Abstract:
A hybrid powertrain is provided that provides engine starting from an electric-only mode with a reduced torque load on the motor/generator. The powertrain includes an engine and a motor/generator, which may be a single motor/generator in a strong or full hybrid, but is not limited to such. The motor provides propulsion torque in an electric-only operating mode, and is configured to apply torque to a transmission input member. A first clutch is selectively engagable to connect the engine output member for common rotation with the transmission input member. An engine starting mechanism is provided that multiplies motor torque used to start the engine so that less is diverted from propelling the vehicle, enabling a smaller motor/generator to be used. A method of controlling such a powertrain is also provided.

Description:
TECHNICAL FIELD 
     The invention relates to a hybrid powertrain with a mechanism for starting an engine and a method of controlling a hybrid powertrain. 
     BACKGROUND OF THE INVENTION 
     Hybrid powertrains have two or more power sources. Some hybrid vehicles are operable in an electric-only operating mode in which drive power is provided exclusively by one or more electric motor/generators that utilize power stored in an energy storage device (ESD) such as a battery. For hybrid vehicles that have an internal combustion engine as the other power source, operation in an electric-only mode improves fuel economy and reduces emissions. When the ESD is discharged to a predetermined level or when additional output torque is required, the engine is started. Starting the engine is typically accomplished using a dedicated starter motor or by engaging a clutch that directly connects the engine and the motor/generator, referred to as a starting clutch. Dedicated starter motors add weight and cost. 
     SUMMARY OF THE INVENTION 
     A hybrid powertrain is provided that provides engine starting from an electric-only mode with a reduced torque load on the motor/generator. The powertrain includes an engine and a motor/generator, which may be a single motor/generator in a strong or full hybrid, but is not limited to such. The motor provides propulsion torque in an electric-only operating mode, and is configured to apply torque to a transmission input member. A first clutch is selectively engagable to connect the engine output member for common rotation with the transmission input member. An engine starting mechanism is provided that multiplies motor torque used to start the engine so that less is diverted from propelling the vehicle, enabling a smaller motor/generator to be used. The engine starting mechanism includes a first and a second gear train, as well as a second clutch having a first portion connected with the transmission input member via the first gear train and a second portion connected with the engine output member via the second gear train. The second clutch is selectively engagable to connect the first and second portions for common rotation, thereby transferring torque from the transmission input member to the engine output member with torque multiplication. The second clutch may be a hydraulic clutch of smaller capacity than the first clutch. In some embodiments, the second clutch is an active material clutch, also referred to as a smart clutch, such as a magnetorheological fluid (MRF) clutch or an electrorheological fluid (ERF) clutch. An MRF is a type of smart fluid with magnetic particles suspended in a carrier fluid. When subjected to a magnetic field, the apparent viscosity of the fluid increases, allowing the fluid&#39;s ability to transmit force to be controlled. Examples of suitable particles include straight iron powders, reduced iron powders, iron oxide powder/straight iron powder mixtures and iron oxide powder/reduced iron powder mixtures. An ERF is a suspension of nonconducting particles in an electrically insulating fluid. The apparent viscosity of an ERF changes in response to an electric field, allowing the fluid&#39;s ability to transmit force to be controlled. 
     The use of torque multiplying gear trains and the second clutch reduces the reserve torque requirement for the motor to start the engine, which reduces the possibility of torque sag, i.e., the diversion of output torque from the driveline when the motor does not have sufficient torque reserve to start the engine. This allows for a smaller motor/generator than with a starting clutch that directly connects the transmission input member and the engine output member. The engine starting mechanism also reduces control challenges associated with starting the engine directly with a typical engine starting clutch which has a slower response time due to fill time of a hydraulic clutch, as well as nonlinear behavior due to variations in the compressibility of fluid caused by entrained air. Finally, start quality is improved by a reduction in driveline torque disturbance as the speed of the engine output member and the speed of the transmission input member may be synchronized before the first clutch is engaged. 
     A method controlling the powertrain described above includes engaging the second clutch to transfer torque from the motor/generator to the engine to start the engine during an electric-only operating mode, and may include releasing the second clutch after the engine is started, synchronizing the speed of the engine output member with the speed of the transmission input member, and then engaging the first clutch to thereby transfer engine torque to the transmission input member. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a hybrid powertrain with an engine start mechanism; 
         FIG. 2  is a flow chart illustrating a method of controlling the powertrain of  FIG. 1 ; and 
         FIG. 3  is a schematic partial cross-sectional side view of an exemplary MRF clutch for the engine start mechanism of  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, wherein like reference numbers refer to like components throughout the several views,  FIG. 1  shows a hybrid powertrain  10  having an engine  12  and a hybrid transmission  14 . The transmission  14  has a transmission input member  16  and a transmission output member  18 . A transmission gearing arrangement  20  transfers torque from the input member  16  to the output member  18 . The transmission gearing arrangement  20  includes a plurality of selectively engagable clutches and brakes, as well as gears that may be planetary gear sets or intermeshing gear trains. The clutches and brakes are engagable in different combinations to affect torque transfer at different torque ratios such as overdrive and underdrive ratios, as is known. 
     A single motor/generator  22  has a rotor  24  mounted for rotation with the input member  16  and a stator  26  grounded to a stationary (i.e., nonrotatable) member  28 , such as a transmission casing. An energy storage device (ESD)  30  holds stored electrical energy that is selectively applied to the stator  26  through a power inverter  32  under the direction of an electronic controller  34 . The ESD  30  and motor/generator  22  are sufficiently sized so that the powertrain  10  may provide sufficient propulsion torque to the output member  18  using only the motor/generator  22 , and not the engine  12 , as a power source under many operating conditions, establishing what is referred to as an electric-only operating mode. 
     The controller  34  receives a variety of input signals indicative of operating conditions via sensors such as a throttle position sensor, wheel speed sensors, etc. Under certain operating conditions, these input signals are equivalent to an engine start request. The controller  34  then operates an engine starting mechanism  38  in order to start the engine  12  and deliver engine torque to the input member  16  without creating appreciable torque sag by motor torque diverted to start the engine  12 . The engine starting mechanism  38  includes an engine starting clutch  40  and a first gear train having a first gear  42  connected for rotation with the transmission input member  16  (i.e., a first shaft) and meshing with a second gear  44  mounted for rotation on a second shaft  46 . Shaft  46  is rotatably supported by the transmission casing  28  by support portions  47  of the casing  28  extending from the casing  28  to the shaft  46  on either side of the second gear  44 , with bearings  49  between the support portions  47  and gear  44 . A second gear train includes a third gear  48  and a fourth gear  52 . The third gear  48  is a spur gear mounted on a third shaft  50  that is rotatably supported by the casing  28  by support portion  47  with bearings  49  between the support portion  47  and the third gear  48 . The fourth gear  52  is rotatably supported by an engine output member  54 , such as a crankshaft that may be turned to start the engine  12 . The fourth gear  52  may be a ring gear or flex plate of the engine  12 . 
     A first portion  64  and a second portion  68  of the engine starting clutch  40  are selectively engagable to transmit torque from the shaft  46  to the shaft  50 , thus creating a powerflow path from the motor/generator  22  to the engine  12  through the gears  42 ,  44 ,  48 ,  52 , which are configured to multiply torque from the transmission input member  16  to the engine output member  54 . The portions  64 ,  68  may be friction plates and reaction plates (in the case of a hydraulic clutch), a drum and rotor (in the case of an MRF clutch), or other engagable portions of known clutch types. Specifically, when operating conditions warrant, the controller  34  provides an actuating signal to the starting clutch  40 . In a preferred embodiment, the gears  42  and  44  are designed to have a gear ratio (ratio of speed of gear  44  to gear  42 ) of approximately 1:1 and the gears  48  and  52  are designed to have a gear ratio (the ratio of speed of gear  52  to speed of gear  48 ) of approximately 2.5:1. That is, motor torque provided through the first gear train  42 ,  44  is multiplied by 2.5 through the second gear train. Thus, the engine output member  54  spins at a lower speed than the rotor  24  of motor/generator  22  to start the engine  12 . Once the engine  12  is started, the fired engine speed is controlled by controller  34 , or by another controller in communication with controller  34 , to bring the speed of the engine output member  54  within a predetermined range or, or equal to, the speed of the input member  16 . Once a controlled speed is reached, the controller  34  signals a valve body  56  to direct hydraulic pressure to a main clutch  60  to fill an apply cavity that engages opposing clutch plates of the clutch  60 , transferring torque from the engine  12  to the transmission  14 . The starting clutch  40  is disengaged when the engine  12  is started prior to engaging the main clutch  60 . Prior to start of the engine  12  and engagement of starting clutch  40 , clutch slip in the clutch  40  is equal in magnitude to the speed of the gear  44 . Once the engine  12  is started and before main clutch  60  is engaged, starting clutch  40  is disengaged. When main clutch  60  is engaged, the slip in the starting clutch  40  increases in magnitude. 
     Referring to  FIG. 2 , a method  100  of controlling a hybrid powertrain is described with respect to hybrid powertrain  10 . First, in block  102 , controller  34  receives an engine start request, which is one or more sensor signals indicative of operating conditions warranting starting the engine  12  during an electric-only operating mode. Next, in block  104 , the controller  34  causes the starting clutch  40  to be engaged bringing the engine  12  up to firing speed. The mechanism by which the controller  34  engages clutch  40  depends on the type of clutch used. 
     Once the engine  12  is firing, the clutch  40  is then disengaged by the controller  34  in block  106 . In block  108 , engine speed is then synchronized with the speed of the motor/generator  22  using speed sensors (not shown) or otherwise, and an engine control module (not shown). The main clutch  60  is then applied in block  110 , completing the transition from an electric-only operating mode to a hybrid operating mode. 
     The clutch  40  may be a hydraulic clutch, or an active material clutch, sometimes referred to as a “smart clutch”. The clutch capacity of clutch  40  is lower than the clutch capacity of main clutch  60  because it need not handle the greater torque load of the engine output member  54 . Thus, even if the clutch  40  is a hydraulic clutch, it will be filled faster, at a lower fill volume, than the main clutch  60 . Alternatively, the clutch  40  may be a smart clutch, such as an MRF or ERF clutch. A smart clutch has the advantage of precise engagement and disengagement times, as the clutch connection is controllable by applying an electric or magnetic field, rather than dependent on hydraulic fluid building to a sufficient pressure. 
     Exemplary Embodiment of an Engine Starting Clutch 
     Referring to  FIG. 3 , an exemplary MRF clutch  40  selectively joins or couples a pair of rotatable members, exemplified herein as the respective input and output members (i.e., shaft  46  and shaft  50 , respectively). A connecting member or sleeve  62  can be directly connected to or interposed between the input member  46  and the first portion  64  of the MRF clutch  40 , referred to as rotatable outer housing or drum  64 , to rotate in conjunction therewith. That is, rotation of the shaft  46  in conjunction with an actuated MRF clutch  40  ultimately rotates the drum  64 , with the MRF clutch  40  operable for selectively transferring or transmitting torque from the shaft  46  to the shaft  50  as described below. 
     The MRF clutch  40  includes a magnetically permeable stator  66  within the drum  64 , the second portion  68 , referred to as rotor  68 , and a magnetic field generator  70 . The rotor  68 , having a rotational degree of freedom with respect to the stator  66 , is journaled, splined, or otherwise directly connected to the shaft  50  to rotate in conjunction therewith about a rotational axis  72 . The rotor  68  includes an axial member  74  which at least partially defines at least a pair of respective inner and outer working gaps  78 A and  78 B as discussed in more detail below, with a volume of MR fluid  80  substantially filling the working gaps  78 A,  78 B. Although not shown in  FIG. 3  for clarity, one or more intermediate working gaps may be disposed between the working gaps  78 A,  78 B. 
     The magnetic field generator  70  is in field communication with the MR fluid  80  in each of the working gaps  78 A,  78 B, with the magnetic field illustrated generally in  FIG. 3  by a set of magnetic flux lines  82 . The stator  66  and the rotor  68  each include respective magnetic or magnetically-permeable portions  84 ,  86  and non-magnetic portions  90 ,  92 , which serve to guide the magnetic field or flux lines  82  in a manner suitable for the purposes disclosed herein. Suitable magnetizable materials for use as the magnetic portions  84 ,  86  and stator  66  can include, but are not limited to, iron, steel, carbonyl iron, etc., or a combination comprising at least one of the exemplary magnetizable materials described above. Suitable non-magnetic materials for use as the non-magnetic materials  90 ,  92  can include, but are not limited to, stainless steel, aluminum, brass, plastics, etc., or a combination thereof Alternatively, an air gap may be employed in place of or in addition to the use of non-magnetic portions, as will be understood by those of ordinary skill in the art. 
     The magnetic field generator  70  can be configured as an electromagnet as shown in  FIG. 3 , including a magnetic core  94  and a field coil  96  that is electrically energized via the ESD  30  of  FIG. 1 . Exemplary fluid seals  98 A,  98 B serve to prevent leakage of the MR fluid  80  from the working gaps  78 A,  78 B. While exemplary fluid seals  98 A,  98 B are depicted in  FIG. 3 , it will be appreciated that other arrangements or sealing devices may also be employed. 
     An exemplary composition for the MR fluid  80  includes magnetizable particles, a carrier fluid, and additives. By way of example, the magnetizable particles of the MR fluid  80  can include paramagnetic, super-paramagnetic, or ferromagnetic compounds or a combination thereof. The magnetizable particles can be comprised of materials such as but not limited to iron, iron oxide, iron nitride, iron carbide, carbonyl iron, chromium dioxide, low carbon steel, silicon steel, nickel, cobalt, or the like, or a combination thereof. The term “iron oxide” can include all forms of pure iron oxide, such as, for example, Fe 2 O 3  and Fe 3 O 4 , as well as those containing small amounts of other elements such as manganese, zinc, barium, etc. Specific examples of iron oxide include ferrites and magnetites. In addition, the magnetizable particles can be comprised of alloys of iron, such as, for example, those containing aluminum, silicon, cobalt, nickel, vanadium, molybdenum, chromium, tungsten, manganese, copper, or a combination thereof. 
     When energized via the ESD  30  or other suitable energy storage device, the magnetic field generator  70  creates a magnetic field (flux lines  82 ), which ultimately passes through the MR fluid  80  filling the working gaps  78 A,  78 B. That is, a magnetic field is electrically induced around the wires of the field coil  96 , radiating outward therefrom to produce the resultant magnetic field (flux lines  82 ). As will be understood by those of ordinary skill in the art, the magnetic field naturally weakens in a direction progressing radially-outward away from the wires of the field coil  96 , with the magnetic field strengthening in closer proximity to the field coil  96 . While shown schematically as a single box in  FIG. 3  for clarity, it is understood that the magnetic field lines of an actual magnetic field are concentrically circular with respect to the magnetic field generator  70 , and the direction of circulation of the magnetic field itself is dependent upon the direction of current flow within the field coil  96 . These factors are at least partially controllable via the controller  34  of  FIG. 1  and the ESD  30 . 
     When the field coil  96  is electrically energized, the magnetic particles suspended in the carrier of the MR fluid  80  will align with the induced magnetic field (flux lines  82 ), thereby increasing the apparent viscosity of the MR fluid  80 . The increase in apparent viscosity increases the shear strength of the MR fluid  80 , resulting in torque transfer from the input member  46  to the output member  50  through the MRF clutch  40 . The output member  50  can then be used directly or indirectly for any suitable purpose, such as to start the engine  12  of  FIG. 1 . Because energizing and responsiveness of fluid  80  thereto is nearly instantaneous, engine starting is carried out quickly and efficiently with the engine starting mechanism  40 . 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.