ACCUMULATOR RACKS

Abstract

A method of charging a hydro-pneumatic energy storage system is described. The system has a first hydro-pneumatic accumulator with a first hollow vessel. Disposed within the first hollow vessel is a first compressible volume containing a first amount of gas. The system has a second hydro-pneumatic accumulator with a second hollow vessel. Disposed within the second hollow vessel is a second compressible volume containing a second amount of gas. The gas contained in the first volume is pre-pressurized to a first hydrostatic pre-charge pressure and the gas contained in the second volume is pre-pressurized to a second hydrostatic pre-charge pressure. The second pre-charge pressure is higher than the first pre-charge pressure. In addition, the gas in the first volume is pressurized by discharging a non-compressible hydraulic fluid into the first vessel while keeping a quantity of non-compressible hydraulic fluid contained in the second vessel constant to keep the pressure of the gas contained in the second volume at the second pre-charge pressure.

Claims

1-13. (canceled)

14. A method of charging a hydro-pneumatic energy storage system, the energy storage system comprising a first hydro-pneumatic accumulator comprising a first hollow vessel and, disposed within the first hollow vessel, a first compressible volume containing a first amount of gas, and the system comprising a second hydro-pneumatic accumulator comprising a second hollow vessel and, disposed within the second hollow vessel, a second compressible volume containing a second amount of gas, the method comprising the steps of: pre-pressurizing the gas contained in the first volume to a first hydrostatic pre-charge pressure and pre-pressurizing the gas contained in the second volume to a second hydrostatic pre-charge pressure, the second pre-charge pressure being higher than the first pre-charge pressure; pressurizing the gas in the first volume by discharging a non-compressible hydraulic fluid into the first vessel while keeping a quantity of non-compressible hydraulic fluid contained in the second vessel constant to keep the pressure of the gas contained in the second volume at the second pre-charge pressure; and, when the pressure of the gas in the first volume reaches the second pre-charge pressure, pressurizing the gas in the second volume by discharging a non-compressible hydraulic fluid into the second vessel.

15. The method of claim 14, further including the step of keeping the quantity of non-compressible hydraulic fluid contained in the first vessel constant to keep the pressure of the gas in the first volume at the first maximum pressure when the pressure of the gas in the first volume reaches a first maximum pressure which is equal to or higher than the second pre-charge pressure.

16. The method of claim 14, wherein pre-pressurizing the gas contained in the first volume to the first pre-charge pressure includes adjusting the first amount of gas, and wherein pre-pressurizing the gas contained in the second volume to the second hydrostatic pre-charge pressure includes adjusting the second amount of gas.

17. The method of claim 14, wherein the non-compressible hydraulic fluid is discharged into the first vessel and into the second vessel through a hydraulic displacement unit driven by an engine or by a vehicle output of an automotive vehicle.

18. A method of discharging a hydro-pneumatic energy storage system, the system comprising a first hydro-pneumatic accumulator comprising a first hollow vessel and, disposed within the first vessel, a quantity of a non-compressible hydraulic fluid and a first volume containing a first amount of gas at a first maximum hydrostatic pressure, and the system comprising a second hydro-pneumatic accumulator comprising a second hollow vessel and, disposed within the second vessel, a quantity of a non-compressible hydraulic fluid and a second volume containing a second amount of gas at a second maximum hydrostatic pressure, the second maximum pressure being higher than or equal to the first maximum pressure, the method comprising the steps of: letting the gas contained in the second volume expand to at least partially displace the non-compressible hydraulic fluid contained in the second vessel out of the second vessel while keeping the quantity of non-compressible hydraulic fluid contained in the first vessel constant; when the pressure of the gas contained in the second volume has fallen to the first maximum pressure, letting the gas contained in the first volume expand to at least partially displace the non-compressible hydraulic fluid contained in the first vessel out of the first vessel; when the pressure of the gas contained in the second volume has fallen to a second pre-charge pressure which is equal to or smaller than the first maximum pressure, halting the expansion of the gas contained in the second volume to keep the pressure of the gas contained in the second volume at the second pre-charge pressure; and when the pressure of the gas contained in the first volume has fallen to a first pre-charge pressure which is smaller than the second pre-charge pressure, halting the expansion of the gas contained in the first volume to keep the pressure of the gas contained in the first volume at the first pre-charge pressure.

19. The method of claim 18, wherein the non-compressible hydraulic fluid displaced out of the first vessel and out of the second vessel drives a hydraulic displacement unit drivingly engaged with an engine or with a vehicle output of an automotive vehicle.

20. The method of claim 18, wherein a ratio of the first maximum pressure less the second pre-charge pressure to the second pre-charge pressure less the first pre-charge pressure is less than or equal to 0.2.

21. The method of claim 20, wherein the ratio of the first maximum pressure less the second pre-charge pressure to the second pre-charge pressure less the first pre-charge pressure is less than or equal to 0.1.

22. The method of claim 18, wherein the first pre-charge pressure is at least 10 bar.

23. The method of claim 22, wherein the first pre-charge pressure is at least 30 bar.

24. The method of claim 23, wherein the first pre-charge pressure is at least 50 bar.

25. The method of claim 18, wherein the second pre-charge pressure is at least twice the first pre-charge pressure.

26. The method of claim 25, wherein the second pre-charge pressure is at least three times the first pre-charge pressure.

27. The method of claim 26, wherein the second pre-charge pressure is at least 3.5 times the first pre-charge pressure.

28. A hydro-pneumatic energy storage system, comprising: a first hydro-pneumatic accumulator comprising a first hollow vessel and, disposed within the first vessel, a first compressible volume containing a first amount of gas; a second hydro-pneumatic accumulator comprising a second hollow vessel and, disposed within the second vessel, a second compressible volume containing a second amount of gas; a feed pipe; at least one control valve selectively fluidly connecting the accumulators to the feed pipe; at least one pressure sensor for measuring a hydrostatic pressure in the feed pipe and/or in the accumulators; and a control unit for controlling the at least one control valve, the control unit having a charge mode and a discharge mode; wherein, when the control unit is in the charge mode, the control unit is configured to selectively fluidly connect the accumulators to the feed pipe and to selectively fluidly disconnect the accumulators from the feed pipe by pre-pressurizing the gas contained in the first volume to a first hydrostatic pre-charge pressure and pre-pressurizing the gas contained in the second volume to a second hydrostatic pre-charge pressure, the second pre-charge pressure being higher than the first pre-charge pressure; pressurizing the gas in the first volume by discharging a non-compressible hydraulic fluid into the first vessel while keeping a quantity of non-compressible hydraulic fluid contained in the second vessel constant to keep the pressure of the gas contained in the second volume at the second pre-charge pressure; and, when the pressure of the gas in the first volume reaches the second pre-charge pressure, pressurizing the gas in the second volume by discharging a non-compressible hydraulic fluid into the second vessel; and wherein, when the control unit is in the discharge mode, the control unit is configured to selectively fluidly connect the accumulators to the feed pipe and to selectively fluidly disconnect the accumulators from the feed pipe by letting the gas contained in the second volume expand to at least partially displace the non-compressible hydraulic fluid contained in the second vessel out of the second vessel while keeping the quantity of non-compressible hydraulic fluid contained in the first vessel constant; when the pressure of the gas contained in the second volume has fallen to the first maximum pressure, letting the gas contained in the first volume expand to at least partially displace the non-compressible hydraulic fluid contained in the first vessel out of the first vessel; when the pressure of the gas contained in the second volume has fallen to a second pre-charge pressure which is equal to or smaller than the first maximum pressure, halting the expansion of the gas contained in the second volume to keep the pressure of the gas contained in the second volume at the second pre-charge pressure; and when the pressure of the gas contained in the first volume has fallen to a first pre-charge pressure which is smaller than the second pre-charge pressure, halting the expansion of the gas contained in the first volume to keep the pressure of the gas contained in the first volume at the first pre-charge pressure.

29. The hydro-pneumatic energy storage system of claim 28, further comprising a hydraulic displacement unit in fluid communication with the accumulators through the feed pipe, the hydraulic displacement unit being drivingly engaged or selectively drivingly engaged with at least one of an engine of an automotive vehicle, a vehicle output of an automotive vehicle and a hydraulic implement.

30. The hydro-pneumatic energy storage system of claim 28, further comprising a compressor in fluid communication with the first volume and with the second volume, the compressor being configured to pre-pressurize the gas contained in the first volume to the first pre-charge pressure by adjusting the first amount of gas, and the compressor being configured to pre-pressurize the gas contained in the second volume to the second pre-charge pressure by adjusting the second amount of gas.

31. The hydro-pneumatic energy storage system of claim 28, wherein the first volume and/or the second volume are formed by one of a closed bladder, an elastic diaphragm and a floating piston.

Description

[0044] Preferred embodiments of the presently proposed system are described in the following detailed description and are depicted in the accompanying drawing in which:

[0045] FIG. 1 shows a hydro-pneumatic energy storage system comprising two hydraulic accumulators in fluid communication with a common feed pipe;

[0046] FIGS. 2a-c shows different stages of a process of charging/discharging two

[0047] FIGS. 3a-c shows different stages of a process of charging/discharging the two hydraulic accumulators of FIG. 1;

[0048] FIG. 4 shows pressure-volume diagrams of a process gas contained in the accumulators of FIGS. 2 and 3;

[0049] FIG. 5 shows energy-pressure diagrams of the process gas contained in the accumulators of FIGS. 2 and 3;

[0050] FIG. 6 shows energy-volume diagrams of the process gas contained in the accumulators of FIGS. 2 and 3;

[0051] FIG. 7 shows a pressure-volume diagram of a polytropic transformation; and

[0052] FIG. 8 shows the effect of precharge pressure assuming adiabatic transformations.

[0053] It is understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification are simply exemplary embodiments of the inventive concepts defined herein. Hence, specific dimensions, directions or other physical characteristics relating to the embodiments disclosed are not to be considered as limiting, unless expressly stated otherwise.

[0054] In a hydraulic hybrid vehicle, the optimal volume and precharge pressure of hydraulic accumulators depend on vehicle and duty cycle characteristics. Normally, accumulator sizing is determined at the design stage and the precharge is defined as a tuning parameter that can be changed at the vehicle setup stage (but not during operation). These parameters are the result of a compromise and may be far from optimal in certain cases.

[0055] The invention as described herein uses a rack of accumulators as energy storage, arranged in order to allow a dynamic modification of total volume and/or precharge level, thus increasing the degrees of freedom for the hybrid powertrain control and improving the overall performance by matching the accumulator parameters to the current operating conditions.

[0056] Hybrid vehicles typically include a reversible energy storage system (RESS) paired with an internal combustion engine. The RESS enables the following typical hybrid vehicle features: [0057] Kinetic energy recuperation during braking [0058] Engine power buffering and optimal management of powertrain operating point

[0059] A hydraulic rechargeable energy storage device (HRESS) is typically comprised of one or more accumulators, of the bladder or piston type, which store energy by compressing a fixed mass of air, thus increasing its pressure. The pressure increase is directly linked to the thermodynamic transformation of the gas in the accumulator, which can be modeled as a polytropic transformation as represented in FIG. 7, where the effect of the transformation type is highlighted. In most cases, the actual transformation is quite close to adiabatic. FIG. 8 shows the effect of precharge pressure assuming adiabatic transformations. The first part of each curve is flatter than the rest, and corresponds to the initial filling of the accumulator.

[0060] One of the most challenging features of hydraulic hybrid systems is the rapid pressure decrease during accumulator discharge, due to the steep change in the pressure-volume curve. This makes the accumulator available only in part of its operating range, where the pressure is sufficiently high, orif the precharge is set to high valuesdecreased the overall system efficiency by introducing losses due to flow or displacement partialization. For these reasons, the ideal hydraulic accumulator would maintain its pressure almost constant, so as to be maintained at the most appropriate pressure for the entire volume range. Ideally, the pressure level should be set according to the application and the operating conditions.

[0061] On the other hand, given a certain volume of oil exchanged, working in the steep range of the pressure-volume curves increases the accumulator energy density.

[0062] In most hybrid applications, the total volume of energy storage is split up into several accumulators, for packaging and cost issues. In the standard configuration, the accumulators are all at the same precharge pressure (at least nominally) and the flow is shared equally among them.

[0063] The proposed invention comprises an arrangement of several accumulators that aims at maximizing the HRESS energy density. The solution is shown in FIG. 1.

[0064] Each accumulator can be precharged at a different level and can be individually connected to the main pressure line, using either hydraulic controls or electronic controls. In the typical operation, the accumulators are charged in order of increasing precharge: when a pressure threshold is reached on the first accumulator, the corresponding valve closes and the second accumulator is connected, thus changing the pressure-volume characteristics because of the different precharge pressure. The process is then repeated with the third accumulator when the pressure threshold in the second is reached, and so on. Even if all accumulators are precharged at the same level, the individual connection allows for dynamic resizing of the total volume.

[0065] The connection/disconnection logic can be implemented as a purely hydraulic control with an appropriate combination of pressure-operated valves, or it can be implemented using electronically controlled valves for greater flexibility. In the first case, the resulting HRESS can be integrated into the overall system as a passive component equivalent to a standard accumulator (only with different operating characteristics), with the advantage of increased energy density, as shown in FIG. 4. In the second case, active HRESS management forms an integral part of the hybrid powertrain energy management strategy, since additional degrees of freedom are introduced.

[0066] FIG. 1 shows a hydro-pneumatic energy storage system 1 in accordance with the present invention. The system 1 comprises a first hydro-pneumatic accumulator 2a and a second hydro-pneumatic accumulator 2b in fluid communication with a common feed pipe 3 through accumulator valves 4a, 4b, respectively. In the simplest case, the accumulator valves 4a, 4b are configured as 2/2-way shut-off valves, respectively.

[0067] The first accumulator 2a includes a first rigid vessel 5a enclosing a first storage space 6a. The first vessel 5a is configured to withstand hydrostatic pressures up to a maximum pressure of at least 400 bar, for example. The first storage space 6a comprises a first compressible volume 7a and a first variable space 8a. The first compressible volume 7a is formed by a closed elastic bladder filled with a first amount of a process gas, for example nitrogen. A volume of the process gas contained in the first compressible volume 7a may be expanded up to a first precharge volume of V.sub.01=10 L. The first accumulator 2a further includes a first pressure sensor 9a configured to determine a hydraulic pressure of non-compressible hydraulic fluid in the first variable volume 8a which is indicative of the pressure of the gas contained in the first volume 7a.

[0068] When a hydrostatic pressure of a non-compressible hydraulic fluid in the feed pipe 3 is higher than a hydrostatic pressure of the process gas contained in the first compressible volume 7a, the non-compressible hydraulic fluid may be discharged into the first variable space 8a by opening the shut-off valve 4a, thereby pressurizing the process gas contained in the first compressible space 7a. Similarly, when the hydrostatic pressure of the process gas contained in the first compressible volume 7a is higher than the hydrostatic pressure of the non-compressible fluid in the feed pipe 3, the non-compressible hydraulic fluid contained in the first variable space 8a may be at least partially displaced out of the first variable space 8a and into the feed pipe 3 by opening the valve 4a and thus letting the process gas contained in the first compressible volume 7a expand, thereby creating a fluid flow. The first variable space 8a may be fluidly disconnected from the feed pipe 3 by closing the valve 4a.

[0069] Here, the second accumulator 2b is identical to the first accumulator 2a. The second accumulator 2b includes a second rigid vessel 5b enclosing a second storage space 6b. The second storage space 6b comprises a second compressible volume 7b formed by a closed elastic bladder filled with a second amount of the process gas, and a second variable space 8b. A volume of the process gas contained in the second compressible volume 7b may be expanded up to a second precharge volume of V.sub.02=10 L. Like the first accumulator 4a, the second accumulator 2b includes a second pressure sensor 9b configured to determine a hydraulic pressure of non-compressible hydraulic fluid in the second variable space 8b which is indicative of the pressure of the gas contained in the second volume 7b.

[0070] The gas contained in the second compressible volume 7b may be pressurized and depressurized by opening and closing the valve 4b, thereby discharging non-compressible hydraulic fluid from the feed pipe 3 into the second variable space 8b or displacing non-compressible hydraulic fluid contained in the second variable space 8b out of the second variable space 8b and into the feed pipe 3, in the same manner as explained above with respect to the first accumulator 2a.

[0071] The feed pipe 3 may be in fluid communication with a hydraulic circuit including at least one hydraulic displacement unit such as a hydrostatic pump and/or a hydrostatic motor. The hydraulic displacement unit may be drivingly engaged or selectively drivingly engaged with an engine of an automotive vehicle or with an output of an automotive vehicle. For example, the feed pipe may provide fluid communication between the hydro-pneumatic energy storage system 1 and the main fluid lines of a hydrostatic circuit of a hydrostatic transmission between the engine of an automotive vehicle and the vehicle output.

[0072] The system 1 further includes a compressor 10 in fluid communication with the first compressible volume 7a and with the second compressible volume 7b through compressor valves 11a, 11b, respectively. Here, the valves 11a, 11b are configured as 2/2-way shut-off valves. Through the compressor 10 and the compressor valves 11a, 11b the first amount of the process gas contained in the first compressible volume 7a and the second amount of the process gas contained in the second compressible volume 7b may be increased or decreased. In particular, the compressor 10 may be used to regulate the first precharge pressure p.sub.01 of the process gas in the first volume 7a and to regulate the first precharge pressure p.sub.02 of the process gas in the second volume 7b. The precharge pressures p.sub.01, p.sub.02 may be regulated independently.

[0073] The system 1 further includes an electronic control unit 12 configured to control the valves 4a, 4b, 11a, 11b and the compressor 10. The control unit 10 may include one or more processors, for example. The control unit 12 may control the valves 4a, 4b, 11a, 11b and the compressor 10 independently. The control unit 12 may control the valves 4a, 4b based on an input from an operator and/or based on pressure signals from the pressure sensors 9a, 9b, the pressure signals being indicative of the hydrostatic pressure of the process gas contained in the first and the second compressible volume 7a, 7b, respectively. In particular, the control unit 12 may be operated in a charge mode and in a discharge mode. When operated in the charge mode, the control unit 12 actuates the valves 4a, 4b, 11a, 11b and the compressor 10 according to a method of charging the system 1. When operated in the discharge mode, the control unit 12 actuates the valves 4a, 4b according to a method of discharging the system 1. The charging method and the discharging method are explained in more detail further below.

[0074] FIGS. 2a-c show two hydraulic accumulators 13a, 13b at different stages of a method of charging the accumulators 13a, 13b known from the prior art. In order to compare the conventional method illustrated in FIGS. 2a-c with the presently proposed improved method, the accumulators 13a, 13b are assumed to be identical to the accumulators 2a, 2b of the system 1 of FIG. 1.

[0075] In a first step shown in FIG. 2a the accumulators 13a, 13b are precharged to the same precharge pressure p.sub.0 of 60 bar. In FIG. 2a, a precharge volume of the process gas contained in compressible bladders 14a, 14b is 10 L, respectively, amounting to a total precharge volume of V.sub.0=20 L.

[0076] According to the known charging method of FIG. 2, the two accumulators 13a, 13b are charged by fluidly connecting the variable spaces 15a, 15b of the accumulators to a common pressurized feed pipe (not shown) and by displacing a non-compressible hydraulic fluid from the feed pipe into the variable spaces 15a, 15b, thereby pressurizing the process gas in the bladders 14a, 14b to a pressure p>p.sub.0, as shown in FIG. 2b. As the accumulators 13a, 13b are simultaneously charged through the same feed pipe, the hydrostatic pressure of the gas in the first bladder 14a is at all times equal to the hydrostatic pressure of the gas in the second bladder 14b.

[0077] In FIG. 2c the accumulators 13a, 13b are each fully charged to a maximum pressure p.sub.max. In a bladder-type accumulator pmax is restricted to p.sub.max4.Math.p.sub.0. That is p.sub.max240 bar in the example of FIG. 2.

[0078] In a known method of discharging the accumulators 13a, 13b of FIG. 2, the above described charging process is simply reversed. In other words, according to the known method of discharging the accumulators 13a, 13b, the non-compressible hydraulic fluid contained in the variable spaces 15a, 15b of the accumulators 13a, 13b of FIG. 2b or 2c is uniformly discharged through the common feed pipe so that the pressure p in the bladders 14a, 14b is at all times equal during the discharge.

[0079] The thermodynamic transformation of a fixed amount of a process gas during a compression or expansion may be described by a polytropic transformation:

pV.sup.k=p.sub.0V0.sup.k. Eq. (1)

[0080] The amount of energy E deposited in or gained from the gas when changing the volume of the gas from V.sub.1 to V.sub.2 is given by the following integral:

[00001]$\begin{array}{cc}E=-p.Math.V={}_1^2.Math.\frac{p_0.Math.V_0^k}{V^k}.Math.V=\frac{p_0.Math.V_0^k}{k-1}.Math.\left(V_2^{1-k}-V_1^{1-k}\right).& \mathrm{Eq}..Math.\left(2\right)\end{array}$

[0081] From Eq. (1) and Eq. (2) it follows that the amount of energy E stored in a gas having a pressure p and a volume V, a precharge pressure p.sub.0 and a precharge volume V.sub.0 can be expressed as:

[00002]$\begin{array}{cc}E\left(p\right)=\frac{p_0.Math.V_0^k}{1-k}.Math.\left(V_0^{1-k}-\frac{p_0^{\frac{1-k}{k}}.Math.V_0^{1-k}}{p^{\frac{1-k}{k}}}\right).Math..Math.\mathrm{or}& \mathrm{Eq}..Math.\left(3\right)\\ E\left(V\right)=\frac{p_0.Math.V_0^k}{1-k}.Math.\left(V_0^{1-k}-V^{1-k}\right).& \mathrm{Eq}..Math.\left(4\right)\end{array}$

[0082] The practically realizable maximum amount of energy that may be deposited in the gas can be determined by replacing the pressure p in Eq. (3) by the maximum feasible pressure. For a bladder-type accumulator that value is given by p.sub.max4.Math.p.sub.0, yielding:

[00003]$\begin{array}{cc}{E}_{\mathrm{ma}.Math..Math.x,\mathrm{bladder}}=\frac{p_0.Math.V_0}{k-1}.Math.\left(1-4^{\frac{k-1}{k}}\right).& \mathrm{Eq}..Math.\left(5\right)\end{array}$

[0083] For nitrogen (k=1.4) that value is given by E.sub.max,bladder1.215.Math.p.sub.0.Math.V.sub.0.

[0084] FIG. 4 shows the pressure p(V.sub.OIL) (reference sign 30) of the gas contained in the bladders 14a, 14b of the accumulators 13a, 13b of FIG. 2 during its compression from the precharge volume V.sub.0=20 L shown in FIG. 2a to the minimum volume shown in FIG. 2c. The pressure p(V.sub.OIL) in FIG. 4 is plotted as a function of the volume V.sub.OIL of the non-compressible hydraulic fluid discharged into the variable spaces 15a, 15b, whereby p(V.sub.OIL) is determined according to Eq. (1) and according to V.sub.OIL=V.sub.0V. The values used are V.sub.0=20 L, p.sub.0=60 bar and the adiabatic index of nitrogen k=1.4. In FIG. 4 the pressure 30 is gradually increased from the precharge pressure p.sub.o=60 bar in FIG. 2a to the maximum feasible pressure of the accumulators 13a, 13b p.sub.max=4.Math.p.sub.0=240 bar shown in FIG. 2c. The area underneath the curve 30 in FIG. 4 represents the amount of energy deposited in the gas contained in the accumulators 13a, 13b during the compression.

[0085] FIG. 5 shows the energy E(p) (reference sign 31) deposited in the gas contained in the bladders 14a, 14b as a function of the uniform pressure p of the gas during the process illustrated in FIGS. 2a-c. E(p) in FIG. 5 is determined according to Eq. (3). Again, V.sub.0=20 L, p.sub.0=60 bar and k=1.4. Again, it can be seen that the pressure p of the nitrogen gas contained in the bladders 14a, 14b is increased to its maximum pressure p.sub.max=4.Math.p.sub.0=240 bar. During this process, the amount of energy deposited in the gas increases until it reaches its maximum value of roughly 150 kJ.

[0086] FIG. 6 shows the energy E(V.sub.OIL) (reference sign 32) deposited in the gas contained in the bladders 14a, 14b as a function of the volume V.sub.OIL of the non-compressible hydraulic fluid discharged into the variable spaces 15a, 15b during the charging process. E(V.sub.OIL) in FIG. 6 is determined according to Eq. (4) and according to V.sub.OIL=V.sub.0V. FIG. 6 does not show the complete charging process. Again, V.sub.0=20 L, p.sub.0=60 bar and k=1.4 in FIG. 6. As the gas is compressed, the amount of energy deposited in the gas increases.

[0087] FIGS. 3a-c show different stages of the presently proposed method of charging the hydro-pneumatic energy storage system 1 of FIG. 1. The method may be initiated by a command from an operator through one or more input devices or by a control algorithm. The method is carried out using a charge mode of a control algorithm that is run on a processor of the electronic control unit 12.

[0088] FIG. 3a shows a first step of the proposed method of charging the system 1. In the first step, the control unit 12 commands the compressor 10 to adjust the amount of nitrogen gas contained in the first volume 7a of the first accumulator 2a through the compressor valve 11a to pre-pressurize the gas in the first volume 7a to a first precharge pressure p.sub.01 of 60 bar at a precharge volume of V.sub.01=10 L. Once the precharge pressure p.sub.01 has been set, the control unit 12 commands the compressor valve 11a to close. Still referring to FIG. 3a, the control unit 12 then commands the compressor 10 to adjust the amount of nitrogen gas contained in the second volume 7b of the second accumulator 2b through the compressor valve 11b to pre-pressurize the gas in the second volume 7b to a second precharge pressure p.sub.02 of 230 bar at a precharge volume of V.sub.02=10 L. Thus, the second precharge pressure p.sub.02 of the nitrogen gas contained in the second volume 7b of the second accumulator 2b is only slightly smaller than the maximum feasible pressure of about 4.Math.p.sub.01=240 bar of the first bladder accumulator 2a. Once the precharge pressure p.sub.02 has been set, the control unit 12 commands the valve 11b to close. The valves 4a, 4b remain closed during the first step of FIG. 3a.

[0089] After the first volume 7a and the second volume 7b have been pre-pressurized, the accumulators 2a, 2b are ready for non-compressible hydraulic fluid to be discharged into the variable space 8a of the first accumulator. The value of the first precharge pressure p.sub.01 is preferably chosen such that it matches a typical operating pressure of the hydraulic circuit to which the system 1 is coupled for the charging process. Depending on the particular situation, different values of the first precharge pressure p.sub.01 may be chosen.

[0090] Once the pressure in the common feed pipe 3 of the accumulators 2a, 2b reaches or exceeds the first precharge pressure p.sub.01 of the first accumulator 2a, the control unit 12 commands the first accumulator valve 4a to open to fluidly connect the first variable space 8a of the first accumulator 2a to the feed pipe 3. At the same time, the second accumulator valve 4b remains closed, thereby fluidly disconnecting the second accumulator 2b from the feed pipe 3.

[0091] Upon opening the first accumulator valve 4a, non-compressible hydraulic fluid, typically oil, is discharged into the first variable space 8a of the first accumulator 2a through the feed pipe 3, thereby compressing the first volume 7a and increasing the hydrostatic pressure p.sub.1 of the nitrogen gas contained in the first volume 7a. The feed pipe 3 may be pressurized by a hydrostatic pump/motor of a hydrostatic transmission of a vehicle which is driven by an internal combustion engine of the vehicle, or by a vehicle output of the vehicle during a regenerative braking procedure, for example. During the charging of the first accumulator 2a, the control unit 12 keeps the second accumulator valve 4b shut to keep the hydrostatic pressure of the second accumulator 4b at the second precharge pressure p.sub.02.

[0092] The first volume 7a is further pressurized until the hydrostatic pressure p.sub.1 of the gas contained in the first volume reaches the second precharge pressure p.sub.02. Up to this point, the second accumulator valve 4b is kept closed. The situation now reached is illustrated in FIG. 3b. The gas contained in both volumes 7a, 7b is pressurized to the second precharge pressure p.sub.02. However, whereas the first volume 7a has been almost fully compressed, the second volume 7b is still at the precharge volume of V.sub.02=10 L.

[0093] In FIG. 4, the pressure p(V.sub.OIL) in the first volume 7a during the compression from p=p.sub.01 in FIG. 3a to p=p.sub.02 in FIG. 3b is represented by a first section 20a of the curve 20. As only the first volume 7a is charged during this stage, the increase in pressure represented by the section 20a of the curve 20 in FIG. 4 is larger than the corresponding increase in pressure in the conventional method caused by the same quantity of oil. In other words, for a given volume of non-compressible hydraulic fluid dischared into the system 1, more energy is deposited in the case of the presently proposed method than in the case of the conventional method.

[0094] This is further illustrated in FIG. 6 where the curve 22 representing the amount of energy E(V.sub.OIL) deposited in the system 1 of FIG. 1 according to the presently proposed method lies above the curve 32 representing the conventional method.

[0095] In FIG. 3b, as the pressure in the first volume 7a and/or in the feed pipe 3 reaches the second precharge pressure p.sub.02, the control unit 12 commands the first accumulator valve 4a to close. Thus, the first accumulator 2a is fluidly disconnected from the feed pipe 3 in order to keep the pressure of the compressed gas in the first accumulator 2a at the second precharge pressure p.sub.02. At the same time, the control unit 12 commands the second accumulator valve 4b to open. Thus, the second accumulator 2b is fluidly connected to the feed pipe 3 and non-compressible hydraulic fluid is discharged into the second variable volume 8b of the second accumulator 2b through the feed pipe 3.

[0096] However, it is conceivable that the disconnection of the first accumulator 2a from the feed pipe 3 and the connection of the second accumulator 2b to the feed pipe 3 do not occur at the same time. For example, once the second accumulator 2b is fluidly connected to the feed pipe 3 the first accumulator 2a could remain fluidly connected to the feed pipe 3 until the gas in the first volume 7a reaches the maximum feasible pressure p.sub.max,14.Math.p.sub.01. In other words, for some time non-compressible hydraulic fluid can be discharged into the two accumulators 2a, 2b in parallel/simulataneously. This may smoothen the transition from the charging of the first accumulator 2a to the charging of the second accumulator 2b.

[0097] In FIGS. 4 and 5, the transition from the charging of the first accumulator 2a to the charging of the second accumulator 2b is visible as a point where the curves 20, 21 are not smooth. In FIGS. 4 and 5 this transition point is indicated at 20c, 21c, respectively. The transition point 20c in FIG. 4 divides the curve 20 in two sections 20a, 20b. Similarly, the transition point 21c in FIG. 5 divides the curve 21 in two sections 21a, 21b. The sudden change in the rate at which p(V.sub.OIL) and E(p) increase at 20c, 21c, respectively, is due to the change in the gas volume coupled to the feed pipe 3 at the transition point. While, at 20c and 21c, the gas volume 7a of the first accumulator 2a is almost fully compressed, the gas volume 7b of the second accumulator 2b is still at its precharge value V.sub.02.

[0098] Once the second accumulator valve 4b is opened, non-compressible hydraulic fluid is discharged into the second variable volume 8b of the second accumulator 2b and the second gas volume 7b of the second accumulator 2b is compressed until it reaches its maximum feasible value of p.sub.max,24.Math.p.sub.02=4.Math.230 bar=920 bar. During this state, the first accumulator valve 4a is kept closed to keep the quantity of oil in the first variable space 8a and the pressure in the first volume 7a constant. The accumulators 2a, 2b in their state of maximum charge are illustrated in FIG. 3c.

[0099] FIGS. 4 and 5 clearly show that whereas in the conventional method (see curves 30, 31 in FIGS. 4 and 5, respectively) the maximum pressure to which the process gas in the accumulators 13a, 13b may be compressed is limited to p.sub.max4.Math.p.sub.0=240 bar, in the presently proposed method (see curves 20, 21 in FIGS. 4 and 5, respectively) the range of the accessible pressure may be extended to a value of up to p.sub.max,24.Math.p.sub.02=920 bar. As a result more energy can be stored when charging the system according to the presently proposed method. In practice, the maximum pressure value may be limited by the maximum pressure of the hydraulic circuit to which the system 1 is connected. For example, in a hydrostatic transmission of an automotive vehicle the maximum operating pressure is typically in the range of 450 bar.

[0100] A skilled person readily understands that the charging method proposed herein and illustrated in FIGS. 3-6 can be extended to more than two accumulators. These may then be pre-pressurized to different precharge pressures and charged sequentially, in analogy to the charging of the two-accumulator system 1 described above. For example, each accumulator of an n-accumulator system can be pre-pressurized up to a precharge pressure p.sub.0,j=p.sub.max,j1=4.Math.p.sub.0,j1, where j1 denotes the accumulator preceding the j.sup.th accumulator.

[0101] From the foregoing description of the method of charging the system 1 it is immediately apparent to a skilled person how the system can be discharged in order to arrive at the precharge state depicted in FIG. 3a. Thus, a corresponding method of discharging the system 1 when the first volume 7a contains an amount of process gas pressurized to a first maximum pressure and when the second volume 7b contains an amount of precess gas pressurized to a second maximum pressure, the second maximum pressure being larger than or at least equal to the first maximum pressure, comprises at least the following steps: [0102] letting the gas contained in the second volume 7b expand to at least partially displace the non-compressible hydraulic fluid contained in the second vessel 5b out of the second vessel 5b while keeping the quantity of non-compressible hydraulic fluid contained in the first vessel 5a constant; [0103] when the pressure of the gas contained in the second volume 7b has fallen to the first maximum pressure, letting the gas contained in the first volume 7a expand to at least partially displace the non-compressible hydraulic fluid contained in the first vessel 5a out of the first vessel 5a; [0104] when the pressure of the gas contained in the second volume 7b has fallen to a second pre-charge pressure which is equal to or smaller than the first maximum pressure, halting the expansion of the gas contained in the second volume 7b to keep the pressure of the gas contained in the second volume 7b at the second pre-charge pressure; and [0105] when the pressure of the gas contained in the first volume 7a has fallen to a first pre-charge pressure which is smaller than the second pre-charge pressure, halting the expansion of the gas contained in the first volume 7a to keep the pressure of the gas contained in the first volume 7a at the first pre-charge pressure.