DESCRIPTION

The present invention relates to a regenerative active hydraulic system for a motor vehicle, in accordance with the preamble of claim 1. In particular, a system and a method are illustrated which are adapted to coordinate the power flow between the motor of a vehicle, a hydraulic energy accumulation device and the wheels of the vehicle.

The braking systems currently employed on motor vehicles essentially consist of friction- type systems wherein two mechanical components, e.g. brake disks and pads housed in the wheels of the vehicle, dissipate the kinetic energy of the vehicle via sliding friction caused by their mutual rubbing action. These systems generate up to 50% of non-exhaust emissions caused by release of particulate matter (PM), such as, for example, PM-10 or PM-2.5. Such systems also generate noise pollution due to the noise, e.g. screeching, generated by the two mechanical components rubbing against each other. The intensity of such noise is higher, in particular, from big vehicles such as, for example, buses, lorries, trailer-trucks and trains. Furthermore, friction-type braking systems transform the kinetic energy of the vehicle into heat, which is dissipated without using any particular measure for recovering such energy. When the vehicle is being driven in such traffic conditions that require repeated braking and acceleration, e.g. in city traffic, excessive energy is wasted, resulting in excessive fuel consumption and pollution due to the burning of the fuel. In order to reduce these problems, other types of braking systems are used in association with friction-type braking systems. For example, in the case of a vehicle equipped with an electric drive motor, the electric generator principle is exploited, i.e. when the vehicle is braking the electric motor acts as a generator, and the vehicle’s kinetic energy is transformed into electric energy and supplied to the vehicle’s battery. A torque is thus generated which counters the rotation of the vehicle’s drive wheels, thereby providing a braking effect. This makes it possible to reduce the usage of friction-type braking systems and to recover some of the vehicle’s kinetic energy while braking. However, the braking systems that utilize the electric generator principle are not very efficient when the speed of the vehicle is low, e.g. in city traffic conditions, and require that the vehicle’s battery is not fully charged; for these reasons, this type of solution can only be used as a complement to a friction-type braking system or other systems.

Another type of braking system known in the art, which does not exploit the electric generator principle, is discussed, for example, in United States patent US7273122. Such document illustrates a regenerative hydraulic system for a vehicle, such system comprising:

- a motor for driving said vehicle,

- transmission means operationally connected to at least one wheel of said vehicle, said transmission means being adapted to transmit mechanical power from said at least one motor to said at least one wheel,

- an energy accumulator adapted to store and release a pressurized fluid (e.g. oil),

- a reversible hydraulic machine operationally coupled to said transmission means, said reversible hydraulic machine being in fluidic communication with said energy accumulator.

This regenerative hydraulic system is configured in a manner such that, during a braking phase of said vehicle, said reversible hydraulic machine retards said at least one wheel of said vehicle by pumping the fluid into the energy accumulator, or in a manner such that, during a driving phase of the vehicle, said reversible hydraulic machine supplies supplementary power to at least one wheel of said vehicle by using the pressurized fluid stored in the energy accumulator.

During the braking or driving phase, mechanical power is transferred to the wheels by transmission means, which may be, for example, drive axles or half axles.

The reversible hydraulic machine works as a hydraulic pump and, when reversed, as a hydraulic motor. The reversible hydraulic machine may be, for example, a high-pressure variable-displacement axial-piston hydraulic motor/pump. Within the accumulator, an elastic container contains nitrogen: the container insulates the fluid (oil) from the nitrogen, which can be pressurized up to 25 times the idle state of the accumulator. The kinetic energy of the vehicle is thus recovered into pressurized nitrogen. When the vehicle’s motion is resumed (driving phase), the pressurized fluid accelerates the vehicle by reversing the direction of the hydraulic pump.

The technical solution proposed by United States patent US7273122, as described above, has a number of drawbacks, which will be highlighted below. A first drawback of the system discussed in the above-mentioned patent lies in the fact that such a system is particularly bulky and heavy because of the components thereof, in particular because of the construction characteristics of the accumulator, which uses containers capable of containing a fluid, like nitrogen, that can be compressed to high pressures. This makes such a solution only feasible for big vehicles, e.g. buses, lorries, trailer-trucks and trains, while it is unfeasible for small vehicles like cars, small vans, etc. Consequently, the system proposed in the above-mentioned United States patent cannot be used as a complementary braking system on small (hybrid or pure) electric vehicles.

A second drawback of the above-mentioned system lies in the fact that such a system, just like other regenerative systems, is passive, i.e. it operates as a function of other factors and/or state variables, such as, for example, the speed of the vehicle and/or the state of the accumulation system.

A further drawback of the above-mentioned system lies in the fact that the elastic container, containing nitrogen, in the accumulator is subject to deterioration. Frequent compression and decompression of the nitrogen, due to repeated vehicle braking and driving phases, apply high mechanical stresses to the container, which may over time suffer small lesions and start leaking the gas contained therein, thus making the system less and less efficient in its braking action and in recovering the kinetic energy of the vehicle.

Yet another drawback of the system described in the above-mentioned patent comes from the fact that such a system must be kept efficient and safe by carrying out much maintenance work on its components, particularly on the vehicle’s kinetic energy accumulator.

It is therefore one object of the present invention to solve these and other problems suffered by the prior art, and in particular to provide a regenerative active hydraulic system that allows braking a vehicle while reducing non-exhaust emissions caused by release of particulate matter (PM), e.g. PM- 10 or PM-2.5.

It is a further object of the present invention to provide a regenerative active hydraulic system which is independent of any external factors and/or state variables, such as, for example, the speed of the vehicle and/or the state of the accumulation system.

It is another object of the present invention to provide a regenerative active hydraulic system which permits reducing the noise pollution caused by the rubbing action of friction-type brake components. It is a further object of the present invention to provide a regenerative active hydraulic system for recovering kinetic energy of a vehicle, which does not utilize the principle of compressing a substantially compressible fluid, e.g. a gas, in order to accumulate such energy.

It is a further object of the present invention to provide a regenerative active hydraulic system for braking and for recovering energy which can be installed on any vehicle regardless of its dimensions and of the type of prime mover used in the vehicle, e.g. an electric motor, a combustion engine or a hybrid drive system.

It is another object of the present invention to provide a regenerative active hydraulic system for braking and for recovering energy which is less subject to wear and easy to maintain.

It is yet another object of the present invention to provide a regenerative active hydraulic system for braking and for recovering energy which allows increasing the promptness and effectiveness of the vehicle’s braking and driving phases.

The above-mentioned objects also apply to a method for coordinating a power flow between the motor and the wheels of a vehicle.

The invention described herein consists of a regenerative active hydraulic system for braking and for recovering kinetic energy, intended for a motor vehicle, and a method for coordinating a power flow between the motor and the wheels of the vehicle. The system and the method illustrated herein can be used on any vehicle regardless of its size and of the type of prime mover in use. The system described in the present invention can be used either as an independent braking system or in synergy with (as a complement to) another braking system; for example, it may replace the current friction-type braking system in pure electric or hybrid vehicles, even small ones.

Further advantageous features of the present invention will be set out in the appended claims, which are an integral part of the present description.

The invention will now be described in detail by means of some non-limiting embodiments with particular reference to the annexed drawings, wherein:

- Figure 1 schematically shows a vehicle comprising a regenerative active hydraulic system according to an embodiment of the present invention;

- Figure 2 schematically shows the regenerative active hydraulic system of the vehicle of Figure 1 ; - Figure 3 shows an illustrative flow chart of a braking phase of the regenerative active hydraulic system of Figure 2;

- Figure 4 shows an illustrative flow chart of a driving phase of the regenerative active hydraulic system of Figure 2;

- Figure 5 shows a simulation made by the Applicant to verify the performance of the regenerative active hydraulic system of Figure 2.

With reference to Figure 1, a vehicle 100 comprises at least one motor 140 for driving said vehicle 100. The motor 140 may be, for example, an internal combustion engine using a fuel, e.g. petrol, diesel oil or LPG, stored in a tank (not shown in the drawing). As an alternative, said motor may be, for example, an electric motor powered by one or more batteries (not shown in the drawing). Said motor may also be a hybrid motor, i.e. an internal combustion engine associated with an electric motor, capable of operating in a synergic (coordinated and/or complementary) fashion.

Said vehicle 100 further comprises transmission means 130 operationally connected to at least one wheel 105 of said vehicle 100. The transmission means 130 receive the mechanical power generated by the motor 140 via an input shaft 145. The transmission means 130 are adapted to transmit the mechanical power generated by the motor 140 to at least one wheel 105, e.g. by means of a first drive shaft 131 and/or a second drive shaft 132. For example, the first drive shaft 131 may transmit mechanical power to a first pair of wheels through a first axle 120, to which the wheels 105 of said first pair of wheels are mechanically connected. The second drive shaft 132 may transmit mechanical power to a second pair of wheels through a second axle 125, to which the wheels 105 of said second pair of wheels are mechanically connected.

Each one of said wheels 105, belonging to said first and second pairs of wheels, may comprise a friction-type braking system 110, which can be activated as necessary, e.g. for use as a supplementary or emergency braking system when other braking systems of the vehicle 100 are insufficient for braking the same or inoperative. The transmission means 130 may be, for example, a gear system through which it is possible to vary the mechanical torque transmitted to the wheels 105 from the motor 140. Said transmission means 130 may include a coupling device adapted to mechanically connect said first drive shaft 131 and/or second drive shaft 132 to a regenerative active hydraulic system 200 by means of an interface shaft 150.

Figure 2 schematically shows the regenerative active hydraulic system 200 of the vehicle 100. Said system 200 may be implemented as a hydraulic circuit comprising a tank 260 capable of containing a substantially incompressible fluid, e.g. oil, at least one energy accumulator 240, e.g. a non-linear spring actuator, adapted to store and release the pressurized fluid, at least one reversible hydraulic machine 250 operationally coupled to the transmission means 130 and in fluidic communication with said at least one energy accumulator 240. The regenerative active hydraulic system 200 further comprises a flow control valve 280, at least one drain valve 270, one or more control valves for the fluid in the hydraulic circuit, such as, for example, a first control valve 230, a second control valve 241 and a third control valve 242, at least one pressure sensor 220 for sensing the pressure of said fluid in the hydraulic circuit. Said system 200 further comprises at least one active hydraulic device 290, e.g. a direct-current hydroelectric pump, adapted to pump the fluid into at least one reversible hydraulic machine 250 and into at least one energy accumulator 240. The regenerative active hydraulic system 200 ultimately comprises a control unit 201 capable of controlling the elements of the regenerative active hydraulic system 200, so as to make it operational.

The fluid control valves 230, 242, 241, e.g. Bosch Rexroth LC04Z valves, are used to either allow or prevent the flow of fluid depending on the operating conditions of the regenerative active hydraulic system 200. For example, when they are in the inactive (idle) state, the control valves 230, 241, 242 may allow the fluid to flow in both directions. Conversely, when they are activated, e.g. by an electric signal, they do not allow the fluid to flow (normally open valves). Alternatively, the control valves 230, 241, 242 may be normally closed, i.e. they do not allow the fluid to flow when they are idle; conversely, when they are activated, e.g. by an electric signal, they allow the fluid to flow in both directions. In another embodiment of the invention, the control valves 230, 241, 242 may be of both types, i.e. the same circuit may include normally open valves and normally closed valves.

The flow control valve 280, e.g. a Bosch Rexroth AZPN valve, allows increasing or decreasing the flow of fluid inside the valve itself, e.g. by increasing or decreasing the cross- section of the internal duct of the valve in which the fluid flows, thereby controlling the pressure downstream of the reversible hydraulic machine 250. Flow regulation may occur, for example, proportionally to an electric reference signal that commands an actuator of the flow control valve 280, said actuator being adapted to adjust the cross-section of the internal duct of the valve.

The drain valve 270, e.g. a Bosch Rexroth 041155X85Z valve, allows delivering the fluid from a point of the hydraulic circuit directly into the tank 260. When it is idle, the drain valve 270 stays closed; if the pressure of the fluid in the circuit exceeds a predefined pressure value, then the drain valve will open and let the fluid flow towards the tank 260. The opening mechanism of the drain valve may be either mechanical or electromechanical, i.e. controlled by an electric signal. The drain valve 270 is useful to protect the hydraulic circuit in the event that the pressure of the fluid in the circuit becomes too high, thus preventing damage to the components that make up the hydraulic circuit of the regenerative active hydraulic system 200. In one embodiment of the invention, the drain valve 270 permits operating the friction- type braking system 110 of the vehicle 100. For example, the drain valve 270 may, when active, send an electric signal to the friction-type braking system 110 and/or signal an excessive pressure in the hydraulic circuit by activating signalling means, e.g. LED lamps or audible alarms.

The energy accumulator 240 allows accumulating the kinetic energy of the vehicle 100 and provides for storing and releasing the pressurized fluid. In one embodiment of the invention, the energy accumulator may be implemented as a hydraulic actuator comprising a non-linear spring and a piston inside a container, e.g. cylindrical in shape, in which the piston forms two sealed chambers, one of which houses the spring, while the other one can contain the pressurized fluid. Compression of the non-linear spring may be effected, for example, by the piston being pushed by pressurized fluid entering the energy accumulator 240. The pressurized fluid of the hydraulic circuit can flow inside the energy accumulator 240, for example, through the aperture of the second fluid control valve 241. In order to use the energy accumulated by the energy accumulator 240, the third fluid control valve 242 may be opened, for example, so as to cause the pressurized fluid to return into the hydraulic circuit of the regenerative active hydraulic system 200. In this case, the fluid will be pressed by the piston, in its turn pushed by the previously compressed spring.

The reversible hydraulic machine 250 is operationally coupled to the transmission means 130 through the interface shaft 150, which can rotate in a given direction of rotation 151, e.g. clockwise or counterclockwise. In this manner, part of the rotation energy of the first drive shaft 131 and/or of the second drive shaft 132 is transferred to the reversible hydraulic machine 250, which allows the fluid to flow in the hydraulic circuit. The fluid flows in the hydraulic circuit in a direction of flow 261 that depends on the direction of rotation 151 of the interface shaft 150. The fluid can flow from the tank 260 through the first control valve 230, through the reversible hydraulic machine 250, into the energy accumulator 240 and through the flow control valve 280, and then re-enter the tank 260. As previously described, the fluid is made to flow in the hydraulic circuit by using part of the energy of the first drive shaft 131 and/or of the second drive shaft 132. For this reason, when the regenerative active hydraulic system 200 is not in operation, the hydraulic circuit can be emptied of the fluid, so as to preserve the rotation energy of the first drive shaft 131 and/or of the second drive shaft 132. When the regenerative active hydraulic system 200 is in operation, the torque T generated by the reversible hydraulic machine 250 on the interface shaft 150 is proportional to the flow rate V of the hydraulic machine itself and to the fluid pressure difference at the input 251 and output 252 of the hydraulic machine, i.e. T = V * (Pi-P ₀ ), where Pi and P ₀ are, respectively, the input pressure 251 and the output pressure 252 in the reversible hydraulic machine 250. When the fluid is flowing in the hydraulic circuit in a stationary condition, the input pressure Pi 251 and the output pressure P ₀ 252 in the reversible hydraulic machine 250 are substantially equal, resulting in a substantially null torque T acting upon the interface shaft 150. Conversely, when the fluid is not in a stationary condition, if the input pressure Pi 251 is higher than the output pressure P ₀ 252 in the reversible hydraulic machine 250, then the torque T acting upon the interface shaft 150 will be positive (driving torque), and in such a case the reversible hydraulic machine 250 will work as a hydraulic motor. Vice versa, if the input pressure Pi 251 is lower than the output pressure P ₀ 252 in the reversible hydraulic machine 250, then the torque T acting upon the interface shaft 150 will be negative (braking torque), and in such a case the reversible hydraulic machine 250 will work as a hydraulic pump.

The active hydraulic device 290 is a hydraulic element that can be actively controlled, e.g. by means of an electric signal, and may be, for example, a hydroelectric pump like the Bosch Rexroth EHPID4012 (48V) model. The active hydraulic device 290 is adapted to pump the fluid into the hydraulic circuit operationally connected to at least one reversible hydraulic machine 250 and at least one energy accumulator 240 of the regenerative active hydraulic system 200. The fluid is taken from the tank 260 and delivered into the hydraulic circuit as necessary, so as to obtain an adequate pressure in the hydraulic circuit in the time interval in which the operating state of the regenerative active hydraulic system 200 changes. This technical measure advantageously permits increasing the effectiveness and promptness of the hydraulic system discussed in the present invention, which can store the kinetic energy for subsequently regenerating it, i.e. reusing it, as necessary with better promptness and effectiveness than the prior art. The presence of the active hydraulic device 290 makes the regenerative active system 200 independent of the speed of the vehicle 100 because, in general, the hydraulic pump does not provide the highest pressure at low speed. In one embodiment of the invention, the flow rate of the active hydraulic device 290 can be determined as a function of the flow rate of at least one reversible hydraulic machine 250 and/or of at least one energy accumulator 240 of the regenerative active hydraulic system 200. For example, if the flow rate V of the reversible hydraulic machine 250 is 100 cnrVrevolution, with a volumetric efficiency h of 90%, then the flow rate of the active hydraulic device 290 can be determined as ( 1 -h) V, i.e. the flow rate of the active hydraulic device 290 may correspond to 10 cmVrevolution.

The control unit 201 is adapted to control the elements comprised in the regenerative active hydraulic system 200, so at to make it operational. The control unit 201 may be, for example, a programmable microcontroller comprising a memory that stores information including the program code and the data necessary for executing it. Such unit may also have inputs that can be connected to, for example, at least one pressure sensor 220, and outputs for controlling, for example, the flow control valve 280, at least one drain valve 270, and one or more fluid control valves 230, 241, 242 of the hydraulic circuit.

With reference to Figure 3, the following will describe and exemplary method for the execution of a braking phase of the vehicle 100 by means of the regenerative active hydraulic system 200.

At step 310, the regenerative active hydraulic system 200 is not operational, in that the vehicle 100 is not in the braking phase yet. In this state, the hydraulic circuit contains no fluid; consequently, the reversible hydraulic machine 250 is operating in the absence of fluid and hence the pressures Pi and P ₀ at the input 251 and output 252, respectively, of the reversible hydraulic machine 250 are substantially equal. The first control valve 230, the second control valve 241 and the third control valve 242 are closed, the drain valve 270 is closed, the flow control valve 280 is open, and the energy accumulator 240 has no charge (it contains no pressurized fluid). The regenerative active hydraulic system 200 may be in this state, for example, when the vehicle 100 is moving at a substantially constant speed or with variable motion, i.e. gradually accelerating or decelerating without using any braking system.

At step 320, the control unit 201 makes the regenerative active hydraulic system 200 operational for the braking phase of the vehicle 100. In this case, the control unit 201 receives the braking command from the driver of the vehicle and/or from the automated driving system of the vehicle 100. During this step, the control unit 201 opens the first control valve 230 to allow the fluid to enter the hydraulic circuit and opens the second control valve 241 to allow the fluid to enter the accumulator 240.

At step 330, the control unit 201 acquires a first value of the pressure P ₀ at the output 252 of the reversible hydraulic machine 250, e.g. by means of the pressure sensor 220. If the first value of the pressure P ₀ of the fluid in the hydraulic circuit is comprised between a first predetermined pressure value and a second predetermined pressure value, the first predetermined pressure value being lower than the second predetermined pressure value, then the control unit 201 will execute step 340, otherwise it will execute step 350.

In a further embodiment of the invention, the control unit 201 may be implemented as a feedback control system, e.g. a proportional integral derivative (PID) system, which generates a second reference value of the pressure P ₀ at the output 252 of the reversible hydraulic machine 250 which is proportional to the braking command, for the purpose of exerting a proper braking action on the vehicle 100. Such second reference value of the pressure P ₀ at the output 252 of the reversible hydraulic machine 250 may depend, for example, on the required braking torque, which in turn will depend on the physical characteristics of the vehicle 100 and of the hydraulic circuit of the regenerative active hydraulic system 200. In this embodiment of the invention, the control unit 201 commands at least one active hydraulic device 290 to pump the fluid into the hydraulic circuit on the basis of the difference between the first value of the pressure P ₀ acquired by the control unit 201 at the output 252 of the reversible hydraulic machine 250, e.g. by means of the pressure sensor 220, and the second reference value of the pressure P ₀ at the output 252 of the reversible hydraulic machine 250.

At step 340, the control unit 201 commands at least one active hydraulic device 290 to pump the fluid into the hydraulic circuit, thus rapidly reaching an adequate operating pressure, which is comprised between the first predetermined pressure value and the second predetermined pressure value. In this manner, the regenerative active hydraulic system 200 will quickly reach full efficiency, thus improving the promptness of the braking action exerted on the vehicle 100. The first predetermined pressure value and the second predetermined pressure value will depend on the physical characteristics of the vehicle 100, such as, for example, top speed and so on, and will also depend on the physical characteristics of the hydraulic circuit and its components.

In another embodiment of the invention, step 330 and step 340 may be replaced with a single step in which at least one active hydraulic device 290 will pump said fluid into said hydraulic circuit when the pressure of said fluid in said hydraulic circuit is comprised between the first predetermined pressure value and the second predetermined pressure value. In this embodiment, the active hydraulic device 290 is not commanded by the control unit 201, but operates autonomously. In this case, the active hydraulic device 290 may comprise, for example, a pressure sensor for sensing the pressure of the fluid in the hydraulic circuit, or may comprise a mechanism that can be activated by the pressure of the fluid in the hydraulic circuit. In other embodiments of the invention, one or more active hydraulic devices may be employed, suitably commanded to pump the fluid at different points of the hydraulic circuit, so that the regenerative active hydraulic system 200 will quickly reach full efficiency, thus improving the promptness of the braking phase of the vehicle 100.

At step 350, the control unit 201 closes the flow control valve 280 in proportion to the received braking command. For example, hard braking will result in a command having the control unit 201 close the flow control valve 280 to reduce the cross-section of its internal duct compared to the cross-section of the valve in the idle condition. Thus, the pressure P ₀ at the output 252 of the reversible hydraulic machine 250, located upstream of the flow control valve 280, will be higher than the pressure Pi at the input 251 of the reversible hydraulic machine 250. As a consequence, a braking torque will be generated on the interface shaft 150, which will retard the first drive shaft 131 and/or the second drive shaft 132 via the transmission means 130 to which the interface shaft 150 is connected. During this step, the energy accumulator 240 accumulates energy by taking in the pressurized fluid flowing through the first control valve 241 previously opened at step 320.

At step 360, the control unit 201 verifies if the pressure P ₀ at the output 252 of the reversible hydraulic machine 250 is greater than or equal to a third predetermined pressure value. If not, the control unit 201 will execute step 370, otherwise it will execute step 380. Such third predetermined pressure value is determined on the basis of the maximum pressure value that the hydraulic circuit can withstand without damage, and is dependent on the physical characteristics of the components that make up the regenerative active hydraulic system 200. At step 370, the control unit 201 terminates the braking phase of the vehicle 100. The control unit 201 closes the first control valve 230 to stop the delivery of fluid to the hydraulic circuit, opens the flow control valve 280, and closes the second control valve 241 to keep the pressurized fluid in the accumulator 240. As a result, the hydraulic circuit is emptied of the fluid, the reversible hydraulic machine 250 operates in the absence of fluid, and hence the pressures Pi and P ₀ at the input 251 and at the output 252, respectively, of the reversible hydraulic machine 250 are substantially equal, so that the difference between P ₀ and Pi is substantially null. Step 390 is then carried out.

At step 380, the fluid pressure exceeds the third predetermined pressure value and the fluid is made to flow towards the tank 260 through the drain valve 270, so as to reduce the pressure in the hydraulic circuit and prevent it from suffering any damage. Also, the control unit 201 closes the first control valve 230 to stop the delivery of fluid into the hydraulic circuit, opens the flow control valve 280, and closes the second control valve 241 to keep the pressurized fluid in the accumulator 240. In one embodiment of the invention, when the fluid pressure exceeds the third predetermined pressure value it is possible to activate at least one friction- type braking device 110 of said vehicle 100, so as to ensure safe braking of the vehicle 100. At step 390, the regenerative active hydraulic system 200 is not operational and the energy accumulator 240 is charged, i.e. it contains pressurized fluid.

With reference to Figure 4, the following will describe an exemplary method for the execution of a driving phase of the vehicle 100 by means of the regenerative active hydraulic system 200.

At step 410, the regenerative active hydraulic system 200 is not operational, in that the vehicle 100 is not in the driving phase. In this state, the hydraulic circuit contains no fluid; consequently, the reversible hydraulic machine 250 is operating in the absence of fluid and hence the pressures Pi and P ₀ at the input 251 and at the output 252, respectively, of the reversible hydraulic machine 250 are substantially null. The first control valve 230, the second control valve 241 and the third control valve 242 are closed, the drain valve 270 is closed, the flow control valve 280 is open, and the energy accumulator 240 is charged (it contains pressurized fluid).

At step 420, the control unit 201 makes the regenerative active hydraulic system 200 operational for the driving phase of the vehicle 100. In this case, the control unit 201 receives the driving command from the driver of the vehicle, e.g. from an accelerator pedal being pressed, and/or receives the driving command from the automated driving system of the vehicle 100. During this step, the control unit 201 opens the first control valve 230 to allow the fluid to enter the hydraulic circuit and opens the third control valve 242 to allow the pressurized fluid to exit the accumulator 240.

In another embodiment of the invention, during this step an additional active hydraulic device will pump said fluid into said hydraulic circuit, e.g. at the input 251 of the reversible hydraulic machine 250, if the pressure Pi of said fluid in said hydraulic circuit is comprised between the first predetermined pressure value and the second predetermined pressure value. In this embodiment, the additional active hydraulic device may act autonomously, as opposed to being commanded by the control unit 201. In this case, the additional active hydraulic device may comprise, for example, a pressure sensor for sensing the pressure of the fluid in the hydraulic circuit, or may comprise a mechanisms that can be activated by the pressure of the fluid in the hydraulic circuit. In this manner, the regenerative active hydraulic system 200 will quickly reach full efficiency, thus improving the promptness and effectiveness of the driving phase of the vehicle 100. The first predetermined pressure value and the second predetermined pressure value will depend on the physical characteristics of the vehicle 100, e.g. mass, top speed, etc., and will also depend on the physical characteristics of the hydraulic circuit and its components.

In other embodiments of the invention, one or more active hydraulic devices may be employed, suitably controlled to pump the fluid at different points of the hydraulic circuit, so that the regenerative active hydraulic system 200 will quickly reach full efficiency, thus improving the promptness and effectiveness of the driving phase of the vehicle 100.

At step 430, the control unit 201 verifies if the pressure Pi at the input 251 of the reversible hydraulic machine 250 is greater than or equal to the third predetermined pressure value. If not, the control unit 201 will execute step 440, otherwise it will execute step 450. As previously specified, the third predetermined pressure value is determined on the basis of the maximum pressure value that the hydraulic circuit can withstand without damage, and may also depend on the physical characteristics of the components that make up the regenerative active hydraulic system 200.

At step 440, the control unit 201 verifies if the energy accumulator 240 has no charge, e.g. by means of suitable pressure sensors; in the affirmative case, it will execute step 460, otherwise it will execute step 430.

At step 450, the fluid pressure exceeds the third predetermined pressure value, and the fluid is made to flow towards the tank 260 through the drain valve 270 so as to reduce the pressure in the hydraulic circuit to prevent it from suffering any damage. Also, the control unit 201 closes the first control valve 230 to stop the delivery of fluid into the hydraulic circuit. Step 270 is then carried out.

At step 460, the control unit 201 terminates the driving phase of the vehicle 100. The control unit 201 closes the first control valve 230 to stop the delivery of fluid into the hydraulic circuit and closes the third control valve 242 to keep the accumulator 240 with no charge. In this manner, the hydraulic circuit is emptied of the fluid, the reversible hydraulic machine 250 operates in the absence of fluid, and hence the pressures Pi and P ₀ at the input 251 and at the output 252, respectively, of the reversible hydraulic machine 250 are substantially null. At step 470, the regenerative active hydraulic system 200 is not operational and the energy accumulator 240 has no charge, i.e. it contains no pressurized fluid.

With reference to Figure 5, the following will describe a simulation carried out by the Applicant to assess the performance of the regenerative active hydraulic system 200. In particular, the Applicant considered, as the vehicle 100, an electric-drive Toyota Prius with a curb weight of 1350 kg, a maximum torque of 220 Nm and a minimum braking distance of 33 feet (from an initial speed of 60 mph), simulating an urban driving cycle according to the ARTEMIS standard (https://www. diesejnet.com/standards/cvcles/artemls.Dhn). Said urban cycle has a duration of 993 seconds, a distance of 4.874 km, an average speed of 17.7 km/h, a maximum speed of 57.3 km/h, 21 stops, 77 low-speed decelerations (between 0 and 50 km/h), 2 medium-speed decelerations (between 50 and 90 km/h).

The results of the simulations are shown in Figure 5, wherein: graph 510 shows the kinetic energy recovered by the electric brake (electric generator) during the whole urban cycle, which amounts to approximately 1600 kJ; graph 520 shows the kinetic energy recovered by the regenerative active hydraulic system 200 according to the present invention, which amounts to approximately 120 kJ; finally, graph 530 shows the kinetic energy dissipated by the friction-type braking system 110, which amounts to approximately 13 kJ.

The recovered kinetic energy, shown in graph 520, is for the most part transformed into mechanical work and is for the least part transformed into heat dissipated by the fluid and by the components of the hydraulic circuit of the regenerative active hydraulic system 200. Such heat can be used, for example, for heating the passenger compartment of a pure electric vehicle. This will increase the mileage of such a vehicle, because part of the battery energy will not be used for heating the passenger compartment. This technical measure provides a considerable increase in the mileage of a pure electric vehicle, and particularly of those vehicles which must operate in very cold environments. The heat dissipated by the regenerative active hydraulic system 200 may, for example, be delivered into the passenger compartment of a vehicle by means of a heating system thermally coupled, e.g. by means of a heat exchanger, to the hydraulic circuit of the regenerative active hydraulic system 200.

If the vehicle 100 did not use the regenerative active hydraulic system 200 according to the present invention, the friction-type braking system 100 would also have dissipate the energy recovered by the regenerative active hydraulic system 200, for a total dissipation of approximately 123 kJ. This would imply excessive wear of the components of the friction- type braking system 110, resulting in increased non-exhaust emissions caused by release of particulate matter (PM). Moreover, the regenerative active hydraulic system 200 makes it possible to reduce the noise pollution caused by friction between the components of the friction-type braking system 110, since it drastically reduces the utilization thereof.

The advantages of the present invention are apparent from the above description.

The regenerative active hydraulic system described in the present invention advantageously permits recovering kinetic energy of a vehicle. This system does not use a compressible fluid, e.g. gas, for accumulating such energy, but uses a device comprising a spring together with a substantially incompressible fluid (oil), which is less bulky than a gas device.

The regenerative active hydraulic system described in the present invention for recovering kinetic energy of a vehicle can advantageously be installed on any vehicle, regardless of its dimensions and the type of prime mover in use, in that it uses an energy accumulator that is smaller than prior-art ones.

The system described in the present invention may advantageously be used either as an independent braking system or in synergy with (as a complement to) another braking system. For example, this system may replace the current friction-type braking system, or anyway reduce its dimensions and usage, in pure electric or hybrid vehicles, even small ones.

The regenerative active hydraulic system described in the present invention advantageously makes it possible to brake a vehicle while reducing non-exhaust emissions caused by release of particulate matter (PM), e.g. PM-10 or PM-2.5, in that the utilization of friction-type brakes is highly reduced on vehicles. Furthermore, the regenerative active hydraulic system according to the present invention advantageously makes it possible to reduce the noise pollution caused by friction between the components of the friction-type braking system, since the utilization thereof is drastically reduced.

The regenerative active hydraulic system for braking and for recovering energy, as described in the present invention, is advantageously less subject to wear and is easy to maintain because it employs an energy accumulator that uses, instead of gas, a substantially incompressible fluid, thus being less subject to wear and easier to maintain than a gas energy accumulator.

The regenerative active hydraulic system for braking and for recovering energy, as described in the present invention, advantageously improves the promptness and effectiveness of the braking and driving phases of the vehicle by using one or more appropriately controlled active hydraulic devices for pumping the fluid at different points of the hydraulic circuit, so that the hydraulic system will quickly reach full efficiency.

Of course, without prejudice to the principle of the present invention, the forms of embodiment and the implementation details may be extensively varied from those described and illustrated herein merely by way of non-limiting example, without however departing from the protection scope of the present invention as set out in the appended claims.