This application is a 35 USC 371 national phase filing of International Application No. PCT/EP2017/068394, filed Jul. 20, 2017, the disclosure of which is incorporated herein by reference in its entirety.

The present invention concerns a method for utilizing a park brake system to improve the deceleration of a vehicle in the event of failure of the service brake system.

In a well-known manner, a vehicle includes a park brake system and a service brake system. The service brake system comprising a brake pedal and at least one air supply circuit. The park brake system and the service brake system include common actuators for each wheel. Typically, each one of these actuators includes a cylinder inside which are arranged two compartments, namely a compartment for the application of the parking brake and a compartment for the application of the service brake. The compartment for the application of the parking brake includes a loaded spring and a pressurized chamber, the loaded spring being capable of providing an axial braking force when the pressurized chamber is vented. The compartment for the application of the service brake includes a diaphragm and a pressurized chamber, the diaphragm being capable of providing an axial braking force when the chamber is pressurized. The diaphragm and the spring act on the same piston rod.

The parking brake system and the service brake system are pneumatically powered and there is at least one air supply circuit for each system. Typically, each air supply circuit of the service brake system includes an air tank for the front axle(s) and another air tank for the rear axle(s). A compressor supplies all of the air supply circuits of the vehicle and valves protect each air supply circuit of the vehicle from an air leak in another air supply circuit.

In the automotive industry, the actual braking homologation requires that in case of single failure in the service brake system, a minimum rate of braking still needs to be achieved, the stopping distance and deceleration are representative parameters of this rate of braking. The idea is to use the park brake system to assist the service brake system and improve the secondary and/or residual braking performance in the event of failure of the service brake system.

Typically, the weight of a tractor vehicle is not evenly distributed because most of the load is supported by the front axle. Therefore, the brakes of the front axle wheels are more efficient than the brakes of the rear axle wheels. Accordingly, the rate of braking (stopping distance and deceleration) highly decreases when a leak occurs in one air supply circuit of service brakes, i.e. when the service brakes of the front axle wheels become ineffective. The assistance of the parking brake system helps then improving the rate of braking.

US 2011/0005874 A1 discloses a method wherein, in the event of failure of one or more of the service brake circuits, the vehicle can still be braked by means of the parking brake device. The parking brake device therefore constitutes an auxiliary braking function for the service brake. When a failure of the service brake is detected, the parking brake device is automatically applied or an alarm signal is output so that the driver can brake the vehicle using an electrical activation device for activating the parking brake or the brake pedal.

US 2014/0054118 A1 discloses a method according to the preamble of claim1. In particular, such method includes a first step consisting in determining whether service brake pressure is below a predetermined amount of pressure, a second step consisting in determining whether the brake pedal is depressed and a third step consisting in applying the parking brake as auxiliary brake if both conditions are met.

In the event of failure of the service brake system, the parking brake is then applied as soon as the driver depresses the brake pedal. The parking brake is deactivated when the driver releases the brake pedal. One imagines that when the parking brake is repeatedly used as auxiliary braking, a high quantity of compressed air is consumed because the parking brake system consumes a higher quantity of compressed air than the service brake system. Also, the application of the parking brake with maximum effort while the vehicle is moving, for example on a highway, may lead to a very dangerous situation.

US 2009/0280959 A1 discloses a method for controlling a vehicle brake system comprising steps consisting in, first, measuring the pressure inside a first reservoir among the reservoirs dedicated to the service brake system, second, in comparing the measured reservoir pressure with a predetermined minimum pressure value and in, third, venting the spring-actuator part of a combination spring-actuated/diaphragm cylinder so as to automatically engage the parking brake. Accordingly, not all service brake failures are considered, meaning that there are configurations wherein no assistance is provided despite the existence of a service brake failure. In addition, the assistance is automatically triggered, i.e. without any human action, which may be dangerous and surprise the driver given that the driver did not necessarily want to slow down the vehicle (ex: vehicle moving on a highway).

WO 2006/053630 A1 discloses a motor vehicle wherein the parking brake may be operated as an auxiliary for the main brake (service brake) and as a sustained-action brake, for example during long downhill drives. The parking brake may be used as an aid for reversing drive when the driving direction is changed, for example in the case of shovel dozer which changes its driving direction frequently. WO 2006/053630 A1 does not tackle the problem of the application of a park brake system in the event of failure of the service brake system.

DE10339245 A1 discloses a method of controlling an engagement of a vehicle parking brake so as to avoid damage of the parking brake. In particular, the service brake is applied instead of the parking brake when the vehicle speed is outside a preferred speed range. Thus, DE10339245 A1 does not tackle the problem of the application of a park brake system in the event of failure of the service brake system.

The aim of the present invention is to propose a method that remedies the abovementioned drawbacks.

To this end, the invention concerns a method.

Thanks to the invention, the parking brake system does not assist the service brake system if the pedal brake does not reach the threshold position, i.e. if the driver does not depress the pedal brake enough. The parking brake assistance is then triggered only in emergency situations, wherein the driver requires strong braking and wherein such braking cannot be performed efficiently in the event of failure of the service brake system. Even in case of a failure, the service brake system always provides a minimum braking torque, which enables in most of the cases to slow down and stop the vehicle. The idea is then to use as much as possible the service brake system and to use the parking brake assistance only in absolute necessity, such as emergency braking. This allows saving pneumatic energy since the parking brake system consumes more energy than the service brake system.

Further advantageous features of the method are specified in the dependent claims.

The invention also concerns a vehicle.

