MOTOR VEHICLE BRAKING SYSTEM WITH BRAKE FLUID VAPORIZATION PROTECTION VALVES

Abstract

Braking system of a motor vehicle comprising at least one hydraulic braking circuit (12, 14) comprising several brake actuators (24) connected together by this circuit (12, 14) which transmits a pressure of a brake fluid to clamp rotating elements (22), at least one brake actuator (24) comprising on its brake fluid inlet a piloted valve (26) which is open under normal driving conditions, and comprising a device for controlling the valve taking 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 have 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 pronounced braking, in particular from a high vehicle speed or in a long mountain descent requiring frequent braking, a significant increase in the temperature of the brake linings can be obtained, transmitted by conduction to the brake fluid entering the actuators, which can reach its boiling point. In particular, the various types of standardized braking fluids originally have a boiling point above 200°C, which can exceed 260°C for certain types, but they are hydrophilic and with a moisture loading over time they can have a boiling point approaching 150°C.

[0004] There is then a risk of formation in a highly heated brake of a vapor lock, called "vapor lock" in English, which will give significant elasticity to the column of liquid in the circuit comprising this brake, preventing the transmission of pressure to the linings. The speed of the vehicle adding a circulation of fresh cooling air around the wheel brakes, this phenomenon can be obtained in particular after prolonged braking having significantly heated the brakes, while the vehicle has a low speed which no longer cools them.

[0005] Motor vehicles generally have two independent circuits starting from the master cylinder equipped with two pistons arranged in tandem, each of which most generally 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 be different between the front and rear of the vehicle. In the case of the "X" brake circuit diagram, which is the most common, if a vapor lock appears on both front brakes then both rear brakes are also inoperative. In the case of an "I+I" diagram for a vapor lock on both front brakes, as on one only, then the rear brakes are still used.

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

[0008] The thermostatic valve opens when it detects too high a temperature of the brake fluid in the actuator, and a non-return valve closes the supply line, which forces this liquid, when draining after stopping braking, to pass through the cooling coil to return to this pipe and then to the master cylinder. This provides a possibility of cooling the fluid, which is not very effective for low vehicle speeds, giving a little ventilation of the coil.

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

[0010] In this way, two different fluids subjected to the same pressure are separated, which makes it possible to provide the second brake fluid in the actuator with better resistance to boiling, while the first liquid 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 type of known braking circuit, presented in particular by document FRAI-2800821, comprises two connection ports for the brake actuator, one incoming and one outgoing, allowing circulation of the brake fluid in this actuator controlled by solenoid valves, which is activated during braking controlled by the brake pedal or by a circulation pump for this liquid. By circulating the fluid in the actuator, an exchange of calories is obtained towards the hydraulic circuit concerned which heats up, and which also requires a cooling device. dissement. In addition, this circuit uses a pump which has additional 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 comprising several brake actuators connected together 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 comprises on its brake fluid inlet a controlled valve which is open under normal driving conditions, and comprises a device for controlling the valve taking 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 subject to excessive heating, the detection of the high temperature of this actuator can immediately trigger the closing of the valve by its control device.

[0016] This provides insulation of the actuator which can contain vapor coming 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 comprising at least one other actuator remaining 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 comprises an element sensitive to the temperature of the brake actuator.

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

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

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

[0022] In addition, each piloted valve may include a non-return valve arranged in parallel, which is open when 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 beyond a temperature below the boiling temperature threshold of the liquid, its valve control device automatically places this valve in a closed position.

[0024] The invention also relates to a motor vehicle comprising any one of the preceding characteristics, remarkable in that it comprises two circuits braking circuits each comprising at least one wheel brake equipped with a controlled valve.

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

[0026] The invention will be better understood and other characteristics and advantages will appear more clearly on reading the description below given by way of example, with reference to the appended drawings in which:

[0027] [Fig. 1] shows 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 significant braking;

[0028] [Fig.2] shows 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.l] shows, during 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 may be a disc or a drum, and the temperature of the brake fluid 4 contained in the actuator of this brake.

[0031] From the time t0 when braking begins, the brake actuator being at ambient temperature T°0, there is firstly a very rapid heating of the brake linings due to the high friction speed which gives significant braking power, then with the reduction in speed a heating which becomes less and less rapid which ends at time t1 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 diffusion of calories around the friction linings by conduction with the elements in contact, and by convection in the ambient air which depends on the speed of the vehicle generating an air flow. 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 rolling of the vehicle until it comes to a complete stop, there is a slow diffusion by conduction of the thermal energy produced by the linings, which affects the brake fluid located nearby in the clamping pistons of these linings. When the vehicle stops at the first time t1 the temperature of the liquid 4 in the brake control has reached a moderate temperature T2 which is lower than its boiling threshold temperature T3

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

[0035] But in parallel the thermal energy contained in the linings and its nearby elements, having a high mass with a high heat capacity, continues to be transmitted to the brake fluid of the actuator which sees its temperature 4 continue to rise. At the second time t2 the temperature of the brake fluid 4 reaches the boiling threshold temperature T3, this liquid begins to vaporize by forming in the brake actuator a volume of compressible gas.

[0036] After the second time t2 the temperature of the linings 2 continues to drop progressively at the same time as that of the brake fluid 4 rises slowly, to arrive at the third time t3 at a convergence of these temperatures which become uniform over the entire brake actuator but still higher than the boiling threshold T3. The brake fluid continues to produce steam.

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

[0038] Between the second time t2 and the fourth time t4, when the driver controls the brake pedal by moving a volume of liquid from a hydraulic circuit of the vehicle with the master cylinder, an elastic compression of the vapor in the relevant actuator of this circuit is obtained, which prevents a rise in pressure of the liquid in the entire circuit. All the actuators of this circuit are no longer operational.

[0039] [Fig. 2] shows 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 function of the wheels of the “ABS” type or a trajectory correction function 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 pilot-controlled safety valve 26 supplying the internal pistons of this actuator.

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

[0042] The pilot valve 26 comprises a temperature-sensitive element 34, directly connected to the actuator 24 to sense its temperature, which above a temperature slightly lower than the boiling temperature threshold T3 expands against a return spring arranged opposite 36 which tends to return this valve to its position open O, by causing a sliding of an internal drawer which moves the valve into its closed position F. The temperature sensitive element 34 may comprise any expanding component such as a vaporizing fluid, a material such as a liquefying wax, or a two-blade device which expands upon changing position.

[0043] With the sliding of the internal slide of the pilot valve 26 giving the closed position F, a closure of the passage between the actuator 24 and its pipe 18 is obtained, which isolates this actuator by preventing the steam produced by its brake fluid contained inside, if its temperature continues to rise to reach the boiling threshold T3, from rising in this pipe. The second wheel brake 20 which is connected by the same hydraulic circuit 12, 14 to the wheel brake comprising the steam, remains operational.

[0044] In the case of a front wheel brake being made safe, for a diagram of the vehicle's hydraulic circuit in "X" the rear brake of this circuit which remains operational is obtained, or for a diagram in "I+I" the second brake of the front axle which remains operational.

[0045] After the fourth time t4 comprising a temperature of the brake fluid 4 which falls below the boiling threshold temperature T3, a progressive retraction of the temperature-sensitive element 34 is obtained, and a return of the piloted valve 26 to its normally open position O under the effect of the return spring 36 which occurs after the complete condensation of the vapor. The wheel brake concerned 20 becomes active again.

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

[0047] During braking, in the event of closure of the pilot valve 26, with the non-return valve 40, a pressure is obtained in the circuit 12 delivered to the brake pipe 18 which does not go towards the gaseous part contained in the relevant actuator 24, making the rest of this circuit operational. And when the brake pedal 8 is released, a guaranteed return of the brake fluid emptying the actuator 24 is obtained via the non-return valve 40, which gives a rapid pressure drop in this brake.

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

Claims

Claims

1. Braking system of a motor vehicle comprising at least one hydraulic braking circuit (12, 14) comprising several brake actuators (24) connected together by this circuit (12, 14) which transmits a pressure of a brake fluid to tighten rotating elements (22), characterized in that at least one brake actuator (24) comprises on its brake fluid inlet a piloted valve (26) which is open (0) under normal driving conditions, and comprises 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 an element sensitive to the temperature of the brake actuator (24).

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

4. A braking system according to claim 3, characterized in that the component comprises a vaporizing fluid, a liquefying material or an expanding blade device.

5. Braking system according to any one of the preceding claims, characterized in that each piloted valve (26) comprises 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 pilot valve (26) comprises a non-return valve (40) arranged in parallel, which is passing when going from the brake actuator (24) towards the circuit (12, 14).

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

8. 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-controlled 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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