AUTOMOBILE BRAKING SYSTEM WITH BRAKE FLUID VAPORIZATION PROTECTION VALVES

Abstract

A motor vehicle braking system comprising at least one hydraulic braking circuit (12, 14) having several brake actuators (24) connected to each other by this circuit (12, 14) which transmits brake fluid pressure to clamp rotating elements (22), at least one brake actuator (24) having on its brake fluid inlet a pilot-operated valve (26) which is open under normal driving conditions, and having a valve control device that takes into account the temperature of the actuator (24) to close this valve (26). Figure 2

Description

Title of the invention: MOTOR VEHICLE BRAKING SYSTEM WITH VALVES FOR PROTECTION AGAINST BRAKE FLUID VAPORIZATION

[0001] The present invention relates to a motor vehicle braking system comprising a hydraulic control fluid, equipped with valves for protection against vaporization of this fluid, as well as a method of operating such a system and a motor vehicle equipped with this braking system.

[0002] Motor vehicles generally include a braking circuit comprising a master cylinder controlled by the brake pedal, transmitting the pressure of a brake fluid to the various wheel brake actuators which brake the rotation of a disc or a drum by the friction of linings generating thermal energy.

[0003] In the event of heavy braking, particularly from high vehicle speeds or during a long mountain descent requiring frequent braking, a significant temperature rise can occur in the brake linings. This temperature is transmitted by conduction to the brake fluid entering the actuators, which can reach its boiling point. In particular, the various types of standardized brake fluids initially have a boiling point above 200°C, which can exceed 260°C for some types, but they are hydrophilic and, with moisture accumulation over time, can have a boiling point approaching 150°C.

[0004] This creates a risk of vapor lock forming in a brake that is overheating. Vapor lock gives the brake fluid column in the circuit significant elasticity, preventing pressure from being transmitted to the brake pads. Since the vehicle's speed increases the circulation of cool air around the wheel brakes, this phenomenon can occur, in particular, after prolonged braking that has significantly heated the brakes, when the vehicle is traveling at a low speed that is no longer cooling them.

[0005] Motor vehicles generally have two independent circuits from the master cylinder equipped with two pistons arranged in tandem, each of which most commonly supplies the brakes of the wheels arranged on the same diagonal of the vehicle in an "X" pattern, or alternatively the two brakes of the same axle in an "I+I" pattern.

[0006] Wheel brake heating is generally similar on the same axle with the same brake types and equivalent loads, but may The braking system can differ between the front and rear of the vehicle. In the case of the "X" configuration, which is the most common, if a steam blockage occurs in both front brakes, then both rear brakes are also inoperative. In the case of an "I+I" configuration, a steam blockage in both front brakes, as well as in one, means the rear brakes remain functional.

[0007] To avoid these problems, a known type of braking circuit, presented in particular by document KR-A-20070065104, includes a brake fluid return loop when the brake is released, which starts from a thermostatic valve fixed on the brake actuator and then passes through a cooling coil with ambient air, and finally returns to the supply line upstream of this actuator.

[0008] The thermostatic valve opens when it detects an excessively high temperature of the brake fluid in the actuator, and a check valve closes the supply line. This forces the fluid, when drained after braking is stopped, to pass through the cooling coil before returning to this line and then to the master cylinder. This provides a means of cooling the fluid, which is not very effective at low vehicle speeds, resulting in minimal ventilation of the coil.

[0009] Another type of known braking circuit, presented in particular by document US-A-4867280, includes a diaphragm or piston disposed in the brake actuator, which receives on one side the supply fluid from the master cylinder to transmit it on the other side to a second brake fluid actuating the two pistons of a caliper clamping a disc.

[0010] In this way, two different fluids subjected to the same pressure are separated, which makes it possible to predict for the second brake fluid in the actuator a better resistance to boiling, while the first fluid arriving from the master cylinder has better qualities for other parameters such as corrosion resistance or lubrication.

[0011] However, the management of two different and complex fluids, in particular in the case of maintenance during the renewal of fluids, or repair, with risks of error in the choice of these fluids.

[0012] Another known type of braking circuit, notably described in document FRAI-2800821, comprises two connection ports for the brake actuator, one inlet and one outlet, allowing brake fluid to circulate within this actuator. This circulation is controlled by solenoid valves and is activated during braking operations initiated by the brake pedal or by a fluid circulation pump. The fluid circulation within the actuator results in heat exchange with the relevant hydraulic circuit, which heats up and also requires a separate cooling device. Furthermore, this circuit uses a pump which adds mass and cost.

[0013] The present invention aims in particular to avoid these problems of the prior art.

[0014] To this end, it proposes a braking system for a motor vehicle comprising at least one hydraulic braking circuit having several brake actuators connected to each other by this circuit which transmits a pressure of a brake fluid to clamp rotating elements, this system being remarkable in that at least one brake actuator has on its brake fluid inlet a pilot-operated valve which is open under normal driving conditions, and includes a valve control device which takes into account the temperature of the actuator to close this valve.

[0015] An advantage of this braking system is that in the event of a risk of boiling of the brake fluid contained in an actuator of the circuit, which is subjected to excessive heating, the detection of the high temperature of this actuator can immediately trigger the closure of the valve by its control device.

[0016] An isolation is obtained of the actuator which can contain vapor from the liquid, which makes it possible to prevent this vapor from rising through its supply pipe into the circuit, thus leaving the rest of the circuit, including at least one other actuator, to remain functional.

[0017] The braking system according to the invention may further comprise one or more of the following characteristics, which may be combined with each other.

[0018] Advantageously, the valve control device includes a temperature-sensitive element of the brake actuator.

[0019] Advantageously, the temperature-sensitive element of the control device includes a component that expands with temperature.

[0020] In this case, in particular the component may include a vaporizing fluid, a liquefying material or a blade device that expands.

[0021] Advantageously, each piloted valve includes a return spring which acts against the control device of that valve to tend to return the valve to its open position.

[0022] In addition, each piloted valve may include a check valve arranged in parallel, which is through going from the brake actuator to the circuit.

[0023] The invention also relates to a method of operating a braking system comprising any one of the preceding characteristics, remarkable in that when a brake actuator heats up to a temperature below the boiling point of the liquid, its valve control device automatically puts this valve in a closed position.

[0024] The invention also relates to a motor vehicle comprising any one of the preceding characteristics, notable in that it includes two circuits braking systems, each comprising at least one wheel brake equipped with a pilot-operated valve.

[0025] In particular, the vehicle may have two braking circuits, each acting on two wheel brakes arranged diagonally on the vehicle.

[0026] The invention will be better understood and other features and advantages will become more apparent upon reading the following description given by way of example, with reference to the accompanying drawings in which:

[0027] [Fig. 1] presents for a brake system according to the prior art a graph of the evolution over time of the temperature of the brake linings of a wheel brake, and of its brake fluid during heavy braking;

[0028] [Fig.2] presents a diagram of a motor vehicle braking circuit according to the invention; and

[0029] [Fig.3] shows the piloted valve of each brake actuator in this circuit.

[0030] Fig. 1 shows, during a powerful braking of a vehicle at high speed, as a function of time t arranged on the horizontal axis, the temperature T° of the clamping linings of the rotating element braked by a brake of the vehicle 2, which can be a disc or a drum, and the temperature of the brake fluid 4 contained in the actuator of this brake.

[0031] From time t0 when braking begins, with the brake actuator at ambient temperature T°0, we first observe a very rapid heating of the brake linings due to the high friction speed which gives a significant braking power, then with the decrease in speed a less and less rapid heating which ends at time tl when the vehicle stops with a maximum temperature of the linings TL In practice the temperature of the linings can exceed 500°C, and even reach 1000°C in extreme conditions.

[0032] During vehicle braking, there is a certain heat diffusion around the friction linings by conduction with the contacting elements, and by convection in the ambient air, which depends on the vehicle speed generating an airflow. The temperature curve of the linings 2 results from the evolution of the thermal power produced by braking and the cooling capacities.

[0033] During this same period of vehicle movement until it comes to a complete stop, there is a slow diffusion by conduction of the thermal energy produced by the brake linings, which affects the brake fluid located nearby in the clamping pistons of these linings. At the first stop of the vehicle, t1, the temperature of the fluid 4 in the brake control system has reached a moderate temperature T2, which is lower than its boiling point T3.

[0034] From the first time t1, the temperature of the linings 2 decreases initially rapidly, due to the significant temperature difference with the ambient air and with the elements in contact, then less and less rapidly as this difference is reduced. This di The gradual reduction of the temperature of the linings is limited by the fact that, with the vehicle stationary, there is no longer any airflow from movement.

[0035] But in parallel, the thermal energy contained in the linings and their surrounding components, which have a high mass and a high heat capacity, continues to be transferred to the brake fluid of the actuator, whose temperature 4 continues to rise. At the second time t2, the temperature of the brake fluid 4 reaches the boiling point temperature T3, and this fluid begins to vaporize, forming a volume of compressible gas in the brake actuator.

[0036] After the second time t2 the temperature of the linings 2 continues to decrease progressively while that of the brake fluid 4 rises slowly, until at the third time t3 a convergence of these temperatures which become uniform over the whole of the brake actuator but still above the boiling point T3. The brake fluid continues to produce vapor.

[0037] Finally, at the fourth time t4, the entire actuator reaches a temperature falling below the boiling point T3, its cooling then allows a progressive condensation of the brake fluid vapor.

[0038] Between the second time step t2 and the fourth time step t4, when the driver operates the brake pedal by displacing a volume of fluid from a hydraulic circuit of the vehicle with the master cylinder, an elastic compression of the vapor occurs in the relevant actuator of this circuit, preventing a pressure increase in the fluid throughout the entire circuit. All the actuators of this circuit are no longer operational.

[0039] Figure 2 presents a braking system according to the invention, comprising the brake pedal 8 which acts on a master cylinder 10 supplying two independent hydraulic circuits 12, 14, each passing through a pressure modulation device 16 of a safety function such as an anti-lock braking system of the "ABS" type or a trajectory correction system of the "ESC" type.

[0040] Each hydraulic circuit 12, 14 extends after the pressure modulation device 16 by an individual pipe 18 connected to a wheel brake 20 comprising a rotating element 22 and a brake actuator 24, passing through a safety pilot valve 26 supplying the internal pistons of this actuator.

[0041] Fig. 3 shows the pilot valve 26 disposed between the individual pipe 18 and the brake actuator 24, in its normally open rest position O which is through.

[0042] The pilot-operated valve 26 includes a temperature-sensitive element 34, directly connected to the actuator 24 to detect its temperature, which, above a temperature slightly below the boiling point threshold T3, expands against a return spring 36 located opposite it, which tends to return the valve to its position open O, causing an internal drawer to slide, which moves the valve into its closed position F. The temperature-sensitive element 34 may include any expanding component such as a vaporizing fluid, a liquefying wax-like material, or a two-blade device that expands by changing position.

[0043] Sliding the inner spool of the pilot-operated valve 26 to the closed position F closes the passage between the actuator 24 and its conduit 18, isolating the actuator and preventing the steam produced by the brake fluid inside it from rising into the conduit if its temperature continues to increase to reach boiling point T3. The second wheel brake 20, which is connected by the same hydraulic circuit 12, 14 to the wheel brake containing the steam, remains operational.

[0044] In the case of a safety shutdown of a front wheel brake, for a hydraulic circuit diagram of the vehicle in "X" the rear brake of this circuit remains operational, or for a diagram in "I+I" the second brake of the front axle remains operational.

[0045] After the fourth stage t4, during which the brake fluid temperature 4 falls below the boiling point temperature T3, the temperature-sensitive element 34 gradually retracts, and the pilot-operated valve 26 returns to its normally open position O under the action of the return spring 36, which occurs after the vapor has completely condensed. The relevant wheel brake 20 then becomes active again.

[0046] A non-return valve 40 can optionally be arranged in parallel with the piloted valve 26, with a passing direction of the actuator 24 towards the individual brake line concerned 18, and a blocking in the other direction.

[0047] During braking, when the pilot valve 26 closes, a pressure is generated in the circuit 12, delivered to the brake line 18, which does not flow to the gaseous portion contained in the relevant actuator 24, thus making the rest of this circuit operational. When the brake pedal 8 is released, the brake fluid returns to the actuator 24 via the check valve 40, resulting in a rapid pressure drop in this brake.

[0048] It should be noted that the braking system according to the invention includes minor modifications affecting only the brake actuators 20, without altering the rest of the circuit, and can therefore be easily implemented on new or existing vehicles. In particular, the pilot-operated safety valve 26 can be installed only on the front brakes of the vehicle, which are subject to greater heating, in order to reduce costs.

Claims

Demands

1. Braking system of a motor vehicle comprising at least one hydraulic braking circuit (12, 14) having several brake actuators (24) connected together by this circuit (12, 14) which transmits a pressure of a brake fluid to clamp rotating elements (22), characterized in that at least one brake actuator (24) has on its brake fluid inlet a piloted valve (26) which is open (0) under normal driving conditions, and has a valve control device (34) taking into account the temperature of the actuator (24) to close this valve (26).

2. Braking system according to claim 1, characterized in that the valve control device (34) comprises a temperature-sensitive element of the brake actuator (24).

3. Braking system according to claim 2, characterized in that the temperature-sensitive element of the control device (34) comprises a component that expands with temperature.

4. Braking system according to claim 3, characterized in that the component comprises a vaporizing fluid, a liquefying material or a blade device that expands.

5. Braking system according to any one of the preceding claims, characterized in that each piloted valve (26) has a return spring (36) which acts against the control device of this valve (34) to tend to return the valve (26) to its open position (0).

6. Braking system according to any one of the preceding claims, characterized in that each piloted valve (26) has a check valve (40) arranged in parallel, which is through going from the brake actuator (24) to the circuit (12, 14).

7. A method of operating a braking system according to any one of the preceding claims, characterized in that when a brake actuator (24) heats up to a temperature below the boiling point threshold of the liquid (T3), its valve control device (34) automatically puts this valve (26) into a closed position (F).

8. A motor vehicle equipped with a braking system according to any one of claims 1 to 6, characterized in that it comprises two braking circuits (12, 14), each comprising at least one brake

9. wheel (20) equipped with a pilot-operated valve (26). Motor vehicle according to claim 8, characterized in that it comprises two braking circuits (12, 14) each acting on two wheel brakes (20) arranged diagonally on the vehicle.

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