Section 1.1: Air Compressors and Governors

Air Compressors and Governors

In a commercial vehicle equipped with an air brake system, the energy required to apply the brakes does not come directly from the driver’s foot. Instead, it is generated by compressed air stored in steel reservoirs. The source of this compressed air is the air compressor, and its operation is regulated by the governor. Understanding how these two components interact, their mechanical design, and the specific regulatory thresholds established by the Insurance Corporation of British Columbia (ICBC) is fundamental to operating a commercial vehicle safely and passing the air brake endorsement exam.

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The Air Compressor: The Heart of the System

The air compressor is the mechanical pump that draws in ambient atmospheric air, compresses it, and forces it into the vehicle's air reservoirs. It is typically mounted directly on the vehicle’s engine.

Drive Systems

Compressors are powered by the engine in one of two ways:

1. Gear-Driven: In most modern heavy-duty commercial vehicles, the compressor is gear-driven. It is bolted directly to the engine block, and its crankshaft is driven by the engine’s timing gears. This design is highly reliable as it eliminates the risk of belt slippage or breakage.
2. Belt-Driven: On some medium-duty vehicles, the compressor is driven by one or more V-belts or a serpentine belt connected to the engine crankshaft pulley. Drivers of belt-driven systems must inspect the belts daily for proper tension, cracking, fraying, and alignment. A loose or broken belt will prevent the compressor from building air, leading to system failure.

Lubrication and Cooling

Because compressing air generates significant friction and heat, the compressor requires continuous lubrication and cooling:

• Lubrication: The compressor is typically lubricated by the engine's oil pressure system. A feed line carries pressurized oil from the engine block to the compressor bearings and cylinders, which then drains back into the engine oil pan. If the compressor's piston rings wear out, oil can pass into the compressed air stream—a condition known as oil carryover. This oil can contaminate the air lines and damage rubber valves downstream.
• Cooling: The compressor generates extreme heat during the compression stroke. To prevent overheating, most compressors are cooled by the engine’s liquid cooling system, with coolant circulating through jackets in the compressor head. Some smaller units may be air-cooled, featuring cooling fins cast onto the compressor body to dissipate heat into the passing air stream.

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The Governor: Controlling the Pressure

The compressor runs continuously whenever the engine is operating. However, the air reservoirs have a finite capacity and must not be over-pressurized. The governor acts as the brain of the pressure control system, regulating when the compressor actively pumps air into the reservoirs.

Cut-In and Cut-Out Pressure Cycles

The governor monitors the pressure inside the air reservoirs (typically via a sensing line connected to the supply or wet tank). It operates between two critical pressure thresholds:

1. Cut-Out Pressure: When the air pressure in the reservoirs reaches a preset maximum limit—120 to 135 psi under ICBC rules—the governor cuts out. It directs pressurized air to the unloader mechanism (unloader pistons or valves) located in the compressor's cylinder head. This air pressure holds the compressor’s intake valves open. While the compressor continues to turn with the engine, it cannot compress air; instead, the air simply passes back and forth through the open intake valves. This is called the unloaded stage, which reduces engine drag and conserves fuel.
2. Cut-In Pressure: As the driver uses the brakes or air accessories, or as normal minor leakage occurs, the reservoir pressure drops. When the pressure falls to the minimum threshold—which must be at least 80 psi under ICBC specifications, but typically ranges from 100 to 115 psi—the governor cuts in. It vents the air pressure holding the unloader valves open, allowing the compressor’s intake valves to close and resume their normal function. The compressor enters the loaded stage, actively pumping air to recharge the reservoirs.

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The Compressor Build-Up Test

A critical part of the daily pre-trip air brake inspection is verifying that the compressor can charge the system efficiently. A worn compressor or a system with major air leaks will fail to build pressure quickly enough to keep up with driving demands.

Step-by-Step Build-Up Test Procedure

Under ICBC guidelines, the compressor's efficiency is evaluated using the compressor build-up test:

1. Prepare the Vehicle: Park on a level surface, chock the wheels, and ensure the spring parking brakes are applied.
2. Reduce Pressure: With the engine off, pump the foot valve (service brake pedal) to exhaust the reservoir pressure down to below 50 psi (typically around 40 psi).
3. Start the Engine: Start the engine and establish a fast idle speed of 1,000 to 1,200 rpm.
4. Time the Build-Up: Using a stopwatch or watch with a second hand, monitor the primary and secondary air pressure gauges on the dashboard. Start timing the instant the pressure needles pass 50 psi.
5. Stop the Timer: Stop timing the instant the pressure needles reach 90 psi.
6. Evaluate the Result: To pass the ICBC safety standard, the air pressure must rise from 50 to 90 psi within 3 minutes at fast idle. If the build-up takes longer than 3 minutes, the vehicle is unsafe to operate and must be serviced immediately.

Diagnosis of Build-Up Failures

If a vehicle fails the build-up test, the technician or operator must investigate several potential issues:

• Air Leaks: The most common cause is a severe leak in the supply line or reservoirs that allows air to escape as fast as the compressor pumps it.
• Worn Piston Rings: Worn rings in the compressor cylinder reduce its volumetric efficiency, preventing it from compressing air effectively and causing excessive oil carryover.
• Clogged Air Filter: A dirty compressor air intake filter restricts the volume of air entering the cylinders, slowing the compression rate.
• Faulty Governor: A misadjusted or malfunctioning governor may prematurely trigger the unloaders or leak air, keeping the compressor in a semi-unloaded state.