Abstract:
A method for power management in an electro-mechanical power-split infinitely variable transmission (eVT) designed to operated within a designated speed ratio range for vehicular applications. The eVT is comprised of an input shaft coupled to the output shaft of a drive engine to receive power, a drive shaft, two electric machines, and a pair of planetary trains each having a sun member, a ring member, a set of planetary members, and a planet carrier. The eVT further contains one or more torque transfer devices to connect or disconnect members of the planetary trains for transferring torque. The drive shaft is coupled with a final drive of a vehicle for delivering or recapturing power to or from the vehicle drive wheels. The two electric machines are interconnected electronically via a power control unit and are coupled respectively with members of the planetary train. The method of power management in the eVT is selected based on the current speed and torque of the input and drive shafts, and upon the desired operating parameters.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     A transmission is an important part of vehicle power train. The primary function of a transmission is to regulate speed and torque to meet the operator demands for speed and acceleration. The major requirements for transmissions are speed ratio range, torque capacity, efficiencies, including transmission and system efficiencies, weight and cost.  
         [0002]     There are two types of transmissions: stepwise and step-less. Stepwise transmissions, using multiple gear sets and clutching devices, are quite popular. The speed ratio change is accomplished in discrete steps. Speed ratio change is often associated with interruptions in both speed and torque. The output speed variation between two speed ratios is realized by varying the input speed. The major disadvantage of stepwise transmission is system efficiency, since the engine cannot always operate at its most efficiency speed. For the same reason, pollution is also a problem for a vehicle with a stepwise transmission.  
         [0003]     Step-less transmissions provide a continuously variable speed ratio change. With a step-less transmission, it is possible to operate an engine at an optimal speed and, therefore, keep the engine at its peak efficiency. Common types of step-less transmissions include friction drives such as belt continuously variable transmissions, traction drives such as toroidal drives and hydrostatic drives.  
         [0004]     Hydrostatic drives are noisy and have low efficiency. They generally are used for low speed applications such as agriculture machines and construction equipment. Friction or traction drives are more efficient, but they are less rugged for handling large torque loads. In addition, many the traction drives are usually quite heavy and costly to manufacture.  
         [0005]     Recent development in step-less transmissions has been in the area of electromechanical transmissions, such as those disclosed in U.S. Pat. Nos. 2,991,683, 3,623,568, 3,699,351, 5,571,058, 5,577,973, 5,603,671, 5,669,842, 5,730,676, 5,851,162, 5,875,691, 5,907,191, 5,914,575, 5,916,050, 5,931,757, 5,935,035, 5,980,410, 6,022,287, 6,053,833, 6,090,005, 6,234,930, 6,248,036, European Granted Patent No. EP 0755818 B1 and Tenberge, P., (1999), “Electric-Mechanical Hybrid Transmission,” Proc. International Congress on Continuously Variable Power Transmission, Eindhoven University of Technology (hereinafter “Tenberge”).  
         [0006]     Most of the newly proposed electromechanical transmissions operate on the so-called power-split concept historically developed for hydrostatic drives. There are two basic power-splitting devices, a single planetary unit and a compounded planetary unit that comprises two nested sub-planetary sets. When properly connected with two electric machines, a single planetary system is capable of producing at least a node point where no power is passing through the electric machines and all power transmitted is passing through a mechanical path. At the mechanical node point, there is no energy conversion from mechanical form to electric form and back to mechanical form Thus, the transmission yields the maximum efficiency. An electromechanical transmission with single planetary train is called single node system. An example of such a system is the Toyota Hybrid System (THS), disclosed in U.S. Pat. Nos. 5,907,191, 5,914,575 and 2,991,683. The THS system is now in limited production.  
         [0007]     However, as the output speed of the transmission moves away from the node point, the power to the electric machines increases significantly. The power that is circulated between the two electric machines can far exceed the power that the transmission is transmitting. Internal power circulation occurs at speeds either above the node point when one motor is connected to the output shaft or below the node point when one motor is connected to the input shaft. Internal power circulation generates heat and power loss and offsets the efficiency benefit otherwise provided by the transmission. For this reason, the effective speed ratio range is limited. To cover a useful speed ratio range, oversized electric machines are often used.  
         [0008]     To reduce or restrict internal power circulation, sophisticated control systems are developed for THS. The strategy is to monitor the torque value of the electric motor and shift the engine to another driving point of higher speed. In other words, the control system limits the speed to the node point or slightly above.  
         [0009]     A compound planetary unit is a four-branch system. When two of its four branches are connected to two electric machines, it can produce at least two mechanical node points where no electric power is passing from the input of the transmission to the output through the electric machines. An electromechanical transmission equipped with a compound planetary train is referred to as two-node system. As with single planetary unit, a two-node system also suffers from the internal power circulation problem. Internal power circulation occurs outside the two node points, below the first node point or above the second node point. But in general, a two-node system has a wider speed ratio range than a single node system.  
         [0010]     To extend speed ratio range and overcome excessive internal power circulation, multi-pass (also called multi-mode) IVTs, analogy to speed ratio shifting in stepwise transmissions, have been proposed.??? 
         [0011]     U.S. Pat. Nos. 5,935,035, 5,577,973, 5,931,757, 6,090,005 and European Granted Patent No. EP 0755818 B1 describe various configurations of variable, two-mode, power split, parallel, hybrid electric transmissions. They all employ a compound planetary set and two electric machines. The two-mode design provides adequate speed ratio range. The first mode covers slow vehicle speed operation and the second mode covers relatively high-speed operation. The mode shifting is realized through clutches and synchronized gear sets. The constructions of these transmissions are complex.  
         [0012]     In the first mode, there exists a pure mechanical node point. In the second mode, there are two mechanical node points. At each mechanical node point, there is no energy conversion from mechanical form to electric form and back to mechanical form. Thus, the transmission yields the maximum efficiency.  
         [0013]     Away from the node points, the power to the electric machines increases. In fact, for most of U.S. Pat. Nos. 5,935,035, 5,577,973, 5,931,757, 6,090,005 and European Granted Patent No. EP 0755818 B1, the power to electric machines increases rapidly as the vehicle&#39;s speed drops below the first node point in the second mode operation. Therefore, the transmission has to go through a mode shifting in order get into slow speed operation. As mentioned before, this shifting requires synchronizing gear sets. Although the shifting is continuous in speed, it is not continuous in torque and power.  
         [0014]     Shifting between different modes presents an interesting challenge. It is often associated with a torque and a power interruption. Various means have been disclosed in prior art to perfect synchronizing mechanisms. To reduce torque interruption due to torque reversals in electric machines, Tenberge presented a means of using electronically controlled hydraulic clutch and brake packs to retain the torque balance and facilitate the mode shifting through differential engagement.  
         [0015]     U.S. Pat. No. 6,203,468 proposed a speed and torque control method to prevent speed and torque fluctuations in mode switching from series drive to parallel drive. The basic strategy is to match the speeds of the two electric machines and reduce engine torque to zero at the switching point. Since the engine at switching point produces zero power, an on-board energy storage device is required for such system.  
       SUMMARY OF PRESENT INVENTION  
       [0016]     The objective of present invention is to provide a simple, compact and low cost solution to continuously variable transmissions and to vehicle hybridization. Another objective is to eliminate internal power circulation and provide smooth, non-interruptive shifting in speed, torque and power between regime or mode changes.  
         [0017]     The invention comprises two planetary trains, two electric machines and at least one torque transfer device that can selectively connect one component to another component or components to transfer torque. Each planetary train has a ring member, a sun member and a plurality of planets that engaged with the ring member and the sun member. Each planetary train has a planet carrier that holds the planets in the annular space between the ring and the sun members. Each electric machine can be operated as a motor to covert electric energy to mechanical energy or as a generator to convert mechanic energy to electric energy. The invention further comprises two external couplers to revive mechanical power from a prime mover or to deliver mechanical power to drive axle.  
         [0018]     At least one member of the first planetary train is operatively connected to one of the electric machines. At least one member of the first planetary train is operatively connected to one of the external couplers.  
         [0019]     At least one member of the second planetary train is operatively connected one of the electric machines. At least one member of the second planetary train is operatively connected to one of the external couplers.  
         [0020]     Means exists that, when desired, provide at least one operative connection between one member of the first planetary train and a member of the second planetary train.  
         [0021]     Means exists (such as a clutch) that, when desired, provide a second operative connection of a second member of the first planetary train to a second member of the second planetary train.  
         [0022]     Means exists (such as a brake) that selectively hold at least one member of the planetary trains stationary.  
         [0023]     One of important features of the invention is the high transmission efficiency over wide speed ratio range, from very low speed, down to vehicle stop, up to very high speed as in highway operation.  
         [0024]     The transmission of current design provides at least two mechanical link points where no power is passing from one external coupler to the other external coupler through the electric machines.  
         [0025]     The transmission of current invention provides smooth reconfigurations of power paths. These reconfigurations, also called regime or mode shafting or change, are continuous in speed, torque and power.  
         [0026]     For the entire designed speed range, from reverse to zero output speed and to highway output speed, the transmission is able, if desired, to restrict the magnitude of power to electric machines below the input power. There is no internal power circulation.  
         [0027]     The transmission blends the series hybrid configuration, parallel hybrid configuration, pure electric drive and pure mechanical drive over entire speed range, leveraging the benefits of series hybrid configuration and pure electric drive at slow speed operation and the benefits of parallel hybrid configuration and pure mechanical drive at high-speed operation.  
         [0028]     The current invention is suitable for a co-centric configuration that is desirable for most popular installations. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]     Referring to  FIG. 1 , an electric hybrid transmission I comprises first and second planetary trains  15 ,  16  each having a first and second sun member  4 ,  10 , a first and second ring member  2 ,  8 , a first and second plurality of planets  3 ,  9  and a first and second planet carrier  5 ,  6 . An input means I is directly connected to the first ring member  2 . An output means  7  is connected to the first planet carrier  5  and to the second planet carrier  6 . A first electric machine  13  is connected to the first sun member  4 , and a second electric machine  14  is connected to the second ring member  8 . A locking clutch  11  selectively connects the first sun member  4  of the first planetary train  15  to the second sun member  10  of the second planetary train  16 . A brake  12  selectively grounds the second sun member  10  of the second planetary train  16  to a fixed, non-rotational member  17  of the transmission I.  
         [0030]     The first electric machine  13  is connected to the second electric machine  14  through a power-regulating device (not shown)(also known as a control unit) such that each electric machine  13 ,  14  can receive power from or deliver power to the other electric machine  13 ,  14 . An energy storage device (not shown) may also be used so that each electric machine  13 ,  14  can receive power and/or deliver power to the energy storage device. In this sense, the transmission I operates not only as a speed regulator—conventional transmission, but also as a power regulator—power-buffering device, for vehicle hybridization.  
       OPERATION OF PREFERRED EMBODIMENT  
       [0031]     A thermal engine or a prime mover (not shown) is operatively connected to the input means  1  of the transmission I. A final drive is operatively connected to the output means  7 . Assuming for a moment that the transmission operates as a speed regulator only, all power received from input will be delivered to the output except internal power losses. Under full throttle, the engine provides a constant speed of 3200 rpm and operates at a constant power of 120 hp for most of its operation.  
         [heading-0032]     Slow-Speed Operation  
         [0033]     During slow speed operation, clutch  11  is disengaged so that the first sun member  4  of the first planetary train  15  is disconnected from the second sun member  10  of the second planetary train  16 . Brake  12  is engaged to ground the second sun member  10 , holding it stationary. The second planetary train  16  serves as a speed reduction device.  
         [0034]     At start up, the vehicle is at stationary. No power is required but torque is needed for the maximum acceleration of the vehicle. The engine delivers zero power at a speed of 3200 rpm (or reduced speed in practical application) by providing zero torque. The torque required to hold or accelerate the vehicle is provided solely by the second motor  14  through the second planetary train  16  which serves as speed reduction gear. The motor torque is amplified by a factor of M output /M motor2 =1+1/K 2  at the output shaft. The motor torque is a fraction of the output torque, M motor2 /M output =K 2 /(1+K 2 ). At this moment, the second electric motor  14  is stationary, consuming no power except internal loss. First electric motor  13  is in reverse rotation, providing zero torque. The state is considered as series hybrid since the electric motor supplies 100% torque as if there were no mechanical link from engine to wheel.  
         [0035]     After start-up, the vehicle accelerates, and the kinetic energy builds. Power to the output is required. The engine provides the power and, as a result, engine torque increases (either by increasing throttle angle under constant speed or by increasing engine speed under full throttle). To balance engine torque, the torque of the first electric motor  13  increases proportionally. The torque of the first motor is 1/K 1  of the input torque. The engine torque increases until the engine produces its maximum torque or power. From hereon the engine operates at constant speed and power.  
         [0036]     After start-up, the torque of the transmission is shared by the engine and second electric motor  14 . In this sense, the second electric motor  14  is operated as a motor and the first electric machine  13  is operated as a generator, supplying electric power to the second electric motor  14 . FIGS.  2  to  4  show the speed, torque and power as functions of output speed under constant engine speed and power.  
         [0037]     As output speed (vehicle speed) increases, the speed of second electric motor  14  increases and the speed of first electric motor  13  decreases in magnitude until the first electric motor  13  comes to a standstill ( FIG. 4 ). At which point, the second planetary train  16  is at “free wheeling” state, no torque is on acting on any members of the second planetary train  16 . Zero current in the second electric maotor  14  indicates this point. Therefore, no power is passing through either electric motor  13 ,  14 . This is the first node point.  
         [0038]     The first node point marks the end of the slow-speed operation regime and the beginning of the high-speed operation regime.  
         [0039]     It can be shown that in the first regime where  
         0   ≤       ω   out       ω     i   ⁢           ⁢   n         ≤       K   1         K   1     +   1         ,       
 
 the power that passes through the electric machines, P electric , is proportional to the power that is being transmitted through the transmission, P transmission . Assuming no net electric power is being drawn from or delivered to the transmission,  
         P   electric     =       [     1   -       (     1   +     1     K   1         )     ⁢       ω   out       ω     i   ⁢           ⁢   n             ]     ⁢       P   transmission     .           
 
 Therefore, the power that passes through the electric machines, P electric , is always less than the power that is being transmitted through the transmission, P transmission  (i.e. P electric ≦P transmission ). There is no internal power circulation. 
 
 High-Speed Operation 
 
         [0043]     At the first node point, once the control unit determines that the vehicle is going to operate in a high-speed regime, clutch  11  engages, thereby connecting the first sun member  4  of the first planetary train  15  with the second sun member  10  of the second planetary train  16 . Brake  12  disengages to release the second sun member  10 . The regime transition is smooth in speed, torque and power as indicated in FIGS.  2  to  4 . This is because both the first and second sun members  4 ,  10  in the two planetary trains  15 ,  16  are at zero speed and the second planetary train  16  is momentarily at free wheeling state.  
         [0044]     As vehicle speed increases, the torque of second electric motor  14  changes direction, and the speed of the second electric motor  14  decreases. The second electric machine  14  becomes a generator, supplying electric power to the first electric machine  13 . Concurrently, the speed of the first electric machine  13  changes direction, and the torque of the first electric machine  13  starts to decrease. Now, the first electric machine  13  becomes a motor, receiving electric power generated from the second electric machine  14 .  
         [0045]     The speed of the second electric machine  14  and the torque of the first electric machine  13  reduce as the vehicle speed further increases. Then the second electric machine  14  comes to a standstill at which point the torque of the first electric machine  13  is zero. This is the second node point where no power passes through both electric machines. When operating between the first and the second node points, it can be shown that the power to the electric machines, P electric , is always less than the power that is being transmitted through the transmission, P transmission . In fact, the maximum power to the electric machine, P max , is only a fraction of the transmission power  
         P   transmission     ,       P   max     =           ϕ     -   1         ϕ     +   1       ⁢     P   transmission         ,       
 
 where φ is the nominal speed ratio range, defined as the ratio of the output speed at the second node point to the output speed at the first node point. 
 
         [0047]     After passing the second node point as the vehicle speed further increases, the torque of the first electric machine  13  and the speed of the second electric machine  14  change their directions. Consequently, the first electric machine  13  operates as a generator again, supplying electric power to the second electric machine  14 . The second electric machine  14  operates as a motor, converting the electric power received from the first electric machine  13  into mechanical power.  
         [heading-0048]     Reverse Operation  
         [0049]     The transmission I can operate in a number of possible modes. Assuming there is an on-board energy storage device such as battery packs, the vehicle can operate in pure electric mode in reverse. Like in slow-speed operation mode, the clutch  11  is disengaged to disconnect the first sun member  4  of the first planetary train  15  from the second sun member  10  of the second planetary train  16 . Brake  12  is engaged to ground the second sun member  10  of the second planetary train  16 .  
         [0050]     When the power control unit determines the vehicle is going into reverse, the first electric machine  13  is switched off and is in a freewheeling state (this can be achieved, for instance, by using switch reluctant motors). Power from the storage device is channeled to the second electric machine  14 , which is now solely powering the vehicle in reverse. In this mode, the engine can either be shut off or be in an idle state.  
         [0051]     It is also possible to reverse the vehicle in series hybrid mode without drawing power from energy storage device. All power supplied comes directly from the engine through a series configuration (engine to generator to motor to wheel). In this case, the energy storage device may or may not be necessary. This mode of operation can be achieved by embodiment IV, as described below.  
       ALTERNATIVE EMBODIMENTS  
       [0052]     Referring to  FIG. 6 , there is shown a second embodiment II of the transmission of the present invention. The second embodiment is a direct derivative of the first embodiment, and both embodiments share many common features. Unlike the first embodiment, the input means  1  is connected to the first sun member  4  of the first planetary train  15 . The first electric machine  13  is connected to the second sun member  10  of the second planetary train  16 . The second electric machine  14  is connected to the first ring member  2  of the first planetary train  15 . The second ring member  8  of the second planetary train  16  is selectively connected to the first ring member  2  of the first planetary train  15  through a clutch  20  or ground to the fixed non-rotating member  17  by a brake  21 .  
         [0053]     With additions of clutches and brakes, the functionality of the basic embodiments can be enhanced. Such enhancements are shown in two additional embodiments III, IV shown in  FIGS. 7 and 8  and described below.  
         [0054]      FIG. 7  shows the third embodiment where an additional clutch  22  and a brake  23  are added to the transmission. The brake  23  can be used to ground the second ring member  8  of the second planetary train  16  when the second electric machine  14  comes to a standstill. It can also be used in conjunction with brake  12  to provide a parking function.  
         [0055]     Clutch  22  is used to disconnect the input means  1  from the first ring member  2  of the first planetary train  13 . It is usefull when both electric machines  13 ,  14  are required to power the vehicle for maximum power in a pure electric drive mode.  
         [0056]      FIG. 8  shows a fourth embodiment of the present invention. Compared with the third embodiment, one can see that a clutch  24  and a brake  25  are added. Clutch  24  is used to selectively connect the first planet carrier  5  of the first planetary train  15  to the second planet carrier  6  of the second planetary train  16 . The brake  25  is used to ground the first planet carrier  5  of the first planetary train  15  when the transmission calls for such an action.  
         [0057]     With the addition of clutch  24  and brake  25 , it is possible to operate the transmission in series hybrid configuration over a wide speed range. In series configuration, clutch  22  is engaged, connecting the input means  1  to the first ring member  2 . Brake  25  is engaged to ground the first planet carrier  5 . Clutch  24  is disengaged to disconnect the first carrier member  5  of the first planetary train  15  from the second planet carrier  6  of the second planetary train  16 . Clutch  11  is also disengaged to disconnect the first sun member  4  of the first planetary train  15  from the second sun member  10  of the second planetary train  16 . Brake  12  is engaged to ground the second sun member  10  of the second planetary train  16 . The two planetary trains  15  and  16  are de-attached from each other. The first planetary train  15  functions as a speed increaser from the input means  1  to the first motor  13 . The second planetary train  16  functions as a speed reducer from the second motor  14  to the output means  7 .  
         [0058]     The mechanical power from the engine drives the first electric machine  13  through the first planetary train  15 . The first electric machine  13  in turn generates electric power to power the second electric machine  14  through the power control unit. The second electric machine  14  then delivers power to the output means  7  through the second planetary train  16 .  
         [0059]     Although the series hybrid configuration can operate over a wide speed range from reverse to forward, it shows distinct advantages when operated in reverse mode—it avoids internal power circulation. The transition from forward to reverse, or vice versa, can be made smooth in speed, torque and power. At zero vehicle speed, the first and second carrier members  5  and  6  in both planetary trains  15 ,  16  are stationary. The first planetary train  15  is at free-wheeling state, and no torque is acting on the first planet carrier  5 .