Abstract:
A method for assisting a motor vehicle with a hill start, the motor vehicle having previously been held stationary when parked by a brake. The method measures inclination of the vehicle, previously establishes a map between engine torque and engine speed, defines, within the map, an authorized zone and a forbidden zone, provides a vehicle engine speed value, provides a vehicle engine torque value, deduces from these coordinates of the engine operating point within the map, and releases the brake automatically only if the coordinates of the operating point lie within the authorized zone of the map.

Description:
BACKGROUND 
     The present invention relates to the field of assisting with hill starts, otherwise called take-offs, for a motor vehicle. 
     More precisely, the invention relates to a method for assisting with a hill start for a motor vehicle previously held stationary by a brake, the method comprising the steps consisting in:
         measuring the inclination of the vehicle, and   automatically releasing the brake.       

     The problem to be solved for such methods is to prevent a possible stalling of the engine. Specifically, for certain engines, in particular low-torque engines, it is possible that the difference between the real engine torque, delivered in particular when the motor vehicle is started, and the engine torque necessary for taking off cannot be compensated for (typically by pressing on the accelerator). 
     To solve such a problem, it is known practice to use multiple sensors on the motor vehicle. 
     However, the use of sensors, in addition to their intrinsic cost, in parallel requires sometimes complex electronic management of the data supplied by these sensors, which may furthermore increase the computing time of the electronic control unit (ECU). 
     BRIEF SUMMARY 
     The object of the present invention is to remedy these drawbacks by proposing a solution that is simple to apply and aiming to minimize the number of sensors necessary, hence in particular the cost of applying such a method. 
     With this objective in mind, the device according to the invention, moreover according to the preamble cited above, is essentially characterized in that it also comprises the steps consisting in:
         previously establishing a chart between the engine torque and the engine speed,   defining in the chart a zone authorized for a hill start and a zone forbidden for such a start,   providing a value of the engine speed of the vehicle,   providing a value of the engine torque of the vehicle,   deducing therefrom the coordinates of the point of operation of the engine in the chart, and   not automatically releasing the brake unless the coordinates of the point of operation are in the authorized zone of the chart.       

     By virtue of this feature, only one sensor of the inclination of the motor vehicle may be necessary. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will appear more clearly on reading the following description given as an illustrative and nonlimiting example made with reference to the appended figures in which: 
         FIG. 1  illustrates an embodiment of a chart according to the invention, and 
         FIG. 2  illustrates an embodiment of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     To illustrate the principle of the invention, consideration is given to a motor vehicle previously held stationary by a brake. 
     The brake may be a parking brake or the footbrake of the motor vehicle. That is to say that the invention may be applied irrespective of the type of engine, irrespective of the engine power, and above all irrespective of the type of gearbox. 
     During a hill start, it is increasingly desirable to obtain assistance with said hill start. In the case of a vehicle parked on an upslope (positive slope) and having to move forward, it is possible, without assistance, for the motor vehicle to move backward for an instant corresponding to the difference between the moment when the driver releases the brake and the moment when the engine torque is sufficient to drive the motor vehicle, and this occurs all the more so when the slope is great and the motor vehicle is laden and therefore has a considerable weight. 
     Moreover, it may happen, particularly for an engine with low engine torque, that the latter stalls. This stalling is due to the inability of the engine to deliver the torque necessary for the take-off of the motor vehicle, combined with a reduction in the speed of said engine. 
     To solve these problems with a minimum of sensors, according to the invention, a chart of the engine is established. Such a chart is illustrated in  FIG. 1  and corresponds to the engine torque CPL delivered according to the engine speed REG. 
     Such a chart is obtained for example on an engine test bench before the vehicle is placed in service, or established during running phases of the motor vehicle. Its form and its values depend on the type of engine, so the graphic units are not indicated in  FIG. 1 . 
     For a given engine, the envelope of its chart, as illustrated in  FIG. 1 , is independent of the vehicle weight and of the slope on which the latter is likely to be. 
     One step of the method according to the invention consists in supplying a preferably instantaneous value of the engine speed of the vehicle. This step is advantageously applied by the measurement of said speed, by any means known to those skilled in the art. 
     Another step of the method according to the invention consists in supplying a preferably instantaneous value of the engine torque of the vehicle. This step is advantageously applied by an estimate of said torque, by any means known to those skilled in the art, including for example by measurement of the injection time. 
     Once the values of torque CPL(t) and of speed REG(t) are established at a given moment t, another step of the method according to the invention consists in deducing therefrom the coordinates of the operating point at the time t of the engine in the chart. 
     The coordinates of the operating point at the time t are advantageously established in real time. 
     According to the invention, one step consists in defining in the chart an authorized zone and a forbidden zone. These zones are described subsequently and correspond in this instance in  FIG. 1  to the subzones Z 1 , Z 2 , Z 3  and Z 4  for the authorized zone, and to the subzones Z 5  and Z 6  for the forbidden zone. 
     The authorized zone corresponds to the zone in which the engine, through the driver of the motor vehicle or through means of control for this purpose, is capable of achieving an engine speed allowing it to deliver sufficient torque for the take-off, that is to say sufficient to overcome the slope and the dispersions (dispersion of the slope sensor, of vehicle weight, of the engine speed and of the engine torque). 
     Once obtained, the coordinates of the operating point then make it possible to position the latter in the chart and to deduce therefrom whether this point is positioned in the authorized zone or in the forbidden zone. 
     If the coordinates of the operating point are in the authorized zone of the chart, the brake is automatically released, for example control means can transmit a signal to this effect. 
     It should be noted that, in one embodiment, the motor vehicle is fitted with a footbrake and with means for controlling said footbrake configured to keep the pressure on said brake, hence hold the motor vehicle, between the moment when the driver releases said brake and the moment when the engine torque is sufficient to drive the motor vehicle. In this embodiment, the command means operate the means for controlling said footbrake. 
     Thanks to the invention, it is possible to use only one sensor, making it possible to measure the inclination of the vehicle. 
     Measuring the inclination of the vehicle, that is to say the slope α on which the latter is situated, advantageously makes it possible to define the authorized zone and the forbidden zone in a dynamic manner as described below. 
     Thanks to the invention, it is possible to apply assistance with take-off, without measuring or estimating the weight of the motor vehicle. 
     However, the weight of the motor vehicle has an influence on the torque needed to be delivered by the engine to allow take-off. 
     Therefore, in a schematic manner, the steeper the slope and the heavier the motor vehicle, the more engine torque must be delivered to allow take-off. 
     To be able to dispense with the need to know the vehicle weight, the authorized zone and the forbidden zone are advantageously defined as described below. 
     Specifically, when the chart is established, in particular its envelope, for example on a test bench, it is not possible to know what will be the laden weight of the motor vehicle, the latter depending on the number and the weight of any passengers, and of any baggage. 
     However, usually in a regulatory manner, a minimal weight of the motor vehicle, called the unladen weight, and a maximal authorized laden weight, called the laden weight, is defined. These weights are known for a given motor vehicle. 
     According to the invention, it is therefore possible to define a maximal take-off torque CPL_P(α) and a minimal take-off torque CPL_V(α), illustrated in  FIG. 1 . 
     The maximal take-off torque CPL_P(α) corresponds to the engine torque necessary to make a motor vehicle take off when laden. 
     The minimal take-off torque CPL_V(α) corresponds to the engine torque necessary to make an unladen motor vehicle take off. 
     As seen above, these two engine torques CPL_P(α) and CPL_V(α) depend on the slope α. 
     Therefore, for two slopes α1 and α2 such that α2&gt;α1, CPL_P(α2)&gt;CPL_P(α1) and CPL_V(α2)&gt;CPL_V(α1). 
     Measuring the slope α, that is to say the inclination of the vehicle, makes it possible to determine the values of the thresholds of laden take-off torque CPL_P(α) and of unladen take-off torque CPL_V(α), which will have been calibrated in advance. 
     Thanks to these threshold values, it is possible advantageously to define dynamically the authorized zone and the forbidden zone in the chart. 
     Accordingly, typically, the forbidden zone comprises a first subzone (Z 5  in  FIG. 1 ) corresponding to any delivered engine torque value that is below the unladen take-off torque CPL_V(α), irrespective of the engine speed. 
     If the coordinates of the operating point correspond to a point in subzone Z 5 , it is then considered, according to the invention, that the delivered engine torque is insufficient for the unladen take-off and that, in these conditions, the brake must not be automatically released. 
     Advantageously, it is therefore possible to subdivide the authorized zone and/or the forbidden zone into a plurality of subzones. 
     Accordingly, the authorized take-off zone comprises in particular a range including any point of operation the coordinates of which are such that the delivered engine torque is greater than the unladen take-off torque CPL_V(α), and the engine speed of which is greater than the minimal nominal engine speed REG_N_min(α) ( FIG. 1 ). 
     This range corresponds to the subzones Z 1 , Z 2  and Z 3  in  FIG. 1 . 
     More precisely, the authorized zone may therefore include a first subzone Z 1  in which the engine torque is greater than the laden take-off torque CPL_P(α) corresponding to the total of the unladen take-off torque CPL_V(α) and the dispersion torque CPL_P(α)-CPL_V(α). 
     The intersection of the envelope of the chart and the value of the laden take-off torque CPL_P(α) defines two engine speeds: a maximal nominal engine speed REG_N_max(α) and a minimal nominal engine speed REG_N_min(α). 
     The minimal nominal engine speed REG_N_min(α) is the engine speed short of which the engine torque is below the laden take-off torque CPL_P(α). 
     The maximal nominal engine speed REG_N_max(α) is the engine speed beyond which the delivered engine torque becomes less than the laden take-off torque CPL_P(α). 
     As seen above, the laden take-off torque CPL_P(α) and the unladen take-off torque CPL_V(α) advantageously depend on the slope α. Consequently, the minimal nominal engine speed REG_N_min(α) and the maximal nominal engine speed REG_N_max(α) depend likewise advantageously on the slope α. 
     The authorized zone may comprise a second subzone Z 2  in which the engine torque is greater than the unladen take-off torque CPL_V(α) (and less than the laden take-off torque CPL_P(α)), and the engine speed between the minimal nominal engine speed REG_N_min(α) and the maximal nominal engine speed REG_N_max(α). 
     The authorized zone may comprise a third subzone Z 3  in which the engine speed is greater than the maximum nominal speed REG_N_max(α) and the engine torque is greater than the unladen take-off torque CPL_V(α). 
     This third subzone Z 3  forms part of the authorized zone because, even through the delivered engine torque is in this subzone below the laden take-off torque CPL_P(α), a simple lowering of the engine speed, by the driver of the motor vehicle or in a controlled manner, easily makes it possible to find an engine speed between the minimal nominal engine speed REG_N_min(α) and the maximal nominal engine speed REG_N_max(α) corresponding to the first subzone Z 1  or to the second subzone Z 2 , hence corresponding to the ability to deliver an engine torque sufficient for the take-off of the motor vehicle. 
     The authorized zone may also comprise a fourth subzone Z 4  in which the engine torque is greater than the total of the unladen take-off torque CPL_V(α), the speed-increase torque and the movement-limit torque, and in which the engine speed is below the minimal nominal engine speed REG_N_min(α). 
     In this subzone Z 4 , although the engine speed is below the minimal nominal engine speed REG_N_min(α), it is considered that, since the delivered engine torque is greater than the total of the unladen take-off torque CPL_V(α), the speed-increase torque, and the movement-limit torque, the torque necessary for the engine speed to reach the minimal nominal engine speed REG_N_min(α) is capable of being delivered by the engine. 
     Conversely, if the delivered engine torque is below the total of the unladen take-off torque CPL_V(α), the speed-increase torque, and the movement-limit torque; and the engine speed is below the minimal nominal engine speed REG_N_min(α), the torque necessary for the engine speed to reach the minimal nominal engine speed REG_N_min(α) is not capable of being delivered by the engine and it is considered that take-off must not be authorized (subzone Z 6 ) because the motor vehicle is likely to stall. 
     The invention can be illustrated also in a flow chart manner in  FIG. 2 . 
       FIG. 2  illustrates an embodiment of the method according to the invention, in particular in algorithmic form. 
     The algorithm according to the invention is advantageously applied by a computer program. The computer program according to the invention comprises program code instructions for the execution of the steps of the method as defined above when said program is executed on a computer. 
     In a first step, a value of the engine torque CPL is supplied. 
     Then, in a second step, the value of the engine torque is compared with the laden take-off torque CPL_P(α). If the difference is positive, take-off is authorized. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 1 . 
     Otherwise, a computation is made to determine whether the engine torque CPL is greater than the unladen take-off torque CPL_V(α), and whether the engine speed is between the minimal nominal engine speed REG_N_min(α) and the maximal nominal engine speed REG_N_max(α). 
     In the affirmative, take-off is authorized. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 2 . 
     In the negative, still with CPL&gt;CPL_V(α), the speed REG is compared with the maximal nominal engine speed REG_N_max(α). 
     If the speed REG&gt;REG_N_max(α), then take-off is authorized. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 3 . 
     If the speed REG&lt;REG_N_min(α), and the engine torque CPL is greater than the total of the unladen take-off torque CPL_V(α), the speed-increase torque, and the movement-limit torque, then take-off is authorized. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 4 . 
     If the delivered engine torque CPL is below the total of the unladen take-off torque CPL_V(α), the speed-increase torque, and the movement-limit torque; and if REG&lt;REG_N_min(α), take-off is forbidden. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 6 . 
     Similarly, if the delivered engine torque CPL is below the unladen take-off torque CPL_V(α), take-off is forbidden. This step makes it possible to determine whether the coordinates of the point of operation are in the subzone Z 5 . 
     The steps mentioned above are not necessarily sequential. If they are sequential, other sequences for the order of the steps are possible.