Principles of Hydraulics and Pneumatics in Vehicles

Understanding Fluids: Liquids and Gases

Fluids are substances that fit the shape of the container where they are found and flow through conduits. They are divided into two main categories:

• Liquid Science (Hydraulics): This includes the study of substances such as oil, fuel, and cooling liquids.
• Gaseous Science (Pneumatics): This involves the study of gases like air, nitrogen, and Freon R-134a.

The primary function of these fluids is to transmit and multiply applied forces. In vehicles, hydraulic circuits are found in the lubrication system, engine cooling, automatic gearboxes, hydraulic brakes, and clutches. The main difference between the two is that gases can be compressed, while liquids cannot.

Key Properties of Fluids

Density

Density is defined as the division between the mass of a substance and the volume it occupies. It is a critical measure for identifying substances. For instance, oil changes from a liquid to a gas at 400 °C, whereas Freon transitions at -26.5 °C.

Viscosity

Viscosity represents the resistance to displacement of a fluid at a certain temperature; higher viscosity means greater resistance. This is the specific magnitude we look for when selecting the correct oil for a car. Viscosity is measured using a viscometer, which consists of a container with a known capacity and a calibrated orifice at its bottom. While different standards exist, the SAE is the most commonly used for measuring oil viscosity.

Measuring Elements in Fluid Circuits

Manometry

Manometers are used to measure pressure in pneumatic or hydraulic circuits. There are two basic types:

• High-pressure: These can reach up to 300 bars and are typically used in hydraulics.
• Low-pressure: These oscillate between 0 and 20 bars and are used in pneumatics.

Gauges feature two types of connections: conical or threaded rapid plugs. The standard unit is the bar, though the PSI scale is also used.

Flowmeter

A flowmeter measures the flow traveling through a given conduit. Its operation is based on a turbine; when fluid passes through, the turbine moves, creating an electric signal.

Pressure Switch (Presostat)

A pressure switch is an element responsible for transforming a pressure level into an electric signal. These are found in circuits to activate or deactivate electric or hydraulic motors. In workshop compressed air systems, the pressure switch measures the pressure in the boiler to regulate the electric motor, starting or stopping it when the boiler reaches maximum pressure.

Fundamental Gas Laws

Boyle-Mariotte Law

This law states that with more volume comes less pressure, and with more pressure comes less volume, provided the temperature remains constant.

Gay-Lussac Law

As temperature increases, the volume of gas increases at the same pressure; conversely, as temperature decreases, the volume decreases. This phenomenon occurs because the collision of gas molecules increases with temperature. All gases share the same coefficient of expansion: 1/273.

Pascal's Principle and Flow Continuity

Pascal's Principle

Exerting pressure on a piston in a particular section increases its strength when placed at the exit section of a larger plunger, always working with the same pressure.

Flow and Continuity

Flow is the amount of fluid that passes through a point per unit of time. It can be measured using different classifications depending on whether the focus is on mass or volume.