2.1 Air Compressors, Drive Systems, and Governor Operation
Key Takeaways
- The compressor turns whenever the engine runs; it stops pumping (unloads) when governor air holds its intake valves open, so no clutch is involved.
- Most compressors are driven directly from the engine's internal gearing; belt-driven add-on compressors need their fasteners, pulleys, and belt tension checked on the daily trip inspection.
- Alberta's check is governor cut-out at 120–135 PSI (828–931 kPa) and cut-in 20–25 PSI (138–172 kPa) below the cut-out pressure.
- Alberta's build-up test requires pressure to rise from 50 to 90 PSI (345–621 kPa) within 3 minutes with the engine at a fast idle of about 1200 RPM; most vehicles take 1 to 1.5 minutes.
- A slow air pressure build-up rate is a defect that must be recorded and reported; it is listed as a minor (not major) defect in the NSC trip inspection schedule.
Air Compressors, Drive Systems, and Governor Operation
Quick Answer: The air compressor is an engine-driven piston pump that turns whenever the engine runs. When pressure reaches the governor cut-out setting, Alberta's inspection standard is 120–135 PSI (828–931 kPa). At that point the governor sends air to unloaders that hold the compressor's intake valves open, so it stops compressing, and the air dryer purges. When pressure falls to the cut-in point, which should be 20–25 PSI (138–172 kPa) below cut-out, the governor exhausts that signal and pumping resumes. Alberta's build-up test requires pressure to rise from 50 to 90 PSI (345–621 kPa) within 3 minutes at a fast idle of about 1200 RPM.
The supply circuit's job
In the Alberta schematics the supply circuit is drawn in blue. It includes the compressor, the governor, the air dryer, the safety valve, and the supply (wet) tank. Its job is to take in atmospheric air, compress it, clean it, and deliver it to the service tanks at a controlled pressure. Every brake application ultimately depends on this circuit, so it is the first thing checked in the supply-circuit step of the trip air brake inspection.
How a piston compressor works
Air brake compressors are reciprocating piston pumps with one or more pistons. Each piston cycles through two strokes:
- Intake stroke. As the piston moves down, it draws air in past spring-loaded intake valves. Depending on the design, this air comes from the engine's air intake system or from the compressor's own filter.
- Compression stroke. As the piston moves up, the intake valves close, the air is compressed, and it is pushed out through the discharge valves into a heat-resistant discharge line leading to the air dryer and supply tank.
Compressing air makes it hot. Most compressors are lubricated by the engine's oil system, and many are cooled by engine coolant as well as by airflow over the housing. As piston rings and valves wear, more engine oil gets past the rings and into the discharge air. This is called oil carryover. The oil ends up in the dryer and tanks, so oily discharge from a drain valve points to a compressor or dryer problem that should be reported.
How compressors are driven, and what to check
| Drive arrangement | Where you see it | Trip inspection points |
|---|---|---|
| Gear-driven (most common) | The compressor is driven directly from the engine's internal gearing and mounted on the engine | Look for oil leaks and for loose mounting fasteners, lines, and the governor |
| Belt-driven add-on | Engines that were never designed to support an air system; the manufacturer adds a compressor on brackets driven by a belt | Shake the top of the compressor to check for loose fasteners and brackets, move the pulleys side to side to check for cracked hubs or loose spindle nuts, and pull the belt at the middle of its longest run to check its tension |
If the belt is loose or glazed, or the brackets are loose, the compressor can slip or stop pumping. The first sign is usually a slow build-up.
Loaded and unloaded: why the compressor never "shuts off"
The compressor is driven by the engine and has no clutch, so it turns whenever the engine turns. It is controlled pneumatically instead:
- Loaded (pumping). No governor signal is present, the intake valves seat normally, and every stroke pushes air toward the tanks.
- Unloaded (resting). At cut-out, the governor sends air pressure to unloader plungers in the compressor head. These hold the intake valves open, so on the up-stroke the air is simply pushed back out of the intake instead of being compressed. The compressor spins freely with little load until pressure falls to cut-in.
The governor
The governor controls when the compressor loads and unloads. It is usually mounted on the top or side of the compressor head but can be mounted remotely. On vehicles with an Air Dryer Integrated Solution (AD-IS) module, it is built into that module. The governor senses supply tank pressure through a small control line, about 1/4 inch (6–7 mm), that runs from the supply tank to the governor.
| Governor event | Alberta check | What happens |
|---|---|---|
| Cut-out | 120–135 PSI (828–931 kPa) | The governor sends air to the unloaders; the compressor stops compressing; the air dryer purges with a noticeable "sneeze" |
| Cut-in | 20–25 PSI (138–172 kPa) less than the cut-out pressure | The governor exhausts the unloader signal; the intake valves seat; the compressor starts pumping again |
Worked example: a governor that cuts out at 125 PSI should cut in between 100 and 105 PSI. If it cut out at 135 PSI, the acceptable cut-in window would be 110–115 PSI. The rule is always relative to the cut-out you actually observed.
If the governor fails to cut out, pressure keeps climbing until the safety valve opens (see Section 2.3). Hearing the safety valve pop off is a sign of a governor or unloader problem that must be reported.
The build-up test
Alberta's supply-circuit check includes a timed build-up:
- Start with pressure low. In the practical sequence, this follows the park control valve check (Section 8.3), so pressure is already low.
- Start the engine and run it at a fast idle of about 1200 RPM.
- Time the rise from 50 PSI to 90 PSI (345 to 621 kPa).
- The standard is within 3 minutes.
The course notes that 3 minutes is generous: most vehicles make the 50-to-90 climb in about a minute to a minute and a half. A vehicle that is close to the limit is telling you something. Common causes of slow build-up include:
- a slipping or loose belt (belt-driven units),
- a restricted compressor air intake,
- worn or carboned compressor valves, or unloaders that stick partly open,
- an air leak in the supply side (dryer, discharge line, supply tank, or drain valve).
In the National Safety Code trip inspection schedule (Schedule 1), a slow air pressure build-up rate is listed as a defect (a minor defect), not a major one. It must be recorded on the trip inspection report and reported to the carrier. A major failure, such as an air loss rate that exceeds the prescribed limit, puts the vehicle out of operation until repaired.
What stops an air brake compressor from compressing air once the governor reaches cut-out?
What does Alberta's compressor build-up test require?
During the supply circuit check, the governor cuts out at 130 PSI. Which cut-in pressure meets Alberta's standard?