Abstract:
The invention relates to a power train for a hybrid vehicle, comprising two hydraulic machines ( 20, 22 ) including variable displacement, both of which are connected to a hydraulic pressure accumulator ( 24 ) which stores energy, a first hydraulic machine ( 20 ) being permanently connected to an internai combustion traction engine ( 2 ), and a second hydraulic machine ( 22 ) being connected, also permanently, to drive wheels ( 8 ) of the vehicle, characterized in that the two hydraulic machines ( 20, 22 ) are also interconnected by a linking means ( 10 ) that can be engaged or disengaged.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the US National Stage, under 35 U.S.C. 371, of International App. No. PCT/FR2010/052150, which was filed on Oct. 11, 2010 and claims priority to French Application No. 0957216, which was filed on Oct. 15, 2009, and which is incorporated herein by reference. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND 
     The present invention relates to a powertrain for a hybrid vehicle using hydraulic power as an energy storage device, and a hybrid vehicle equipped with such a powertrain. 
     A conventional hybrid vehicle uses an internal combustion engine as the main motorization, and a hydraulic machine connected to the combustion engine, which can function as a motor or a pump to either charge hydraulic pressure accumulators, or to supply mechanical power to drive wheels of the vehicle by drawing energy from the accumulators. 
     In this way, the hydraulic machine can be used as a pump during the braking phases to recharge the pressure accumulators, thereby recovering the kinetic energy of the vehicle. During the acceleration phases, the hydraulic machine can also be used as a motor by drawing pressurized fluid from the accumulator to supply mechanical power in addition to the power supplied by the combustion engine. 
     The use of the energy storage device optimizes the operation of the combustion engine and reduces its consumption and polluting emissions. 
     A known refinement of these hybrid vehicles, in particular disclosed in document US-A1-20090036248, includes a first hydraulic machine connected to the combustion engine, and a second hydraulic machine connected to one element of a planetary gear system, while one of the two other elements of the planetary gear system is connected to the drive wheels of the vehicle and the other element to the combustion engine. 
     In this arrangement, the two hydraulic machines can be used in parallel operating mode, whereby the hydraulic machines and the combustion engine can simultaneously drive the drive wheels of the vehicle through a mechanical power transmission. 
     However, this arrangement is not conducive to serial operating mode because the two hydraulic machines are permanently mechanically coupled to each other. On the one hand, this allows recharging of the pressure accumulators by the combustion engine through a first hydraulic machine. On the other hand, this allows use of the stored energy in independent manner to provide traction to the vehicle through the second hydraulic machine at suitable rotational speeds for each machine, which can be regulated independently to optimize the operating points. 
     SUMMARY 
     The present invention eliminates the disadvantages of the prior art, and provides a simple and effective solution for creating a powertrain for a hybrid vehicle, allowing for the use of the two hydraulic machines in serial or parallel mode. 
     To this end, a powertrain for a hybrid vehicle includes two variable displacement hydraulic machines, both of which are connected to a hydraulic pressure accumulator which stores energy. The first hydraulic machine permanently connects to the driving combustion engine, and the second hydraulic machine permanently connects to the drive wheels of the vehicle. In addition, the two hydraulic machines connect to each other with a coupling element that can be engaged and disengaged. 
     One advantage of the powertrain is that a first parallel type operating mode can be obtained by engaging a coupling element, so that the drive wheels are driven directly by the combustion engine, with parallel assistance from the hydraulic machines. A second serial type operating mode disengages the coupling element so that the two hydraulic machines work independently. 
     In addition, the powertrain can include one or more of the following characteristics, which can be combined with each other. 
     Advantageously, the two hydraulic machines connect to each other by a two-piece shaft, which connect through the coupling element. The first part of the shaft connects to the combustion engine, and the second part connects to the drive wheels. 
     Advantageously, the coupling element includes an engagement device, such as a mechanical coupling or clutch, actuated by a controlled actuator and controlled by a control unit. 
     Advantageously, a control unit controls the variable displacement of the hydraulic machines and the combustion engine. 
     The control unit adjusts the variable displacement of each hydraulic machine used as pump or motor, as a function of the desired torque on the shaft and of the pressure difference between high and low pressure supply circuits of the hydraulic machines. 
     Advantageously, the high pressure circuit connects to a hydraulic motor with the variable displacement controlled by the control unit to drive accessories. 
     A method for operating a powertrain including any of the preceding characteristics is also disclosed. In this method, the coupling element is disengaged in serial operating mode and the variable displacements of the first hydraulic machine are used as a pump, and of the second hydraulic machine used as motor is controlled by the control unit. 
     Furthermore, the powertrain operates in a parallel operating mode by engaging the coupling element and controlling with the control unit the variable displacements of the hydraulic machines. 
     In the parallel operating mode, the operating method uses the two hydraulic machines simultaneously to supply either high traction torque or high braking force to the drive wheels. 
     A hybrid vehicle with a powertrain comprising two variable displacement hydraulic machines is also disclosed. Both hydraulic machines are connected to a hydraulic accumulator for storing energy. In addition, the powertrain includes any of the preceding characteristics noted above. 
     The foregoing and other features, and advantages of the disclosure as well as embodiments thereof will become more apparent from the reading of the following description in connection with the accompanying drawings. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings which form part of the specification: 
         FIG. 1  is a diagram of a powertrain comprising a hydraulic circuit of a hybrid vehicle; 
         FIG. 2  is a partial diagram of the powertrain in a serial operating mode, partially showing the hydraulic circuit, and arranged to supply torque to the drive wheels; 
         FIG. 3  is a partial diagram of the powertrain in a serial operating mode, configured for recovering energy, or for starting the combustion engine; 
         FIG. 4  is a partial diagram of the powertrain in a parallel operating mode, arranged for supplying torque to the drive wheels, and for recharging the pressure accumulators; 
         FIG. 5  is a partial diagram of the powertrain in a parallel operating mode, configured to supply torque to the drive wheels starting from the combustion engine and the pressure accumulator; and 
         FIG. 6  is a partial diagram of the powertrain in a parallel operating mode, configured to recover energy. 
     
    
    
     Corresponding reference numerals indicate corresponding parts throughout the several figures of the drawings. 
     DETAILED DESCRIPTION 
     The following detailed description illustrates the claimed invention by way of example and not by way of limitation. The description clearly enables one skilled in the art to make and use the claimed invention, describes several embodiments, adaptations, variations, alternatives, and uses of the claimed invention, including what is presently believed to be the best mode of carrying out the claimed invention. Additionally, it is to be understood that the claimed invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The claimed invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
       FIG. 1  shows a schematic of a powertrain  1  of an automotive vehicle, having a combustion engine  2  driving a mechanical transmission  6  with a motor shaft  4 . The mechanical transmission has a differential distributing the rotational movement to the drive wheels  8  of an axle. The drive wheels can be either front or rear wheels of the vehicle. 
     The motor shaft  4  includes a coupling element  10  that engages and disengages by a controlled actuator  12  controlled by a control unit  40 , which can divide the motor shaft into an upstream part permanently connected to the combustion engine  2 , and a downstream part also permanently connected to the drive wheels  8 . 
     The coupling element  10  includes an engagement device, such as a mechanical coupling or clutch that actuates by controlled actuator  12 . 
     The upstream part of motor shaft  4  permanently drives a first hydraulic machine  20 , and the downstream part of the motor shaft permanently drives a second hydraulic machine  22 . The two hydraulic machines  20 ,  22  that have variable displacement, are controlled by the control unit  40 , and can be operated in either a pump mode or a motor mode. 
     The hydraulic machines  20 ,  22  are connected together to a high pressure circuit including a pressure accumulator  24 , which maintains permanent pressure thanks to a membrane containing a gas under pressure. The high pressure circuit includes a high pressure sensor  26  measuring the charge pressure of the pressure accumulator  24 . 
     The high pressure circuit connects to a hydraulic motor  28  with variable displacement that is controlled by the control unit  40 , which runs at a predetermined speed and torque for optimization of the energy consumption of accessories  30 . These accessories  30  include, in particular, an alternator, or a climate control compressor. 
     The hydraulic machines  20 ,  22  and the hydraulic motor  28  for the accessories connect to a low pressure circuit having a hydraulic reservoir  32  pressurized by the high pressure circuit. The pressurized hydraulic reservoir  32  maintains a minimum low pressure to eliminate cavitation problems in the aspiration of hydraulic machines  20 ,  22  when they are used as pumps. 
     In addition, the low pressure circuit includes a low pressure sensor  34 . This allows the control unit to determine the quantity of energy available in the pressure accumulator  24 , and the torque applied to the shaft of each hydraulic machine  20 ,  22  by determining the difference with the high pressure sensor  26 . 
     The low pressure circuit also includes a filter  36  ensuring a certain purity level of the hydraulic fluid, and a heat exchanger  38  for cooling of the hydraulic fluid. 
     A pressure control valve  42  is interposed between the high and low pressure circuits to limit the maximum pressure difference between these two circuits. 
       FIG. 2  shows the power train in a serial operating mode, used generally for low power, such as about a few kilowatts. In the serial operating mode, the coupling element  10  disengages via the controlled actuator  12 , thereby separating the two parts of the motor shaft  4 . 
     The combustion engine  2  supplies power to the first hydraulic machine  20 , which is used as pump to charge the pressure accumulator  24  as indicated by arrow  50 . At the same time, the pressure accumulator  24  supplies a flow of pressurized fluid, indicated by arrow  52 , thereby feeding the second hydraulic machine  22 , which is used as motor, and which drives the drive wheels  8  of the vehicle with the transmission  6 . 
     The control unit  40  permanently controls the displacement of the first hydraulic machine  20 , and the operational characteristics of the combustion engine  2  through the intermediary of its engine controller, to optimize the operating point of the engine in order to obtain the best efficiency. In particular, combustion engine  2  is used intermittently for efficient delivery of high power, which is stored in the pressure accumulator  24  to be used afterwards in a progressive manner. 
     The high pressure sensor  26  indicates the maximum pressure in accumulator  24  whereby the combustion engine  2  and the charging of the accumulator are stopped, or a minimum pressure for restarting the engine and the recharging of the accumulator. 
     Similarly, the control unit controls the variable displacement of the second hydraulic machine  22  as a function of the required torque on the drive wheels  8 , and the pressure difference between the high and low pressure circuits generating the torque. 
     When the combustion engine  2  stops, driving continues in pure hydraulic mode due to the flow supplied by the pressure accumulator  24 , which is indicated by arrow  52 , without emission of polluting gas. 
       FIG. 3  is a diagram of the powertrain in a serial operating mode, arranged for recovering energy, or for starting the combustion engine  2 . 
     During a deceleration of the vehicle, or when driving downhill, the kinetic energy of the vehicle can be recovered by adjusting the variable displacement of the second hydraulic machine  22  used as pump, which is driven by the drive wheels  8 , to recharge the pressure accumulator  24 , as indicated by arrow  62 . 
     The variable displacement of the second hydraulic machine is adjusted as a function of the pressure difference between high and low pressure circuits. In this way, a braking torque is applied on the drive wheels  8 , the power of which is modulated by regulation of the variable displacement hydraulic machine  22 . 
     Energy also draws independently, as indicated by arrow  60 , from the pressure accumulator  24  for driving the first hydraulic machine  20  used as motor, and for starting the combustion engine  2  when the vehicle starts driving, or automatically after stopping when the engine operates in a “Start-Stop” mode. 
     The variable displacement of the first hydraulic machine  20  also adjusts as a function of the pressure difference available between high and low pressure circuits, to obtain the necessary traction torque. 
       FIG. 4  shows the powertrain in a parallel operating mode, used generally for high power, such as greater than about a few kilowatts. The coupling element  10  is engaged by controlled actuator  12 , connecting the two parts of motor shaft  4 . 
     The combustion engine  2  supplies power directly to the drive wheels  8  through the motor shaft  4 , as indicated by arrow  70 . This direct mechanical link is used for high power with good efficiency. The transmission  6  can include several speed ratios, to adapt to the speed of the combustion engine  2  and improve the efficiency. 
     In parallel to the direct driving of the drive wheels  8  by motor shaft  4 , the combustion engine  2  recharges the pressure accumulator  24  through the first hydraulic machine  20  used as pump, as indicated by arrow  72 , and if necessary also through the second hydraulic machine  22  used as pump, as indicated by arrow  74 . 
     The control unit  40  regulates the variable displacements of the hydraulic machines  20 ,  22  when operated as pumps to obtain the required power from these pumps. The control unit  40  also controls the torque of the combustion engine  2  in order for the engine to deliver the total power necessary for pumps  20 ,  22 , and for the transmission  6  and the drive wheels  8 . 
     When the pressure accumulator  24  is fully charged, the variable displacements of pumps  20 ,  22  are progressively brought to zero so that no fluid flows. In parallel, the torque required from the combustion engine  2  reduces in order not to disturb the smooth driving of the vehicle. 
       FIG. 5  shows an alternate embodiment of the parallel operating mode the powertrain operating in, where the motor shaft  4  receives, besides the power supplied by the combustion engine  2 , complementary power delivered by one or both hydraulic machines  20 ,  22 , which are functioning as motors. The energy draws from the pressure accumulator  24  as indicated by arrows  80 ,  82  and the variable displacements of the hydraulic motors  20 ,  22  are regulated based on the difference between high and low pressure circuits, and the power to be obtained. 
     The operating mode shown in  FIG. 5  provides high drive power to the drive wheels  8 , since the combustion engine  2  and the two hydraulic machines  20 ,  22  are adding their power to give the vehicle the highest acceleration. This operating mode occurs when energy is available in the pressure accumulator  24 , and the torque requirement is higher than what the combustion engine  2  alone can deliver. 
       FIG. 6  shows another alternate embodiment of a parallel operating mode of the powertrain, where the driver wants to brake the vehicle. 
     One of the two hydraulic machines  20 ,  22  is used as a pump, or in case of heavy braking, both hydraulic machines  20 ,  22  are used as a pump. In this operating mode, the hydraulic machines have their variable displacement adjusted in order to obtain a torque on motor shaft  4  as a function of the difference between high and low pressure circuits, which brakes the drive wheels as needed. 
     In this case, the combustion engine  2  is also driven, and delivers a braking torque which is additive to the torque of the hydraulic machines  20 ,  22 . 
     When the pressure in the accumulator  24  rises, and for constant braking requirements, the displacement of the hydraulic machines  20 ,  22  gradually and proportionally reduces to maintain the braking torque. When the pressure accumulator  24  is fully charged, the control unit gradually brings the displacements to zero volume, and actuates the wheel brakes of the vehicle, which take over automatically in progressive manner to maintain comfort and safety. 
     When the pressure accumulator  24  is fully charged, and in case of accidental overcharging of the accumulator, the pressure control valve  42  sends the high pressure fluid through a bypass to the low pressure circuit. 
     In general, the control unit  40  continuously adapts the variable displacements of the hydraulic machines  20 ,  22  to continually adjust as a function of the difference between high and low pressure circuits, and the desired torque on the shafts of the hydraulic machines when functioning as motors or pumps. These adaptations are always coordinated with the actions of the combustion engine  2  to set the torque supplied by the engine, or on the wheel brakes to set the braking levels, so as to obtain smooth driving and reduce polluting emissions. 
     The control unit  40  can be a separate unit linked to the control unit of the combustion engine  2 , or can be integrated in the control unit of the combustion engine. The combustion engine is designed to be used in a reduced operating range, without any particular dynamic demands. It can be simplified and optimized for this range, and its cost can be reduced. 
     Since the braking of the vehicle is in part assured by the hydraulic machines, the wheel braking means dissipate less energy and have less wear. They can also be simplified. 
     Changes can be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.