Patent Publication Number: US-2006014608-A1

Title: Continuous variable control methods for hydraulic powertrain systems of a vehicle

Description:
RELATED APPLICATION  
      The present application claims priority to U.S. Provisional Application Ser. No. 60/587,575, filed Jul. 13, 2004, entitled “Energy Optimization of a System”, which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD  
      The present invention relates to vehicle, hybrid, and hydraulic drive powertrain control systems. More particularly, the present invention is related to the efficient and simultaneous control of a vehicle engine and a continuous variable transmission, which may include hydraulic pumps and hydraulic drive motors.  
     BACKGROUND OF THE INVENTION  
      Conventional powertrains operate with significant energy loss, produce significant emissions, and have limited potential in fuel economy improvement. Much of the energy loss is due to a poor match between engine power capacity and average power demand. The load placed on the engine at any given instant is directly determined by the total road load at that instant, which varies between high and low load. To meet acceleration requirements, the engine must be more powerful than the average power required to propel the vehicle. The efficiency of an internal combustion engine varies significantly with load, being best under high loading and worst under low loading. Since engine operation experienced in normal driving is nearly always at the low end of the spectrum, the engine typically operates inefficiently.  
      Hybrid vehicle systems have been investigated as a means to mitigate the foregoing inefficiencies. A hybrid vehicle system provides a “buffer” between the power required to propel the vehicle and the power produced by the internal combustion engine in order to moderate the variation of power demand experienced by the engine. The effectiveness of a hybrid vehicle system depends on its ability to operate the engine at peak efficiencies and on the capacity and efficiency of the buffer medium. Typical buffer media include electric batteries, mechanical flywheels and hydraulic accumulators.  
      Although hybrid vehicle systems have provided some improvement in operating efficiencies, there is an opportunity for further improvement. Thus, there exists a need for a powertrain system having improved efficiency and thus fuel economy that is feasible for various vehicle applications.  
     SUMMARY OF THE INVENTION  
      One embodiment of the present invention provides a method of controlling a powertrain of a vehicle. The method includes the generation of an engine torque versus engine revolutions per minute (RPM) reference for an engine. A current engine speed is determined. A fuel input signal and a continuous variable transmission control signal are generated in response to the engine torque versus engine RPM reference and the current engine speed to maintain an approximately constant engine speed for various engine loading conditions.  
      Another embodiment of the present invention provides a powertrain system control circuit that includes a memory with a stored engine torque versus engine rpm reference. An engine speed sensor generates an engine speed signal. A controller is coupled to the memory and the engine speed sensor and generates a fuel input signal and a continuous variable transmission control signal in response to the engine torque versus engine rpm reference and the engine speed signal to maintain an approximately constant engine speed for various engine loading conditions.  
      The embodiments of the present invention provide several advantages. One such advantage is the provision of maintaining a constant engine RPM for various loading conditions. This allows for an engine to be operated at a low RPM for multiple loading conditions and to provide efficient fuel consumption.  
      Another advantage provided by an embodiment of the present invention is the provision of maintaining operation of an engine at a maximum load for a predetermined and constant RPM for multiple loading conditions. In doing so, the stated embodiment continuously maintains the engine operating at a peak fuel efficiency level during both low and high loading conditions.  
      The present invention itself, together with further objects and attendant advantages, will be best understood by reference to the following detailed description, taken in conjunction with the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of this invention reference should now be had to the embodiments illustrated in greater detail in the accompanying figures and described below by way of examples of the invention wherein:  
       FIG. 1  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system in accordance with an embodiment of the present invention.  
       FIG. 2  is a schematic and block diagrammatic view of the air injection portion of the powertrain system of  FIG. 1 .  
       FIG. 3  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a sample single turbocharger configuration in accordance with another embodiment of the present invention.  
       FIG. 4  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a sample single supercharger configuration in accordance with yet another embodiment of the present invention.  
       FIG. 5  is a logic flow diagram illustrating a method of operating a vehicle hydraulic powertrain system in accordance with an embodiment of the present invention.  
       FIG. 6  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a non-gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 7  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a four-wheel drive configuration in accordance with another embodiment of the present invention.  
       FIG. 8  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a dual axle non-gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 9  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a rear dual axle gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 10  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a six-wheel drive gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 11  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a six-wheel drive gearset/non-gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 12  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system illustrating a six-wheel drive non-gearset configuration in accordance with another embodiment of the present invention.  
       FIG. 13  is a schematic and block diagrammatic view of a vehicle hydraulic powertrain system incorporating a multi-input powered gearset.  
       FIG. 14  is a block diagrammatic and schematic view of a powertrain system in accordance with another embodiment of the present invention.  
       FIG. 15  is a block diagrammatic and schematic view of a powertrain control circuit in accordance with another embodiment of the present invention.  
       FIG. 16  is a logic flow diagram illustrating a method of controlling a powertrain of a vehicle in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION  
      The present invention is disclosed herein primarily in the context of a roadway vehicle such as a truck equipped with a continuously variable hydrostatic drive. However, it will be understood that the invention is also useful both in other vehicular applications and in non-vehicular applications such as power generation stations.  
      In the following description, various operating parameters and components are described for one constructed embodiment. These specific parameters and components are included as examples and are not meant to be limiting.  
      The present invention includes an engine, such as a turbocharged diesel engine, in which a high flow of above-atmospheric pressure air is injected into the engine exhaust manifold at distributed locations to simultaneously improve engine power output, exhaust emissions and fuel efficiency. In a sample embodiment, the injected air is provided by a supercharger, at a flow rate of approximately 100-250 cubic feet per minute (CFM). The injected air provides greatly increased exhaust airflow at low engine speeds to dramatically increase the turbocharger boost pressure, which increases engine power output. Improved low speed power output is beneficial in nearly any application including applications, such as a vehicle hydrostatic drive applications, in which the engine is operated at a low and substantially constant speed. The engine exhaust emissions are improved because the injected air: (1) reduces the gas temperature in the exhaust manifold well below the temperature at which NOx emissions are formed; (2) promotes more complete combustion of the air/fuel mixture in the engine to reduce soot; and (3) promotes secondary combustion in the exhaust manifold to reduce other exhaust emissions such as carbon monoxide (CO) and hydrocarbons (HC). The reduction of exhaust emissions through secondary combustion, in turn, allows the engine air fuel ratio to be operated closer to the ideal stoichiometric air/fuel ratio for improved thermodynamic efficiency. The engine fuel efficiency is further improved in constant speed applications, such as in continuously variable hydrostatic drive applications, where losses associated with the acceleration and the deceleration of the engine is minimized.  
      Referring now to  FIG. 1 , the reference numeral  10  generally designates a hydraulic powertrain system that includes an engine (ENG)  12  and a hydrostatic drive  14 . The engine  12  may be in the form of a diesel engine, a combustion engine, a hydraulic engine, an electric engine, or other engines or motors known in the art. The hydrostatic drive  14  couples the power output of the engine  12  to a drive arrangement that includes a driveshaft  16 , a differential gearset (DG)  18 , drive axles  20 ,  22  and drive wheels  24 ,  26 .  
      The hydrostatic drive  14  primarily includes a variable capacity main hydraulic pump (HP)  28  that is driven by the engine  12 , a hydraulic drive motor (DM)  30  is coupled to the driveshaft  16 , and to a hydraulic valve assembly (HVA)  32 . The DM  30  includes two or more hydraulic motors that are ganged together. The ganging of the motors to each other and the coupling of the motors between the DG  18  and the HP  28  provides efficient energy transfer to the drive axles  20 ,  22 . The hydraulic motors may be in a dual arrangement, a tandem arrangement, or in a sequencing arrangement. A dual arrangement refers to the use of two hydraulic motors as primarily described herein. A tandem arrangement refers to the direct coupling of the hydraulic motors in series. A sequencing arrangement refers to the ability to select one or more of the hydraulic motors for operation in any combination and the ability to control the timing thereof.  
      In one embodiment, the DM  30  includes a first drive motor  31  and a second drive motor  33  that are ganged together in series without use of a gearset. The PCM  42  may control the timing between the drive motors  31 ,  33  relative to each other to provide efficient coupling therebetween and to prevent undesired harmonic generation due to improper synchronization. The first drive motor  31  is mounted to the second drive motor  33  via an adaptor block  35 . The first drive motor  31  is configured and designed for high torque, low speed operation, while the second drive motor  33  is designed for low torque, high speed operation. The drive motors  31 ,  33  may be operated separately or in combination, such as to provide increased torque at low speeds or when starting from rest or from a zero velocity state. The drive motors  31 ,  33  may be controlled electronically and/or in response to hydraulic fluid received therefrom. The drive motors may be variable displacement motors.  
      In a sample embodiment of the present invention, a first drive motor operates in response to an electrical signal received from a controller internal or external to the DM  30  and a second drive motor operates in response to hydraulic fluid received from the first drive motor. The electrical signal may be generated in response to engine speed, throttle position, and vehicle speed. The controller may be the below described PCM  42 , may be part of the DM  30 , or may be some other vehicle controller. The engine speed, throttle position, and vehicle speed may be acquired from the sensors  61 , also described below. Each drive motor within the DM  30  may have an associated controller for controlling displacement thereof.  
      In another sample embodiment, a first drive motor is operated continuously throughout translation of the corresponding vehicle, such as during both low-speed and high-speed operation, and a second drive motor is selectively operated as desired. This provides increased torque at “take-off” or low speeds when under increased load. This minimizes the amount of activation and deactivation of drive motors and provides desired fuel efficiency.  
      In general, the HP  28  supplies fluid to the DM  30  by way of HVA  32 , while directing a portion of the fluid to a reservoir  34 . Note that the DM  30  is not supplied by high-pressure hydraulic fluid stored within a high-pressure accumulator. The hydraulic powertrain system  10  in not using a high-pressure accumulator provides an efficient hydraulic powertrain system that is lighter and can provide improved fuel efficiency. High-pressure hydraulic fluid stored in a high-pressure accumulator is generally or approximately at a fluid pressure greater than 1000 psi. The HP  28 , the DM  30 , and the HVA  32  are operated by the powertrain control module (PCM)  42 . The combination of the HP  28 , the HVA  32 , the DM  30 , and the PCM  42  may be referred to as a hydrostatic continuously variable transmission. The HVA  32  includes a number of solenoid-operated valves that are selectively energized or deenergized to control fluid flow.  
      The reservoir  34  is a low-pressure reservoir and is used to store and hold hydraulic fluid. The hydraulic fluid within the reservoir  34  is at a pressure of approximately less than 100 psi. The reservoir  34  may be a single reservoir as shown or may be divided up into multiple stand-alone reservoirs that may be in various vehicle locations. An example dual reservoir system is shown with respect to the embodiment of  FIG. 3  in which a first reservoir  34   a  and a second reservoir  34   b  are shown.  
      The PCM  42  is powered by a vehicle storage battery  44 , and may include a micro-controller for carrying out a prescribed control of the DM  30  and the HVA  32 . The PCM  42  is also coupled to hydraulic pump  28  for controlling its pumping capacity, and to an engine fuel controller (EFC)  48  for controlling the quantity of fuel injected into the cylinders (not shown) of the engine  12 . In a particularly advantageous mechanization, PCM  42  controls the capacity of hydraulic pump  28  to satisfy the vehicle drive requirements, while controlling EFC  48  to maintain a low and substantially constant engine speed such as 1000 RPM. The PCM  42  may control the HP  28  and the DM  30  independently, individually, simultaneously, or otherwise to provide a desired or predetermined torque output for a given engine speed for desired traction of the wheels  24 ,  26 .  
      The PCM  42  and the EFC  48  may be microprocessor based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The PCM  42  and the EFC  48  may be application-specific integrated circuits or may be formed of other logic devices known in the art. The PCM  42  and the EFC  48  may be a portion of a central vehicle main control unit, an interactive vehicle dynamics module, a control circuit having a power supply, may be combined into a single integrated controller, or may be stand-alone controllers as shown.  
      The PCM  42  continuously monitors various inputs of the engine  12 , the HP  28 , and the DM  30  including the speed and torque of the engine  12  and the hydrostatic transmission  14  to electronically manage and simultaneously operate the powertrain system  10  using the lowest energy input. The PCM  42  controls several outputs in response to the inputs including fuel input of the engine  12 , displacement of the HP  28 , displacement of the DM  30 , efficiency curve information, percent engine load, accelerator pedal position, pressures of the HP  28  and DM  30 , as well as other various parameters of the powertrain system  10 . It is desired that the engine  12  operate at a maximum engine load for a given rpm. The HP  28  and the DM  30  are efficient at their maximum swash plate positions and at desired pressure ranges. The PCM  42  provides such control to achieve desired efficiencies. The configuration of the powertrain system  10 , the components utilized therein, and the control methodology provided within the PCM  42  allow for efficient system operation at start, stop, and through various drive modes that allow for the non-use of a high-pressure accumulator.  
      The hydrostatic drive  14  additionally includes first and second charge pumps (CP)  52 ,  54  that are ganged together with the HP  28 . The charge pumps  52 ,  54  are driven by the engine  12 . The first charge pump  52  supplies control pressure to HP  28  and DM  30  from reservoir  34 , and the second charge pump  54  supplies hydraulic fluid from reservoir  34  to an auxiliary hydraulic drive motor (ADM)  56 , described below. The charge pumps supply hydraulic fluid at moderate pressures approximately between 100-1000 psi. The charge pumps  52 ,  54  prevent cavitation of and maintain low friction operation of the HP  28 , the DM  30 , and the ADM  56 . Although two charge pumps are shown any number of charge pumps may be utilized.  
      The PCM  42  is also coupled to a display  57 , which may be operated via a display controller  59 , and to sensors  61  and memory  63 . The display  57  may be used to indicate to a vehicle operator system pressures, temperatures, maintenance information, warnings, diagnostics, and other system related information. The maintenance information may, for example, include oil life, filter life, pump performance parameters, hydraulic motor performance parameters, engine performance parameters, and other maintenance related information. The display  57  and the display controller  59  may also indicate or provide data logging and historical data for diagnostics including system pressure, system temperature, oil life, maintenance schedule information, system warnings, as well as other logging and historical data.  
      The display controller  59  displays the stated information in response to data received from the sensors  61  or retrieved from the memory  63 . The memory  63  may store the above stated information, as well as other vehicle systems related information known in the art. The memory  63  may be in the form of RAM and/or ROM, may be an integral portion of the PCM  42  or the display controller  59 , may be in the form of a portable or removable memory, and may be accessed using techniques known in the art.  
      The display may be in the form of one or more indicators such as LEDs, light sources, audio generating devices, or other known indicators. The display may also be in the form of a video system, an audio system, a heads-up display, a flat-panel display, a liquid crystal display, a telematic system, a touch screen, or other display known in the art. In one embodiment of the present invention, the display  57  is in the form of a heads-up display and the indication signal is a virtual image projection that may be easily seen by the vehicle operator. The display  57  provides real-time image system status information without having to refocus ones eyes to monitor a display screen within the vehicle.  
      The display controller  59  may, for example, be in the form of switches or a touch pad and be separate from the display  57 , as shown. The display controller  59  may be an integral part of the display  57  and be in the form of a touch screen or other display controller known in the art. The display controller  59  may also be microprocessor based such as a computer having a central processing unit, memory (RAM and/or ROM), and associated input and output buses. The display controller  59  may be application-specific integrated circuits or may be formed of other logic devices known in the art. The display controller  59  may be a portion of a central vehicle main control unit, such as the PCM  42 , an interactive vehicle dynamics module, a control circuit having a power supply, may be combined into a single integrated controller, or may be a stand-alone controller as shown.  
      The sensors  61  may include pressure sensors, temperature sensors, oil sensors, flow rate sensors, position sensors, engine speed sensors, vehicle speed sensors, throttle position sensors, as well as other vehicle system sensors known in the art. In one embodiment of the present invention a pressure sensor, a temperature sensor, and a flow rate sensor are used to indicate the pressure, temperature, and flow rate of the hydraulic fluid received by the DM  30 .  
      The hydrostatic system  14  may also include a heat exchanger  65  for cooling of the hydraulic fluid within return line  67 . Cooling of the hydraulic fluid aids in providing efficient operation of the hydrostatic system  14  and increases operating life of the components and devices contained therein. The heat exchanger  65  may be of various types and styles and may be located in various locations within a vehicle. The heat exchanger  65  may be in the form of an air-to-oil heat exchanger or a liquid-to-oil heat exchanger. Thus, the heat exchanger may be cooled by air and/or by a liquid coolant, such as water, propylene glycol, or other coolant or a combination thereof. The heat exchanger  65  may be associated solely with the cooling of hydraulic fluid within the return line  67  or may be used for cooling of other fluids. In one embodiment of the present invention, the heat exchanger  65  is shared and is used to cool hydraulic fluid within the hydrostatic system  14 , as well as oil within the engine  12 . The heat exchanger  65  may be in the form of a radiator and may be cooled by a fan (not shown).  
      The hydrostatic system  14  may further include particulate filters with various pressure ratings. In the embodiment shown a low-pressure return line filter  69  is coupled between the reservoir  34  and the heat exchanger  65  and is used to filter the hydraulic fluid in return line  67 . Charge pump filters  71  are coupled between the charge pumps  52 ,  54  and the HP  28 , the DM  30 , and the ADM  56 , respectively, and are used to filter hydraulic fluid entering the HP  28 , the DM  30 , and the ADM  56 . The charge pump filters  71  are rated for higher fluid pressures than that of the low-pressure filter  69 . Although a specific number of filters are shown, any number of filters may be utilized.  
      Referring now also to  FIG. 2 , the engine  12  includes an intake manifold  12   a  that receives intake air. An exhaust manifold  12   b  collects the engine cylinder exhaust gases.  FIG. 2  illustrates the exhaust manifold  12   b  of a typical diesel engine having an in-line cylinder configuration. The cylinder exhaust gases are discharged into the left and right portions or runners of the exhaust manifold  12   b , and are channeled toward a central collection plenum  12   c  with one or more exit ports  12   d . In a typical application, the left-hand and right-hand portions of the exhaust manifold  12   b  may be separate castings that are individually bolted to the engine  12 . In any event, the exhaust gas exit ports  12   d  lead to the impeller section ( 1 )  60   a  of an exhaust-driven turbocharger  60  en route to an exhaust pipe or header  62 . The impeller section  60   a  drives a compressor section (C)  60   b  of the turbocharger  60 , which compresses atmospheric pressure air for delivery to the intake manifold  12   a . The inlet atmospheric pressure air passes through an inlet air filter (IAF)  64 , and is delivered to the compressor section  60   b  via low-pressure conduit  66 . The high-pressure air at the outlet of compressor section  60   b  is passed though an intercooler  68  by the conduits  70 ,  72  en route to the intake manifold  12   a.    
      In a conventional turbocharged diesel engine, the gas temperature in the exhaust manifold is well above 1700° F., the temperature above which NOx emissions are readily formed. Moreover, since a conventional turbocharger produces little boost at low engine speeds, the air/fuel ratio in the engine cylinders becomes too rich when the fuel delivery is increased to accelerate the engine. As a result, partially consumed fuel is discharged into the exhaust manifold, producing objectionable levels of soot until the engine speeds up and the turbocharger produces sufficient boost. The high levels of soot formation and the low speed power deficiency can be addressed by some external means that speeds up the turbocharger impeller. The increased speed of the turbocharger impeller provides the intake air boost needed, but at the expense of increased NOx formation due to high cylinder and exhaust manifold temperatures and long residence times. The embodiment described below with respect to  FIG. 2 , on the other hand, provides an approach that not only achieves low speed soot and power improvements, but also achieves significant improvements in NOx emissions and fuel economy.  
      A mechanically driven supercharger (SC)  74  delivers high-pressure air to the exhaust manifold  12   b  at distributed locations along its length. The inlet air is passed through an inlet air filter  64  (which may be the same inlet air filter used by the turbocharger  60 , or a different inlet air filter), and is delivered to the supercharger inlet  75  by a conduit  76 . The supercharger outlet  77  is coupled to a high-pressure plenum  78  from which a number of branches  78   a  inject the air into distributed locations of the exhaust manifold  12   b , at an approximate flow rate of 100-250 CFM. In one embodiment, the number of branches  78   a  is equal to the number of engine cylinders discharging exhaust gases into the manifold  12   b , and the air is injected in proximity to the points at which the exhaust gases are discharged into the manifold  12   b . The temperature of the air injected into exhaust manifold  12   b  by supercharger  74  is approximately 307° F., effectively cooling the exhaust gasses to approximately 350° F., which is well below temperatures at which NOx emissions are readily formed. Interestingly, this also has the effect of reducing the required cooling capacity of the liquid coolant that is circulated through the engine  12 , thereby reducing the engine power requirements for coolant pumping and radiator airflow.  
      In the illustrated embodiment, the supercharger  74  is driven by a hydraulic accessory drive motor (ADM)  56  powered by hydraulic fluid from charge pump  54  as mentioned above. This is particularly advantageous in the context of a hydrostatic vehicle drive since the additional hydraulic fluid pressure for powering the supercharger  74  is available at very little extra cost, and the capacity of ADM  56  can be controlled by the PCM  42  as indicated to optimize the rotational speed of the supercharger  74  regardless of the engine speed. Furthermore, the supercharger  74  may be located remote from the engine  12  as implied in  FIGS. 1-2 , which allows the supercharger  74  to be mounted in a location that provides cooler inlet air and easier mounting and routing of the air conduits. Of course, the supercharger  74  can alternatively be driven by a different rotary drive source such as an electric or pneumatic motor, or the engine  12 .  
      In summary, the air injection system of the present invention simultaneously contributes to improved exhaust emissions, engine power output and fuel efficiency, and allows a turbocharged diesel engine to be well suited to highly efficient low constant speed operation in a hydrostatic vehicle drive.  
      Referring now to  FIG. 3 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  110 ′ illustrating a sample single turbocharger configuration in accordance with another embodiment of the present invention is shown. The powertrain system  110 ′ is similar to the powertrain system  10 , however the turbocharger  60  is replaced with a high-efficiency turbocharger  60 ′, which eliminates the need for the supercharger  74  and associated componentry. The turbocharger has impeller  60   a ′ and compressor  60   b ′. The turbocharger  60 ′ may be configured for efficient operation at low constant engine speeds. The engine speed is controlled by the PCM  42  such that a low constant speed is maintained.  
      Referring now to  FIG. 4 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  10 ″ illustrating a sample single supercharger configuration in accordance with another embodiment of the present invention is shown. The powertrain system  10 ″ is also similar to the powertrain system  10 . However a supercharger  74 ′ is utilized in replacement of the supercharger  74  and is configured to supply air to the intake manifold  12   a . In supplying air to the intake manifold  12   a  the turbocharger  60  is not utilized and is thus removed. Also, since the supercharger  74 ′ does not draw air from the exhaust manifold  12   b ′ the intercooler  68  is also eliminated. The plenum  78 ′ includes an additional branch  80  over that of the plenum  78 , which supplies the air to the intake manifold  12   a . The exhaust manifold  12   b ′ is also modified to couple directly to the header or exhaust pipe  62 .  
      Referring now to  FIG. 5 , a logic flow diagram illustrating a method of operating a vehicle hydraulic powertrain system in accordance with an embodiment of the present invention is shown. Although steps  200 - 222  are described primarily with respect to the embodiments of  FIGS. 2 and 3 , the method of  FIG. 4  may be easily modified for other embodiments of the present invention.  
      In step  200 , an engine is activated, such as the engines  12 . The engine may be activated via the PCM, or by other methods known in the art.  
      In step  202 , a main hydraulic pump, such as the HP  28 , is operated or driven directly off of the engine. The main hydraulic pump may be coupled to a crankshaft of the engine and receive rotational energy therefrom.  
      In step  204 , a first charge pump, such as the CP  52 , is also operated off of the engine. The first charge pump may be ganged to the main hydraulic pump and also operate in response to rotation of a crankshaft of the engine. In step  206 , the first charge pump supplies control pressure to the main hydraulic pump and to a main hydraulic motor, such as the DM  30 . In steps  204  and  206 , the first charge pump may be operated and the control pressure may be adjusted by a PCM, such as the PCM  42 . The control pressure may also be adjusted mechanically within the charge pump.  
      In step  208 , one or more main hydraulic motors, such as the motors of the DM  30 , are operated off of high-pressure hydraulic fluid received from the main hydraulic pump. The flow direction of the high-pressure hydraulic fluid may be adjusted by a hydraulic valve assembly, such as the hydraulic valve assembly  32 .  
      In step  210 , a driveshaft followed by components of an axle assembly and the corresponding wheels of a vehicle are rotated in response to rotational energy received from the main hydraulic motors. Components of an axle assembly may refer to, for example, the DG  18  and the axles  20  and  22 . With respect to the embodiment of  FIG. 1 , the DM  30  rotates the driveshaft  16 , the DG  18 , the axles  20 ,  22 , and the wheels  24 ,  26  for translation of the corresponding vehicle in a forward or reverse direction.  
      In step  212 , a second charge pump, such as the CP  54 , is operated similarly as the first charge pump. In step  214 , the second charge pump supplies hydraulic fluid to an auxiliary drive motor, such as the ADM  56 , at a controlled pressure, which may also be adjusted by the a PCM or internally controlled.  
      In step  216 , the auxiliary drive motor is activated and operated utilizing the hydraulic fluid received from the second charge pump. The auxiliary drive motor may also be activated and operated via a PCM, such as the PCM  42 .  
      In step  218 , a supercharger, such as the supercharger  218 , is operated off of the auxiliary drive motor. In step  220 , the supercharger draws air through an intake filter and injects it into an exhaust manifold. In step  222 , a turbocharger, such as the turbocharger  60 , is operated in response to exhaust received from the exhaust manifold. The turbocharger directs and or injects exhaust gas into an intake manifold and into an exhaust pipe.  
      The above-described steps are meant to be illustrative examples; the steps may be performed sequentially, synchronously, simultaneously, or in a different order depending upon the application.  
      The hydraulic drive motors and the hydraulic wheel motors of  FIGS. 6-13  described below may each include one or more hydraulic motors similar to the DM  30 . When more than one hydraulic motor is utilized they may be ganged as described above with respect to DM  30 .  
      Also, the heat exchanger  65  and the filters  69  and  71  are not shown in  FIGS. 6-12  for simplicity of illustration. The heat exchanger  65 , the filters  69  and  71 , and other similar devices may be incorporated within the embodiments of  FIGS. 6-12  as desired. Also, in  FIGS. 6-12  the signal control lines between the PCMs and the hydraulic drive motors and the hydraulic wheel motors are also not shown for simplicity of illustration, but may be included and are designed for control efficiency.  
      Additionally, the term “wheel pair axle” refers to a set of front end or rear drive components that include a pair of wheels that are positioned laterally relative to each other and are approximately in the same fore and aft position on a vehicle. For example, a standard four-wheel vehicle has two front wheels and two rear wheels. The front wheels are part of a first wheel pair axle and the two rear wheels are part of a second wheel pair axle. The term wheel pair axle does not imply that the wheels contained in that pair are on or rotated by the same axle. However, the wheels within a wheel pair axle may be rotated by one or more driveshafts, by one or more hydraulic drive motors, such as one or more of DM  30 , or by a pair of hydraulic wheel motors, as shown in  FIGS. 1 and 3 - 4  described above, as well as in  FIGS. 6-12  described below.  
      Note also that although in  FIGS. 6-13  a single charge pump is shown as supplying hydraulic fluid to multiple hydraulic drive motors and to multiple hydraulic wheel motors, any number of charge pumps may be utilized.  
      Referring now to  FIG. 6 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  300  illustrating a non-gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  300  has a hydrostatic transmission  302  that includes the HP  28 , an HVA  304 , a first hydraulic wheel motor (WM)  306 , a second hydraulic wheel motor  308 , and a PCM  310 . The HVA  304  and the PCM  310  are similar to the HVA  32  and the PCM  42 , respectively, and are configured for the WMs  306 ,  308 . The WMs  306 ,  308  are coupled to and rotate the axles  310 ,  312 , which in turn rotate the wheels  24 ,  26 . The WMs  306 ,  308  may be separated by the axles  310 ,  312  or by a vehicle suspension (not shown). The WMs  306 ,  308  may also be ganged together or may be coupled via a transfer case or gearbox. The combination of the WMs  306 ,  308 , the axles  310 ,  312 , and the wheels  24 ,  26  form a single rear wheel pair axle  314 . The charge pump  316  is similar to the CP  52 , but is also configured for the WMs  306  and  308 .  
      Referring now to  FIG. 7 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  330  illustrating a four-wheel drive configuration in accordance with another embodiment of the present invention is shown. The powertrain system  330  has a hydrostatic transmission  332  that includes the HP  28 , the HVA  334 , the first hydraulic DM  30 , the second hydraulic drive motor  336 , and the PCM  338 . The DM  30  is coupled to the first driveshaft  16 , which rotates components within a rear wheel pair axle  338 . The rear wheel pair axle  338  includes the axles  20 ,  22 , and the wheels  24 ,  26 . The second DM  336  is coupled to a second driveshaft  338 , which rotates components within a front wheel pair axle  340 . The front wheel pair axle  340  includes axles  342 ,  344 , and wheels  346 ,  348 . The HVA  334  and the PCM  338  are similar to the HVA  32  and the PCM  42 , respectively, and are configured for the DMs  30 ,  336 . The charge pump  349  is similar to the CP  52 , but is also configured for the DMs and  336 .  
      In the sample embodiment of  FIG. 7 , multiple reservoirs are shown. A first reservoir  350  supplies hydraulic fluid to the CPs  54  and  349  and receives hydraulic fluid from the HP  28 , the ADM  56 , and the DM  30 . A second reservoir  352  also supplies hydraulic fluid to the CPs  54  and  349 , but receives hydraulic fluid from the HP  28 , the ADM  56 , and the DM  336 . The reservoirs  350  and  352  allow for shorter supply lines and are generally smaller than the reservoir  34 .  
      Referring now to  FIG. 8 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  360  illustrating a dual axle non-gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  360  has a hydrostatic transmission  361  and is similar to the powertrain system  300 , but includes the first rear wheel axle  314  and a second rear wheel axle  362 . A second rear wheel axle  362  includes the WMs  364 ,  366 , axles  368 ,  370 , and wheels  372 ,  374 . The WMs  364 ,  366  may also be separately utilized, as shown, ganged together, or coupled via a transfer case or gearbox. The HVA  376  and the PCM  378  are similar to the HVA  304  and the PCM  310 , respectively, and are configured for the WMs  306 ,  308 ,  364 ,  366 . The charge pump  380  is similar to the CP  316 , but is also configured for the WMs  306 ,  308 ,  364 ,  366 .  
      Referring now to  FIG. 9 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  400  illustrating a rear dual axle gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  400  has a hydrostatic transmission  402  that includes the HP  28 , the HVA  404 , the first DM  30 , the second DM  406 , and the PCM  408 . The powertrain system also includes a first rear wheel pair axle  410  and a second rear wheel pair axle  412 . The first wheel pair axle  410  includes a gearset  414 , the axles  20 ,  22 , and the wheels  24 ,  26 . The second wheel pair axle  412  is coupled to the first wheel pair axle  410  via the second DM  406  and a second driveshaft  416 . The second wheel pair axle  412  includes a second gearset  418 , axles  420 ,  422 , and wheels  424 ,  426 . The first gearset  414  is configured to couple the first driveshaft  16  and the second DM  406 . This configuration aids in maintaining synchronization of the DMs  30 ,  406 , such that the wheels  24 ,  26 ,  424 ,  426  rotate in agreement. The first gearset  414  may not be coupled to the second DM  406  and timing between the DMs  30 ,  406  may be controlled by the PCM  408 . The HVA  404 , the PCM  408 , and the charge pump  430  are configured for the DMs  30 ,  406 .  
      Referring now to  FIG. 10 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  450  illustrating a six-wheel drive gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  450  has a hydrostatic transmission  452  and is similar to the powertrain system  400 , but also includes a front wheel pair axle  454 . The front wheel pair axle  454  includes a third drive motor  456 , a third driveshaft  458 , a third gearset  460 , axles  462 ,  464 , and wheels  466 ,  468 . The HVA  470 , the PCM  472 , and the charge pump  474  are configured for the DMs  30 ,  406 , and  456 .  
      Referring now to  FIG. 11 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  500  illustrating a six-wheel drive gearset/non-gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  500  has a hydrostatic transmission  502  and is similar to the powertrain system  360 , but like powertrain system  450  also includes the front wheel pair axle  454 . The HVA  504 , the PCM  506 , and the charge pump  508  are configured for the WMs  306 ,  308 ,  368 ,  370 , and the DM  456 .  
      Referring now to  FIG. 12 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  520  illustrating a six-wheel drive non-gearset configuration in accordance with another embodiment of the present invention is shown. The powertrain system  520  has a hydrostatic transmission  522  and is also similar to the powertrain system  360 , but further includes a front non-gearset wheel pair axle  524 . The front non-gearset axle  524  includes a fifth hydraulic wheel motor  526 , a sixth hydraulic wheel motor  528 , corresponding axles  530 ,  532 , and wheels  534 ,  536 . The WMs  526 ,  528  may also be separately utilized, as shown, ganged together, or coupled via a transfer case or gearbox. The HVA  538 , the PCM  540 , and the charge pump  542  are configured for the WMs  306 ,  308 ,  368 ,  370 ,  526 ,  528 .  
      Referring now to  FIG. 13 , a schematic and block diagrammatic view of a vehicle hydraulic powertrain system  550  incorporating a multi-input powered gearset  552  is shown. The powertrain system  550  includes a pair of hydraulic drive motors  554  and  556 . The drive motors  554 ,  556  may include one or more drive motors ganged together, similar to the DM  30 . The drive motors  554 ,  556  are coupled and supply power to the multi-input gearset  552  via driveshafts  558  and  560 , respectively. The multi-input gearset  552  rotates a pair of axles  562  and  564 , which in turn rotate two pairs of wheels  566 . Wheel transfer axels  568  reside between each pair of the wheels  566 . Although 4 wheels are shown in a dual axel configuration, any number of wheels may be utilized.  
      The PCM  42  is coupled to the multi-input gearset  552  and selects the amount of power to be received by the wheels  566  via a power divider  570  of the multi-input gearset  552 . The power divider  570  may be in the form of, for example, one or more solenoids and selects one or more of the drive motors  554 ,  556  to receive power therefrom. The power divider  570  may receive power from one or both of the drive motors  554 ,  556 . The power divider  570  may be variable in design in that it may adjust the level of power received from each of the drive motors  554 ,  556 . The power divider  570  performs such selection in response to a signal received from the PCM  42 .  
      In another embodiment, the power divider  570  may systematically and dynamically select and adjust the amount power received from the drive motors  554 ,  556  without receiving a signal from the PCM. The power divider  570  may be a “smart” device and contain logic or other electrical and mechanical devices for performing such selection and adjustment. The selection and adjustment, for example, may be performed in response to vehicle speed or engine rpm.  
      Use of the power divider  570  and multiple drive motors, which are separately coupled via associated driveshafts and/or ganged together, provides a wider range of operation without “weak spots”. Weak spots refer to temporary periods or transitions when a decreased amount of torque is available. The use of the power divider  570  also eliminates the need for a clutch to disengage one or more of the drive motors, thus minimizing system components and complexity.  
      The embodiment with respect to  FIG. 13  allows for hydraulic drive motors of different size, having different displacement and power characteristics, to be incorporated and coupled to a single gearset without the direct coupling or ganging of the drive motors.  
      As an example, each of the drive motors  554 ,  556  may be utilized from a rest position to aid in accelerating the vehicle from rest. As the vehicle speed increases one of the motors  554  or  556  may be deactivated. The first drive motor  554  may be a high-speed/low-torque motor and the second drive motor  556  may be a low-speed/high-torque motor. As the vehicle speed increases the second drive motor  556  may be deactivated. The second drive motor  556  may be entirely deactivated at a predetermined vehicle speed or the second motor may be gradually deactivated as the vehicle speed increases. As an example, the second drive motor  556  may be deactivated at a wheel speed of approximately 200-260 rpm. The PCM  42  or the power divider  570  may utilize vehicle speed or wheel speed tables to determine when and to what extent to deactivate the second drive motor  556 .  
      Referring now to  FIG. 14 , a block diagrammatic and schematic view of a powertrain system  600  in accordance with an embodiment of the present invention is shown. The powertrain system  600  includes a prime mover  602 , a transmission  604 , and a delivery system  606 . The prime mover  602  provides an input torque to drive the transmission  604 . The transmission  604  converts the input from the prime mover  602  to an output torque for driving the delivery system  606  to perform work as designed. The transmission  604  may be a hydrostatic or continuously variable transmission, such as the transmissions  14 ,  302 ,  332 ,  361 ,  402 ,  452 ,  502 , and  522 . The powertrain system  600  may, for example, drive wheels for vehicular movement, pump fluids and/or gases, actuate lifting equipment, or perform other types of work. A controller  608  is coupled to the prime mover  602 , the transmission  604 , and the delivery system  606  and simultaneously controls the various inputs and outputs of the stated devices to maintain a minimal energy input to the transmission  604  to perform the tasks desired. The controller  608  may be in the form of or be used in replacement of one of the controllers  42 ,  310 ,  338 ,  378 ,  408 ,  472 ,  506 , and  540 .  
      The prime mover  602  may be any suitable machine or device that provides input torque for the transmission  604 . Examples of a suitable prime mover include an electric motor, an internal combustion engine, and a hydraulic and/or air (pneumatic) motor. The transmission  604  may be any suitable device, which can alter the input torque of the prime mover  602  to a desired output torque for driving the delivery system  606 .  
      An example of a suitable powertrain system includes the use of an internal combustion engine that may function as the prime mover and variable hydraulic rotary axial pumps and variable hydraulic rotary axial piston motors that may function in combination as the transmission. The hydraulic pumps are driven by the prime mover  602 . The hydraulic motors function as the delivery system and are attached to a drive axle or the wheels of the vehicle. The transmission  604  decouples the prime mover or engine speed from the road or vehicle speed and allows the engine to operate at low speeds.  
      The controller  608  continuously monitors the inputs to the primary mover  602  and to the transmission  604  and in response thereto electronically and simultaneously manages and adjusts the speed and torque of the primary mover  602  and the transmission  604  to operate the system with the minimum energy input from the primary mover  602 . This allows for reduced engine revolutions per minute (and per mile driven), fewer combustion events per mile driven, lower fuel consumption, lower emissions of undesirable by products of combustion per mile driven, lower engine temperatures, which are also a byproduct of engine combustion, and increased engine operating life.  
      Referring now to  FIG. 15 , a block diagrammatic and schematic view of a powertrain control circuit  650  in accordance with an embodiment of the present invention is shown. The powertain control circuit  650  may be used to control operation of both an engine  651  and a hydrostatic or continuously variable transmission  653  having one or more hydraulic pumps and hydraulic drive motors. An example engine  12  and hydrostatic transmissions  14 ,  302 ,  332 ,  361 ,  402 ,  452 ,  502 ,  522 , hydraulic pumps,  28 ,  52 ,  54 ,  316 ,  349 ,  380 ,  430 ,  508 , and drive motors  30 ,  31 ,  33 ,  306 ,  308 ,  336 ,  364 ,  366 ,  406 ,  456 ,  526 ,  528 ,  554 , and  556  are described above. The powertrain control circuit  650  includes a controller  652 , such as the controller  608  or the like, which has multiple inputs  654  and multiple outputs  656 . The controller  652  also includes or is coupled to a memory  658 .  
      The inputs  654  include various signals from the memory  658  and from a sensor complex  660 . The controller  652  is coupled to a vehicle speed sensor  662 , an accelerator sensor  664 , an engine speed sensor  666 , hydraulic pump sensors  668 , hydraulic drive motor sensors  670 , and may be coupled to other sensors known in the art. The sensors  662 ,  664 ,  666 ,  668 ,  670 , and  671  may or may not be part of the sensor complex  660 . The vehicle speed sensor may be of various type and styles known in the art and may, as a couple of examples, include a drive shaft rotation sensor or a wheel speed sensor. The accelerator sensor  664  may be coupled to both the controller  652  and to the fuel injectors  672  of the engine  651 , as shown, or simply to the controller  652 . The accelerator sensor  664  may be in the form of an accelerator pedal position sensor, a drive-by-wire acceleration sensor, or some other accelerator sensor known in the art. The engine speed sensor  666  is coupled to the engine  651  or elsewhere and provides an indication of a current actual engine speed. The engine speed sensor  666  may be a rotary sensor, an optical sensor, a camshaft or crankshaft sensor, a flywheel sensor, or other engine speed sensor known in the art.  
      The hydraulic pump sensors  668  and the hydraulic drive motor sensors  670  may be used to sense pressures, displacements, and speeds of hydraulic pumps  674  and hydraulic drive motors  676  within the hydrostatic transmission  653  and powertrain system  650 . The sensors  668  and  670  provide status information in the form of feedback signals to the controller  652 . Various types and styles of hydraulic pump sensors and the hydraulic drive motor sensors may be used. The sensors  668  and  670  may be coupled to or within the hydraulic pumps  674  and drive motors  676 , coupled to hydraulic fluid lines (not shown) that extend to and from the hydraulic pumps  674  and the drive motors  676 , coupled to reservoirs or hydraulic fluid tanks (not shown), or elsewhere in the powertrain system  650 . The sensors  668  and  670  may also be coupled to a hydraulic valve assembly  678  or to various hydraulic valves therein or elsewhere and provide valve position feedback to the controller  652 . The sensors  668  and  670  may be used to detect the position of hydraulic pump displacement actuators  680  and of hydraulic motor displacement actuators  682 .  
      The memory  658  stores various efficiency references  684  and may also include various parameter relationships  686 . For example, the efficiency references  684  may include efficiency curves  688  or efficiency tables  690  relating various parameters to allow the controller  652  to generate output signals to the fuel injectors  672 , the hydraulic pumps  674  and the drive motors  676 . One example efficiency curve is that of engine torque plotted in relation to engine RPM. The controller  652  may plot or compare engine torque to engine RPM to obtain a desired efficient operation of the engine  651 . Other example efficiency curves include hydraulic pump and drive motor pressure, displacement, and speed curves as can be determined by one skilled in the art and tend to be specific to a particular pump, motor, and hydrostatic system used. The individual efficiencies of the hydraulic pumps  674  and the drive motors  676  may be plotted or compared against a swash plate angle, of a hydraulic pump, and hydraulic pressure and/or accelerator pedal positioning to obtain a desired efficient operation. As another example, the controller  652  or memory  658  may have stored a relationship for percent engine load, which can be determined from accelerator position and engine RPM.  
      Referring now to  FIG. 16 , a logic flow diagram illustrating a method of controlling a powertrain of a vehicle is shown. Although the method of  FIG. 16  is primarily described with respect to the control circuit  650  and embodiment of  FIG. 15 , it may be applied to other control circuits and embodiments of the present invention. Also, the following steps  700 - 708  do not address control of charge pumps, such as charge pumps  52 ,  54 ,  316 ,  349 ,  380 ,  430 , and  508 , which may be incorporated herein.  
      In step  700 , the controller  652  receives various generated input signals from the sensors  662 ,  664 ,  666 ,  668 ,  670 , and  671 . In step  700 A, the controller  652  receives a vehicle speed signal generated from the vehicle speed sensor  662 . In step  700 B, the controller  652  receives an accelerator signal generated from the accelerator sensor  664 . The accelerator signal is directly related to the desired speed, change in speed, and torque requested from a vehicle operator or as systematically determined by the controller  652 , such as for autonomous vehicle control. In step  700 C, the controller  652  receives an engine speed signal generated from the engine speed sensor  666 . In step  700 D, the controller  652  receives hydraulic pump signals generated from the hydraulic pump sensors  668 . In step  700 E, the controller  652  receives hydraulic drive motors signals generated from the drive motor sensors  670 . In step  700 F, the controller  652  receives valve position signals generated from the hydraulic valve sensors  671 .  
      Steps  700 A- 700 F are stated herein to provide some example sensor signals that may be utilized as inputs to the controller  652 . Of course, other inputs may be provided.  
      In step  702 , the controller  652  receives or generates various efficiency references and/or parameter relationships  684  and  686  from the memory  658 . The controller  652  receives or generates an engine torque versus engine RPM reference.  
      In step  704 , the controller  652  may determine percent engine load in response to the accelerator signal and the engine RPM signal. Note that the engine load may vary for a single engine RPM depending upon the accelerator position or, in other words, the amount of fuel being supplied to the engine. Steps  700 - 704  may be performed simultaneously.  
      In step  706 , the controller  652  maintains a constant engine speed. In step  706 A, the controller  652  generates a fuel input signal in response to the engine torque versus engine RPM reference and the current engine speed to maintain an approximately constant engine speed. The constant engine speed is set to provide sufficient torque to power the transmission  653  and to maintain a minimal engine speed. The controller  652  utilizes proportional-integral control logic to maintain the constant engine speed. The controller  652  compares the actual engine RPM with the desired engine RPM in order to determine the engine speed error and the direction of change. The controller  652  maintains the constant engine speed for various engine-loading conditions including during low, normal, and high loading conditions. The constant engine speed is maintained during steady state or cruising modes, during acceleration, and/or during hauling or trailering of heavy loads.  
      The controller  652  may utilize setpoint variables for the hydraulic pumps  674  and drive motors  676 . The setpoint variables adjust the operating speed of the engine, unlike the hydraulic pump sensor and the drive motor sensor signals, which provide feedback for closed-loop control. In an example embodiment, the hydraulic pumps  674  are set at a minimum displacement and the drive motors  676  are set at a maximum displacement. During acceleration of the vehicle, the drive motors  676  are maintained at the maximum displacement and the displacement of the hydraulic pumps  674  is increased until the pumps are at full displacement. The transition from minimum displacement to full displacement of the hydraulic pumps  674  may occur at approximately 20 mph depending on pump size and motor and/or gear ratios. Upon full displacement of the hydraulic pumps  674  and maximum displacement of the drive motors  676  the motor displacements are monitored to maintain the desired engine speed.  
      In step  706 B, the controller  652  generates a continuous variable transmission control signal in response to the engine torque versus engine RPM reference and said current engine speed to maintain the approximately constant engine speed. In step  706 B 1 , the controller  652  generates hydraulic pump control signals, which may include desired pressures, displacements, and operating speeds of the hydraulic pumps  674 . In step  706 B 2 , the controller  652  generates drive motor control signals, which may also include desired pressures, displacements, and operating speeds of the drive motors  676 . The pump and motor control signals are generated to adjust the transmission according to and in proportion to the engine speed error and the direction of adjustment desired. The pump and motor control signals are inversely related to the desired increase and decrease in engine speed. In order to increase engine speed, both the pump and motor control signals are decreased to provide additional load on the engine. Conversely, to decrease engine speed, both the pump and motor control signals are increased.  
      Step  706  is incorporated herein to provide examples of some output signals that may be generated by the controller  652  in response to the received input signals and the efficiency references and relationships stored. Steps  700 - 706  are performed continuously and reiterated to maintain the engine speed at an approximately constant value or within a desired range. The constant value or desired range may be predetermined based on the fuel efficiency and output of the engine. Although not shown in  FIG. 16 , steps  700 - 706  may also be performed continuously and reiterated to maintain the operation of the engine at maximum load for the stated constant value or desired range. The maximum load or maximum load value may also predetermined and stored in the memory  658 . In general, engines operate most fuel efficiently at a maximum engine load at a given engine RPM. Although not shown in  FIG. 16 , steps  700 - 706  may also be performed accordingly to operate the hydraulic pumps  674  and the drive motors  676  at or near peak efficiency levels. In general, hydraulic pumps and drive motors have peak efficiency operation when operated at their maximum swash plate position and over a desired pressure range. Steps  700 - 706  may be performed many times per second or as often as the controller  652  allows.  
      Although it may not be possible to provide the optimum operating condition or to operate both the engine and the hydrostatic transmission at peak efficiencies and achieve operator desired speed and acceleration at constantly changes grades, road conditions, and load conditions, the above-described control techniques allow a powertrain system to maintain these peak efficiencies for a significant portion of operation and to approximate these peak efficiencies during the remainder.  
      The present invention provides a method of managing system parameters to use a minimum amount of input energy or fuel consumption to provide a desired input torque to a transmission at generally all times of operation or during a specified duration of operating time. The system continuously monitors the inputs and the output controls for peak efficiency settings and operation of an engine and hydrostatic transmission. The process of continuously controlling the hydraulic pumps and drive motors provides an extremely smooth and “shift-free” acceleration.  
      The present invention also provides a hydraulic powertrain system that eliminates the need for a high-pressure accumulator, which reduces weight and can increase fuel economy of a vehicle. This is particularly advantageous in vehicle applications, such as refuse truck applications, where small changes in vehicle weight can effect the hauling capacity and thus the profitability of a vehicle. The present invention further provides multiple efficient hydraulic motor configurations for various vehicular applications.  
      While the invention has been described in reference to the illustrated embodiments, it should be understood that various modifications in addition to those mentioned above will occur to persons skilled in the art. Accordingly, it will be understood that systems incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.