Air Brake System: Compressor and Control

Air Compressor Operation

• Piston Mechanism: The air compressor uses a piston with crescent rings to create a tight seal against the cylinder wall.

• Two-Stroke Cycle:

  • Downstroke: Creates a low-pressure area (vacuum) above the piston, drawing air in through the inlet port. The inlet valve opens against spring pressure, allowing air to fill the cavity.

  • Upstroke: Once the piston hits bottom dead center, the inlet valve closes due to spring pressure, trapping air. As the piston moves up, it compresses the trapped air. This compressed air cannot escape back to the intake side because spring pressure and air pressure keep the inlet valve closed. Instead, it is forced out through the discharge valve (which opens when compressed air pressure overcomes its spring tension) into the air system.

• Valves: Both inlet and discharge valves are spring-loaded closed and are opened by air volume and mass working against spring pressure.

• Constant Operation: The compressor runs continuously as long as the engine is running; its movement cannot be stopped.

Air System Pressure Control: The Governor

• Function: The governor controls the air pressure within the system by telling the compressor when to stop pumping air.

• Unloader Port: The governor is connected to the compressor's unloader port via a signal line.

• Cut-Out Pressure: When the system reaches a preset pressure, typically $125 ext{ PSI}$, the governor activates.

  • A piston inside the governor, working against a spring, moves up when air pressure exceeds the spring tension. This allows air movement from the governor to the unloader port on the compressor.

  • Unloader Piston: The incoming air pressure from the governor acts on an unloader piston within the compressor. This piston lifts a plunger that holds the compressor's inlet valve open.

  • Unloaded Cycle (Cut-Out): By holding the inlet valve open during both the downstroke and upstroke, the compressor cannot build pressure. Air is drawn in on the downstroke but is simply pushed back out the intake on the upstroke, preventing it from reaching the discharge line. The compressor continues to operate mechanically but does not add air to the system.

• Cut-In Pressure: The compressor remains in the unloaded (cut-out) state until the system air pressure depletes. This usually occurs when the pressure drops to approximately $20 ext{ PSI}$ below the cut-out pressure, typically around $105 ext{ PSI}$.

  • At this point, the spring in the governor overcomes the reduced air pressure holding its piston up, causing the governor's piston to move down.

  • This action allows the air pressure holding the unloader piston up to exhaust to the atmosphere, releasing the unloader piston.

  • The unloader piston then falls, allowing the inlet valve to close, and the compressor resumes building air pressure, sending it to the system.

• Governor Location: Governors can be mounted remotely, on the air dryer, or directly on the compressor. They receive a signal (air pressure) directly from the supply tank.

• Adjustability: The cut-out pressure of the governor can be adjusted by altering the spring tension inside. Loosening a jam nut and turning a flat blade screwdriver slotted screw can either increase (more tension, higher cut-out) or decrease (less tension, lower cut-out) the required air pressure to move the piston.

• Terminology: "Unloaded" means the compressor is not pumping air; "Loaded" means the compressor is pumping air.

Air System Components and Safety

• Pressure Relief Valve:

  • Function: A safety device designed to prevent catastrophic failure from over-pressurization.

  • Set Point: Typically set to open and release excess air pressure to the atmosphere at $150 ext{ PSI}$.

  • Operation: It is spring-operated. When air pressure reaches $150 ext{ PSI}$, it unseats a ball against spring tension, allowing air to escape. It re-seats automatically at a slightly lower pressure (e.g., $140 ext{ PSI}$) once the excess pressure is relieved.

  • Location: Found as close to the compressor as possible, potentially on the compressor itself, the air dryer, or the wet tank. Multiple relief valves may be present.

• Turbocharged Engine Regulator: On turbocharged engines, a small regulator restricts the pressurized air flow into the air compressor's inlet side to $<br>10 ext{ PSI}$. This prevents damage and loss of turbo boost (which can be between $30$ and $50 ext{ PSI}$ at maximum) if the compressor were to draw too much air from the turbocharger, leading to intermittent power complaints.

• Pressure Protection Valves (PPVs):

  • Function: Normally closed, pressure-sensitive valves that only open when specific conditions (e.g., a system pressure of $<br>80 ext{ PSI}$) are met.

  • Application: Protects crucial air consumers, like air suspension airbags, ensuring they do not receive air until sufficient system pressure is available. This also prevents loss of air from the main system if there's a leak in the airbag system.

Air Tanks and Management

• Material: Air tanks can be made from aluminum or steel.

• Mandatory Drain Valves: Every air tank is legally mandated to have a drain valve.

  • Manual Drain Valves: Requires manual operation.

  • Automatic Drain Valves: Operates based on pressure differential to expel moisture and contaminants.

    • A diaphragm has equal pressure on both sides until air pressure on the top side depletes (e.g., during a brake application). The remaining higher pressure (e.g., $120 ext{ PSI}$) on the bottom side of the diaphragm causes it to move, opening the valve and expelling a small amount of air and moisture.

    • This often triggers when the parking brakes are released, as it uses a significant volume of air from the tanks, causing a pressure drop.

• Tanks Terminology and Flow:

  • Wet Tank (Supply Tank): The first tank to receive air from the air compressor (after the air dryer). Most moisture should settle here.

  • Dry Tanks (Primary/Secondary): These tanks receive air from the wet tank and should contain less moisture.

  • Air Flow Sequence: Compressor $<br>\rightarrow$ Air Dryer $<br>\rightarrow$ Supply/Wet Tank $<br>\rightarrow$ Primary Dry Tank $<br>\rightarrow$ Secondary Dry Tank.

• Check Valves: Air tanks typically have a one-way check valve to protect the air pressure already sent into them. Some tanks may have internal dividers, effectively creating two tanks in one (e.g., a wet tank and a primary tank) with built-in check valves internally.

Air Dryer

• Purpose: To remove moisture and oil from the compressed air before it enters the air tanks and other components.

• Mechanism: Contains desiccant, which are small silica beads that absorb water.

• Maintenance: The desiccant filter should be replaced annually. It's often a spin-on canister.

• Safety Precaution: Always drain all air tanks before opening any component of the air brake system, including spinning off the air dryer canister, to prevent exposure to high pressures ($125 ext{ PSI}$).

General Safety during Service

• Always drain air tanks before working on any air system component.

• Wear appropriate Personal Protective Equipment (PPE) such as steel-toed boots, high-visibility clothing, safety glasses, and hard hats. They are crucial particularly when dealing with high-pressure systems up to $125 ext{ PSI}$.