Method and a system for estimating the water content of brake fluid in a brake system

1. A method for estimating a water content of brake fluid in a brake system comprising a plurality of brake calipers and a brake fluid container, each of the plurality of brake calipers and the brake fluid container comprising a sensor configured to detect a parameter indicative of the water content of the brake fluid, the method comprising:

arranging the brake system in a controlled environment;

for a predetermined time period, exposing the brake system to a controlled air temperature and air humidity for a predetermined period of time such that an increase in water content in the brake fluid is accelerated in comparison to an increase in water content during normal operating conditions of a vehicle;

determining the water content of the brake fluid in the brake fluid container and in each brake caliper as a function of time during the predetermined time period; and

based on the determined water content as a function of time, establishing a model describing the relation between the water content of brake fluid in a brake caliper and the water content in the brake fluid in the brake fluid container.

2. The method according to claim 1 , further comprising repeatedly increasing and decreasing the air temperature and the air humidity according to a predetermined cycle during the predetermined time period.

3. The method according to claim 1 , wherein a maximum air temperature is in the range of 60 to 100° C.

4. The method according to claim 1 , wherein a maximum air humidity is in the range of 80 to 100%.

5. The method according to claim 1 , wherein the predetermined time period is in the range of 50 to 100 days.

6. The method according to claim 1 , wherein establishing a model comprises establishing a model describing the water content of brake fluid in each of the plurality of brake calipers as a function of the water content of the brake fluid in the brake fluid container.

7. The method according to claim 1 , wherein determining the water content of the brake fluid comprises measuring a conductivity, a density, a viscosity and/or a refraction of the brake fluid.

8. The method according to claim 1 , further comprising, in a brake system in a vehicle comprising a plurality of brake calipers and a brake fluid container, the brake fluid container comprising a sensor configured to detect a parameter indicative of a water content of the brake fluid, determining a water content in the brake fluid container and estimating a water content in a brake caliper based on the established model.

9. The method according to claim 8 , further comprising estimating a water content of the brake caliper assumed to have a highest water content for a given water content of the brake fluid in the brake fluid container according to the predetermined model.

10. The method according to claim 8 , further comprising estimating a water content of the brake caliper having a longest fluid connection between the brake caliper and the brake fluid container.

11. The method according to claim 8 , further comprising estimating a water content of the brake caliper at the rear wheel on a side opposite of a driver side of the vehicle.

12. A brake system for a vehicle, the brake system comprising:

a plurality of brake calipers;

a brake fluid container comprising a sensor configured to detect a parameter indicative of a water content of the brake fluid in the brake fluid container; and

a control unit configured to determine a water content of brake fluid in the brake fluid container based on the detected parameter and to estimate a water content of the brake fluid in a brake caliper based on the determined water content of brake fluid in the brake fluid container using a predetermined model describing the relation between the water content of brake fluid in a brake caliper and the water content in the brake fluid of the brake fluid container;

wherein the predetermined model is established based on a determined water content as a function of time by:

arranging the brake system in a controlled environment;

for a predetermined time period, exposing the brake system to a controlled air temperature and air humidity for a predetermined period of time such that an increase in water content in the brake fluid is accelerated in comparison to an increase in water content during normal operating conditions of the vehicle; and

determining the water content of the brake fluid in the brake fluid container and in each brake caliper as a function of time during the predetermined time period.

13. The brake system according to claim 12 , wherein the control unit is further configured to determine a water content of the brake caliper having a highest water content for a given water content of the brake fluid in the brake fluid container according to the predetermined model.

14. The brake system according to claim 12 , wherein the control unit is further configured to estimate a water content of the brake caliper having a longest fluid connection between the brake caliper and the brake fluid container.

15. The brake system according to claim 12 , wherein the control unit is further configured to estimate a water content of the brake caliper at the rear wheel on a side opposite of the driver side of the vehicle.