Abstract:
A method for detecting sustained combustion in the engine of a hybrid electric powertrain that includes a starter/generator driveably connected to the engine, a transmission for driving a load, and an input clutch for opening and closing a drive connection between the electric machine and the transmission, includes the steps of using the starter/generator to produce torque and crank the engine, preparing the engine to produce combustion, producing torque capacity across the input clutch while slipping the clutch, and continuing use of the starter/generator until a sum of the crankshaft torque applied by the starter/generator and the crankshaft torque applied by the transmission is less than some torque threshold for a predetermined period length.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a powertrain for a hybrid electric vehicle and, in particular, to detecting the start of sustained combustion in an internal combustion engine (ICE). 
         [0003]    2. Description of the Prior Art 
         [0004]    In a vehicle powertrain where a starter/generator has a continuous drive connection to an engine and a selective connection to a transmission through an input clutch whose torque capacity is variable, the vehicle is frequently driven solely by the starter/generator without the engine operating. The engine is frequently stopped to prevent use of engine fuel and is frequently restarted when engine torque is required to augment power produce by the starter/generator. 
         [0005]    Detecting that an internal combustion engine has been successfully started is confirmed upon observing stable, sustained combustion in the engine&#39;s combustion chamber. A reliable technique for determining that the engine has been started or restarted is required in a powertrain of this type. 
         [0006]    It is conventional to start an engine in a vehicle powertrain in which the starter/generator does not have a full time connection to the engine. In such cases, the starter electric motor is connected to the engine by a one-way torque transmitting device, a one-way clutch, -such that the starter motor can only add torque to the engine, and the engine is permitted to rotate freely faster than the starter motor speed. In such configurations, engine start detection criteria consists of considering the engine speed relative to the idle reference and/or the starter motor speed over some period of time. An engine speed threshold condition and a duration threshold are used. 
         [0007]    A speed based engine start detection scheme is not reliable is a powertrain having of the full-time connection between the engine and the electric machine that starts the engine. Any torque disturbance resulting from an engine start is difficult to observe in the speed domain because the electric machines are continually connected to the engine and the speed reference is regulated by the electric machine. Engine speed cannot exceed the starter/generator speed. 
         [0008]    Furthermore, aggressively close engine start and transmission events can distort engine start detection mechanism when there is overlap of transmission events during the engine start process when using conventional start detection techniques. 
       SUMMARY OF THE INVENTION 
       [0009]    A method for detecting sustained combustion in the engine of a hybrid electric powertrain that includes a starter/generator driveably connected to the engine, a transmission for driving a load, and an input clutch for opening and closing a drive connection between the electric machine and the transmission, includes the steps of using the starter/generator to produce torque and crank the engine, preparing the engine to produce combustion, producing torque capacity across the input clutch while slipping the clutch, and continuing use of the starter/generator until a sum of the torque applied to the crankshaft by the starter/generator and the torque applied to the crankshaft by the transmission is less than some torque threshold during start for a predetermined period length. 
         [0010]    The method uses the estimated transmission load on the crankshaft and accounts for this in the detection process because the starter/generator torque estimate alone does not indicate that the engine has been started. 
         [0011]    The method provides robust engine start detection for hybrid electric vehicle configurations when there may be some overlap of mechanical transmission events during the engine start process. Furthermore, since the estimation of transmission load on the crankshaft can be accurately determined only while slipping the input clutch, engine start detection is a necessary condition before operating the transmission with full engagement of the input clutch. 
         [0012]    The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0013]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0014]      FIG. 1  is a schematic diagram showing a powertrain for a hybrid electric vehicle; 
           [0015]      FIG. 2  is a schematic diagram showing details of the transmission shown in  FIG. 1 ; 
           [0016]      FIG. 3  is a graph showing the variation of the sum of torque produced by the CISG and torque capacity of the input clutch while starting the engine shown in  FIG. 1 ; and 
           [0017]      FIG. 4  is a diagram of the steps for detecting the start of sustained combustion in the engine. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    As shown in  FIGS. 1 and 2 , a HEV powertrain  12  includes a power source, such as a diesel or gasoline engine  14 ; a transmission  16 , such as dual input clutch powershift transmission or a manual transmission; a second power source  18 , such as a crank integrated starter/generator (CISG) or a belt integrated starter/generator (BISG), driveably connected to the engine crankshaft  22  and connected to the transmission input  20 . The powertrain  12  may include another power source or load  24 , such as an electric motor or electric rear axle drive (ERAD) driveably connected to the transmission output  26 . Electric machine  18  provides starter/generator capability. An electric storage battery. 28 , which is electrically connected to electric machine  18 , supplies power to crank the engine  14  when the engine is being started with the electric machine  18  in starter mode. 
         [0019]    When electric machine  18  operates in generator mode, the state of charge of battery  28  can be replenished when machine  18  is driven by the engine or by the wheels of the vehicle through the transmission  16 . An IESG controller  30  controls the magnitude of electric power carried on lines  32 ,  34  between electric machine  18  and battery  28 . 
         [0020]      FIG. 2  illustrates a first input clutch  40 , which selective connects the input of transmission  16  alternately to the even-numbered gears  42  associated with a first shaft  44 ; and a second input clutch  46 , which selective connects the transmission input  20  alternately to the odd-numbered gears  47  associated with a second shaft  49 . 
         [0021]    An electronic transmission control module (TCM)  50  includes a microprocessor accessible to electronic memory and containing control algorithms expressed in computer readable code, which are executed repeatedly at frequent intervals. TCM  50  controls engagement, disengagement and slip across the input clutches  40 ,  45  by issuing command signals sent to solenoid-actuated servos  52 ,  54 , which operate the input clutches. A speed sensor  56  produces an electronic signal representing the speed of a shaft  44 , which signal is transmitted to as input to TCM  50 . A speed sensor  58  produces an electronic signal representing the speed of a shaft  49 , which signal is transmitted as input to TCM  50 . Engine speed NE is also supplied as input to TCM  50  by a speed sensor  60  on the engine crankshaft  22 . The torque capacity of each input clutch  40 ,  45  is determined by TCM  50 . Slip across the respective input clutch is determined by TCM  50  from the speed of crankshaft  22  and the speed of the respective transmission shaft  44 ,  49 . 
         [0022]    An electronic engine control module (ECM)  62 , which communicates with the TCM  50 , also includes a microprocessor accessible to electronic memory and containing control algorithms expressed in computer code, which are executed repeatedly at frequent intervals. ECM  62  controls operation of engine  14  in response to input signals produced by various sensors representing engine and driveline parameters, such as engine speed NE, engine throttle position TP, air mass flow rate MFR in the engine intake manifold  64 , etc. ECM  62  controls engine operation by issuing control commands, which vary the engine ignition spark timing, air-fuel ratio and other engine control parameters. 
         [0023]    In a powertrain whose engine  14  has a continuous connection with an electric machine  18 , such as a CISG or BISG, detecting the start of sustained combustion in the internal combustion engine  14  should be performed in the torque domain because changing the torque capacity of the input clutch  40 ,  45  can impose a friction and/or inertia torque disturbance on the crankshaft  22 , particular when such transmission events occur close to the point in time when the engine starts. As a result of the friction and/or inertia torque, the starter/generator torque does not provide a reliable indication of sustained engine combustion. For example, when the engine starts with a quick transmission engagement, the transmission input clutch can stroke or partially engage at a load that is similar to that of motoring the engine (i.e., without fuel) pumping and friction losses. When operating in this condition, it is impossible to determine that the engine has started by monitoring starter/generator torque alone, because starter/generator torque is constant before, during and after sustained engine combustion occurs. 
         [0024]    In the engine starting routine represented in  FIG. 3 , the proper indication that the engine has started is instead the algebraic sum of the torque applied by the starter/generator  18  and the torque applied by the transmission  16  on the crankshaft  22 . If the transmission is fully engaged, the torsional load due to other torque sources and loads  24  is transmitted also by transmission  16  to the crankshaft  22  through the electric machine  18 . Since road load is difficult to estimate accurately, start detection with the transmission fully engaged is not a reliable method. Hence, start detection is a necessary criterion to enable transmission engagement. 
         [0025]    The magnitude of torque produced by the starter/generator  18  is determined by the ECM  62  either from commands for IESG torque  70  issued by the ECM to an IESG controller  30 , or from the magnitude of electrical power carried on lines  32 ,  34  between starter/generator  18  and battery  28 . The magnitude of electric power can be determined accurately from the current supplied to starter/generator  18 , the voltage across the machine, and its power loss. 
         [0026]    While the oncoming input clutch  40 ,  45  is slipping, the torque load imposed by transmission  16  on crankshaft  22  is represented by the input clutch torque capacity. TCM  50  determines the torque capacity of the oncoming input clutch from parameters including slip across the clutch; temperature of the clutch; pressure applied to the friction surfaces  66 ,  68  of the clutch by the actuating servo  56 ,  58 ; effective radius of the mating friction surfaces of the clutch from the axis of rotation  20 ; coefficient of friction of the mating friction surfaces; gain of the clutch. The magnitudes of these data are either stored in electronic memory or determined from input data from the sensors. After the oncoming input clutch is fully engaged, it is impossible to accurately estimate the torque load on crankshaft  22  imposed by transmission  14 . Therefore, full engagement of the input clutches  40 ,  45  occurs only after the engine has started. 
         [0027]    In  FIG. 3 , both input clutches  40 ,  45  are open at  70 . Vertical line  72  represents the point in time where torque capacity of the oncoming input clutch begins. Vertical line  74  represents the point in time where the oncoming input clutch is fully engaged. Line  76  represents the variation of CISG torque during the engine starting procedure. Line  78  represents the variation of torque load transmitted to crankshaft  22  determined or estimated with reference to the torque capacity of the oncoming input clutch. Line  80  represents the algebraic sum of the torque applied to crankshaft  22  from starter/generator  18  and the torque applied to crankshaft  22  from transmission  16 . Line  82  represents the variation of engine torque during the engine starting procedure. Vertical line  84  represents the point in time when torque output by starter/generator  18  begins. 
         [0028]    During the period before the oncoming input clutch is activated and after torque is produced by starter/generator  18 , engine torque  82  is negative representing an inertia and friction/pumping load on starter/generator  18 . Thereafter, engine torque increases as the engine begins to produce torque, and it rises rapidly after the engine starts in zone  86 . 
         [0029]    Transmission crankshaft torque  78  is zero until the subject input clutch is activated at  72 ; thereafter, it becomes a large negative torque load on the starter/generator  18 . As algebraic sum  80  of the torque applied to crankshaft  22  from starter/generator  76  and the torque  78  transmitted to crankshaft  22  from transmission  18  declines in zone  86 , and sustained combustion of engine  14  occurs, as evidenced by the increase in engine torque  82  that occurs after zone  86 . 
         [0030]    In  FIG. 4 , a request  92  to begin the engine start procedure is produced by a vehicle controller and is sent to the ECM  62 . 
         [0031]    At  94 , a test is made by controller ECM  62  to determine whether engine  14  is ready to crank. If the result of test  94  is logically false, control returns to  92 . But if the result of test  94  is true, control advances to  96  where engine  14  is cranked by starter/generator  18  to a reference engine speed. 
         [0032]    At  98 , the ECM  62  actuates ignition spark, throttle, fuel and another engine parameters to start the engine. 
         [0033]    At  100 , a test is made to determine whether the algebraic sum of the crankshaft torque applied by the starter/generator and the torque applied to crankshaft  22  from transmission  16  is less than some torque threshold, which is a calibrated constant value stored in electronic memory. If the result of test  100  is logically false, control returns to  96 . But if the result of test  100  is true, control passes to  96  where a counter is started and periodically incremented to measure time following the start of the counter. 
         [0034]    At  104 , a test is made to determine whether the count  102  has reached a reference count. If the result of test  94  is logically false, control returns to  96 . But if the result of test  104  is true, at  106  the engine is determined to have been started. 
         [0035]    At  108 , starter/generator  18  operates normally without the engine starting procedure control. At  110 , TCM  50  enables input clutches  40 ,  45  to operate in accordance with a transmission control algorithm and to become fully engaged. At  112 , the engine starting procedure is terminated. 
         [0036]    In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.