Method for controlling a refilling motor that drives a hydraulic pump

Abstract

Method for controlling a refilling motor (2) that drives a hydraulic pump (4) that refills at least one pressure accumulator (8), this accumulator thereafter delivering the hydraulic pressure (P) to a hydraulic traction machine (20) of a hybrid vehicle, characterized in that it takes account both of the rate of discharge of the pressure accumulator (8) and of the time to start (i) the refilling motor (2) so as to manage this start in such a way that this accumulator more or less reaches its minimum operating pressure level (PI) at the end of this start (t1).

Claims

1. A method for controlling the starting of a refilling motor of a hydraulic traction drive; the hydraulic traction drive comprising said refilling motor, at least one pressure accumulator, and a hydraulic pump; a said hydraulic pump being in fluid communication with the pressure accumulator, and said refilling motor being operatively connected to the hydraulic pump to drive the hydraulic pump in order to refill the at least one pressure accumulator; the at least one pressure accumulator delivering the hydraulic pressure to a hydraulic power machine of a hybrid vehicle, the at least one pressure accumulator varying between a minimum operating pressure (P1) and a maximum pressure (P2); the method comprising: monitoring the rate of discharge of pressure from the pressure accumulator (dP/dt); determining a start-up pressure (P1); said start-up pressure being greater than the minimum operating pressure (P1) and being the pressure of the at least one pressure accumulator at a time t1t, wherein t1 is the calculated start-up time at which the pressure in the accumulator will be at said minimum operating pressure (P1) and t is the time required to start the refilling motor; whereby, said start-up pressure (P1) is determined as a function of dP/dt and t; and starting the refilling motor when the pressure in the accumulator drops to the start-up pressure (P1), such that when the pressure accumulator reaches approximately its minimum operating pressure level (P1) at the end of the start-up time (t1), the refilling motor will be started and ready to begin recharging the at least one pressure accumulator.

2. The control method according to claim 1, including a step of computing an available energy level contained in the pressure accumulator from the continually measured pressure in the pressure accumulator and from the minimum operating pressure level (P1), and a step of comparing the available energy level with the hydraulic power consumed by the hydraulic power machine in order to determine the start-up pressure corresponding to the start-up of the refilling motor.

3. The control method according to claim 2, wherein the power consumed is computed by measuring the variation of pressure of the accumulator with respect to time (dP/dt).

4. The control method according to claim 1, wherein, for an accumulator containing a pressurized gas, the minimum pressure level (P1) of the accumulator is greater than the residual gas pressure (P0) corresponding to complete discharging of the fluid pressure in the hydraulic circuit.

5. A power train for a hybrid vehicle which includes a refilling motor that drives a hydraulic pump in fluid communication with at least one pressure accumulator in order to refill the at least one pressure accumulator; said at least one pressure accumulator supplying power to a hydraulic power machine of this vehicle; wherein the vehicle includes a controller which computes an available energy level (E) contained in the pressure accumulator as a function of the continually measured pressure (P) in this accumulator and the minimum operating pressure (P1), and a comparator which compares the available energy level (E) with the hydraulic power consumed (Pconso) by the hydraulic power machine in order to determine a start-up pressure (P1) of the pressure accumulator corresponding pressure of the pressure accumulator at a period of time At before the time t1 when, based on the power consumption, the pressure accumulator is estimated to reach said minimum operating pressure P1; wherein t1 is the calculated start-up time at which the pressure in the accumulator will be said minimum operating pressure (P1) and At is the time required to start the refilling motor; and said controller being operable to start the refilling motor when the pressure in the pressure accumulator reaches the start-up pressure (P1) such that the refilling motor will be started and ready to begin recharging the pressure accumulator when the pressure in the accumulator reaches the minimum operating pressure P1 at time t1.

6. The power train according to claim 5 wherein the pump and the hydraulic machine have variable displacements.

7. The power train according to claim 5 wherein the refilling motor is an internal combustion engine.

8. The power train according to claim 5 wherein the accumulator contains a pressurized gas.

9. A hybrid vehicle having a power train according to claim 5.

Description

DESCRIPTION OF THE FIGURES

(1) The invention will be better understood and other characteristics and advantages will appear more clearly upon reading of the description below given as an example and in a non-limiting manner, in reference to the appended drawings in which:

(2) FIG. 1 is a diagram of a power train that implements the control method according to the invention;

(3) FIG. 2 presents three views in axial section of a hydraulic accumulator of this power train, successively including three filling levels A, B and C;

(4) FIG. 3 is a time-based graph presenting the start-ups of the refilling motor; and

(5) FIG. 4 is a block diagram of the control method.

DETAILED DESCRIPTION

(6) FIG. 1 presents a power train 1 of a vehicle which includes an internal combustion engine 2 which drives a variable displacement pump 4. The variable displacement pump 4 pumps the hydraulic fluid from a tank 6 in order to fill a pressure accumulator 8.

(7) The pressure accumulator 8 has a flexible membrane 12 separating a pressurized gas 10 generally containing nitrogen from the fluid which is maintained continually at a pressure approximately equivalent to that of the gas.

(8) The pressurized fluid is then used by a variable displacement hydraulic machine 20 operating as motor, which is connected to a differential 22, and which drives the two driving wheels 24 of a given axle of the vehicle, distributing the power between these wheels. In contrast, during braking of the vehicle, the driving wheels 24 can drive the hydraulic machine 20 operating as pump, in order to refill the pressure accumulator 8 and thus to recover kinetic energy of the vehicle during the braking phases or when going downhill.

(9) In order to optimize the functioning of the internal combustion engine 2, particularly in order to reduce the number of start-ups of this engine, and to deliver high power during its periods of operation, one seeks to store the largest possible quantity of energy in the pressure accumulator 8 and to use the largest possible quantity of energy in a single time.

(10) It should be noted that in the case of an electric motor instead of the internal combustion engine, reducing the frequency of start-ups of this electric motor can be sought as well, particularly in order to reduce the number of start-ups leading to current consumption peaks.

(11) FIG. 2 presents a hydraulic pressure accumulator 8 which, in a first case A, is completely discharged, the hydraulic pressure circuit being, for example, open and put directly in connection with the tank 6.

(12) In this case, the gas 10 maximally expands, and the membrane 12 descends until coming in contact with a lower stop 14 which limits the expansion of the membrane and beyond which the membrane cannot expand. The internal pressure of the gas 10 is then the residual pressure P0, and its maximum volume V0 represents the nitrogen capacity of this accumulator 8.

(13) In a second case B, the pressure accumulator 8 is slightly filled by the pressure of the fluid, the membrane 12 being in its low operating position, which gives a minimum pressure threshold P1 of the gas 10, and a maximum operating volume V1 of this gas.

(14) The low operating position of the membrane 12 is the lowest that is continually acceptable without risk of deterioration of the membrane, in order to guarantee the reliability of the accumulator 8. The minimum operating pressure of the hydraulic circuit is therefore approximately equal to P1.

(15) In a third case C, the pressure accumulator 8 is highly filled by the pressure of the fluid, the membrane 12 being in its maximum high operating position, which gives a maximum pressure P2 in the gas 10, and a minimum volume V2 of this gas.

(16) The gas 10 then undergoes, between these two volumes, maximum volume V1 and minimum volume V2, a volume difference V which, multiplied by the corresponding pressure, represents stored energy that can be restored.

(17) In practice, the pressure of the gas 10 should oscillate between its minimum value P1 and its maximum value P2, going if possible from one extreme value to the other during operation of a cycle of refilling and discharging of the pressure accumulator 8, in such a way as to store and then use in a single time the greatest possible quantity of energy.

(18) The graph of FIG. 3 presents, as a function of time t, in its upper part, the change 30 of the pressure P of gas in the pressure accumulator 8, and in its lower part, the start-ups of the internal combustion engine 2, noted A for stop and D for start-up.

(19) With the accumulator 8 filled initially to its maximum pressure P2, it is discharged regularly during powering of the vehicle driven by the hydraulic machine 20 which is used as a pump. The internal combustion engine is at a stop A.

(20) A computer for management of the power train 1, that implements the control method, measures both the pressure P in the pressure accumulator 8 and its pressure variation over time or rate of discharge dP/dt, which is connected with the use of the fluid by the hydraulic machine 20 leading to a flow of this fluid.

(21) It should be noted that the fluid flow varies particularly as a function of the pressure P in the accumulator 8, of the adjustment of the displacement of the hydraulic machine 20, of the speed of the vehicle, of the torque on the driving wheels requested by the driver, and of the resisting torque due to the running conditions. The management computer must continually check the rate of discharge dP/dt of the accumulator 8, which can change.

(22) At time t0, the pressure being P1 slightly greater than P1, the method detects that, with the current rate of discharge dP/dt of the accumulator 8, its pressure will reach the minimum threshold P1 at time t1 corresponding to the end of the start-up D of the internal combustion engine 2, the pump 4 beginning in effect to refill the accumulator. The time difference t between times t0 and t1 corresponds to the time necessary for the start-up of this internal combustion engine 2.

(23) Starting from the time t1 when the pressure has descended to the minimum pressure threshold P1, there is then a rise again of the pressure up to the maximum pressure P2, corresponding to time t2 when the internal combustion engine will again be at a stop A.

(24) The discharging of the pressure accumulator 8 can then begin again with, for the next cycle, an identical time difference t between times t3 and t4, but with a start of time t3 corresponding to a pressure P1 greater than P1, because in this case, the lowering of pressure with respect to time dP/dt is greater than the preceding one, the hydraulic machine 20 consuming more fluid flow.

(25) The block diagram of FIG. 4 presents a means for implementation of the control method.

(26) A controller or computation means 42 computes, from the continually measured pressure P in the accumulator 8, and from the minimum threshold pressure P1 recorded in memory, the available energy level E contained in this accumulator.

(27) Then a comparator 44 compares the available energy level E in the accumulator 8 with the power consumed Pconso by the hydraulic machine 20 at that time, which represents a variation of energy E in time, in order to determine the start-up pressure P1 that will trigger the start-up of the internal combustion engine 2. It is thus ensured that, taking into account the power consumed Pconso, the time difference t necessary for the start-up of the internal combustion engine 2, which is recorded in memory, is complied with.

(28) An optimization of the functioning of the power train 1 is thus achieved in a simple, robust and effective manner. Moreover, by integrating the control method in the management computer for the power train 1, and by using existing sensors, particularly for the measurement of the pressure of the fluid, this method can be carried out economically.

(29) As a variant, a similar method for controlling can be used for the management of the start-up of the internal combustion engine 2, in order to refill several pressure accumulators connected in parallel.