Abstract:
A method to permit a start time of a heat engine of a vehicle to be controlled. The heat engine is mechanically coupled to a polyphase rotary electrical machine connected to an on-board electrical network. The method is of the type consisting of carrying out pre-fluxing by establishing an excitation current in the inductor for a predetermined pre-fluxing time (Tpref) before the phase currents are established. In accordance with the method, the predetermined pre-fluxing time (Tpref) is a function of the voltage (Vbat+X) of the on-board electrical network. Typically, the predetermined pre-fluxing time (Tpref) is increased when the voltage (Vbat+X) of the on-board electrical network reduces within a nominal voltage range (V  1 , V 2 ).

Description:
This application claims foreign priority benefit under 35 U.S.C. §119 of French patent application no. 08/53425, filed on May 27, 2008, which application is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and a device for controlling the start time of a vehicle heat engine. 
     The invention also relates to a starter-alternator which can carry out the method and comprises said device. 
     2. Description of Related Art 
     Energy saving and pollution reduction considerations, particularly in urban surroundings, have prompted automotive vehicle manufacturers to equip their models with an automatic heat engine stop/restart system such as the system known as the “Stop and Go” system. 
     As discussed in VALEO EQUIPEMENTS ELECTRIQUES MOTEUR&#39;s French patent document FR 2 875 549, vehicles are rendered capable of functioning in “Stop and Go” mode by dint of a reversible electrical machine, or starter-alternator, coupled to a heat engine supplied by an AC/DC converter in “starter” mode. 
     Under certain conditions, using a starter-alternator in the “Stop and Go” functional mode consists of causing the heat engine to stop completely when the vehicle itself is stopped, then restarting the heat engine subsequent, for example, to an action by the driver which is interpreted as a restart command. 
     A typical “Stop and Go” situation is that of stopping at a red light. When the vehicle stops at the light, the heat engine is automatically stopped, then when the light turns green, the engine is restarted using the starter-alternator following detection by the system of the clutch pedal being depressed by the driver, or of any other action which is interpreted as meaning that the driver intends to restart the vehicle. 
     Clearly, the automatic restart function carried out by a starter-alternator system is a function which must be as transparent as possible for the driver of the vehicle. 
     In starter-alternators constituted by a polyphase rotary electrical machine, the phase currents and the excitation current are generally supplied simultaneously by the power circuits at the instant of restarting. 
     US. Pat. No. 6,335,609 states that under such circumstances, the engine torque can only be produced with a perceptible delay. 
     That delay is due to a magnetic flux being established in the rotor, and it proposes pre-fluxing the inductor before the phase currents are established, in order to reduce the time necessary for the heat engine to attain a predetermined rotation speed. 
     However, the method is carried out by controlling the excitation current for a fixed period, and it does not appear to be adapted to starter-alternators which are supplied by an on-board variable voltage electrical network of the “14+X” type. 
     As a result, there is a need for a method and a device which can keep the start time within limits which are acceptable to the driver in the case of an automatic stop/restart system architecture of the micro-hybrid type, where the voltage of the on-board electrical is network depends on the charge state of the ultracapacitor. 
     SUMMARY OF THE INVENTION 
     The present invention aims to satisfy this need and more precisely, it provides a method for controlling the start time for a heat engine of a vehicle, which is mechanically coupled to a polyphase rotary electrical machine connected to an on-board electrical network. 
     The method is of the type consisting of pre-fluxing by establishing an excitation current in the inductor for a predetermined pre-fluxing time before the phase currents are established. 
     According to the invention, the predetermined pre-fluxing time is, in remarkable manner, a function of a variable voltage (termed 14+X volt) of the on-board electrical network. 
     Highly advantageously, said predetermined pre-fluxing time is increased when the voltage of the on-board electrical network drops within a nominal voltage range. 
     In accordance with a further characteristic of the method of the invention, the predetermined pre-fluxing time is in the range from a minimum value below which the start time is greater than a reference threshold, to a maximum value corresponding to magnetic saturation of the inductor. 
     Preferably, the predetermined pre-fluxing time is tabulated for the reference threshold of the start time as a function of the on-board electrical network voltage during a prior step by means of a two-dimensional linear interpolation of the variations in the start time as a function of the pre-fluxing time and a discrete set of values for this voltage. 
     The on-board electrical network is advantageously connected to the terminals of at least one ultracapacitor or the like and for this reason, in accordance with the method of the invention, in remarkable manner, the predetermined pre-fluxing time is also a function of the voltage at the terminals of said ultracapacitor. 
     The invention also concerns a device for controlling the start time of a heat engine of a vehicle adapted to carrying out the method described above. 
     In a manner which is known per se, said heat engine is mechanically coupled to a polyphase rotary electrical machine supplied by power circuits connected to an on-board electrical network and controlled by a control circuit. 
     The device of the invention is remarkable in that it comprises means for determining a pre-fluxing time as a function of a voltage of the on-board electrical network. 
     Preferably, said means are included in the control circuit and comprise a memory containing a tabulation of the pre-fluxing time as a function of the voltage of the on-board electrical network for a reference threshold of the start time. 
     The on-board electrical network is advantageously connected to the terminals of at least one ultracapacitor or the like. 
     A starter-alternator for a heat engine of a vehicle having a regenerative braking function of the micro-hybrid type also highly advantageously includes the control device of the invention described above. 
     These few essential specifications will make apparent to the skilled person the advantages of the invention over the prior art. 
     The detailed specifications of the invention are given in the description below which is made with reference to the accompanying drawings. It should be noted that these drawings are solely for the purposes of illustrating the text of the description and do not in any way constitute a limitation upon the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation of an automatic micro-hybrid type stop/restart system using a start time control device in accordance with the invention; 
         FIG. 2  shows the start-up time of a heat engine in an automatic stop/restart system analogous to that shown in  FIG. 1 , as a function of the voltage of the on-board electrical network, in the absence of the device of the invention; 
         FIG. 3  shows the variations in the start time as a function of the pre-fluxing time and of a discrete set of voltages for the on-board electrical network in an automatic stop/restart system analogous to that shown in  FIG. 1 , in the absence of the device of the invention. 
         FIG. 4  shows the variations in the pre-fluxing time as a function of the voltage of the on-board electrical network in accordance with the method of the invention; 
         FIGS. 5   a  and  5   b  show a constant start time under application of the method of the invention for two different voltages for the on-board electrical network. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the invention concerns vehicles equipped with a starter-alternator with a regenerative braking device of the micro-hybrid type as shown diagrammatically in  FIG. 1 . 
     FIG. I shows a starter-alternator  1  coupled to a vehicle heat engine  2 . 
     Said starter-alternator  1  comprises a reversible excitation type polyphase electrical machine  3  coupled to the engine  2  by means of a belt-and-pulley transmission  4 . 
     The electrical machine  3  comprises a rotor  5  which is fixed to an output pulley  6  at the end of a shaft  7 . The rotor  5  has an inductor  8  supplied via a rotary commutator by an excitation circuit  9 . 
     The machine  3  also includes phase windings  10 , or inductors, supplied via an AC/DC converter  11 . 
     A control circuit  12  controls the power circuits of the machine  3 , constituted by the AC/DC converter  11  and the excitation circuit  9 , depending on information supplied by a position sensor  13  for the rotor  5  and on control signals generated by an electronic control unit for the vehicle. 
     The electronic control unit receives the operating parameters for the engine  2  and other relevant information via dedicated wired connections or via an onboard CAN type data bus. 
     The AC/DC converter  11  is preferably constituted by a chopper circuit for the on-board electrical network voltage Vbat+X generating pulses the frequency and width of which are controlled by the control circuit  12  when the starter-alternator  1  functions in electric motor mode, 
     This chopper circuit  12  is a reversible AC/DC converter which functions in synchronous rectifier mode when the starter-alternator  1  functions in alternator mode. 
     In the micro-hybrid type architecture shown in  FIG. 1 , an on-board electrical network is connected to the terminals of an ultracapacitor  14  instead of being supplied directly by an on-board battery  15 , as is the case for a conventional architecture. 
     When functioning in generator mode, the electrical machine  3  charges the ultracapacitor  14  via the reversible AC/DC converter  11  functioning in rectifier mode and supplies the on-board electrical network with a voltage Vbat+X which is higher than the voltage Vbat in the battery. 
     The on-board electrical network including a DC/DC converter  16  allows electrical energy to be exchanged between the on-board battery  15  and the ultracapacitor  14 . 
     It is proposed, in accordance with a general principle of the invention, in the context of a system carrying out automatic restart functions, to maintain the start time of the heat engine  2  constant regardless of the voltage Vbat+X of the on-board electrical network. 
     As can be seen in  FIG. 2 , in the absence of appropriate corrective measures, the start time Tdem of the heat engine  2  depends on the voltage Vbat+X of the on-board electrical network, i.e. the charge state of the ultracapacitor  14 . 
     The measurements  17  were taken over a fixed pre-fluxing time corresponding to magnetic saturation of the inductor  8 , and for a constant angle profile. 
     When the charge on the ultracapacitor  14  is low, then under these conditions the start time Tdem, defined as the time interval between the time when the electrical machine  3  applies a torque to the heat engine  2  and the time when it reaches a reference rotation speed, may reach unacceptable values having regard to the desired aim of transparency of the system. 
     Thus, a weighting function is proposed which adjusts the start parameters to ensure a mean start time Tdem for a whole range of nominal operational voltages. 
     In the case of an EDLC (electrochemical double layer capacitor) type ultracapacitor  14  comprising, for example, 10 elementary 1500 F cells mounted in series and thus having a resultant capacity of 150 F, and a working voltage of 25 V, the nominal operational range is assumed to be in the range 18 to 24 V. 
       FIG. 3  shows the results of tests carried out on a micro-hybrid system analogous to that shown in  FIG. 1  without a start time control device, varying the pre-fluxing time Tpref and for a plurality of voltages (18 V, 20 V, 22 V and 24 V) for the on-board electrical network Vbat+X. 
     The pre-fluxing time Tpref varies between a minimum value Tpref-min below which the start time is always above a reference threshold Tdem-ref, i.e. below which the start-up function is degraded even at maximum charge of the ultracapacitor  14 , and a maximum value Tpref-max above which magnetic saturation of the inductor  8  is observed. 
     If this reference threshold Tdem-ref is selected to be the start time Tdem which is to be kept constant, then a two-dimensional linear interpolation can be employed to calculate the pre-fluxing time Tpref corresponding to each value for the voltage of the on-board electrical network Vbat+X included in the nominal voltage range. 
       FIG. 4  shows the interpolated values  18  for the pre-fluxing time Tpref which can retain a constant start time Tdem equal to Tdem-ref for any value of the voltage Vbat+X of the on-board electrical network included between the minimum voltage V 1  (minimum charge of ultracapacitor  14 ) and the maximum voltage V 2  (maximum charge of the ultracapacitor  14 ). 
     The consequences of carrying out the method for controlling the start time Tdem of the invention in the automatic micro-hybrid type stop/restart system shown in  FIG. 1  are shown to good effect in timing diagrams  19 ,  20  of figures Sa and  5   b  which indicate the rotation speed N of the starter-alternator  1  as a function of time during the start-up phase. It will be noted in this example that the rotation speed N of the starter-alternator  1  is linked to that Nm of the heat engine  2  by a ratio of approximately 2.4, corresponding to a “starter-alternator/heat engine shaft” pulley ratio for the mechanical transmission. 
     Since the heat engine  2  is stopped, the excitation current is supplied to the rotor  5  from the initial restart command time. The phase currents are only supplied to the stator by the converter  11  at a time to following a delay equal to the minimum value Tpref-min for the pre-fluxing time Tpref in the case in which the voltage of the on-board electrical network Vbat+X is the maximum voltage V 2  ( FIG. 5   a ). 
     The rotation speed N increases from time t 0  to reach a reference rotation speed N 0  at time t 1 , i.e. by definition after a delay t 1 -t 0  equal to the reference start time Tdem-ref. 
     In the case in which the on-board electrical network voltage Vbat+X is the minimum voltage V 1  ( FIG. 5   b ), the phase currents are only provided by the AC/DC converter  11  at a time t 2  after a delay equal to the maximum value, Tpref-max, for the pre-fluxing time Tpref. 
     The rotation speed N reaches the reference rotation speed N 0  at a time t 3  after the same interval, Tdem-ref, as in the preceding case. 
     The analogous timing diagrams show that the interval between the time at which the phase currents are provided and the time at which the reference rotation speed N 0  is reached is constant when the voltage of the on-board electrical network Vbat+X varies across the whole range of nominal voltages V 1 , V 2  provided that the pre-fluxing time Tpref follows the variation law shown in  FIG. 4 . 
     This law is tabulated in a memory of the control device  12  of the starter-alternator  1 , which determines the appropriate pre-fluxing time Tpref depending on the supply voltage Vbat+X which is applied to it. 
     Clearly, the invention is not limited to the single preferred embodiment described above. 
     The measurements and test results are given purely by way of example for a type 144/5 starter-alternator (stator diameter: 144 mm; number of coils: 5) and a 150F/25V EDLC ultracapacitor. 
     The minimum value, Tpref-min, of the pre-fluxing time, Tpref, and the maximum value, Tpref-max, which are suitable for this model are respectively approximately 50 ms and 150 ms when the voltage of the on-board electrical network Vbat+X varies between 18V and 24V; the reference threshold Tdem-ref of the start time Tdem which is retained, which allows the starter-alternator  1  to reach a reference rotation speed N 0  of approximately 1800 rpm, is approximately 440 ms. This reference rotation speed N 0  of approximately 1800 rpm corresponds to a reference rotation speed Nm 0  of the heat engine  2  of approximately 750 rpm (idling speed of the heat engine). 
     The above description is applicable to other starter-alternator models  1  or to other types of energy stores, for example a Ni-MH battery replacing the ultracapacitor  14 , simply by employing numerical values for the parameters which differ from those indicated. 
     Thus, the invention encompasses any possible variation in embodiment which falls within the scope defined by the accompanying claims.