Why are air bubbles dangerous in a hydraulic brake system?

One of the most necessary conditions for a hydraulic system to function properly is that the hydraulic fluid must be incompressible.

Effect of air bubbles on a hydraulic system:-
  1. Loss of bulk modulus -Air is a compressible fluid. When air bubbles get into a hydraulic system, the force does not get transmitted properly.Also,free or entrained air in the hydraulic system reduces substantially the effective bulk modulus of the system. That is, an air-oil mixture appears to increase the compressibility of the fluid, making the system spongy. 
  2. Loss of horsepower — When an air pocket is present in an actuator, it is alternately compressed and relaxed as the actuator is cycled. Since the air pocket must first be compressed before the fluid can cause the actuator to move, power is consumed. Upon relaxation, the air pocket expands and rives fluid out. The stored power, therefore, is expended in driving fluid back into the reservoir and not in moving the actuator.
  3. Spongy control — Because fluids are considered to be basically incompressible, we expect great stiffness in a hydraulic system. That is, the positioning of an actuator should be immediate (rapid response) and precise. The larger the amount of free or entrained air, the spongier (softer, less stiff) the system.
  4. Loss of system fluid — One of the most serious conditions that can occur in a hydraulic system is the loss of reservoir fluid. The fluid level must be kept high enough to insure enough fluid for the pump intake, otherwise cavitation begins.

In that case, on pressing the brake pedal, one would feel the pedal like a jello. It would be as if you are pressing a sponge. The wheels would not lock instantly and the car would travel considerably long distance before the required braking effect is obtained.

No comments:

Post a Comment