CDL · Air Brakes · 5 min read · Open lesson
Making and Storing Compressed Air
- Identify the three individual systems that make up an air brake system and describe the function of each.
- Explain how the engine-driven air compressor and governor produce and regulate compressed air within the storage tanks.
- Describe the operation and maintenance requirements for air storage tanks, drain valves, alcohol evaporators, safety relief valves, and supply pressure gauges.
The Three Brake Systems in Commercial Air Brakes
Air brakes use compressed air to make the brakes work. When they are maintained and used properly, air brakes provide a safe method for stopping large and heavy vehicles. Operating a heavy commercial motor vehicle requires an understanding of how compressed air is generated, managed, and monitored. Air brakes use compressed air, and the storage tanks hold enough air for several brake applications even if the compressor stops working.
An air brake system is not just a single mechanism; it is made up of three different braking systems: the service brake system, the parking brake system, and the emergency brake system. Each of these three systems serves a distinct purpose during vehicle operation, parking, or unexpected mechanical trouble.
The service brake system applies and releases the brakes when the driver uses the brake pedal during normal driving. This is the system you rely on to manage your speed and bring the vehicle to a halt under ordinary driving conditions. The parking brake system applies and releases the parking brakes when the driver uses the parking brake control, keeping the vehicle securely held in place when parked. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle if the brake system fails. These systems provide braking during normal driving, parking, and a brake-system failure, but the brakes must be maintained and used properly.
The Air Compressor and Governor
Before air pressure can be used to stop a vehicle, it must first be produced and regulated. The air compressor pumps air into the air storage tanks, which are also called reservoirs. The air compressor is connected to the vehicle engine through gears or a v-belt. The engine drives the compressor, and the governor controls when the compressor pumps air into the storage tanks.
Air compressors require cooling and lubrication to function reliably. Depending on how the vehicle is built, the air compressor may be air cooled, or it may be cooled by the engine cooling system. Lubrication methods also depend on the vehicle design: the air compressor may have its own oil supply, or it may be lubricated by engine oil. When preparing for a trip, a driver must know which setup the vehicle uses. If the air compressor has its own oil supply, the driver must check the oil level before driving.
The air compressor governor controls when the compressor pumps air into the storage tanks. When air storage tank pressure reaches the cut-out level, which is around 125 pounds per square inch (psi), the governor stops the compressor from pumping air. As the driver operates the vehicle and uses air, the pressure in the storage tanks begins to decrease. When air tank pressure falls to the cut-in pressure, which is around 100 psi, the governor allows the compressor to start pumping again.
Air Storage Tanks and Moisture Drainage
Once air is compressed, it must be stored until the brakes require it. The number and size of air storage tanks varies among different vehicles, depending on the requirements of the vehicle. Regardless of their configuration, air storage tanks hold enough compressed air to allow the brakes to be used several times if the compressor stops working.
Compressed air usually contains some water and compressor oil, which can collect at the bottom of the air tanks. Water and compressor oil in the compressed air system are bad for air brakes. Water and compressor oil tend to collect in the bottom of an air storage tank. In cold weather, water in the air brake system can freeze. Freezing water inside the air brake system can cause brake failure. To prevent these dangerous conditions, air storage tanks must be drained completely.
Each air storage tank is equipped with a drain valve located at the bottom. Air tank drain valves are either manually operated or automatic. A manual air tank drain valve can be operated by turning it a quarter turn or by pulling a cable. A driver must drain manual air tanks completely at the end of each day of driving. Automatic drain valves expel water and oil automatically. Air tanks equipped with automatic drain valves may also be equipped for manual draining. In addition, air tanks with automatic drains are available with electric heating devices, which help prevent automatic drain valves from freezing in cold weather.
| Drain Valve Type | Method of Operation | Driver Maintenance Responsibility |
|---|---|---|
| Manual Drain Valve | Operated by turning a quarter turn or by pulling a cable | The driver must drain manual air tanks completely at the end of each day of driving. |
| Automatic Drain Valve | Expels water and compressor oil automatically; may include electric heating devices | May also be equipped for manual draining; electric heating devices help prevent freezing in cold weather. |
Alcohol Evaporators and Safety Relief Valves
Winter driving creates unique risks for air brake systems because ice inside the air brake system can make the brakes stop working. To combat this hazard, some air brake systems are equipped with an alcohol evaporator. An alcohol evaporator introduces alcohol into the air brake system. This alcohol reduces the risk of ice forming in valves and other parts during cold weather.
Having an alcohol evaporator requires routine driver maintenance. The driver must check the alcohol evaporator container and fill it as necessary every day during cold weather. Drivers must also remember that the presence of an alcohol evaporator does not replace regular tank drainage: daily air tank draining is still required on vehicles with an alcohol evaporator unless the system has automatic drain valves.
Protection from over-pressurization is equally critical. A safety relief valve is installed in the first tank the air compressor pumps air into. The safety valve protects the air tank and the rest of the system from excessive air pressure. This safety relief valve is usually set to open at 150 psi. If the safety valve releases air, something in the air brake system is wrong. When this happens, the driver must have a mechanic fix the fault.
Monitoring Tank Air Pressure with Gauges
A driver must always be aware of the amount of air pressure available in the storage reservoirs. For this reason, all vehicles equipped with air brakes have a supply pressure gauge connected to the air tank. Supply pressure gauges tell the driver how much air pressure is stored in the air tanks.
Depending on how the vehicle is designed, the gauge layout may vary. On vehicles with a dual air brake system, there will be a pressure gauge for each half of the system or a single gauge with two needles. The supply pressure gauge shows how much pressure is in the air tanks; the governor normally stops pumping at around 125 psi and allows pumping again at around 100 psi.
Common mistakes
- Assuming all air compressors use engine oil: If the air compressor has its own oil supply, the driver must check the oil level before driving.
- Believing that an alcohol evaporator eliminates the need to drain air tanks: Daily air tank draining is still required on vehicles with an alcohol evaporator unless the system has automatic drain valves.
- Confusing governor cut-in and cut-out pressures: The governor cut-out level stops pumping at around 125 psi, while the cut-in pressure starts pumping again at around 100 psi.
- Ignoring a safety valve that vents air: A safety valve is usually set to open at 150 psi; if it releases air, something is wrong and a mechanic must fix the fault.
Summary
- Air brakes use compressed air to safely stop large and heavy vehicles when maintained and used properly, operating through service, parking, and emergency brake systems.
- The air compressor is driven by gears or a v-belt, is air cooled or cooled by engine coolant, and may be lubricated by engine oil or have its own oil supply that must be checked before driving.
- The governor controls tank pressure by stopping pumping at the cut-out level (around 125 psi) and resuming pumping at the cut-in pressure (around 100 psi).
- Air storage tanks hold enough compressed air to allow several brake applications if the compressor stops, but water and oil can collect in them; the water can freeze in cold weather and cause brake failure.
- Drain tanks with manual drain valves completely at the end of each day of driving by turning the valve a quarter turn or pulling a cable; automatic drain valves expel water and oil automatically.
- If the vehicle has an alcohol evaporator, check its container daily during cold weather and fill it as necessary; continue draining tanks with manual drain valves daily.
- The safety relief valve is installed in the first tank and is usually set to open at 150 psi to protect that tank and the rest of the system; supply pressure gauges show how much pressure is in the tanks.
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