Abstract:
A method for restarting a vehicle engine that is stopped at a known crank angle includes actuating a clutch located in a torque path between a starting motor and the engine with desired pressure related to the known crank angle during the restart, and using the starting motor to drive the engine during the restart.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates generally to a method for starting an engine of a hybrid electric vehicle (HEV) by transmitting torque from a starting motor through a clutch and damper to the engine. 
         [0003]    2. Description of the Prior Art 
         [0004]    A modular hybrid transmission (MHT) is an arrangement of powertrain components that includes an internal combustion engine, torsion damper, disconnect clutch, electric motor/generator and torque converter arranged in series ahead of an automatic transmission. The electric machine operates as a motor to crank the engine during starting using a high voltage battery as a source of power for the motor. 
         [0005]    In an MHT system a key objective is to start the engine consistently and quickly using the least amount of starting motor reserve torque. Using a one-size fits all disconnect clutch pressure profile may produce engine starts with varying accelerations, which presents problems such as fuel/air cranking calibrations, possible no-starts, etc. Increased starting torque requires more reserve starting torque from the starting motor. 
         [0006]    Varying the operative pressure profile of the disconnect clutch based on speed is likely difficult to control due its reliance on information that becomes available too late for ideal application to the engine stating procedure. Furthermore, a vehicle system controller (VSC) controls electric machine torque and possibly speed. A disconnect clutch pressure controller adjusts clutch actuation pressure based on engine speed or acceleration, thereby producing the potential for engine speed control difficulty. A VSC receives input from the vehicle operator, coordinates the engine and electric machine, and may disconnect the clutch and transmission. 
       SUMMARY OF THE INVENTION 
       [0007]    A method for restarting a vehicle engine that is stopped at a known crank angle includes actuating a clutch located in a torque path between a starting motor and the engine with desired pressure related to the known crank angle during the restart, and using the starting motor to drive the engine during the restart. 
         [0008]    The method uses different open loop pressure profiles for the disconnect clutch pressure control based on the position of the engine when it stops. 
         [0009]    The disconnect clutch pressure profile established how much electric machine torque will be directed to cranking the engine. If the disconnect clutch pressure profile changes based on the stopping position of the engine, a reduction of torque required to crank the engine may be realized or anticipated. 
         [0010]    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 
         [0011]    The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
           [0012]      FIG. 1  is a schematic diagram showing an arrangement of components related to the powertrain for a HEV; 
           [0013]      FIG. 2  is graph showing the relation between engine speed and time while starting an engine that had been stopped at 60 degrees with starting torque low and high; 
           [0014]      FIG. 3  is graph showing the relation between engine speed and time while starting an engine that had been stopped at 10 degrees with starting torque low and high; 
           [0015]      FIG. 4  is graph showing the relation between initial crank position and time for the engine to reach 300 rpm; 
           [0016]      FIG. 5  shows various disconnect clutch pressure profiles and the corresponding engine speed variation during an engine restart; and 
           [0017]      FIG. 6  shows a variation of one of the pressure profiles of  FIG. 5  and the corresponding engine speed variation during an engine restart. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]      FIG. 1  illustrates an MHT configuration of powertrain  10  components that includes an internal combustion engine  12 , an engine disconnect clutch  14 , a high voltage battery  16 , a high voltage to low voltage DC/DC converter  18 , low voltage battery  20 , low voltage starter  22 , torsion damper  24 , electric machine  26 , torque converter  28 , torque converter bypass clutch  30 , transmission gear box  32 , driveshaft  34 , final drive gearing  36 , halfshafts  38 ,  40 , and driven wheels  42 ,  44 . 
         [0019]    The torsion damper  24  comprises a coiled spring or a mechanism that includes multiple coiled springs, wherein torsion applied to the damper causes displacement of the spring mechanism. Torsional energy is dissipated by the damper  24  due to frictional contact between the moving springs and the walls of a damper casing containing the springs. 
         [0020]    A main transmission pump  46 , driven by the engine  12 , supplies pressurized hydraulic fluid to the hydraulic system of the transmission  32  and the torque converter  28 . An auxiliary oil pump, driven by an electric motor (not shown), supplies pressurized hydraulic fluid to the hydraulic system of the transmission  32  and the torque converter  28  when the engine is off. 
         [0021]    The internal combustion engine (ICE)  12  is connected to the electric machine  26  and transmission  32  through the disconnect clutch  14 , which can engage and disengage the engine from the powertrain to satisfy operational requirements of the hybrid vehicle in different modes. 
         [0022]    The high voltage electric machine  26  is secured to the impeller shaft  50  of the torque converter  28 . The electric machine  26  is powered by the high voltage battery  16 . 
         [0023]    The HEV powertrain  10  could share the same transmission hardware with conventional vehicles but different control algorithm, e.g. a regular step ratio transmission could be used in the powertrain to drive the vehicle. 
         [0024]    The torque converter  28  used in this configuration is preferably identical to the torque converter used in conventional automatic transmissions. When bypass clutch  30  is open, differential speed between the transmission input shaft  52  and the impeller shaft  50  is possible. When the bypass clutch  30  is closed the torque converter impeller and turbine are mechanically connected, in which case the speed of the electric machine  26  and transmission input  52  are substantially identical. 
         [0025]    Alternatively, other types of automatic transmissions can be used in the powertrain  10 , e.g. a continuously variable transmission (CVT) having a drive belt engaged with a two pulleys, or an automatic manual transmission, or other HEV technologies. The overall hybrid operation is similar but details of the mechanism disconnecting the motor from the transmission are different. 
         [0026]    The torsion damper  24  is a mechanical component having the primary function of modulating or eliminating high frequency torsional vibration from the powertrain  10 . The engine  12  is cranked to start by the high voltage motor  26 . 
         [0027]    Engine cranking torque required to pull up an engine varies significantly base on the position of engine at crank. Less torque is required to start an engine when an engine piston  70  is advancing close to top dead center in its cylinder than when the cylinder is farther from, but approaching top dead center. 
         [0028]    The torque required to overcome the first and second compression strokes of an engine, when engine speed is low and compression energy is lost, i.e., does not drive the engine crankshaft during the expansion stroke, will change based on the crank angle at which the engine is stopped. The crank angle varies between 0 degrees and 720 degrees for a four stroke engine. 
         [0029]      FIG. 2  shows that for an engine stopped at 60 degrees BTDC, the first few compression strokes of a starting engine waste energy and provide no compression help on the expansion stroke. When starting torque is low  82 , the period length for engine speed to reach 300 rpm is longer than when starting torque is higher  84 . 
         [0030]      FIG. 3  shows that for an engine stopped at 10 degrees BTDC, after the second compression stroke energy from the compressed air-fuel mixture on the expansion stroke increases reducing the period length required for engine speed to reach 300 rpm. 
         [0031]      FIG. 4  shows that over a range of engine crank positions when a relatively low magnitude of cranking torque is applied, the engine may not accelerate. 
         [0032]      FIG. 5  shows a pressure profile  90  for disconnect clutch  14  when the engine  12  is stopped at 60 degrees BTDC, as determined from an electronic signal representing an engine crank angle produced by sensor  91 . When hydraulic pressure of 56.5 psi is supplied to clutch  14 , the torque transmitting capacity of the clutch is 73 lb-ft. Curve  92  shows the corresponding increase of engine speed during a period  102  required for engine speed to reach 300 rpm using clutch pressure profile  90 . 
         [0033]    Similarly,  FIG. 5  shows a pressure profile  94  for disconnect clutch  14  when the engine  12  is stopped at 10 degrees BTDC. When hydraulic pressure of 52.5 psi is supplied to clutch  14 , the torque transmitting capacity of the clutch is 65 lb-ft. Curve  96  shows the corresponding increase of engine speed during the period  102  required for engine speed to reach 300 rpm using clutch pressure profile  94 . 
         [0034]    The clutch pressure profile  98  for disconnect clutch  14  when the engine  12  is stopped at 60 degrees BTDC with hydraulic pressure of 62.5 psi supplied to clutch  14 , produces 85 lb-ft of clutch torque transmitting capacity. Curve  104  shows that the engine speed corresponding to clutch pressure profile  98  increases rapidly to 300 rpm. 
         [0035]    The engine start produced by pressure profile  98  is premature, i.e., occurs over a period  106  that is too short for the operating conditions or vehicle operator&#39;s expectations, and wastes energy, which is supplied by starting motor  26 . 
         [0036]    Curve  108  shows that an alternate engine start that is produced by pressure profile  98  is delayed, i.e., requires a period  110  that is too long for the engine speed to reach 300 rpm, particularly so when the engine start is initiated by the vehicle operator&#39;s depressing the accelerator pedal. Preferably the period  102  for engine speed to reach 300 rpm has a consistent length. 
         [0037]    Each of the disconnect clutch pressure profiles  90 ,  94 ,  98  determines how much electric machine torque will be directed to cranking the engine  12 . If the disconnect clutch pressure profile changes based on the stopping position of the engine, a reduction of torque required to crank the engine may be realized or anticipated. 
         [0038]      FIG. 6  shows a variation  112  of the disconnect clutch pressure profile  90  of  FIG. 5  and the corresponding engine speed variation  114  during an engine restart. The desired pressure profile  11 , applicable when the engine  12  is stopped at 60 degrees BTDC, provides a stepwise increase in clutch pressure when needed at  114  instead of the linear increase of pressure profile  90  whose peak magnitude supplied to clutch  14  is of 56.5 psi. Curve  114  shows the corresponding increase of engine speed during a period  112  required for engine speed to reach 300 rpm. 
         [0039]    The open loop pressure profiles for disconnect clutch pressure control are selected and applied to crank and start engine  12  with reference to the angular position of the engine, i.e., the crank angle of the stopped engine, and the basis for a command to restart the engine. 
         [0040]    For example, if the vehicle is operating in electric mode with the engine stopped, and the state of charge of battery  16  is low, the powertrain controller will issue a command to restart the engine using the electric machine  26 . An engine restart under such condition is preferably smooth, of high quality and occurs over a consistent length  102 . The engine restart occurs at relatively low cranking torque with the desired disconnect clutch pressure profile being  90  or  94 , depending on the crank angle position of engine  12  while stopped. 
         [0041]    But if the vehicle operator initiates an engine restart, such as by depressing the accelerator pedal  124 , the engine restart occurs over a relatively short period  106  at relatively high cranking torque. Under such operating conditions the engine restart may be less smooth and of shorter duration and the desired disconnect clutch pressure profile is  98  depending on the crank angle position of engine  12  while stopped. 
         [0042]    In order to facilitate sustained engine combustion following engine cranking, the magnitude of pressure applied to clutch  14  decreases. After combustion becomes sustained in engine  12 , the magnitude of pressure applied to clutch  14  increases to a magnitude that is able to transmit engine torque through the electric machine  26 , torque converter  28 , transmission gearing  32  and final drive  36  to the driven wheels  42 ,  44 . 
         [0043]    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.