Hydraulic systems in motor vehicles may use differenUKt principles of brake operation. On this basis, we distinguish the basic hydraulic brake commonly used in passenger cars, as well as the pneumatic brake used mainly in trucks and heavy trailers. How do they differ from each other and what are their advantages and disadvantages?
In the hydraulic braking system in cars and other vehicles Pascal’s law is used, and the most important element of the system is the master cylinder. When the driver presses the brake pedal, the piston of the master cylinder is pressed simultaneously and the pressure of the fluid in the entire closed hydraulic system increases. The front face of the piston located in the cylinder pushes the friction brake components together – disc brake pads. The pressure force is proportionally greater than the pressure exerted by the pedal.
The brake fluid in the hydraulic braking system flows through rigid and flexible pipes.
In trucks, pneumatic brake systems are used. If the truck has a trailer, the pneumatic two-wire braking system can be used. It affects the front and rear axle of the vehicle separately, due to the possession of two circuits.
The pneumatic brake system uses a set of brake valves, separate for each circuit, placed under the brake pedal. It has been supplemented with valves that accelerate the outflow of air from the circuits, at the end of the braking process. This results in a faster release of the friction lining pressure.
On the rear axle of the brakes, an automatic brake switch and a manual emergency brake valve are installed. In such systems, as in the case of hydraulic brakes, friction elements are used – traditionally they are drum constructions in which the jaws covered with friction lining cooperate with the inner surface of the drum. They are pressed down by a rotating press, powered by a pneumatic cylinder, to compensate for the wear of the linings that affect the delay of the brake operation time.
More and more often disc brakes are used for pneumatic braking systems, which increases braking efficiency. In general, a ventilated disc is used to efficiently discharge a large amount of heat generated in the friction process.
It activates the valves of individual pneumatic systems, opening a continuous supply of compressed air to the cylinders located directly at the wheels.
The basic elements of an air brake include:
- air compressor,
- pressure regulator,
- basic brake valve,
- parking brake valve,
- four-circuit safety valve,
- air tank,
- drainage valve,
- mechanical braking force regulator,
- brake cylinder,
- double-acting brake cylinder,
- the direction of force.
Hydraulic and Pneumatic brake – Comparison
Hydraulic brakes are usually used in passenger cars of all kinds, while pneumatic brakes in heavy trucks and trailers. The working element in the hydraulic system is fluid, while in the pneumatic one – compressed air. Therefore, in the first type of brakes, the hydraulic pump works, and in the second type the compressor, most often driven by the vehicle’s engine.
The advantage of pneumatic braking systems is that after stopping the engine for various reasons, the driver can stop the vehicle, and if the pneumatic conduits break, the brake will start working immediately and immobilize the vehicle. On the other hand, in the hydraulic system, breaking the brake lines supplying brake fluid will cause the brakes to fail and the vehicle can not be stopped in this way. However, they provide a short braking distance, simpler construction and easier replacement of their individual components.
Causes and types of faults occurring in the brakes
Car brakes may break down, increasing the risk of a traffic accident. Therefore, you must prevent excessive wear. The cause of the failure may be worn friction linings of brake pads (they should not have a thickness less than 5 mm), wiping discs, missing or too little brake fluid, breaking the brake cable and airing the brakes as a result of air bubbles entering the brake lines.
As a result of wear and tear on individual components of the braking system or its aeration, the braking distance, lack of vehicle adhesion, one-way descent during cornering and braking, uneven tire wear and a greater risk of collision may occur.
In extreme cases, failure of the braking system leads to the fact that the car can only be stopped using a handbrake or engine, thanks to the reduction of gears. In the event of a failure, the pneumatic brake stops the vehicle immediately, which is a great advantage in relation to hydraulic brakes.