Fig. 14.2. Scheme of interaction of brake system elements: 1 – pump; 2 – BTCS valve; 3 – front wheel; 4 – rear wheel; 5 – rear wheel; 6 – front wheel; 7 – hydraulic inlet valves
The main brake cylinder together with the brake booster is located in the engine compartment. Since 1996, the diameter of the brake booster has been increased to 229 mm (instead of 203 mm), the gain has increased from 4 to 4.5. The diameter of the main brake cylinder has also been increased. The pistons compress the brake fluid contained in the main brake cylinder, the hydraulic pressure created in the brake system is transmitted through pipes and hoses to the calipers of all four wheels (Fig. 14.2). In the calipers of the front wheels, the pistons press the brake shoes to the brake discs. In the rear wheels, the pistons of the wheel brake cylinders press the brake shoes to the brake drums.
The parking brake acts via cables on the brake mechanisms of the rear wheels, compared to older models it is more effective and requires less effort. In modern Fiesta models, the parking brake adjustment is carried out in the car, which, among other things, significantly reduces maintenance costs.
The Fiesta is equipped with self-adjusting disc and drum brakes, with the rear axle being adjusted by an expensive mechanism.
The most important elements of the braking system
Brake system with two brake circuits.
The vehicle is equipped with a service brake system with diagonally divided circuits, i.e. each circuit provides braking of the front and diagonally located rear wheel. If one of the circuits fails, the second circuit is used, ensuring that the vehicle stops with sufficient efficiency.
Master brake cylinder.
Converts the force applied to the brake pedal into hydraulic pressure. When the brake pedal is released, it provides a rapid reduction in pressure in the brake system.
Vacuum brake booster.
The brake booster reduces the force required to press the brake pedal by up to 60%, making it easier to control the vehicle. The brake booster operates by using the vacuum in the intake manifold or created by a vacuum pump (on vehicles with diesel engines). When braking, the membrane connected to the main brake cylinder rod reacts to the difference between atmospheric air pressure and the pressure in the intake manifold.
Wheel brake cylinder.
The brake fluid pressure in the wheel brake cylinder can reach 120 bar. The pistons of the wheel hydraulic cylinder transmit the force to the brake pads of the disc or drum brake.
TECHNICAL DICTIONARY
Brakes
Front brakes
Brake disc.
Rotates synchronously with the axle hub and removes heat generated by friction during braking.
Support.
Fig. 14.3. Front wheel brake mechanism: 1 – brake disc; 2 – brake shoe guide; 3 – support
The steel brake disc rotates freely with the wheel, and the brake caliper covers the disc on both sides (Fig. 14.3). When you press the brake pedal, the caliper piston presses the brake pads against the brake disc - the friction between them causes the car to brake.
When braking, kinetic energy is converted into thermal energy due to friction, which is removed from the friction zone by the brake disc and caliper. Since heat is transferred through one caliper cylinder, the brake fluid heats up slightly.
Pressing the brake pedal.
When you press the brake pedal, the rod connected to the pedal presses on two pistons located one behind the other in the master cylinder. The pistons compress the brake fluid contained in the master cylinder, and the hydraulic pressure thus created is transmitted through pipes and hoses to the calipers of all four wheels. In the calipers, the pistons press the brake pads against the brake discs, resulting in the braking of the car.
Releasing the brake pedal.
The pressure in the hydraulic system instantly decreases, and the caliper piston, together with the brake pads, moves away from the brake disc. A gap is formed between the brake pads and the disc, the brake disc begins to rotate freely, and the braking process stops.
Rear brakes
Drum brakes.
Fig. 14.4. Rear wheel brake mechanism: 1 – wheel brake cylinder; 2 – brake shoe; 3 – hub axle; 4 – parking brake cable; 5 – brake drum
The Fiesta has single-acting brakes on the rear axle, consisting of two semicircular brake shoes (Fig. 14.4). The thickness of the front brake shoe lining (running brake shoe) is greater than the thickness of the rear (pressed-on brake shoe). A two-piston wheel brake cylinder presses both shoes to the brake drums. They are installed on a support pin located on the opposite side from the wheel brake cylinder. The pin is riveted to the protective casing of the rear brake.
TECHNICAL DICTIONARY
Anti-lock braking system
ABS control unit.
Fig. 14.5. Location of anti-lock braking system components on a vehicle: 1 – brake booster; 2 – brake fluid reservoir; 3 – main brake cylinder; 4 – front brake caliper; 5 – electro-hydraulic unit; 6 – electrical circuit connecting the wheel speed sensors to the electro-hydraulic unit; 7 – hydraulic brake drive lines
Continuously processes information about the rotation speed of the vehicle's wheels received from the sensors and compares it with the programmed values (Fig. 14.5). If the rotation speed values differ, indicating an imminent danger of one or more wheels locking, the control unit activates the hydraulic unit - the brake fluid pressure on the locking wheel decreases until it starts rotating again synchronously with the other wheels. This cycle of pressure change occurs in a matter of milliseconds. In addition to processing the signals and subsequent control of the valves, the control unit also performs a diagnostic function. It checks and records faults in the electrical circuits or elements of the anti-lock system, the operating voltage in the vehicle's on-board network, warning the driver by lighting up a control lamp in the instrument cluster.
Electrohydraulic unit.
It consists of an electric pump and a valve block with electromagnetic valves. When the brake pedal is pressed, the pistons in the main brake cylinder squeeze the brake fluid through the valve block to the wheel brake mechanisms. In this case, the valve block regulates the pressure in the brake circuits that diagonally connect the front wheel to the rear wheel. If the ABS system comes into action, the control unit issues a signal to reduce the pressure and the brake fluid is supplied directly from the valve block to the brake fluid reservoir. If the brake pressure needs to be increased again, the brake fluid is supplied from the reservoir through the hydraulic pump directly to the corresponding brake hydraulic circuit. The operation of the pump can be noticed by a slight pulsation of the brake pedal. The cycle of braking and free rotation of the wheel occurs very quickly and continues until the car stops or until the brake pedal is released.
Wheel speed sensor.
Speed sensors, one on each wheel, measure the speed of rotation of the wheel, and based on their signals, the electronic control unit calculates an average speed, approximately corresponding to the speed of the vehicle. By comparing the speed of rotation of each individual wheel with the average calculated speed, the electronic control unit determines the slip state of the individual wheel and thus determines which wheel is in a pre-lock state.
Fig. 14.6. Location of the front wheel ABS sensor: 1 – ABS sensor; 2 – sensor mounting bolt; 3 – toothed rotor
Fig. 14.7. Location of the rear wheel ABS sensor: 1 – ABS sensor; 2 – sensor mounting bolt; 3 – hole for installing the sensor; 4 – brake drum
The rotation speed sensor, consisting of a magnetic anchor and a coil, is installed with a small gap from the toothed rotor rotating together with the wheel (Fig. 14.6, 14.7). Each tooth of the rotor, passing by the sensor, induces a short voltage pulse in it. Thus, the sensor produces an alternating voltage, the frequency of which increases or decreases in accordance with the rotation speed of the wheel, and transmits it to the control unit.
Longitudinal acceleration sensor.
It consists of two reciprocating switches that are installed under the central console in the car's interior. During normal driving and minor acceleration or braking, they are inoperative. As soon as the braking or acceleration parameters programmed in the control unit are exceeded, the sensors send a corresponding signal to the control unit.
Malfunctions of the brake system with ABS.
The ABS indicator lamp in the instrument cluster lights up after the ignition is turned on and goes out 2 seconds after the engine is started. If the indicator lamp lights up continuously, the ABS is faulty. If the ABS system is faulty, the brakes are still in good working order and function as without ABS. To fix the problem, you must contact a car repair shop. In any case, you must check the reliability of the connectors to the control unit and wheel speed sensors.
(The original source of the publication is on the website: «Fordbook.ru»)