BRAKES 13 A. BRAKING WITHOUT ABS IN OPERATION: In case of normal braking operation when no wheel speed differences are detected and neither ABS nor EBD are active, when the driver presses the brake pedal, brake pressure developed in tandem master cylinder (TMC) reaches to each wheel through hydraulic control unit (HCU). During this phase, both inlet & outlet solenoid valves are switched OFF (or not excited) by the ECU. Hence the inlet valve is open and outlet valve remains closed allowing the brake fluid to flow freely through the inlet valve and to the relevant wheel. When the driver releases brake pedal, the brake fluid flows back from the wheel, through the respective one way valve towards TMC. Solenoid valve Electricity status Valve open Open- close channel close INLET OFF OPE N Master cylinder Wheel cylinder OUTLET OFF CLOS E Wheel cylinder Reservoir B. BRAKING WITH EBD INTO OPERATION: In case of a braking where the ABS ECU detects a slip difference between the front and rear wheels, EBD comes into operation. If the difference in wheel slip between the front and rear wheels exceeds a predetermined threshold the ABS activates the pressure inlet valves for the rear wheels, closing the valves and preventing further pressure build up at that rear axle. If during the braking event, this slip ratio between the front and rear axle reduces below the threshold level the inlet valves will be de-activated and switch back to their normally open state allowing further pressure increase at the rear axle. In this way the rear axle can follow more closely to the Ideal braking force distribution curve for the rear axle. If the driver releases the brake, the inlet valves will be deactivated and fluid can return to the TMC via the inlet valve and the one way valve. EBD control only acts upon the rear axle and can only limit pressure being applied to a rear wheel, it cannot perform pressure reductions. Solenoid valve Electricity status Valve open Open- close channel close INLET ON CLOSE Master cylinder Wheel cylinder OUTLET OFF CLOSE Wheel cylinder Reservoir Fig: Pressure increase on rear axle following ideal brake force distribution curve.
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