulic motor return hose  131 , and the hydraulic reservoir supply hose  139 , in a configuration similar to the plumbing diagram in  FIG. 12 . Each representative line is shown connected to the simplified representations of the appropriate ports. The lines representing the hydraulic motor return hose  131  and the hydraulic pump supply hose  138  are shown emphasized with double arrows, representing high pressure flow and the direction thereof. The lines representing the hydraulic pump return hose  128 , the hydraulic motor supply hose  130 , and the hydraulic reservoir supply hose  139  are shown emphasized with single arrows, representing low pressure flow and the direction thereof. In this manner, the functioning of control valve  115  during the operating condition referred to previously as “engine braking” is disclosed. 
   Other permutations of the invention are possible without departing from the teachings disclosed herein, provided that the function of the invention is to provide a simple installation of an hybrid four wheel drive system by utilizing a conventional front drive steering driven by an hydraulic motor. Other advantages to a vehicle  101  equipped with an hybrid four wheel drive system may also be inherent in the invention, without having been described above.